matxsx.tex

\section{MATXSR}
\label{sMATXSR}
\index{MATXSR|textbf}

\hypertarget{sMATXSRhy}{The}
MATXS\index{MATXS format} material cross section format is
a generalized CCCC-type\index{CCCC format} interface format for
neutron, photon, and charged-particle data, including cross sections,
group-to-group matrices, temperature variations, self-shielding,
and time-dependence.  The CCCC standards are discussed in more
detail in the CCCCR chapter of this manual and in the
CCCC-III\index{CCCC format!CCCC-III} and CCCC-IV\index{CCCC format!CCCC-IV}
reports\cite{CCCC3,CCCC4}.  MATXS libraries
can be used with the TRANSX\index{TRANSX} code\cite{TRANSX,TRANSX2}
to produce effective cross sections for a wide variety of application
codes.

This chapter describes the MATXSR module in NJOY2016.0.

\subsection{Background}
\label{ssMATXSR_back}

Even the very best nuclear cross section processing code would be useless
if it were unable to deliver its products to users.  This is the role of
the interface file.  There have been interface files since the beginning
of calculational neutronics; examples include the DTF format (see the
\hyperlink{sDTFRhy}{DTFR} chapter of this manual) that was devised
for the early discrete-ordinates transport code DTF-IV\cite{DTF},
\index{DTF format} and the CCCC ISOTXS\index{ISOTXS}
format\cite{CCCC4} (see the \hyperlink{sCCCCRhy}{CCCCR}
section of this manual).  Both of these interface formats are still
in use today, but both of them have problems and show their age.  Some
of these problems result from the increase in the capabilities of
computer systems (capabilities that allow us to consider much more
complex problems), some arise from the many new kinds of nuclear systems
that are being studied today, and some come from 20/20 hindsight, which
makes it easy to see the design flaws in earlier formats.

Based on the problems seen with existing interface files, an
ideal interface file should be

\begin{description}
\begin{singlespace}
\item[extendable,]
   so that new cross section types, new incident or secondary particle
   types, or new energy ranges are easy to add without changing the
   basic format;

\item[comprehensive,]
   in order to be able to handle as many of the kinds of data produced
   by the processing code as possible (results should not be lost just
   because there are no places for them);

\item[generalizable,]
   to allow common methods to be used for similar kinds of data (for
   example, $nn$ matrices and $\gamma\gamma$ matrices) in order
   to transfer the experience gained in one field to another, and in
   order to simplify coding by allowing components to be reused;

\item[self-contained,]
   because it should not be necessary to provide additional information
   that is not in the file in order to use or interpret the file;

\item[compact,]
   because nuclear data often have many zeros or very small numbers in
   tables (for example, threshold reactions, scattering matrices),
   and these zeros must be removed effectively for economic storage
   and fast transfer of libraries; and

\item[efficient,]
   thus implying that binary mode should be used, that the records have
   a well-defined maximum size, and that there is a minimum number of
   records to reduce the number of I/O operations.
\end{singlespace}
\end{description}

Comparing the DTF format to these principles gives the following
results: it is fairly {\it extendable} because it has no fixed
particles, energy limits, or reaction types; it is not very {\it
comprehensive} because it can only transmit the total scattering
matrix; it is fairly {\it generalizable} because of the lack of fixed
types; it is not at all {\it self-contained} in that it requires
outside definitions like table length, position of the total, group
structure, and identity of edit cross sections; it is not very {\it
compact} because most zeros must be given explicitly in the tables; and
it is not very {\it efficient} because it uses coded card-image
records.

Similarly, studying the ISOTXS format gives the following results:  it
is not {\it extendable} because it works for neutrons only and allows
only very limited types of reactions to be included; it is not {\it
comprehensive} because it works for neutrons only and allows only very
limited types of reactions to be included; it is not {\it
generalizable} because of its specialization to fast-reactor problems
(as proof, note that the CCCC files for $\gamma$ cross sections use
completely different formats); it is reasonably {\it self-contained}
because all the parameters are internal, the group structure is given,
and all names needed for labeling an interpreted listing are well
determined; it is fairly {\it compact} because many zeros are removed
(but too many still remain); it is fairly {\it efficient} because it
uses binary mode, but record sizes are poorly predictable and very
nonuniform, thereby needlessly increasing the size of application codes
and the number of I/O operations.

\subsection{The MATXS Format}
\label{ssMSTXSR_format}

Following these general principles, the MATXS material cross section
file was designed to extend and generalize the existing interface
formats while still using the CCCC approach for efficiency and
familiarity (see the \hyperlink{sCCCCRhy}{CCCCR} section of this
report for more details).  The
first design principle was that all information would be identified
using lists of Hollerith names.  As an example, if the list of reactions
included in the file contains entries such as \cword{nf}, \cword{ng},
and \cword{n2n}, it is trivial to add additional reactions such
as \cword{kerma} or \cword{dpa}.  This approach is much more extendable
than the fixed set of reaction flags used in ISOTXS.  The second design
principle was that the file would be designed to hold sets of vectors
and rectangular matrices and that the same format would be used
regardless of the contents of the vectors and matrices.  As a
consequence, once a code can handle $n{\rightarrow}n$, it can also
handle $\gamma{\rightarrow}\gamma$; once a code can handle
$n{\rightarrow}\gamma$, it can also handle $\gamma{\rightarrow}n$,
$n{\rightarrow}\beta$, or even $d{\rightarrow}p$.  This approach
is an example of generalization.  Each material is now divided into
data types identified by input and output particle.  As an example,
$n{\rightarrow}\gamma$ is a data type characterized by input particle
equals neutron, and output particle equals photon.  The matrices for
this data type contain cross sections for producing photons in photon
group $\gamma$ due to reactions of neutrons in group $n$.  The
vectors, if any, would contain photon production cross sections
versus neutron group.  The use of completely general data types helps
make the format comprehensive.

The names for materials are written in the forms \cword{u235},
\cword{fe56}, \cword{tinat}, or \cword{h2a}.  Note that ``\cword{nat}''
is used explicitly for elements; names like \cword{be} or \cword{c}
should be avoided. Suffixes ``\cword{a}'', ``\cword{b}'', or
``\cword{c}" are used to label different versions of a material in a
library.  In order to keep names to six characters, isomers should be
identified by incrementing the ``thousands'' digit in the atomic number
field; for example, \cword{nb193a} would be the second version of
the first isomer of $^{93}$Nb.  The standard names for MATXS particles
are given in Table~\ref{sparts}.
\index{MATXS!particle names}

\begin{table}[t]
\caption[Standard MATXSR particle names]{Standard Particle Names}
\begin{center}
\begin{tabular}{ll}
Name & Particle \\ \hline
\cword{n} & neutron \\
\cword{g} & gamma \\
\cword{p} & proton \\
\cword{d} & deuteron \\
\cword{t} & triton \\
\cword{h} & $^3$He nucleus \\
\cword{a} & alpha \\
\cword{b} & beta \\
\cword{r} & residual or recoil (heavier than $\alpha$) \\ \hline
\end{tabular}
\label{sparts}
\end{center}
\end{table}

The standard names for the data types (\cword{htype}) are mostly based
on these particle names; the use of the terms \cword{scat}, \cword{dk},
\cword{therm} are exceptions.  Table~\ref{stypes} illustrates the scheme used.
\index{MATXS!data type names}

\begin{table}[b]
\caption[Standard MATXSR data-type names]{Standard Data-Type Names}
\begin{center}
\begin{tabular}{ll}
Name & Data Type \\ \hline
\cword{nscat} &  neutron scattering \\
\cword{ng} & neutron-induced gamma production \\
\cword{np} & neutron-induced proton production \\
\cword{nr} & neutron-to-recoil matrix \\
\cword{gscat} & gamma scattering \\
\cword{pscat} & proton scattering \\
\cword{pn} & proton-induced neutron production \\
$\cdots$ & $\cdots$ \\
\cword{ntherm} & thermal scattering data \\
\cword{dkn} & delayed-neutron data \\
\cword{dkhg} & decay heat and gamma data \\
\cword{dkb} & decay beta data \\ \hline
\end{tabular}
\label{stypes}
\end{center}
\end{table}

Reactions names are constructed in MATXSR from the ENDF MT number, the
LR flag (if present), and the incident particle name.  Examples of the
standard names are given in Tables~\ref{sname3}--\ref{sname11}.
\index{MATXS!reaction names} Note that the first \cword{n}
is omitted from the last three reactions in Table~\ref{sname3}.  It
is implicit in the data-type name.  This convention saves space in
the name for possible breakup products (see Table~\ref{sname4}).
The first \cword{n} is also implicit in the reactions of Table~\ref{sname4}.
No multiplicity is used in the breakup product strings in order to avoid
possible confusions with the discrete-level number;  just count the
like letters to get the multiplicity.  The names used for the most
common neutron absorption reactions are given in Table~\ref{sname5},
and the names used for the fission reactions are given in
Table~\ref{sname6}.  MATXS libraries typically give the
total fission cross section and all the partial cross sections (when
available) in the vector blocks, but they do not give the total
fission matrix when the partial matrices are available.

\begin{table}[t]
\caption[MATXSR neutron emitting reaction names]{Simple Neutron-Emitting
Reactions}
\begin{center}
\begin{tabular}{lll}
Name & MT & Description \\ \hline
\cword{nelas} & 2 & neutron elastic scattering \\
\cword{nnonel} & 3 & neutron nonelastic (MT=1--MT=2) \\
\cword{ninel} & 4 & neutron inelastic sum (MT=51--91) \\
\cword{n2n}   & 16 & (n,2n) \\
\cword{n3n}   & 17 & (n,3n) \\
\cword{nna}   & 22 & (n,n$'\alpha$) \\
\cword{nnp}   & 28 & (n,n$'$p) \\
\cword{n01}   & 51 & (n,n$_1$) \\
\cword{n02}   & 52 & (n,n$_2$) \\
\cword{ncn}   & 91 & (n,n$'$) to continuum \\ \hline
\end{tabular}
\label{sname3}
\end{center}
\end{table}

\begin{table}[b]
\caption[MATXSR breadkup reaction (LR flag) names]{Breakup Reactions (LR flags)}
\begin{center}
\begin{tabular}{llll}
Name & MT & LR & Description \\ \hline
\cword{n07a} & 57 & 22 & (n,n$_7$)$\alpha$  \\
\cword{n51p} & 65 & 28 & (n,n$_{15}$)p  \\
\cword{n02aa} & 52 & 29 & (n,n$_2$)$2\alpha$ \\
\cword{ncnaaa} & 91 & 23 & (n,n$'$)$3\alpha$ \\
\cword{n06na} & 56 & 24 & (n,n$_5$)n$\alpha$ \\
\cword{n01ee} & 51 & 40 & (n,n$_1$)ee \\ \hline
\end{tabular}
\label{sname4}
\end{center}
\end{table}

\begin{table}[t]
\caption[MATXSR neutron absorption reaction names]{Neutron-Absorption Reactions}
\begin{center}
\begin{tabular}{lll}
Name & MT & Description \\ \hline
\cword{nabs} & 101 & total absorption \\
\cword{ng} & 102 & radiative capture \\
\cword{np} & 103 & (n,p) \\
\cword{na} & 107 & (n,$\alpha$) \\ \hline
\end{tabular}
\label{sname5}
\end{center}
\end{table}

\begin{table}[b]
\caption[MATXSR fission reaction names]{Fission Reactions}
\begin{center}
\begin{tabular}{lll}
Name & MT & Description \\ \hline
\cword{nftot} & 18 & total fission \\
\cword{nf} & 19 & (n,f) first-chance fission \\
\cword{nnf} & 20 & (n,n$'$f) second-chance fission\\
\cword{n2nf} & 21 & (n,n2f) third-chance fission\\
\cword{n3nf} & 38 & (n,n3f) fourth-chance fission\\
\cword{nudel} & 455 & delayed-neutron yield (MF=3) \\
\cword{chid} & 455 & delayed-neutron spectrum (MF=5) \\ \hline
\end{tabular}
\label{sname6}
\end{center}
\end{table}

Table~\ref{sname7} gives some additional reaction names, some of which
are special NJOY names.  As discussed in the
\hyperlink{sGROUPRhy}{GROUPR}\index{GROUPR}
chapter of this report, the total cross section can be averaged with
the $\ell$th order of the flux to obtain the multigroup total
cross section components $\sigma_{t\ell,g}$.  These total cross
section components and the corresponding Legendre fluxes are given
names like the first four shown in Table~\ref{sname7}.  Related
names with first letters \cword{g}, \cword{p}, \cword{d}, {\it etc.},
will also be found in MATXS libraries. The average inverse velocities
are defined to preserve the time term of the time-dependent
Boltzmann equation:

\begin{equation}
  \Big< \frac{1}{v} \Big> = \frac{\displaystyle\int_g
     (1/v)\phi(E)\,dE}{\displaystyle\int_g \phi(E)\,dE} \,\,.
\end{equation}

\begin{table}[t]
\caption[Special MATXSR NJOY names]{Special NJOY Names}
\begin{center}
\begin{tabular}{lll}
Name & MT & Description \\ \hline
\cword{ntot0} & 1 & P$_0$ total cross section \\
\cword{ntot1} & 1 & P$_1$ total cross section \\
\cword{nwt0} & 1 & P$_0$ weight function (flux) \\
\cword{nwt1} & 1 & P$_1$ weight function (flux) \\
\cword{mubar} & 251 & scattering $\bar{\mu}$ \\
\cword{xi} & 252 & scattering $\xi$ \\
\cword{invel} & 259 & inverse velocity (sec/m) \\
\cword{heat} & 301 & energy-balance heat production \\
\cword{kerma} & 443 & kinematic KERMA factor \\
\cword{dame} & 444 & damage-energy production \\ \hline
\end{tabular}
\label{sname7}
\end{center}
\end{table}

\begin{table}[b]
\caption[MATXSR gas production names]{Gas-Production Reactions}
\begin{center}
\begin{tabular}{lll}
Name & MT & Description \\ \hline
\cword{n.neut} & 201 & total neutron production \\
\cword{n.gam} & 202 & total $\gamma$ production \\
\cword{n.h1} & 203 & hydrogen production \\
\cword{n.h3} & 205 & tritium production \\
\cword{n.HE4} & 207 & helium production \\ \hline
\end{tabular}
\label{sname8}
\end{center}
\end{table}

\noindent
The meaning of the terms energy-balance heat production and kinematic
KERMA\index{KERMA} factor are discussed in more detail in the
\hyperlink{sHEATRhy}{HEATR}\index{HEATR} chapter of this
manual.  Briefly, the
energy-balance heating (\cword{mt}=301) is computed by subtracting the
energy carried away by neutrons and photons from the
energy available for a reaction.  The result should be the energy
deposited by charged particles and the recoil nucleus, that is, the
local heating.  Unfortunately, problems with the energy-balance
consistency of evaluations, the difficulty of determining the
available energy for elements, and the inaccuracy in the difference
between relatively large numbers sometimes cause this value to have
unphysical values (for example, negative heating).  These values do
have the property of always conserving energy for large systems.  The
kinematic value (\cword{mt}=443) is computed from reaction kinematics alone.
It is very accurate at low energies, but when three or more particles
are involved in the reaction, it begins to fail.  The results in
\cword{kerma} are always larger than the correct heating value.
Comparing the two estimates for the local heating can reveal problems
in the evaluations\cite{ebal,KAOS5}.  The MATXS user is free to
choose whichever number is more appropriate for the problem. The
reaction \cword{dame} is also generated using data from
\hyperlink{sHEATRhy}{HEATR}.  As discussed in the
\hyperlink{sHEATRhy}{HEATR} section of this report, this ``damage-energy
production'' cross section can be used to obtain the DPA\index{DP}
(displacements per atom) parameter used in radiation damage studies.

The gas-production reaction names given in Table~\ref{sname8} can
also appear with other particle names before the decimal point.  The
names for reactions induced by incident charged particles follow the
neutron names in most cases, except that the first letter is changed to
indicate the incident particle type.  Discrete-level scattering
reactions are exceptions; \cword{n01} is used for both (n,n$_1$)
and (p,n$_1$).  Also note the \cword{n00} cannot be used for
incident neutrons; the name \cword{nelas} is used instead.  Similarly,
\cword{p00} is not used for incident protons.

As discussed in more detail below, scattering from thermal moderators
is treated like {\it materials} in ENDF/B libraries, but it is treated
like {\it reactions} on the GENDF files.  The free-gas scattering
reaction can appear in any material, but the other thermal MT numbers
can only appear in the material corresponding to the dominant
scattering isotope.  For example, \cword{hh2o} only appears in $^{1}$H.
There are two versions of \cword{zrhyd}; one appears in $^{1}$H and the
other in Zr.   The coherent and incoherent terms in the thermal
cross section are kept separate for the convenience of applications;
all the coherent names  end with \$.  Note that MATXS files contain
two different representations for the scattering cross sections at
low energies:

\begin{description}
\begin{singlespace}
\item[static,] where the cross section and group-to-group matrix are
  obtained from \cword{nscat}, which is derived from \cword{mt}=2 on the
  ENDF evaluation. (This is scattering for ``static'' nuclei;
  energy loss from recoil is included.); and

\item[thermal,] where the cross section and group-to-group matrix are
  obtained from one of the thermal reactions in the \cword{ntherm}
  data type. (The scattering nuclei are in motion with a distribution
  described by the Maxwell-Boltzmann law; both energy loss and energy
  gain events are possible.)
\end{singlespace}
\end{description}

\noindent
The TRANSX code gives the user the choice of static or thermal
scattering, and it also allows the user to choose which binding
state is desired for a particular moderator material.

\begin{table}[t]\small
\caption[MATXSR incident proton reaction names]{Incident-Proton Reactions}
\begin{center}
\begin{tabular}{lcl}
Name & MT & Description \\ \hline
\cword{pelas} & 2 & proton elastic scattering \\
\cword{p01} & 601 & discrete-level (p,p$_1$) scattering \\
\cword{n00} & 50 & discrete-level (p,n$_0$)  \\
\cword{n01} & 51 & discrete-level (p,n$_1$) \\
\cword{p2n}   & 16 & (p,2n) \\
\cword{pg}   & 102 & (p,$\gamma$) \\
\cword{pt}   & 104 & (p,t) \\ \hline
\end{tabular}
\label{sname9}
\end{center}
\end{table}

\index{MATXS!thermal cross sections}
\begin{table}[thb]\small
\caption[MATXSR thermal material names (ENDF/B-VII)]{Thermal Material Names
for ENDF/B-VII}
\begin{center}
\begin{tabular}{lll}
Name & MT & Description \\ \hline
\cword{free} & 221 & free-gas scattering \\
 \cword{hh2o} & 222 & H in H$_2$O \\
\cword{poly} & 223 &  H in polyethylene (CH$_2$) incoherent \\
 \cword{poly\textdollar} & 224 & H in polyethylene (CH$_2$) coherent \\
\cword{hzrh} & 225 & H in ZrH incoherent \\
 \cword{hzrh\textdollar} & 226 & H in ZrH coherent \\
\cword{benz} & 227 & Benzene incoherent \\
 \cword{dd2o} & 228 & D in D$_2$O \\
\cword{graph} & 229 & C in graphite incoherent \\
 \cword{graph\textdollar} & 230 & C in graphite coherent \\
\cword{be} & 231 & Be metal incoherent \\
 \cword{be\textdollar} & 232 & Be metal coherent \\
\cword{bebeo} & 233 & Be in BeO incoherent\\
 \cword{bebeo\textdollar} & 234 & Be in BeO coherent \\
\cword{zrzrh} & 235 & Zr in ZrH incoherent \\
 \cword{zrzrh\textdollar} & 236 & Zr in ZrH coherent \\
\cword{obeo} & 237 & O in BeO incoherent \\
 \cword{obeo\textdollar} & 238 & O in BeO coherent \\
\cword{ouo2} & 239 & O in UO$_2$ incoherent \\
 \cword{ouo2\textdollar} & 240 & O in UO$_2$ coherent \\
\cword{uuo2} & 241 & U in UO$_2$ incoherent \\
 \cword{uuo2\textdollar} & 242 & U in UO$_2$ coherent \\
\cword{al} & 243 & Al metal incoherent \\
\cword{al} & 244 & Al metal coherent \\
\cword{fe} & 245 & Fe metal incoherent \\
\cword{fe} & 246 & Fe metal coherent \\ \hline
\end{tabular}
\label{sname10}
\end{center}
\end{table}

The ENDF representation of photoatomic reactions was described in the
\hyperlink{sGAMINRhy}{GAMINR}\index{GAMINR} chapter.  The
\cword{gheat} reaction, constructed in
\hyperlink{sGAMINRhy}{GAMINR}, represents the local heating from atomic
recoil and photo-electric electron production.  Fluorescence
photons from photoelectric interactions are assumed to deposit their
energy locally.
\index{photoatomic}

\index{MATXS!photoatomic cross sections}
\begin{table}[t]\small
\caption[MATXSR photoatomic cross section names]{Photoatomic Cross Sections}
\begin{center}
\begin{tabular}{lll}
Name & MT & Description \\ \hline
\cword{gtot0} & 501 & P$_0$ total \\
\cword{gwt0} & 501 & P$_0$ weight function (flux) \\
\cword{gcoh} & 502 & coherent scattering \\
\cword{ginch} & 504 & incoherent scattering \\
\cword{gpair} & 516 & pair production $(\gamma,2\gamma)$ \\
\cword{gabs} & 522 & photoelectric absorption \\
\cword{gheat} & 525 & heating \\ \hline
\end{tabular}
\label{sname11}
\end{center}
\end{table}

\hyperlink{sGROUPRhy}{GROUPR} and MATXSR
are capable of supporting a new experimental
capability for generating nuclide production cross sections.  This
capability is most useful for radionuclides and isomers, but it
is general enough to handle all the possible heavy products of a
nuclear reaction.  The input GENDF file may contain several different
sections that produce a given nuclide.  MATXSR
adds them up into a single named reaction.  The naming convention
used for capture reactions is \cword{cZZAAA}, where Z and A are
the charge and mass numbers for the nuclide.  Isomers are
handled by incrementing the first postion of the ``AAA''
field.  Products of other reactions
are named using the pattern \cword{rZZAAA}, with the same convention
used for isomers.  The reason that capture products are distinguished
from those from other reactions is that the former may have to be
self shielded.

The CCCC standards have always used 6-character Hollerith strings
for names.  These kinds of names are represented as ``\cword{REAL*8}''
double precision variables on 32-bit machines (IBM, VAX, Sun, {\it etc.})
and as single-precision variables on 60- to 64-bit machines (CDC, Cray).
However, a double-precision variable on a short-word machine can
hold 8 characters.  So can single-precision variables on CDC and
Cray machines.  There do not seem to be any computer systems
currently in use that require 6-character words.  Therefore, the
latest versions of the MATXS format and the MATXSR module
have been written to handle 8-character names.

The formal format specification for the MATXS material cross
section file follows, using the standard CCCC presentation
(except for the ! before the c):
\index{MATXS format}

\small
\begin{ccode}

!          Standardized CCCC format listing for MATXS file
!c
!c**********************************************************************
!c               proposed 09/09/77
!c                       (modified 09/80)
!c                       (nomenclature changed 06/88)
!c                       (modified for const sub-blocks 06/90)
!c                       (ordering changed 10/90)
!c     c                       (bcd format changed 12/21/91)
!c
!cf           matxs
!ce           material cross section file
!c
!cn                       this file contains cross section
!cn                       vectors and matrices for all
!cn                       particles, materials, and reactions;
!cn                       delayed neutron spectra by time group;
!cn                       and decay heat and photon spectra.
!c
!cn           formats given are for file exchange only
!c
!c**********************************************************************
!c
!c
!c----------------------------------------------------------------------
!cs          file structure
!cs
!cs              record type                       present if
!cs              ==============================    ===============
!cs              file identification                 always
!cs              file control                        always
!cs              set hollerith identification        always
!cs              file data                           always
!cs
!cs   *************(repeat for all particles)
!cs   *          group structures                    always
!cs   *************
!cs
!cs   *************(repeat for all materials)
!cs   *          material control                    always
!cs   *
!cs   * ***********(repeat for all submaterials)
!cs   * *        vector control                      n1db.gt.0
!cs   * *
!cs   * * *********(repeat for all vector blocks)
!cs   * * *      vector block                        n1db.gt.0
!cs   * * *********
!cs   * *
!cs   * * *********(repeat for all matrix blocks)
!cs   * * *      matrix control                      n2d.gt.0
!cs   * * *
!cs   * * * *******(repeat for all sub-blocks)
!cs   * * * *    matrix sub-block                    n2d.gt.0
!cs   * * * *******
!cs   * * *
!cs   * * *      constant sub-block                  jconst.gt.0
!cs   * * *
!cs   *************
!c
!c----------------------------------------------------------------------
!c
!c
!c----------------------------------------------------------------------
!cr           file identification
!c
!cl    hname,(huse(i),i=1,2),ivers
!c
!cw    1+3*mult
!c
!cb    format(4h 0v ,a8,1h*,2a8,1h*,i6)
!c
!cd    hname         hollerith file name  - matxs -  (a8)
!cd    huse          hollerith user identifiation    (a8)
!cd    ivers         file version number
!cd    mult          double precision parameter
!cd                       1- a8 word is single word
!cd                       2- a8 word is double precision word
!c
!c----------------------------------------------------------------------
!c
!c
!c----------------------------------------------------------------------
!cr           file control
!c
!cl    npart,ntype,nholl,nmat,maxw,length
!c
!cw    6
!c
!cb    format(6h 1d   ,6i6)
!c
!cd    npart       number of particles for which group
!cd                   structures are given
!cd    ntype       number of data types present in set
!cd    nholl       number of words in set hollerith
!cd                    identification record
!cd    nmat        number of materials on file
!cd    maxw        maximum record size for sub-blocking
!cd    length      length of file
!c
!c----------------------------------------------------------------------
!c
!c
!c----------------------------------------------------------------------
!cr           set hollerith identification
!c
!cl    (hsetid(i),i=1,nholl)
!c
!cw    nholl*mult
!c
!cb    format(4h 2d /(9a8))
!c
!cd    hsetid      hollerith identification of set (a8)
!cd                 (to be edited out 72 characters per line)
!c
!c----------------------------------------------------------------------
!c
!c
!c----------------------------------------------------------------------
!cr          file data
!c
!cl    (hprt(j),j=1,npart),(htype(k),k=1,ntype),(hmatn(i),i=1,nmat),
!cl   1(ngrp(j),j=1,npart),(jinp(k),k=1,ntype,(joutp(k),k=1,ntype),
!cl   2(nsubm(i)i=1,nmat),(locm(i),i=1,nmat)
!c
!cw    (npart+ntype+nmat)*mult+2*ntype+npart+2*nmat
!c
!cb    format(4h 3d ,4x,8a8/(9a8))    hprt,htype,hmatn
!cb    format(12i6)                  ngrp,jinp,joutp,nsubm,locm
!c
!cd    hprt(j)     hollerith identification for particle j
!cd                     n         neutron
!cd                     g         gamma
!cd                     p         proton
!cd                     d         deuteron
!cd                     t         triton
!cd                     h         he-3 nucleus
!cd                     a         alpha (he-4 nucleus)
!cd                     b         beta
!cd                     r         residual or recoil
!cd                               (heavier than alpha)
!cd    htype(k)     hollerith identification for data type k
!cd                     nscat     neutron scattering
!cd                     ng        neutron induced gamma production
!cd                     gscat     gamma scattering
!cd                     pn        proton induced neutron production
!cd                       .          .
!cd                       .          .
!cd                       .          .
!cd                     dkn       delayed neutron data
!cd                     dkhg      decay heat and gamma data
!cd                     dkb       decay beta data
!cd    hmatn(i)    hollerith identification for material i
!cd    ngrp(j)      number of energy groups for particle j
!cd    jinp(k)     type of incident particle associated with
!cd                   data type k.  for dk data types, jinp is 0.
!cd    joutp(k)    type of outgoing particle associated with
!cd                   data type k
!cd    nsubm(i)    number of submaterials for material i
!cd    locm(i)     location of material i
!c
!c----------------------------------------------------------------------
!c
!c
!c----------------------------------------------------------------------
!cr          group structure
!c
!cl    (gpb(i),i=1,ngr),emin
!c
!cc    ngr=ngrp(j)
!c
!cw    ngrp(j)+1
!c
!cb    format(4h 4d ,8x,1p,5e12.5/(6e12.5))
!c
!cd    gpb(i)      maximum energy bound for group i for particle j
!cd    emin        minimum energy bound for particle j
!c
!c----------------------------------------------------------------------
!c
!c
!c----------------------------------------------------------------------
!cr          material control
!c
!cl    hmat,amass,(temp(i),sigz(i),itype(i),n1d(i),n2d(i),
!cl   1locs(i),i=1,nsubm)
!c
!cw    mult+1+6*nsubm
!c
!cb    format(4h 5d ,a8,1p,2e12.5/(2e12.5,5i6))
!c
!cd    hmat        hollerith material identifier
!cd    amass       atomic weight ratio
!cd    temp        ambient temperature or other parameters for
!cd                    submaterial i
!cd    sigz        dilution factor or other parameters for
!cd                    submaterial i
!cd    itype       data type for submaterial i
!cd    n1d         number of vectors for submaterial i
!cd    n2d         number of matrix blocks for submaterial i
!cd    locs        location of submaterial i
!c
!c----------------------------------------------------------------------
!c
!c
!c----------------------------------------------------------------------
!cr          vector control
!c
!cl    (hvps(i),i=1,n1d),(nfg(i),i=1,n1d),(nlg(i),i=1,n1d)
!c
!cw    (mult+2)*n1d
!c
!cb    format(4h 6d ,4x,8a8/(9a8))      hvps
!cb    format(12i6)                    iblk,nfg,nlg
!c
!cd    hvps(i)     hollerith identifier of vector
!cd                      nelas     neutron elastic scattering
!cd                      n2n       (n,2n)
!cd                      nnf       second chance fission
!cd                      gabs      gamma absorption
!cd                      p2n       protons in, 2 neutrons out
!cd                         .          .
!cd                         .          .
!cd                         .          .
!cd    nfg(i)      number of first group in band for vector i
!cd    nlg(i)      number of last group in band for vector i
!c
!c----------------------------------------------------------------------
!c
!c
!c----------------------------------------------------------------------
!cr          vector block
!c
!cl    (vps(i),i=1,kmax)
!c
!cc    kmax=sum over group band for each vector in block j
!c
!cw    kmax
!c
!cb    format(4h 7d ,8x,1p,5e12.5/(6e12.5))
!c
!cd    vps(i)      data for group bands for vectors in block j.
!cd                block size is determined by taking all the group
!cd                bands that have a total length less than or equal
!cd                to maxw.
!c
!c----------------------------------------------------------------------
!c
!c
!c----------------------------------------------------------------------
!cr        scattering matrix control
!c
!cl    hmtx,lord,jconst,
!cl   1(jband(l),l=1,noutg(k)),(ijj(l),l=1,noutg(k))
!c
!cw    mult+2+2*noutg(k)
!c
!cb    format(4h 8d ,4x,a8/(12i6))      hmtx,lord,jconst,
!cb                                     jband,ijj
!c
!cd    hmtx        hollerith identification of block
!cd    lord        number of orders present
!cd    jconst      number of groups with constant spectrum
!cd    jband(l)    bandwidth for group l
!cd    ijj(l)      lowest group in band for group l
!c
!c----------------------------------------------------------------------
!c
!c
!c----------------------------------------------------------------------
!cr          scattering sub-block
!c
!cl    (scat(k),k=1,kmax)
!c
!cc    kmax=lord times the sum over all jband in the group range of
!cc            this sub-block
!c
!cb    format(4h 9d ,8x,1p,5e12.5/(6e12.5))
!c
!cw    kmax
!c
!cd    scat(k)     matrix data given as bands of elements for initial
!cd                groups that lead to each final group.  the order
!cd                of the elements is as follows:  band for p0 of
!cd                group i, band for p1 of group i, ... , band for p0
!cd                of group i+1, band for p1 of group i+1, etc.  the
!cd                groups in each band are given in descending order.
!cd                the size of each sub-block is determined by the
!cd                total length of a group of bands that is less than
!cd                or equal to maxw.
!cd
!cd                if jconst.gt.0, the contributions from the jconst
!cd                low-energy groups are given separately.
!c
!c----------------------------------------------------------------------
!c
!c
!c
!c----------------------------------------------------------------------
!cr          constant sub-block
!c
!cl    (spec(l),l=1,noutg(k)),(prod(l),l=l1,ning(k))
!c
!cc    l1=ning(k)-jconst+1
!c
!cw    noutg(k)+jconst
!c
!cb    format(4h10d ,8x,1p,5e12.5/(6e12.5))
!c
!cd    spec        normalized spectrum of final particles for initial
!cd                particles in groups l1 to ning(k)
!cd    prod        production cross section (e.g., nu*sigf) for
!cd                initial groups l1 through ning(k)
!cd
!cd         this option is normally used for the energy-independent
!cd         neutron and photon spectra from fission and radiative
!cd         capture usually seen at low energies.
!c
!c----------------------------------------------------------------------

\end{ccode}
\normalsize
\vspace{6 pt}

The MATXS format is intended to communicate multigroup cross sections
and matrices for all reaction types, incident particles, and outgoing
particles from a nuclear data processing code to applications.  It
also includes temperature and self-shielding effects, delayed-neutron
data, and a limited format for decay heat and delayed photon or particle
emission.  As shown in the ``File Structure'' presentation above, the
main loop is over material.  Materials are subdivided into submaterials,
which usually correspond to different data types, temperatures, and
background cross section ($\sigma_0$) values.  Each submaterial can
contain a series of ``Vector Blocks'' giving cross section versus energy
for one of the allowed group structures and incident particles, and it
can contain a series of matrix blocks and subblocks giving the cross
sections for group-to-group transfers.

The ``File Identification'' record is the same for all CCCC files.  It
gives the Hollerith name for the file (which is always \cword{matxs}),
a version number \cword{ivers}, which can be used to distinguish
between different libraries in this format, and a Hollerith
identification string \cword{huse}, which can be used for entries
like ``\cword{T2 LANL NJOY}''.

The ``File Control'' record contains parameters that are needed to
compute the lengths of the following records.  The meaning of the
various names is well explained in the format specification, and the
values of the parameters are obtained from the user's input.
The purpose of \cword{maxw} is to tell application codes how much
memory they will need to read through the records on these MATXS files.
It is used for both vector blocks and matrix subblocks when deciding
how to break them up.  The MATXSR value is 5000 words.  The code tries
to make as many records as possible that have nearly this size in order
to minimize the number of I/O operations.  The parameter \cword{length} is
used to help find the end of the file when appending a new material
to an existing file.  Its units are left unspecified in the format.
It is usually the length in records, in which case record skipping
can be used to find the end.  Or it could be the length in words on
computer systems that allow direct word-addressed I/O operations
(this used to be possible using CTSS on Cray computers).

The ``Set Hollerith Identification'' record comes next.  It contains
an arbitrary amount of Hollerith text to describe the contents of the
library.  The description comes from the user's input.

The ``File Data'' record contains a number of important arrays that
define the structure of data types and the location of materials.  The
parameter \cword{hprt} contains the standard names for the \cword{npart}
particles.  The standard names for particles were discussed above.  The
standard names for the data types and materials (\cword{htype} and
\cword{hmatn}) were also discussed above.  The next three parameters
are used to complete the specification of the data types included in
this MATXS library.  \cword{ngrp} just gives the number of groups used
for each particle type; for example, the traditional Los Alamos
30$\times$12 library (with 30 neutron groups and 12 photon groups)
would have \cword{ngrp(1)}=30 and \cword{ngrp(2)}=12.  Using the same
example, the \cword{nscat} data type would have \cword{jinp(1)}=1
and \cword{joutp(1)}=1; the \cword{ng} data type would have
\cword{jinp(2)}=1 and \cword{joutp(2)}=2; and the \cword{gscat} data
type would have \cword{jinp(3)}=2 and \cword{joutp(3)}=2.  The
information for these 6 arrays is given in the user's input.

The final two parameters in the ``File Data'' record are
\cword{nsubm} and \cword{locm}.  The value of \cword{nsubm} depends
on the number of data types, the number of temperatures, and
the number of background cross sections found on the input
GENDF tapes.  The value for \cword{locm(i)} is usually the record
index for material \cword{i}.  A code can then jump directly to a desired
material using record skipping (forward or backward).  However, the
units for \cword{locm} have been left unspecified to allow direct
random access for systems that use word-addressable random-access
I/O operations.

The ``Group Structure'' records give the energy bounds for
\cword{npart} group structures.  Following the normal convention for
application codes, the energy bounds are given in the order of
decreasing energy.  The numbers are obtained from \cword{mf}=1,
\cword{mt}=451 on the GENDF tape.  The \hyperlink{sGROUPRhy}{GROUPR}
module currently uses one group structure for
all particles (n, p, $\alpha$, {\it etc.}), and another for photons ($\gamma$).

Inside the material loop, there is a ``Material Control'' record for
each material.  The choice of names for \cword{hmat} was discussed above.
The \cword{amass} parameter is the same as the ENDF AWR parameter;
that is, it is the ratio of the target mass to the neutron mass.
Temperatures \cword{temp} are given in degrees Kelvin, and background
cross sections for self-shielding codes (\cword{sigz}, or $\sigma_0$)
are given in barns. The parameter \cword{itype} tells which data-type
each submaterial belongs to using the data type codes defined by
\cword{htype}.  Although it is not specified in the format description,
the order that MATXSR loops through
submaterials is as follows:
the outer loop is over data type, the next loop is over temperature,
and the innermost loop is over background cross section.  The number of
cross section vector reactions and matrix reactions for each submaterial
are given in \cword{n1d} and \cword{n2d}.  Finally, \cword{locs(i)}
normally gives the record index for submaterial \cword{i}.  A code can
search the arrays \cword{temp}, \cword{sigz}, and \cword{itype} for a
desired submaterial, and then jump right to the desired submaterial
using record skipping.  Alternatively, a version that uses word
addresses in \cword{locs} could use random-access methods to jump to
the desired submaterial.

Each submaterial that contains vectors (\cword{n1d}$>$0) starts with a
``Vector Control'' record.  This record gives a list of reaction names
in \cword{hvps}.  MATXSR constructs these
reaction names automatically
based on the MT number, ENDF ``LR flag'' (if any), and the incident
particle type.  Examples of these names were given above.  The
parameters \cword{nfg} and \cword{nlg} are used to remove unnecessary
leading or trailing zeros in reaction cross section vectors. The zeroes
are usually due to thresholds.  For example, an (n,2n) reaction might only
have nonzero cross sections for groups 1 through 5 out of an 80-group
structure.  Storing only the 5-element band will save 75 words on the
MATXS file.  Even with the zeros removed, the number of words of vector
cross section data for a submaterial can be quite large.  Therefore,
the MATXS format provides a way to break the vector data into a number
of vector blocks.  The idea is to sum up the bandwidths for each
reaction (that is, \cword{nlg(i)}-\cword{nfg(i)}+1) in order to find the
largest number of reactions that will fit within a block of length
less than or equal to \cword{maxw}.  The data for this block are
written to the output file, and then the next group of reactions is
found.  This continues until all the vector data have been written out
as a series of ``Vector Block'' records with lengths less than
\cword{maxw} (5000 words in MATXSR).  This method minimizes
the number of records on the file while allowing the application codes
that read MATXS libraries to allocate space for reading the records
economically.

Most submaterials will contain \cword{n2b} ``Scattering Matrix Control''
records.  The convention for the reaction names used in \cword{hmtx}
were discussed above.  \cword{lord} gives the {\it number} of
Legendre orders present for this reaction; that is, \cword{lord}=4
for a P$_3$ matrix.  Matrices are compacted for efficient storage
and data transfer using two techniques.  First, unnecessary leading
and trailing zeros for group-to-group transfers into a particular
final group are removed by banding.  \cword{jband(i)} gives the
number of incident groups in the band for final group \cword{i},
and \cword{ijj(i)} gives the group index for the lowest-energy group
(highest group index) in the band for group \cword{i}.  For example,
consider an isotropic (n,2n) reaction with a threshold in group 3 of a
30-group structure.  The values for \cword{jband} and \cword{ijj}
for group 20 might well be 3 and 3, respectively.  That is, groups
1 through 3 scatter into group 20.  The order of storage for this
matrix would be as follows:

\begin{center}
\begin{tabular}{cccc}
Band & Element & \cword{jband} & \cword{ijj} \\ \hline
  1  &  1$\rightarrow$1 & 1 & 1 \\
  2  &  2$\rightarrow$2 & 2 & 2 \\
     &  1$\rightarrow$2 &   &   \\
  3  &  3$\rightarrow$3 & 3 & 3 \\
     &  2$\rightarrow$3 &   &   \\
     &  1$\rightarrow$3 &   &   \\
  4  &  3$\rightarrow$4 & 3 & 3 \\
     &  2$\rightarrow$4 &   &   \\
     &  1$\rightarrow$4 &   &   \\
 $\cdots$ & $\cdots$ & & \\ \hline
\end{tabular}
\end{center}

Note that this scheme is more efficient than the similar one used for the
CCCC ISOTXS file, because that method required that the ingroup element
had to be included in the band.  For anisotropic matrices, the
\cword{lord} Legendre components are stored with the source-group
elements.  Thus, there are \cword{lord(i)}*\cword{jband(i)} elements
in each band,
and they are stored in the following order (assuming a P$_1$ matrix):

\begin{center}
\begin{tabular}{ccccc}
Band & Element & Order & \cword{jband} & \cword{ijj} \\ \hline
  1  &  1$\rightarrow$1 & 0 & 1 & 1 \\
     &  1$\rightarrow$1 & 1 &   &   \\
  2  &  2$\rightarrow$2 & 0 & 2 & 2 \\
     &  1$\rightarrow$2 & 0 &   &   \\
     &  2$\rightarrow$2 & 1 &   &   \\
     &  1$\rightarrow$2 & 1 &   &   \\
  3  &  3$\rightarrow$3 & 0 & 3 & 3 \\
     &  2$\rightarrow$3 & 0 &   &   \\
     &  1$\rightarrow$3 & 0 &   &   \\
     &  3$\rightarrow$3 & 1 &   &   \\
 $\cdots$ & $\cdots$ & & & \\ \hline
\end{tabular}
\end{center}

This kind of matrix block can be subdivided into subblocks using a method
similar to the one described above for vector cross sections.  The code
starts summing the product of \cword{jband} and \cword{lord} for the
final energy groups until the data for the next band will cause the sum
to exceed \cword{maxw} (5000 words in MATXSR).  These data are
then written out as a matrix subblock.  The code then repeats the process
for the rest of the group ranges.  The result is a minimal number of
``Scattering Sub-Blocks,'' none of which has a size larger than the
file limit.

The second method used to compact scattering matrices is based on the
observation that the shape of the outgoing neutron or photon spectrum
from fission and radiative capture reactions tends to be independent of
energy at low neutron energies. \hyperlink{sGROUPRhy}{GROUPR}
determines the group where
significant energy dependence begins.  Below this point, it computes a
single spectrum to describe the outgoing neutron or photon distribution
and a production cross section to go with it.  At high energies, it
produces a group-to-group matrix.  This method can lead to appreciable
reductions in storage requirements.  For example, consider the 187-group
structure, which has many low-energy groups.  The fission spectrum
for $^{235}$U doesn't begin to show significant energy dependence until
an energy of about 9 keV.  This means that there are  118 constant
groups.  A 187$\times$187 matrix is thereby reduced to a 187$\times$69
matrix, and 187-group vector, and a 118-group vector --- a 62 \% reduction in
storage requirements.  Even larger reductions in size are obtained
in more favorable cases.  The parameter \cword{jconst} gives the number
of low-energy groups having a constant spectrum.  If \cword{jconst}$>$0,
a single ``Constant Sub-Block'' record will be given after the regular
``Scattering Sub-Block'' records.  This record will contain the spectrum
\cword{spec} and the production cross section \cword{prod} needed to
reconstruct the low-energy part of the full matrix.  In mathematical form,

\begin{equation}
   \sigma_{x,g{\rightarrow}g'} = \chi^{LE}_{g'}
      \,\sigma^{LE}_{Px,g} + \sigma^{HE}_{x,g{\rightarrow}g'} \,\,,
\end{equation}
\vspace{1 pt}

\noindent
where LE stands for low energy, HE stands for high energy, $\chi^{LE}$ is
the constant spectrum, $\sigma^{LE}_{Px}$ is the production cross section
for reaction $x$, and $\sigma^{HE}$ is the normal group-to-group matrix.

\subsection{Historical Notes}
\label{ssMATXSR_history}

The original version of the MATXS specification was constructed in
September, 1977.  The ``Matrix Control'' record in this original
version contained the names and banding parameters for every group of
every reaction.  As a result, the record could become very large for
libraries with many groups.  The MATXS format was modified in September
of 1980 to have a different ``Matrix Control'' record for each reaction.
For several years following this date, NJOY contained both MATXSR and
NMATXS modules, and two different versions of TRANSX were in use.  All
traces of the original version of the format have now disappeared.

Changes beginning in the late 1980's were introduced to make MATXSR
able to handle data types with either incoming or outgoing charged
particles.  Actually, the format wasn't changed.  The particle,
data type, and reaction name conventions described in
Tables~\ref{sparts} -- \ref{sname11} were chosen, and these new names
required some corresponding changes in the code.  The TRANSX code
also had to be modified to recognize the new names and to work with
multiparticle coupled sets.

The concept of using constant subblocks to reduce the size of GENDF and
MATXS files is actually quite old.  Various versions of updates to
install the scheme were written over about a 5-year period.  It was
finally decided to permanently install the scheme in June of 1990.
The changes required to \hyperlink{sGROUPRhy}{GROUPR}
were the easier part of the job; corresponding changes were required
in \hyperlink{sDTFRhy}{DTFR}, \hyperlink{sCCCCRhy}{CCCCR},
\hyperlink{sMATXSRhy}{MATXSR}, \hyperlink{sPOWRhy}{POWR},
and \hyperlink{sWIMSRhy}{WIMSR}.  In addition, TRANSX had
to be changed to accept the new
constant subblocks.  Unfortunately, this change was large enough to
impact all MATXS users.

If many users were to be impacted, other nagging problems with the code
could also be corrected at that time.  With the original format, it was
always very difficult to add, replace, or extract a material.  The data
type loop was outside the material loop, and the pieces for a given
material were scattered throughout the file.  Therefore, the ordering
of the file was changed to put the data type loop inside the material
loop along with the other submaterials.  With this change, it was easy
to rewrite the BBC library maintenance code from the TRANSX package
to be able to insert a new material at any point in the library.
\index{BBC}
It was also easy to give BBC the capability of extracting a short
library containing only selected materials from a large MATXS
library.  Using the short library can significantly speed
up TRANSX runs to prepare data for reactor design problems.  The only
disadvantage of the new ordering is that identical photoatomic data
blocks have to be given for each isotope of an element.  However,
these data are not too bulky, and the additional overhead is manageable.
This arrangement might make it easier to add photonuclear data in the
future.

Finally, the current scheme of dividing the vector and matrix data into
subblocks was added.  The goal was to keep the record size below some
fairly large maximum value (5000 words is currently being used).  This
kind of limit makes it easier to design application codes that make
efficient use of memory.  In addition, using as few records as possible
reduces the number of I/O operations needed for a trip through the
library, thereby improving execution time.  The new subblocking scheme
is also very simple.

\subsection{MATXS Libraries}
\label{ssMATXSR_libraries}

The normal process for preparing a MATXS library using NJOY starts with
a series of runs to prepare PENDF tapes for each of the materials of
interest. For incident neutrons, these PENDF runs normally involve
running the modules \hyperlink{sRECONRhy}{RECONR},
\hyperlink{sBROADRhy}{BROADR}, \hyperlink{sUNRESRhy}{UNRESR},
\hyperlink{sHEATRhy}{HEATR}, and \hyperlink{sTHERMRhy}{THERMR}.  For
incident photons and charged particles, only
\hyperlink{sRECONRhy}{RECONR} is needed.  The
next step is to run \hyperlink{sGROUPRhy}{GROUPR} for
each material and incident particle.  Each
\hyperlink{sGROUPRhy}{GROUPR} run can produce data
for all outgoing particles and for
photons.  As an example, consider the problem of producing data for a
coupled neutron-photon-proton library.  \hyperlink{sGROUPRhy}{GROUPR}
would be run using the
ENDF-6 incident-neutron data and requesting \cword{mfd} values of 3 (cross
sections), 6 (neutron matrices, $nn$), 16 (photon production matrices,
$n\gamma$), and 21 (proton production matrices,
$np$).  \hyperlink{sGAMINRhy}{GAMINR} would be
run to produce the photoatomic cross sections and $\gamma\gamma$
matrices.  \hyperlink{sGROUPRhy}{GROUPR} can be used to
produce $\gamma n$ and $\gamma p$
matrices.  \hyperlink{sGROUPRhy}{GROUPR} would be run again
for the ENDF-6 format proton
sublibrary, requesting \cword{mfd} values of 3 (cross sections), 6 (neutron
production, $pn$), 16 (photon production, $p\gamma$), and 21 (proton
scattering and production, $pp$).

Once all the GENDF fragments from all these
\hyperlink{sGROUPRhy}{GROUPR} and
\hyperlink{sGAMINRhy}{GAMINR} runs
are available, they can be merged into multimaterial GENDF tapes using
\hyperlink{sMODERhy}{MODER}.  For the example above,
three multimaterial GENDF tapes should
be prepared: one for incident neutrons, one for incident photons, and
one for incident protons.  The input description in the following
section shows how the unit numbers for these three GENDF files are
given to MATXSR.

When the MATXSR run is made, the
code scans through all the input GENDF
tapes searching for the specified materials.  For each material, it
extracts all the data types requested in the
MATXSR input and appearing
on the input tapes.  In addition, it extracts every temperature,
$\sigma_0$ value, reaction, and Legendre order found on the input tapes.
The following paragraphs describe various special features of the
formatting process.

\paragraph{Normal Neutron and Photon Data.}
Normal, infinitely dilute cross sections and group-to-group matrices
for neutron reactions, photon production, and photonuclear reactions
are read from the input tapes and stored in the MATXS file using the
formats described above.  Almost all partial reactions are kept.
Reaction names are constructed automatically; a number of examples were
given above in Tables~\ref{sname3} -- \ref{sname11}.  The total cross
section and weighting flux from
\hyperlink{sGROUPRhy}{GROUPR} are given in the section \cword{mf}=3,
\cword{mt}=1 with both P$_0$ and P$_1$ components.  All four vectors are
written to the MATXS file.  A similar treatment is used for \cword{mt}=501
in the photoatomic case, except that only P$_0$ terms are saved.

The treatment of fission is also special.  ENDF libraries sometimes
use only \cword{mt}=18, the total fission reaction, for both cross section and
emission spectrum data; sometimes the partial fission cross sections
\index{partial fission}
(n,f), (n,n$'$f), (n,2nf), and (n,3nf) are also given in File 3
(\cword{mt}=19, 20, 21, and 38); and sometimes the partial reactions are also
used to describe the neutron emission matrix.  If the last case is found,
the \cword{mt}=18 matrix may not  be complete above the threshold for
second-chance fission, and it should be ignored.  Normally, it would
\index{second-chance fission}
not be processed in
\hyperlink{sGROUPRhy}{GROUPR}, but just in case, MATXSR will ignore it.
Delayed-neutron spectra processed in
\hyperlink{sGROUPRhy}{GROUPR} are written into the section
labeled \cword{mf}=5, \cword{mt}=455 on the GENDF tape, and this
section includes data
for all time groups.  MATXSR
adds up these separate time-group spectra
to produce a single delayed-neutron spectrum for the MATXS vector data
blocks.  Application codes can use the fission data in the MATXS library
to construct steady-state fission vectors as follows:
\index{fission, steady-state}

\begin{equation}
   \bar{\nu}^{SS}_g=\frac{\displaystyle\sum_{g'}\sigma^{HE}_{fg{\rightarrow}g'}
     +\sigma^{LE}_{Pfg} + \bar{\nu}^D_g\sigma_{fg}}
     {\sigma_{fg}} \,\,,
\end{equation}
\vspace{0.5 pt}

\noindent
where $\sigma^{HE}$ is the matrix part of the prompt fission reaction,
$\sigma^{LE}_{Pfg}$ is the low-energy production cross section for
fission neutrons, $\bar{\nu}^D$ is the total delayed-neutron yield,
\index{delayed-neutron spectra}
\index{delayed-neutron yield}
\index{fission matrix}
and $\sigma_{fg}$ is the fission cross section.  If partial fission
matrices are available, the first two terms in the denominator
would also have to be summed over the reactions present. Continuing,

\begin{equation}
   \chi^{SS}_{g'}= \frac{\displaystyle\sum_g\sigma^{HE}_{fg{\rightarrow}g'}
      \phi_g + \chi^{LE}_{g'}\sum_g\sigma^{LE}_{Pfg}\phi_g
       +\chi^D_{g'}\sum_g\bar{\nu}^D_g\sigma_{fg}\phi_g}
         {\rm NORM} \,\,,
\end{equation}
\vspace{0.5 pt}

\noindent
where $\chi^{LE}$ is the constant-spectrum part of the fission reaction,
$\chi^D$ is the total delayed-neutron spectrum, and NORM is the quantity
that normalizes $\chi^{SS}$, namely, the sum of the numerator over
all $g'$.  The weighting flux $\phi_g$ would normally be problem- and
\index{fission chi}
region-dependent in the application code.

\paragraph{Self-Shielding Data.}
If higher temperatures and $\sigma_0$ values are found on the input GENDF
file, MATXSR automatically prepares
submaterials for them.  This
self-shielding information can be used by application codes to prepare
effective cross sections for mixtures of materials in various geometrical
arrangements using equivalence theory.  This approach is often called
the Bondarenko method\cite{Bondarenko}.
\index{self-shielding}
\index{Bondarenko method}
\index{$\sigma_0$}
\index{equivalence theory}

As discussed in more detail in the
\hyperlink{sGROUPRhy}{GROUPR} section of this report, this
system is based on using a model flux for isotope $i$ of the form

\begin{equation}
   \phi^i_\ell(E,T)=\frac{C(E)}{[\sigma^i_0+\sigma^i_t(E,T)]^{\ell+1}}\,\,,
\end{equation}
\vspace{0.5 pt}

\noindent
where $C(E)$ is a smooth weighting flux, $\sigma^i_t(E,T)$ is the total
cross section for material $i$ at temperature $T$, $\ell$ is the Legendre
order, and $\sigma^i_0$ is a parameter that can be used to account for
the presence of other materials and the possibility of escape from the
absorbing region (heterogeneity).  \hyperlink{sGROUPRhy}{GROUPR}
uses this model flux to
calculate effective multigroup cross sections for the resonance-region
reactions (total, elastic, fission, capture) for several values of
$\sigma_0$ and several values of $T$.

When $\sigma_0$ is large with respect to the highest peaks in
$\sigma_t$, the flux is essentially proportional to $C(E)$.  This is
called infinite dilution, and the corresponding cross sections are
appropriate for an absorber in a dilute mixture or for a very thin
sample of the absorber.  As $\sigma_0$ decreases, the flux $\phi(E)$
develops dips where $\sigma_t$ has peaks.  These dips cancel out part
of the effect of the corresponding peaks in the resonance cross
sections, thereby reducing, or self-shielding the reaction rate.
In the MATXS format, these effects are represented by  differences
instead of the  f-factors used in earlier formats (see the description
of the BRKOXS file in the \hyperlink{sCCCCRhy}{CCCCR}
section of this report).  That is,
\index{CCCCR}
\index{BRKOXS}
the submaterial corresponding to temperature $T$ and background cross
section $\sigma_0$ contains the differences between the cross sections
found on the GENDF tape for those parameters and the infinitely dilute
values found in the first submaterial (normally, T=300K and
$\sigma_0{=}1{\times}10^{10} {\rm barns}$).

This approach has two advantages.  First, data for groups with no
self-shielding are automatically removed from MATXS vectors and matrices
by the banding process without having to violate the principle
of {\it generalization} and define a special format for neglecting
values of ``1.0'' instead of values of ``0.0.'' Second, effective
cross sections can be accumulated by simply adding the self-shielding
effects multiplied by appropriate interpolation weights to an
accumulating sum that starts out equal to the infinitely dilute
cross section.  With f-factors, it is necessary to save the infinitely
dilute cross sections during each step so that they can be multiplied
by the f-factors.  Thus, using differences is more economical in coding
and in storage space requirements.  The TRANSX code makes good use of
this feature.

\paragraph{Thermal Data.}
ENDF-6 thermal data comes from a thermal sublibrary.  In this sublibrary,
the various material configurations are handled as separate materials.
However, after the ENDF data have been processed by the
\hyperlink{sTHERMRhy}{THERMR} module,
the different thermal materials are handled as reactions.
Table~\ref{sname10} gives the correspondence between the special
NJOY thermal MT numbers and the actual thermal materials.  These are
the names that work for ENDF/B-VII\cite{ENDF7}.  If MATXSR is
used for ENDF/B-VII (or earlier) materials, the name for BeO will
be slightly wrong, but it could be fixed by hand editing of the
MATXS file.
\index{thermal cross sections}
\index{thermal MT numbers}

Each of these thermal reactions corresponds to a particular dominant
material.  For example, \cword{mt}=222 gives the thermal cross
section for hydrogen bound in water.  It will only appear in the
material H1 in a MATXS library.  Similarly, \cword{mt}=223-227
will only appear in $^{1}$H, \cword{mt}=228 will only appear
in $^{2}$H, \cword{mt}=229-230 will only appear in $^{nat}$C,
\cword{mt}=231-234 will only appear in $^{9}$Be, and
\cword{mt}=235-236 will only appear
in $^{nat}$Zr.  Free-gas scattering (\cword{mt}=221) will appear
in all materials.

Many of these materials describe the scattering from one atom of a
compound bound in that compound; for example, H in H$_2$O, D in D$_2$O,
or Zr in ZrH.  The application code using this data is expected to add
on the effects of scattering from the other atoms of the compound.
For water, the scattering from free oxygen is added to the
``H in H$_2$O'' scattering.  For zirconium, the scattering for
``H in ZrH'' is added to the scattering from ``Zr in ZrH''.  There
are two exceptions in the existing ENDF/B evaluations.  The benzene
data set contains the entire scattering from the C$_6$H$_6$ molecule
normalized to the hydrogen cross section.  Therefore, if an application
code specifies H1 with the benzene scattering option and the correct
density for H1 in the system, the result will contain all of the benzene
scattering effect.  No additional scattering is to be added for the
carbon atoms.  Similarly, BeO contains all the scattering from the
compound normalized to the beryllium cross section.  Check the
\hyperlink{sTHERMRhy}{THERMR}
section of this report for more information.

Six of these materials have names ending with \$.  These reactions
represent coherent elastic scattering from crystalline powdered
\index{coherent elastic}
materials (C, Be, BeO) or incoherent elastic scattering from solids
containing hydrogen (polyethylene, ZrH).
\index{incoherent elastic}
These reactions contain ingroup elements only in the scattering
matrices; that is, they cause angular redistribution without
energy loss in scattering.  Each \$ reaction should be added
to the corresponding inelastic reaction by the application code.
This part of the scattering can lead to difficulties with
transport corrections in discrete-ordinates transport codes.

The final unique aspects of the thermal data are that the matrices
show upscatter, and they are only defined below some maximum
\index{upscatter}
energy.  The energy range aspect is easily handled by the banding
method used to reduce the size of the vector blocks.  The upscatter
aspect is handled by \cword{jband} and \cword{ijj}.  The order of
storage for a simple thermal case with only 2 upscatter groups would
be as follows:

\begin{center}
\begin{tabular}{cccc}
Band & Element & \cword{jband} & \cword{ijj} \\ \hline
 $\cdots$ & $\cdots$ & & \\
 27  &  27$\rightarrow$27 & 2 & 27 \\
     &  26$\rightarrow$27 &   &   \\
 28  &  29$\rightarrow$28 & 3 & 29 \\
     &  28$\rightarrow$28 &   &   \\
     &  27$\rightarrow$28 &   &   \\
 29  &  30$\rightarrow$29 & 3 & 30 \\
     &  29$\rightarrow$29 &   &   \\
     &  28$\rightarrow$29 &   &   \\
 30  &  30$\rightarrow$30 & 2 & 30 \\
     &  29$\rightarrow$30 &   &   \\ \hline
\end{tabular}
\end{center}

\paragraph{Charged-Particle Data.}
The treatment of data for incident charged particles is very similar
\index{charged-particle cross sections}
to that used for neutron data, except for charged-particle elastic
scattering.   The elastic channel has  contributions from Coulomb
scattering that become infinite as the scattering angle goes to zero.
\index{Coulomb scattering}
In nature, this singularity is removed by electronic screening, but
some other approach is needed for a data set that concentrates on
isolated nuclear reactions.  The approach selected is used in some
existing applications.  The elastic scattering distribution is broken
up into two parts: (1) a normal angular distribution for angles from
some low cutoff, say 20$^\circ$, back to 180$^\circ$, and (2) a
straight-ahead continuous slowing-down contribution to represent the
effects of angles below the cutoff.  The continuous slowing-down part
is closely related to the normal ``stopping power'' for charged particles.
\index{stopping power}
The large-angle part can be converted into a normal scattering matrix
(see the discussion of charged-particle elastic scattering in the
\hyperlink{sGROUPRhy}{GROUPR} chapter of this manual for more details).

Discrete-ordinates transport codes can often be modified to handle
charged-particle data in this form, but they require an effective
total cross section.  The ENDF-6 format for charged-particle data
does not define a total cross section because of the singularity in
the elastic contribution.  However, for application purposes, a
reasonable definition of an effective total cross section is that it
is the sum of all the partial reaction cross sections including the
cross section obtained for the truncated elastic scattering reaction.
This is the method that MATXSR
uses to compute the quantities
\cword{ptot0}, \cword{dtot0}, \cword{ttot0}, {\it etc}.

\paragraph{Delayed-Neutron Data.}
Delayed-neutron yields, spectra, and decay constants by time group are
required by reactor kinetics codes.  The previous CCCC format for these
data was DLAYXS (see the \hyperlink{sCCCCRhy}{CCCCR} chapter
of this manual).  Because of the
\index{delayed-neutrons}
\index{DLAYXS}
generalization inherent in the MATXS format, these data can be added
without any changes in the structure of the file.  This option has not
yet been added to MATXSR, and the
procedures used will be described
in a future version of this report.

\paragraph{Decay-Photon and Decay-Heat Data.}
Many nuclear reactions leave radioactive products.  These products
may be simple daughter isotopes that decay in a few steps to a
stable final state with the emission of a few photons and electrons
(or heat), or they may be a complex array of fission product isotopes
that emit a complex spectrum of photons and electrons (or heat) showing
many time constants.  Procedures to store these data in MATXS
libraries will be included in a future version of this report.

\subsection{User Input}
\label{ssMATXSR_inp}

The user input specifications below were copied from the comment
cards at the beginning of the MATXSR module.  It is always a good
idea to check the comment cards in the current version of the
code for possible changes.
\index{MATXSR!MATXSR input}
\index{input!MATXSR}

\small
\begin{ccode}

   !---input specifications (free format)---------------------------
   !
   ! card 1 units
   !   ngen1     input unit for data from groupr
   !   ngen2     input unit for data from gaminr
   !   nmatx     output unit for matxs
   !   ngen3     incident proton data from groupr (default=0)
   !   ngen4     incident deuteron data from groupr (default=0)
   !   ngen5     incident triton data from groupr (default=0)
   !   ngen6     incident he3 data from groupr (default=0)
   !   ngen7     incident alpha data from groupr (default=0)
   ! card 2 user identification
   !   ivers     file version number (default=0)
   !   huse      user id (up to 16 characters, delimited by ',
   !             ended by /) (default=blank)
   ! card 3 file control
   !   npart     number of particles for which group
   !                structures are given
   !   ntype     number of data types in set
   !   nholl     number of cards to be read for hollerith
   !             identification record.
   !   nmat      number of materials desired
   ! card 4 set hollerith identification
   !   hsetid    hollerith identification of set
   !             (each line can be up to 72 characters,
   !             delimited with ', ended by /)
   ! card 5 particle identifiers
   !   hpart     hollerith identifiers for particles
   !             (up to 8 characters each)
   ! card 6 energy groups
   !   ngrp      number of groups for each particle
   ! card 7 data type identifiers
   !   htype     hollerith identifiers for data types
   !             (up to 8 characters each)
   ! card 8 input particle ids
   !   jinp     input particle id for each data type
   ! card 9 output particle ids
   !   joutp    output particle id for each data type
   ! card 10 material data (one card per material)
   !   hmat      hollerith material identifier
   !             (up to 8 characters each)
   !   matno     integer material identifier
   !             (endf mat number)
   !   matgg     mat number for photoatomic data
   !             (default=100*(matno/100) as in endf-6)
   !
   !-------------------------------------------------------------------

\end{ccode}
\normalsize

Card 1 is used to specify the units for the input GENDF tapes
and the output MATXS library.  As usual, the sign of a GENDF unit
number is used to determine its mode; negative numbers mean
binary, and positive numbers mean coded ({\it i.e.}, ASCII).
The output file is always binary, and its sign is ignored.
The most common MATXSR
runs are for neutron and photon data only.
In these cases, card 1 can be truncated after the \cword{nmatx}.
If incident charged-particle GENDF files are available, any of
the units \cword{ngen3} through \cword{ngen7} can be assigned.

Card 2 is used to control the MATXSR
print option \cword{iprint}
and to provide the information for the MATXS ``User Identification''
record.  An example for Card 2 follows:

\small
\begin{ccode}

  0  17  'T2 LANL NJOY'/

\end{ccode}
\normalsize

\noindent
Card 3 is used to input the counts for the ``File Control'' record and
to tell the user input routine how many quantities to read in subsequent
input cards.  Card 4 is repeated \cword{nholl} times to give the Hollerith
description for the library.  Each line must be delimited by quote
characters and terminated by /.  For example,

\small
\begin{ccode}

 ' MATXS17                                27 FEB 91 '/
 ' 69-GROUP THERMAL LIBRARY FROM ENDF/B-VI          '/
 '   THIS LIBRARY INCLUDES NEUTRON, PHOTON, AND     '/
 '   THERMAL SCATTERING DATA FOR 133 MATERIALS.     '/

\end{ccode}
\normalsize

\noindent
The lines are written out on the MATXS file as given, except that
the maximum length for each line is 72 characters.

Card 5 is used to read in the Hollerith names for the \cword{npart}
particles for this library.  The standard names for the particles
were given above in Table~\ref{sparts}.  The number of groups desired
for each particle are given on card 6.  The data-type names (see
Table~\ref{stypes}) are given on Card 7.  Cards 8 and 9 are used to specify
the input and output particles for each data type.  The user
should take care that the number of groups given for each particle
is consistent with the input data, and that the particle assignments
to data type names (\cword{jinp} and \cword{joutp}) are consistent
with the  names.  The code does not check.

Card 10 is repeated \cword{nmat} times to specify the materials
to be written out on the library.  The rules for constructing the
material names \cword{hmat} were given in Section~\ref{ssMSTXSR_format}.  The
\cword{matno} parameter is the ENDF MAT number for this material
as used on the input GENDF files.  For ENDF/B-VI, the MAT number
is the same for all sublibraries (that is, for incident neutrons,
photons, protons, {\it etc.}), and only one value is needed to specify the
desired material.  However, photoatomic data are
\index{photoatomic}
atomic in character, and the MAT numbers always refer to the
element.  For example, MAT=2600 for the photoatomic data of
iron.  \hyperlink{sMATXSRhy}{MATXSR} reads a
second MAT number field, \cword{matgg},
for the photoatomic data.  Its default value is given by

\newpage
\small
\begin{ccode}

  100*(MATNO/100)

\end{ccode}
\normalsize

\noindent
and it is, therefore, not usually needed.  The normal form
for card 10 would be as follows:

\small
\begin{ccode}

  FE56  2631/

\end{ccode}
\normalsize

However, the photoatomic libraries for earlier versions of the
ENDF/B files used MAT numbers like 26 for elements.  The \cword{matgg}
parameter can be used to process data from these older libraries.
Note that delimiting quote characters are not required for Hollerith
names that are single words and start with a letter.

MATXSR always processes all
submaterials found on all the nonzero GENDF
units \cword{ngen1} through \cword{ngen7}.  It processes every
temperature and $\sigma_0$ value found.  It processes (almost) every
reaction cross section and matrix found, and it processes every Legendre
order given.  The only way to control the contents of the MATXS library
is through the input to \hyperlink{sGROUPRhy}{GROUPR} or
\hyperlink{sGAMINRhy}{GAMINR} and by the materials included
when building the input GENDF files.  A convenient way to handle
this task is to assemble the results of a number of single-material
\hyperlink{sGROUPRhy}{GROUPR} runs into composite
GENDF tapes using \hyperlink{sMODERhy}{MODER}.

\subsection{Coding Details}
\label{ssMATXSR_details}

Subroutine \cword{matxsr} is the only public call for module
\cword{matxsm}.  The module has a number of global variables and
arrays defined.  One key set of variables and arrays provides the
area for accumulating the MATXS data.  It provides a set of
equivalenced arrays so that integers, reals, and Hollerith strings
can all be stored in the same binary records.  It sets up both
integers and reals to be 4-byte quantities.  Hollerith words
(with up to 8 characters each) are 8-byte quantities.  The CCCC
\cword{mult} value is set to 2.  See a(200000), ia(200000), and
ha(100000).  The parameter \cword{isiza}=200000 defines the size of
these equivalenced arrays in 4-byte units.

The internal representation for all the elements of the MATXS file
and for GENDF data uses 8-byte quantities.

The next step in subroutine \cword{matxsr}\index{matxsr@{\ty matxsr}}
is to read in the unit numbers for the input and output files and
to open the files.  Note that the sign for \cword{nmatx} is ignored.
 It will always be binary.  Several binary scratch files are also
opened.  Next, the user input for the ``File Identification'' record
is read (see card 2 in the user input description), and the
\cword{cmatxs}\index{cmatxs@{\ty cmatxs}} routine is called to read
the input GENDF files and construct the output MATXS file.  When
\cword{cmatxs} returns, the main routine closes the units, prints its
final timer line, and returns to the NJOY program.

Subroutine \cword{cmatxs}\index{cmatxs@{\ty cmatxs}}
starts by defining three constants:

\begin{description}
\begin{singlespace}

\item[\cword{nsubmx}=100] is the maximum number of submaterials
   allowed for a material, including data types, temperatures, and
   background cross sections;
\item[\cword{maxord}=5] is the maximum Legendre order (that is,
   P$_5$ allowed for group-to-group matrices; and
\item[\cword{maxw}=5000] is the maximum size for vector block
   and matrix subblock records on the MATXS file.

\end{singlespace}
\end{description}

\noindent
It continues by calling \cword{ruinm}\index{ruinm@{\ty ruinm}}
to read the user's input.  Some of the quantities read by \cword{ruinm}
are stored directly into \cword{ia}, \cword{a}, and \cword{ha} using
pointers like \cword{icont} and \cword{iholl}.  Note the use of
\cword{mult}=2 to compute the jump in the index when Hollerith
items or stored, and note the the variable \cword{next} keeps track
of the current location in the MATXS equivalenced arrays.  Subroutine
\cword{ruinm} returns to \cword{cmatxs} after loading the ``File Data''
record.

The next step is to call subroutine
\cword{mtxdat}\index{mtxdat@{\ty mtxdat}} to read the input
GENDF data and write it to the scratch tapes used later in this
subroutine.  The first step is to set up an area at pointer
\cword{igrup} for the \cword{npart} group structures and to calculate
its length.  Note that the number of words reserved is forced to be
even in order to avoid possible word-alignment problems for these
4-byte words.  With the current version of
\hyperlink{sGROUPRhy}{GROUPR}, all particles are
assumed to use the same group structure.  Photons have a different one.
It is only necessary to look through the unit numbers from input card 1
and find the first one that contains data.  The group structures are
then read in from \cword{mf}=1,\cword{mt}=451 and reversed to have
the conventional decreasing-energy order.  The subroutine now
reserves space for the ``Material Control'' record starting at
\cword{imatc} with length \cword{nwmc} and begins the material
loop (see \cword{do 700 im=1,nmat}).
Note how the Hollerith material name is loaded into the ``Material Control''
record storage area by making use of the \cword{mult} parameter and
the equivalence between the arrays \cword{a} and \cword{ha}.

The first loop inside the material loop is the data-type loop (see
\cword{do 600 i=1,} \cword{ntype}).  The same MAT number is used for
all particles,
but a different one is used for photoatomic data.  The input unit to
be used depends on the identity of the incident particle.  Since these
comparisons use constants like \cword{hprot} with values like
``\cword{6hp     },'' it is important to use the standard names in
the user input (see Table~\ref{sparts}).  The identity of the
outgoing particle for the data type determines the MF number for
the matrices on the GENDF tape, \cword{mfm}.  As described in the
\hyperlink{sGROUPRhy}{GROUPR} section of this report,
the MF assignments are as follows:

\begin{center}
\begin{tabular}{cl}
MF value & Outgoing Particle \\ \hline
6 & neutrons \\
16 &  photons \\
21 & protons \\
22 & deuterons \\
23 & tritons \\
24 & $^3$He particles \\
25 & alphas \\
26 & photoatomic data \\ \hline
\end{tabular}
\end{center}

\noindent
The MF number for the vector data (\cword{mfv}) is 3, except for the
photoatomic case, where it is 23.

The next step is to search for the desired material \cword{imat} on
the current input tape.  When the first occurrence of \cword{imat} has
been found, \cword{mtxdat}\index{mtxdat@{\ty mtxdat}} reads in its
head record, and then it sets up a loop over all the temperatures
for this material (the loop goes through statement number 300).  As
it reads through each temperature, it copies the results to scratch
file \cword{nscr}.  The data for the first temperature is also copied
to scratch file \cword{iref}.  File \cword{iref} will be used for
the higher temperature and $\sigma_0$ values to calculate the
delta-sigma values.  While the data are being copied, \cword{mtxdat}
counts the number of one-dimensional reactions, \cword{noned}, and
the number of two-dimensional reactions, \cword{ntwod},
for each value of the background cross section, $\sigma_0$.  When the
scratch tapes are complete, the subroutine starts a loop over $\sigma_0$
for this material and temperature; it calls
\cword{vector}\index{vector@{\ty vector}} to produce the vector
cross sections, it calls \cword{matrix}\index{matrix@{\ty matrix}} to
produce the matrix cross sections, and it loads the information on
this submaterial into the ``Material Control'' record.  When the
$\sigma_0$ loop is complete, the code jumps back to statement
number 300 to get the next temperature.  When all the temperatures
have been processed, it reaches statement number 600 and goes back
through the entire process again for the next data type.  When the
``\cword{do 600}'' loop exits, the entire ``Material Control' block
has been filled in, and the subroutine writes the record to scratch
file \cword{nscrt6}.

At this point, the processing for material \cword{imat} is complete.
The ``\cword{do 700}'' loop continues until all the requested materials
have been found and processed.

Subroutine \cword{findg}\index{findg@{\ty findg}} is used to search
GENDF file \cword{ntape} for a specified section (\cword{mat},
\cword{mf}, \cword{mt}).  If it does not find the requested section,
a fatal error message is issued.  Materials (\cword{mat}) do not have to be
in order on a GENDF tape.  Therefore, this routine simply reads forward
until it comes to the first record matching the requested \cword{mat},
\cword{mf}, and \cword{mt} values.  If the section is not found,
it rewinds once and searches again.  When the section is found, it
backspaces by one record so that the next record read will be
the desired record.

Subroutine \cword{hname}\index{hname@{\ty hname}} is used to construct
a Hollerith reaction name \cword{hreact} from an ENDF MT number
(\cword{mt}), an ENDF ``LR flag'' (\cword{lr}), and an incident
particle name (\cword{hp}).   The definitions of the names were
discussed above in connection with Tables~\ref{sname3}--\ref{sname11},
and the actual MT numbers and strings used are given in
parameter arrays in this subroutine.  If no preset name is found for an
MT number, a default name of the form \cword{MTnnn} is constructed.

Subroutine \cword{vector}\index{vector@{\ty vector}} reads the
cross section vectors for one submaterial (that is, one data type,
one temperature, and one $\sigma_0$).  It starts by clearing
some flags that are used to detect the occurrence of particular
reactions.  For example, \cword{k107} is set if the (n,$\alpha$)
reaction (\cword{mt}=107) is found.  Pointers are assigned for the
vector control and vector data arrays (see \cword{ivcon}, \cword{ivdat}).
Note that there must be enough container memory available to hold all
the vector data (\cword{ning}*\cword{n1d} words).  The routine now starts up a
loop over sections on the input scratch tape (the loop goes through
statement number 115).  The sections that it processes are determined
by the value of \cword{mfd}, which will be 3, except for photoatomic
data, when it will be 23.  Delayed fission $\chi$ is a special case;
it will be found in the section with an MT value of \cword{mfd}+2.  As
each interesting section is found, the Hollerith name for the reaction
is constructed, and the location of the reaction relative to
\cword{ivdat} is computed.  When the total cross section is
found (\cword{mt}=1),
names and locations are set up for the P$_\ell$ components of the weight
function (for example, \cword{nwt0} and \cword{nwt1}) and for the
P$_\ell$ components of the total cross section (\cword{ntot0} and
\cword{ntot1}).  If appropriate, the first letter of these names might
be \cword{p}, \cword{d}, {\it etc.}, depending on the incident particle.
When the total photoatomic cross section is found (\cword{mt}=501), names
and locations are defined for the $\gamma$ weight and the P$_0$ cross
section (\cword{gwt0} and \cword{gtot0}).  When the charged-particle
elastic cross section is found for particle \cword{x} (\cword{mt}=2), names and
locations are set up for the weight function \cword{xwt0} and the
effective charged-particle total cross section \cword{xtot0}.  Once
the name and location have been selected, the code reads the data for
all groups from file \cword{nscr} and stores them at the selected
location.  If this submaterial corresponds to a higher temperature
or $\sigma_0$, the corresponding data is also read from file
\cword{iref} and subtracted from the \cword{nscr} data.  As a result,
self-shielding information in the MATXS file is given as differences
rather than f-factors as in the previous CCCC self-shielding file,
BRKOXS.  The delayed-neutron spectra are read from
\cword{mf}=5, \cword{mt}=455; the
spectra from the time groups on the GENDF tape are added to obtain
a single spectrum $\chi_d$.

There is a special feature in \cword{vector} for incident charged-particle
reactions.  Although discrete-ordinates transport codes require a
total cross section, it is not actually possible to compute a
total cross section for charged-particle reactions because Coulomb
scattering is singular for straight-ahead scattering (at least in
the absence of electronic screening).  The solution to this problem
used by \hyperlink{sGROUPRhy}{GROUPR} is to exclude
a range of forward angles from the
charged-particle elastic matrix (this angular range is handled by
continuous-slowing-down theory in the application codes).  Therefore,
MATXSR can construct an effective
total cross section by adding all
the reaction cross sections found to this truncated elastic cross section.
However, it must be careful to watch for possibly redundant
cross sections that must be omitted from the sum; for example, if
the total ($x$,p) cross section (\cword{mt}=103) is present
for incident particle
$x$, the discrete-level ($x$,p$_n$) cross sections and the
($x$,p) continuum reactions (\cword{mt}=600-649) must be omitted from the sum.
This is the function of flags like \cword{k103}.

There is another special feature for nuclide production data.  When
using this experimental ENDF format, there may be a number of different
sections on the GENDF file producing the same product.  These
separate contributions are added up into one single production cross
section in the block of coding starting at statement number 410.

When all the reactions have been entered, subroutine \cword{vector}
thins the vector data just loaded by removing unneeded leading or
trailing zeros (actually, numbers less than \cword{small=1.e-30}) from
each reaction.  These zeros can arise from thresholds and upper limits
for thermal-range reactions.  In addition, since data for the higher
temperatures and $\sigma_0$ values are stored as differences, zero
(or very small) elements can imply that there is no significant
temperature or $\sigma_0$ dependence.  The subroutine simply steps
through the \cword{ivdat} data block to find the band limits \cword{nfg}
and \cword{nlg} for each reaction, to squeeze out the zeros, and to
record the band limits in the vector control block at \cword{ivcon}.
When all of the \cword{n1d} reactions have been processed, the resulting
vector data is written out as a series of ``Vector Block'' records on
scratch tape \cword{nscr3}.  The subblocking is controlled by \cword{maxw}
(5000 words).  The code just steps through the cross section
bands until the next reaction will cause the sum to exceed \cword{maxw}.
It then writes these reactions to the scratch file, and it repeats the
process for the next group of reactions.  Each record will have fewer
than \cword{maxw} words, and the minimum number of records will be
produced within the restriction that no reaction is broken up between
records.  The last step in the subroutine is to write the ``Vector
Control'' record for this submaterial to scratch file \cword{nscrt2}.

Subroutine \cword{matrix}\index{matrix@{\ty matrix}} reads the
group-to-group matrix data for this submaterial, converts it
to MATXS format, and writes the results to a scratch file.
 The pointer \cword{imcon} is used to accumulate matrix control
information, pointer \cword{icdat} is used for the low-energy
constant spectrum data (if any), \cword{ijgll} is used for
banding information, and pointer \cword{imdat} for the
actual matrix data.  The main loop is over the \cword{n2d} matrix
reaction types.  The head card for the reaction is read in from
\cword{nscr}, and for higher temperatures and $\sigma_0$ values, from
\cword{iref} also.  Subroutine \cword{hname} is called to construct the
Hollerith reaction name, subroutine \cword{band} is called to read
through the data and determine the banding parameters used to remove
excess zero elements, and subroutine \cword{shufl} is called to read
through the input scratch file again and load the data into memory
in its final compact form.

Subroutine \cword{band}\index{band@{\ty band}} is fairly simple.  It
loops over all the groups for this reaction on \cword{nscr} and/or
\cword{iref}.  When the loop is finished, \cword{jg1lo(i)} and
\cword{jg1hi(e)} contain lowest and highest initial-group indices
found for each final group \cword{i}.  Note that the actual
cross section for a matrix element (or the cross section
difference when \cword{iref} is being used) is not checked;
it is assumed that \hyperlink{sGROUPRhy}{GROUPR} has
done a good job removing excess zeros.
This should be improved in a future version.  Before returning, these
limiting group indices are used to compute the MATXS parameters
\cword{jband} and \cword{ijj}, and to insert them into the accumulating
matrix control block at pointer \cword{imcon}).

Subroutine \cword{shufl}\index{shufl@{\ty shufl}} starts by
initializing a loop through statement number 100 that will be used
to produce ``Matrix Sub-Block'' records.  Inside this loop, the
code sums the bandwidths found by \cword{band}\index{band@{\ty band}}
to find out how many final-energy group bands can fit in
a subblock with length less than or equal to \cword{maxw} words.  The
result is a pair of group indices \cword{i0} and \cword{imax} that
define the range of groups to be included in the subblock.  Subroutine
\cword{shufl} now makes a pass through scratch file \cword{nscr}, and
also through scratch file \cword{iref} for higher temperatures and
$\sigma_0$ values.  It stores away the constant subblock data
(\cword{ig}=0 for the constant spectrum and \cword{ig2lo}=0 for the
production cross section), if found.  For each group-to-group element,
it computes the final group index \cword{jg2}.  If the group is in the
allowed range for this subblock, it computes the output location
\cword{noloc} and stores the element in memory.  For higher temperatures
and $\sigma_0$ values, it subtracts the base value found on file
\cword{iref}.  (This step is normally needed for neutron elastic
scattering only.) When the group loops for \cword{nscr} and \cword{iref}
are complete, the subroutine writes out the ``Matrix Sub-Block'' record
on a scratch file, resets the \cword{ng2z} parameter to the top group
of the subblock, and continues the loop through statement number 100 to
produce the rest of the subblocks for this reaction.  The resulting
series of subblock records on the scratch file are all less than
\cword{maxw} words in length, and the number of records is minimal
within the restriction that bands are not split between records.

Returning to \cword{matrix}, data already in memory are used to complete
the ``Matrix Control'' record, which is written out onto \cword{nscrt2}.
If no matrix data were found by \cword{band} and \cword{shufl}, a dummy
matrix control record is written on \cword{nscrt2} and a dummy matrix
data record is written on \cword{nscrt3}.  Finally, the constant subblock
data saved in \cword{shufl} (if any) is written to \cword{nscrt3}.

Returning to subroutine \cword{cmatx}, after the call to \cword{mtxdat},
the data stored in the \cword{a,ia,ha} equivalenced arrays are
written to the output file for the ``File Identification,''
``File Control,'' ``Set Hollerith Identification,'' ``File Data,''
and ``Group Structure'' records.  In a loop over materials, the
``Material Control'' record is copied from \cword{nscrt6} to the output
file.  Then, in a loop over the submaterials for this material, the
``Vector Control'' records are copied from \cword{nscrt2}, the
``Vector Blocks'' are copied from \cword{nscrt3}, the ``Matrix
Control'' blocks are copied from \cword{nscrt2}, and the ``Matrix
Subblocks'' are copied from \cword{nscrt3}.  Subroutine \cword{cmatx}
then returns to the main MATXSR
routine, files are closed, and the
MATXSR run is complete.

\subsection{Error Messages}
\label{ssMATXSR_err}

The fatal-error and warning messages generated by
MATXSR are given
below, along with suggested actions to alleviate the problem.

\begin{description}
\begin{singlespace}

\item[\cword{error in mtxdat***input error (nin=0)}] ~\par
   No input GENDF tape has been given.  Check Card 1 of the
   user input.

\item[\cword{error in mtxdat***too many submaterials}] ~\par
   The code is currently limited to 100 submaterials per material.
   See \cword{nsubmx}=100 in \cword{cmatxs}.

\item[\cword{error in findg***mat or mf or mt le 0 not allowed}] ~\par
   Subroutine \cword{findg} has been asked to search for an
   illegal section on the GENDF tape.

\item[\cword{error in findg***mat=nnnn mf=nn mt=nnn not on tape}] ~\par
   The requested section was not found on the input GENDF tape.
   Check that the correct tape was mounted.

\item[\cword{error in vector***exceed input data array size.}] ~\par
   Check the parameter \cword{maxb}=30000 in \cword{vector}.

\item[\cword{error in band***input too large}] ~\par
   Check the parameter \cword{maxb}=30000 in \cword{matrix}.

\item[\cword{error in shufl***input too large}] ~\par
   Check the parameter \cword{maxb}=30000 in \cword{shufl}.

\item[\cword{error in lst1io***storage exceeded}] ~\par
   See \cword{nbmax}=2000 in \cword{mtxdat}, the size of the
   array \cword{b}.

\end{singlespace}
\end{description}

\cleardoublepage