4. How to Run the XCOM Program
There are two ways to use the XCOM database. The text-based version outputs a
basic text table of data. The other version gives the user more options and
features (e.g., file-uploading, graphing, and graphical tables). The rest of
the information in this document is appropriate for both avenues of output.
There are two forms to be completed to retrieve data. The first form produces
the appropriate second form. The first form is concerned with general
information (type of material: element, compound, or mixture)
(Z ≤ 100). The second is more specific (energy values
and graphing options). The specific input fields are described below.
4.2. The First Form
The material for which cross sections are to be computed can be designated as
an element, compound or mixture. The program will compute values for standard
energies, but the user may also input additional energies. This can be done by
inputing the values by hand or by using
your browser must be file-upload compatible). This information is used to
prepare a specific form with options described below.
Elements may be selected by their atomic number, or by their chemical symbol.
Only elements 1 through 100 are available.
Chemical formulas for compounds should be entered in standard chemical notation,
with appropriate upper and lower case. However, because of hardware limitations,
subscripts must be written on line. For example, the formula for calcium
tungstate must be entered as CaWO4. Parentheses, spaces and dots may not be
used. For example, the formula for calcium phosphate must be entered as Ca3P2O8
(and not as Ca3(PO4)2).
Substances consisting of molecules with only a single species of atoms can be
designated as either elements or compounds. For example, molecular nitrogen
could be treated as an "element" with symbol N, or as a "compound"
with formula N2.
Mixtures may consist of "elemental" components and/or
"compound" components. This is a matter of convenience since it does
not change the results.
The user must specify the chemical symbol or formula (as specified above),
as well as the fraction by weight for each component. The program then uses
these input data to compute the fractions by weight of the individual atomic
constituents, as well as the sum of these fractions. If, in the input data, the
sum of the fractions by weight does not add up to unity, the input data is
accepted, and the program renormalizes all of the fractions by weight so that
they add up to unity.
4.6. The Optional Output Title
This title, which may include embedded blanks, will appear at the top of the
4.7. The Energy List
Users can 1) limit output to the standard energy grid, 2) add
energies of their choice to the standard grid, or 3) limit output to the
set of energies selected by them. In case 2), the additional energies are
merged into the standard energy grid according to magnitude. They are also
indicated by a different color in the output table to distinguish them from the
standard grid values. In case 3), the energies will be sorted numerically, and
duplicates will be removed.
If additional energies are entered by the user, this can be done either from
the keyboard or from a previously prepared input file. This file (stored in any
desired directory on a floppy or hard disk) should contain a list of energies,
with items separated by Return or
Enter. Warning: No more than 75 energies may
be added for a single element. Even less may be unacceptable if there are
multiple constituents. The standard grid alone may contain too many energies if
there are many constituents.
4.8. Output Units
For elements, the user is given three choices: 1) all quantities in
cm2/g; 2) all quantities in barns/atom, where
1 barn = 10-24 cm2;
3) partial interaction coefficients in barns/atom and total attenuation
coefficients in cm2/g. For compounds and mixtures, all quantities
are in cm2/g.
4.9. The Graph
Any number of the seven coefficients may be graphed simultaneously. If the
"none" option is checked it will over-ride any other selections and
no graph will be displayed. Note: The data can be displayed faster if
the graphing is not utilized, and more curves take longer to graph than
Zooming in is possible by redefining the energy range. However, the minimum
energy range (maximum zoom) is one order of magnitude (e.g., 0.01 to 0.1). The
buttons on the right can be used to change the size of the image, which may be
useful for printing. The values given are for the width of the image in pixels.
4.10. The Output Table
The atomic numbers and fractions by weight of the atomic constituents are given
above the table. The main body of the table is supplied with enough headings
to be self-explanatory. The left-most column gives the designations of the
absorption edges (K, L1, L2, L3,
M1, M2, ...) as well as the atomic number of the pertinent
atomic constituent. Data for energies immediately below and above each edge,
are given on two lines. It should be noted that the standard energy grid
automatically includes at least one other energy between any two successive
absorption edges. For materials of atomic number Z ≤ 10, there are no
absorption edges above the minimum energy, 1 keV.