NIST Physical Measurement Laboratory XCOM Photon Cross Sections Database

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4. How to Run the XCOM Program

4.1. Overview

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 file-uploading (note: your browser must be file-upload compatible). This information is used to prepare a specific form with options described below.

4.3. Elements

Elements may be selected by their atomic number, or by their chemical symbol. Only elements 1 through 100 are available.

4.4. Compounds

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.

4.5. Mixtures

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 output page.

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 fewer. 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.

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