**23. FILE 23. \"SMOOTH\" PHOTON INTERACTION CROSS SECTIONS**

**23.1. General Comments on Photon Interaction**

Photon interaction data are divided into two files. File 23 is analogous
to File 3 and contains the \"smooth\" cross sections. File 27 contains
the coherent scattering form factors and incoherent scattering functions
(see Chapter 27).

Electron interaction data are divided into two different files. The
smooth cross sections for elastic scattering, bremsstrahlung,
excitation, and the ionization of different atomic subshells are given
in File 23. File 26 is used to give the angular distribution for
elastically scattered electrons, the outgoing photon spectra and energy
loss for bremsstrahlung, the energy transfer for excitation, and the
spectra of the scattered and recoil electrons associated with subshell
ionization.

Both photo-atomic and electro-atomic reactions can leave the atom in an
ionized state. See Section 28 for a description of the atomic relaxation
data needed to compute the outgoing X-ray and electron spectra as an
ionized atom relaxes back to neutrality.

**23.2. General Description**

This file is for the integrated photon and electron interaction
cross‑sections. The reaction type (MT) numbers for photon and electron
interaction are in the 500 series. Several common photon and electron
interactions have been assigned MT numbers:

+-----------+----------------------------------------------------------+
| # MT      | **Reaction Description**                                 |
+-----------+----------------------------------------------------------+
| 523       | Photo-excitation cross sections                          |
+-----------+----------------------------------------------------------+
| 526       | Electro-atomic elastic scattering                        |
+-----------+----------------------------------------------------------+
| 527       | Electro-atomic bremsstrahlung                            |
+-----------+----------------------------------------------------------+
| 528       | Electro-atomic excitation cross section                  |
+-----------+----------------------------------------------------------+
| 533       | Atomic relaxation data (see Section 28)                  |
+-----------+----------------------------------------------------------+
| 534‑572   | Photo-electric or electro-atomic subshell ionization     |
+-----------+----------------------------------------------------------+

Photon cross sections, such as the total cross section, coherent elastic
scattering cross section, and incoherent (Compton) cross section, are
given in File 23, which has essentially the same structure as File 3.
These data are given as a function of energy, E~γ~, where E~γ~ is the
energy of the incident photon (in eV). The data are given as
energy‑cross section pairs.

Similarly, electron cross sections, such as elastic scattering,
bremsstrahlung, ionization, and excitation, are given in File 23. These
data are given as a function of the electron energy in eV, and they are
also given as energy-cross section pairs.

Each section in File 23 contains the data for a particular reaction type
(MT number). The sections are ordered by increasing MT number.

**23.3. Formats**

The following quantities are defined:

+----------+-----------------------------------------------------------+
| #        | Standard material charge and mass parameters.             |
| # ZA,AWR |                                                           |
+----------+-----------------------------------------------------------+
| **EPE**  | Subshell binding energy (equal to the photoelectric edge  |
|          | energy) in eV. The value is zero if MT is not in 534-599. |
+----------+-----------------------------------------------------------+
| **EFL**  | Fluorescence yield (eV/photoionization).                  |
|          |                                                           |
|          | Value is zero if not a photoelectric subshell ionization  |
|          | cross section.                                            |
+----------+-----------------------------------------------------------+
| **NR,NP, | Standard TAB1 parameters (see Section 1.3.1).             |
| E~int~** |                                                           |
+----------+-----------------------------------------------------------+
| **σ(E)** | Cross section (barns) for a photon or electron of energy  |
|          | E given as a table of NP energy‑cross section pairs.      |
+----------+-----------------------------------------------------------+

> The structure of a section is

\[MAT, 23, MT/ ZA, AWR, 0, 0, 0, 0\] HEAD

\[MAT, 23, MT/ EPE, EFL, 0, 0, NR, NP/ E~int~ / σ(E)\] TAB1

\[MAT, 23, 0/ 0.0, 0.0, 0, 0, 0, 0\] SEND

**23.4. Procedures**

1.  Values are usually for elements; hence, except for mono-isotopic
    elements, ZA=Z × 1000.0; also, AWR should be for the naturally
    occurring element.

2.  Photoelectric edges will not be multi-valued. The edge will be
    defined by two energies differing in the fourth or fifth significant
    figure.

3\. Interpolation is normally log‑log (INT=5).

4\. Kerma factor (energy deposition coefficients) libraries will
normally be local because there is no universal definition. The
application will determine whether annihilation or other radiation
fractions are subtracted.