What's the difference between mcdf-xcom and xcom photon cross section in low energy beams?

[rtownson]

This is a repost from reddit, to save the useful discussion. Thanks to /u/heidarloo for the question.

"I have simulated a geometry in the energy range of 100 keV using EGsnrc's user code. At the low photon energied, the photoelectric effect is the more important interaction in term of the transfered to secondary electrons . In the EGSnrc, I want to use default photon cross section (xcom), and renormalized photoelectric cross sections (mcdf-xcom or epdl-xcom). I have some questions:

1. What's the difference between mcdf-xcom and xcom photon cross section in low energy beams? Which are more suitable for simulation in the photoelectric ranges? Which of them more accurate output?

2. What's the difference between mcdf-xcom and epdl-xcom photon cross section in low energy beams?

3. General question: What's the difference between unnirmalized and normalized photon cross section in low energy beams?"

[rtownson]

Thanks to @mainegra for providing the following answer:

This topic has not been completely resolved in my opinion. EPDL and XCOM produce identical results in every case I have tested. They use the same photoelectric cross sections but slightly different atomic edge energies.

mcdf-xcom uses re-normalized photoelectric cross sections to account for multi-configuration effects due to neighbouring electrons using Pratt's high-energy normalization while xcom assumes interaction with an independent electron. Clearly mcdf-xcom is theoretically more sound, but there is lack of experimental evidence supporting this.

Here is a paper introducing the theory and providing a good historical background: Theory and calculation of the atomic photoeffect by Sabbatucci and Salvat (2016)

ICRU report 90 (2016) describes the current status: Key data for ionizing-radiation dosimetry: measuring standards and applications

The paper below (open access) performs a statistical evaluation of different photoelectric cross section compilations by comparing to a huge number of published experimental data (large uncertainties) from the early 1900's to date in a large energy range (majority below 1 keV) for a large number of elements (no compounds or mixtures). It also provides a great review of the current existing models. It might also be worth it to read their preceding study:

Evolutions in Photoelectric Cross Section Calculations and Their Validation

Their conclusion is that:

No statistically significant improvements are observed with respect to the calculation method identified in the previous paper as the state of the art fort he intended purpose, encoded in the EPDL97 data library. Some of the more recent computational methods exhibit significantly lower capability to reproduce experimental measurements than the existing alternatives.

A recent study (open access) estimates type B uncertainties in brachytherapy dose calculations arising from differences in computer codes and photoelectric cross section compilations used.

It is important to provide enough experimental evidence and make a definitive decision as to which model to use for energies below 100 keV. In the energy range 1 keV to 100 keV, kerma in air or water can differ by 3% to 5%!

I also recommend the references in the above papers for further illumination!