EGSnrc transport parameters not all most accurate out of the box

[rtownson]

Reminder: Fred just mentioned the default brems cs is BH, but more accurate is NRC. I think we already changed this default to be NRC, but we should double check.

[ftessier]

Yes, and also turn on electron-impact ionization by default, so that EGSnrc is most accurate out of the box. Also consider including Compton radiative corrections by default?

[ftessier]

By default, the brems sampling is option is BH, according to egsnrc.macros. A review of default parameters is in order, especially since some applications override the defaults

Default parameters ought to be the same and the most exact across the board, with documentation on possible efficiency improvement using alternative parameters in each application.