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Update electron/positron bremss #827

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Update electron/positron bremss #827

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18 changes: 15 additions & 3 deletions core/include/detray/materials/interaction.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -68,9 +68,20 @@ struct interaction {

scalar_type stopping_power{0.f};

// Only consider electrons at the moment
// Only consider electrons and positrons at the moment
// For middle-heavy particles muons, the bremss is negligibe
if (ptc.pdg_num() == electron<scalar_type>().pdg_num()) {
//
// Also, over 10 MeV, positron and electron bremss might be similar.
// In ICRU 37, it is written that "In our tabulations, the radiative
// stopping power for positrons has been assumed to be the same as that
// for electrons, which is a good approximation at energies above, say,
// 10 MeV. However, it should be mentioned that exploratory calculations
// by Feng et at. (1981), employing the same method as that previously
// used by them for electrons, indicate significant differences between
// positrons and electrons in regard to the differential bremsstrahlung
// cross sections in oxygen and uranium at 500, 50, and 10 keV"
if (ptc.pdg_num() == electron<scalar_type>().pdg_num() ||
ptc.pdg_num() == positron<scalar_type>().pdg_num()) {
// Stopping power ~ E/X (B. B. Rossi, High-energy particles,1952)
// This approximation gets poor in low energy below 10 MeV
stopping_power = rq.m_gamma * ptc.mass() / mat.X0();
Expand Down Expand Up @@ -148,7 +159,8 @@ struct interaction {

scalar_type derivative{0.f};

if (ptc.pdg_num() == electron<scalar_type>().pdg_num()) {
if (ptc.pdg_num() == electron<scalar_type>().pdg_num() ||
ptc.pdg_num() == positron<scalar_type>().pdg_num()) {
// Stopping power ~ E/X = gamma * m/X
// Derivative = dgamma/dqop * m/X = -beta^2 gamma/qop * m/X
// (Eq (D.5) of arXiv:2403.16720)
Expand Down
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