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McXtrace Single_crystal: compute absorption length directly, use runtime NA #2937

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Merged
willend merged 1 commit into from
Oct 30, 2026
Merged

McXtrace Single_crystal: compute absorption length directly, use runtime NA #2937

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22 changes: 6 additions & 16 deletions mcxtrace-comps/samples/Single_crystal.comp
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Original file line number Diff line number Diff line change
Expand Up @@ -1490,7 +1490,7 @@ TRACE
double V0; /* Volume of unit cell */
double l_full; /* photon path length for transmission */
double l; /* Path length to scattering event */
double abs_xsect, abs_xlen; /* Absorption cross section and length */
double abs_xlen; /* Absorption length */
double inc_xsect, inc_xlen; /* Incoherent scattering cross section and length */
double coh_xsect, coh_xlen; /* Coherent cross section and length */
double tot_xsect, tot_xlen; /* Total cross section and length */
Expand Down Expand Up @@ -1539,23 +1539,14 @@ TRACE
ki = sqrt (kx * kx + ky * ky + kz * kz);
event_counter = 0;

/*absorption cross-section */
abs_xsect = 0;
V0 = hkl_info.V0;
/* absorption: sigma [cm^2] = atomic_weight [g/mol] / NA [1/mol] * mu [cm^2/g],
then * 1e26 -> [fm^2] and / V0 [Angs^3] -> [1/m]. Independent of the barns flag. */
abs_xlen = 0;
if (abs_info.muc) {
double mu = Table_Value (abs_info.table, ki * K2E, abs_info.muc);
abs_xsect = (abs_info.atomic_weight / (6.02214076 * 1E23)) * mu; // this is in cm^2
// atomic weight = [g . mol^-1]
// Avogadro number = [mol^-1]
// mu = [cm squared . g⁻1]
// therefore convert to barns if barns set to 1, otherwise to fm^2
if (hkl_info.flag_barns) { // convert cm^2 into barns
abs_xsect *= 1E24;
} else { // convert cm^2 into fm^2, else we assume fm^2
abs_xsect *= 1E26;
}
abs_xlen = abs_info.atomic_weight / NA * mu * 1e26 / V0;
}
V0 = hkl_info.V0;
abs_xlen = abs_xsect / V0;

/*look into Compton scattering*/
inc_xsect = hkl_info.sigma_i;
Expand All @@ -1564,7 +1555,6 @@ TRACE
if (hkl_info.flag_barns) {
/*If cross sections are given in barns, we need a scaling factor of 100
to get scattering lengths in m, since V0 is assumed to be in AA*/
abs_xlen *= 100;
inc_xlen *= 100;
} /* else assume fm^2 */
L = hkl_list;
Expand Down
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