Enhancement: McXtrace: add fluorescence sample

[farhi]

Use X-ray lib to simulate the energy and probability of fluorescence lines:

• http://lvserver.ugent.be/xraylib-web/index.php?xrlFunction=FluorYield&Element=26&ElementOrCompound=FeSO4&Compound=Ca5%28PO4%293&AugerTransa=K&AugerTransb=L2&AugerTransc=M3&Linename1a=K&Linename1b=L3&Linename2=KA1_LINE&LinenameSwitch=ALL&NISTcompound=Gadolinium+Oxysulfide&RadioNuclide=55Fe&Shell=ALL&Energy=10.0&Theta=1.5707964&Phi=3.14159&MomentumTransfer=0.57032&CKTrans=FL12_TRANS&Density=1.0&PZ=1.0&submit=Go%21&Language=C

In any case there are questions to assess:

• scattering direction (4Pi ?, cone ?, etc)
• time delay/decay ?
• etc

[farhi]

A link with xraylib (libxrl) is a straight-forward solution. An implementation with Compton scattering shows how libxrl can be used:

• https://github.com/McStasMcXtrace/McCode/blob/master/mcxtrace-comps/contrib/union/Compton_xrl_process.comp

In principle, fluo, Compton, etc can be used the same. The list of all cross sections is available:

• https://github.com/tschoonj/xraylib/wiki/The-xraylib-API-list-of-all-functions#cross-sections

[farhi]

For instance:

double CS_Energy(int Z, double E, xrl_error **error);

Given an element Z and an energy E, returns the mass-energy absorption cross section in cm2g

double CS_Total_CP(const char compound[], double E, xrl_error **error);
double CS_Photo_CP(const char compound[], double E, xrl_error **error);
double CS_Rayl_CP(const char compound[], double E, xrl_error **error);
double CS_Compt_CP(const char compound[], double E, xrl_error **error);
double CSb_Total_CP(const char compound[], double E, xrl_error **error);
double CSb_Photo_CP(const char compound[], double E, xrl_error **error);
double CSb_Rayl_CP(const char compound[], double E, xrl_error **error);
double CSb_Compt_CP(const char compound[], double E, xrl_error **error);
double CS_Total_Kissel_CP(const char compound[], double E, xrl_error **error);
double CSb_Total_Kissel_CP(const char compound[], double E, xrl_error **error);
double CS_Energy(const char compound[], double E, xrl_error **error);

Identical to the earlier mentioned functions, but require a chemical formula or a NIST compound name compound as first argument.

A C implementation is:

• https://github.com/tschoonj/xraylib/blob/master/example/xrlexample1.c