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| Geant 3.10 | GEANT User's Guide | PHYS460 ##
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Author(s) : F.Carminati Submitted: 20.12. 85
Origin : H.C.Fesefeldt Revised: 19.12.92
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|CALL GMUNUI |
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GMUNUI computes and stores in the appropriate bank the value of the muon-nucleus cross-section for a given material. It is called at initialization time by GPHYSI.
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|CALL GMUNU |
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GMUNU is called by GTMUON every time a muon-nucleus interaction has to happen. It generates the final state particles as well as the outgoing muon. A call to GUHADR is performed if IMUNU (which is the variable set by the MUNU data record) is equal to 1. If the GHEISHA interface is used, an inelastic interaction is forced (which could also be a fission in case of heavy materials). The secondaries from the pi nucleus interaction are always generated if IMUNU is equal to 1, irrespectively of the value of IHADR.
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| CALL GMUSIG (E,E1,COSTET) |
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This routine returns the value of the differential cross-section in millibarns for a muon of energy E to generate a nuclear interaction and an outgoing muon of energy E1 at an angle the cosine of which is COST.
This set of routines generates the interactions of muons with the nuclei of the tracking material. The code is a straight translation into the GEANT style of the corresponding code from the GHEISHA Monte Carlo Program. The GHEISHA routines CASMU and CALIM have been used [bib-GHEI].
The information contained in this chapter is mainly taken from the Gheisha manual (see note) to which the user is referred. The muon-nucleus inelastic cross-section is taken as 0.0003 mb for a nucleon when the energy of the incoming muon is E<30 GeV, and slowly increases for E>30 GeV according to the law:
0.25
sigma =0. 3*(E/30) [mub]
The energy and angle of final state muon is generated according to the ``free quark parton model''. If E is the energy of the incoming muon and Omega and W the angle and the energy of the outgoing muon, the differential cross-section can be written as:
((dsigma)/ (dOmegadW))= gamma(sigma + epsilonsigma )
T S
where:
Gamma = ((kalpha)/ (2pi))((W)/(E))((1)/(1- epsilon))
2 2 2 2 2
epsilon = [1 +2((Q +nu )/ (Q ))tan theta ]
2 Q and nu are the normal scaling variables expressed by:
2 2 2
Q =-q =2(EW- |p||p'|costheta-m ) and nu= E-W
-mu
here sigma and sigma are the photo-absorbtion cross-sections for T S transverse and longitudinal photons respectively for which the relation used is:
2
sigma =0. 3(1-((1)/(1. 868))Q nu)sigma
S T
and sigma is assumed to be constant sigma = 0.12 mb. For the incident T T flux K of the photons Gillman's convention is used:
2
K =nu +Q / 2nu
A three-dimensional importance sampling in the variables E, W and theta is performed each time an interaction has to occur.
The hadrons are generated in an approximate way. The virtual photon is replaced by a real pion of random charge with the same kinetic energy. Then the GUHADR routine is called to generate a pion-nucleus inelastic scattering. While the final state generated this way gives a good approximation for calorimetric purposes, the kinematics of the final state may be a rather poor approximation of reality. The muon-nucleus interactions are activated by the MUNU data record of GEANT. After a muon-nucleus interaction the muon will still be the current particle. If MUNU 1 has been specified, secondaries coming from the interaction of the virtual photon with the nucleus will be in the GEANT temporary stack. If MUNU 2 has been specified, then the secondary particles will not be generated and the energy lost by the muon will be added to DESTEP. For each material a table of muon-nucleus cross-sections is stored at initialization time. See material bank structure for details.