include/Geant4/G4LPMFunction.hh

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0025 //
0026 //
0027 // -------------------------------------------------------------------
0028 //
0029 // GEANT4 Class header file
0030 //
0031 //
0032 // File name: G4LPMFunction
0033 //
0034 // Author: Mihaly Novak
0035 //
0036 // Creation date: 08 October 2025
0037 //
0038 // Modifications:
0039 //
0040 // Class Description:
0041 //
0042 // The `G(s)` and `\phi(s)` Landau Pomeranchuk Migdal (LPM) suppression
0043 // functions, utilised both in some bremsstrahlung and pair-production models
0044 // when computing the actual LPM suppression effect, are factored in this util.
0045 // The functions are pre-computed and interpolated over the `s \in [0,2)`
0046 // intervall while approximated at `s > 2` values when they converge to unity.
0047 //
0048 // `G(s)` and `\phi(s)` are the spin flip and no flip suppression functions of
0049 // Migdal (Migdal PR,1956) including some slowly converging series to which
0050 // approximate expressions were derived by Stanev (Stanev at al. PRD,1982).
0051 // (See e.g. `G4eBremsstrahlungRelModel::ComputeLPMGsPhis` at Geant4-v.11.3.0
0052 //  on the details of the computation.)
0053 //
0054
0055 // -------------------------------------------------------------------
0056 //
0057
0058 #ifndef G4LPMFunction_h
0059 #define G4LPMFunction_h 1
0060
0061 #include "G4Types.hh"
0062
0063 namespace G4LPMFunction {
0064   // Precomputed G(s) and Phi(s) LPM functions.
0065   // Grid: s \in [0, 2.0], ds = 0.05 -> N = 41 points, interleaved (G(s),Phi(s))
0066   inline constexpr G4double kFuncLPM[] = {
0067     0.0000E+00, 0.0000E+00,  6.9163E-02, 2.5747E-01,  2.0597E-01, 4.4573E-01,
0068     3.5098E-01, 5.8373E-01,  4.8095E-01, 6.8530E-01,  5.8926E-01, 7.6040E-01,
0069     6.7626E-01, 8.1626E-01,  7.4479E-01, 8.5805E-01,  7.9826E-01, 8.8952E-01,
0070     8.4003E-01, 9.1338E-01,  8.7258E-01, 9.3159E-01,  8.9794E-01, 9.4558E-01,
0071     9.1776E-01, 9.5640E-01,  9.3332E-01, 9.6483E-01,  9.4560E-01, 9.7143E-01,
0072     9.5535E-01, 9.7664E-01,  9.6313E-01, 9.8078E-01,  9.6939E-01, 9.8408E-01,
0073     9.7444E-01, 9.8673E-01,  9.7855E-01, 9.8888E-01,  9.8191E-01, 9.9062E-01,
0074     9.8467E-01, 9.9204E-01,  9.8695E-01, 9.9321E-01,  9.8884E-01, 9.9417E-01,
0075     9.9042E-01, 9.9497E-01,  9.9174E-01, 9.9564E-01,  9.9285E-01, 9.9619E-01,
0076     9.9379E-01, 9.9666E-01,  9.9458E-01, 9.9706E-01,  9.9526E-01, 9.9739E-01,
0077     9.9583E-01, 9.9768E-01,  9.9632E-01, 9.9794E-01,  9.9674E-01, 9.9818E-01,
0078     9.9710E-01, 9.9839E-01,  9.9741E-01, 9.9857E-01,  9.9767E-01, 9.9873E-01,
0079     9.9790E-01, 9.9887E-01,  9.9809E-01, 9.9898E-01,  9.9826E-01, 9.9909E-01,
0080     9.9840E-01, 9.9918E-01,  9.9856E-01, 9.9926E-01
0081   };
0082
0083   // Obtain the `G(s)` and `\phi(s)` LPM suppression functions at any `s >= 0`.
0084   inline void GetLPMFunctions(G4double& lpmFuncG, G4double& lpmFuncPhi, G4double sVar) {
0085     // sanity check (s should be >= 0)
0086     if (sVar < 0.0) {
0087       lpmFuncG = 0.0;
0088       lpmFuncPhi = 0.0;
0089       return;
0090     }
0091     // case of `s in [0, 2)` use the precomputed functions and interpolate
0092     const G4double lpmSLimit =  2.0; // max_s:=2
0093     const G4double lpmISDelt = 20.0; // deta_s:=0.05, 1/delta_s=20
0094     if (sVar < lpmSLimit) {
0095       G4double  val = sVar*lpmISDelt;
0096       G4int    ilow = static_cast<G4int>(val);
0097       val       -= ilow;
0098       ilow      *= 2;
0099       lpmFuncG   = (kFuncLPM[ilow+2] - kFuncLPM[ilow]  )*val + kFuncLPM[ilow];
0100       lpmFuncPhi = (kFuncLPM[ilow+3] - kFuncLPM[ilow+1])*val + kFuncLPM[ilow+1];
0101       return;
0102     }
0103     // asymptotic case: G(s), Phi(s) goes to 1.0
0104     G4double ss = 1.0/(sVar*sVar);
0105     ss *= ss;
0106     lpmFuncG   = 1.0 - 0.0230655*ss;
0107     lpmFuncPhi = 1.0 - 0.01190476*ss;
0108   }
0109 };
0110
0111 #endif //  G4LPMFunction_h