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G4SmpTerrell.cc
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51 // Copyright (c) 2006 The Regents of the University of California.
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53 // UCRL-CODE-224807
54 //
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56 // $Id: G4SmpTerrell.cc 69860 2013-05-16 14:39:02Z gcosmo $
57 //
58 
59 #include <cmath>
60 #include "G4fissionEvent.hh"
61 
62 #define TWOPI 6.283185307
63 #define SQRT2 1.414213562
64 #define BSHIFT -0.43287
65 #define WIDTH 1.079
66 
67 G4double G4fissionEvent::G4SmpTerrell(G4double nubar) {
68 /*
69  Description
70  Sample Fission Number from Terrell's modified Gaussian distribution
71 
72  method uses Red Cullen's algoritm UCRL-TR-222526
73 */
74 
75 /*
76  Input
77  nubar - average number of neutrons per fission
78  Output
79  G4SmpTerrell - sampled multiplicity
80 
81 */
82 
84  G4double temp1, temp2, expo, cshift;
85  G4double rw, theta, sampleg;
86 
87 
88  if (nubar < WIDTH) G4fissionerr(6, "SmpTerrell", "fission nubar out of range");
89 
90  width = SQRT2 * WIDTH;
91  temp1 = nubar + 0.5;
92  temp2 = temp1/width;
93  temp2 *= temp2;
94  expo = std::exp(-temp2);
95  cshift = temp1 + BSHIFT * WIDTH * expo/(1. - expo);
96 
97  do {
98  rw = std::sqrt(-std::log(fisslibrng()));
99  theta = TWOPI * fisslibrng();
100  sampleg = width * rw * std::cos(theta) + cshift;
101  } while (sampleg < 0.0);
102 
103  return std::floor(sampleg);
104 }