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TPC: take the charge of a stopping step from the energy deposit #16131

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TPC: take the charge of a stopping step from the energy deposit #16131

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19 changes: 11 additions & 8 deletions Detectors/TPC/simulation/src/Detector.cxx
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Original file line number Diff line number Diff line change
Expand Up @@ -210,6 +210,11 @@ Bool_t Detector::ProcessHits(FairVolume* vol)
Int_t numberOfElectrons = 0;
// I.H. - the type expected in addHit is float

// ---| Stepsize in cm and beta*gamma after the step |---
const double stepSize = fMC->TrackStep();
const double betaGamma = momentum.P() / fMC->TrackMass();
constexpr double minBetaGamma = 7.e-3; // validity limit of the collision model below

// use Geant4 energy deposit directly for ionisation (Kr-83m calibration simulations)
if (detParam.UseGeant4Edep) {
// We have multiple collisions and add fluctuations: smear nel using
Expand All @@ -229,16 +234,14 @@ Bool_t Detector::ProcessHits(FairVolume* vol)
// The number of electrons is stored as a float in the HitGroup: maximum cap at
// 2^24 (16777216) ==> largest integer a IEEE-754 float can represent exactly
numberOfElectrons = TMath::Min(numberOfElectrons, 16777216);
} else if (betaGamma < minBetaGamma && stepSize > 0.) {
// a finite step ending below the validity limit, e.g. a track that ranged out:
// ionisation electrons from the energy deposited in this step, Nel = Edep / Wion
numberOfElectrons = static_cast<int>(fMC->Edep() / static_cast<double>(gasParam.Wion));
} else {
// ---| Stepsize in cm |---
const double stepSize = fMC->TrackStep();

double betaGamma = momentum.P() / fMC->TrackMass();
betaGamma = TMath::Max(betaGamma, 7.e-3); // protection against too small bg

// ---| number of primary ionisations per cm |---
// ---| number of primary ionisations per cm (zero-length steps give no collisions) |---
const double primaryElectronsPerCM =
gasParam.Nprim * BetheBlochAleph(static_cast<float>(betaGamma), gasParam.BetheBlochParam[0],
gasParam.Nprim * BetheBlochAleph(static_cast<float>(TMath::Max(betaGamma, minBetaGamma)), gasParam.BetheBlochParam[0],
gasParam.BetheBlochParam[1], gasParam.BetheBlochParam[2],
gasParam.BetheBlochParam[3], gasParam.BetheBlochParam[4]);

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