CSCGasCollisions.cc

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// This implements the CSC gas ionization simulation.
// It requires the GEANT3-generated tables in the input file
// (default) MuonData/EndcapData/collisions.dat

// Outstanding problems to fix 15-Oct-2003
// - ework must be re-tuned once eion or ework are removed before delta e range
// estimation.
// - logic of long-range delta e's must be revisited.
// - The code here needs to have diagnostic output removed, or reduced.
// PARTICULARLY filling of std::vectors!
// - 'gap' is a misnomer, when simhit entry and exit don't coincide with gap
// edges
//  so the CSCCrossGap might better be named as a simhit-related thing.
// 22-Jan-2004 Corrected position of trap for 'infinite' loop while
//    generating steps. Output files (debugV-flagged only) require SC flag.
//    Increase deCut from 10 keV to 100 keV to accomodate protons!
// Mar-2015: cout to LogVerbatim. Change superseded debugV-flagged code to flag
// from config.

#include "DataFormats/GeometryVector/interface/LocalVector.h"
#include "FWCore/MessageLogger/interface/MessageLogger.h"
#include "FWCore/ParameterSet/interface/FileInPath.h"
#include "FWCore/Utilities/interface/Exception.h"
#include "SimMuon/CSCDigitizer/src/CSCGasCollisions.h"

#include "CLHEP/Random/RandExponential.h"
#include "CLHEP/Random/RandFlat.h"

// Only needed if file writing code is activated again
// #include <iostream>
#include <cassert>
#include <fstream>

// stdlib math functions
// for 'abs':
#include <cstdlib>
// for usual math functions:
#include <cmath>

// stdlib container trickery
// for 'for_each':
#include <algorithm>
// for 'greater' and 'less':
#include <functional>
// for 'accumulate':
#include <iterator>
#include <numeric>

using namespace std;

/* Gas mixture is Ar/CO2/CF4 = 40/50/10
We'll use the ionization energies
Ar    15.8 eV
CO2   13.7 eV
CF4    17.8
to arrive at a weighted average of eion = 14.95 */

CSCGasCollisions::CSCGasCollisions(const edm::ParameterSet &pset)
    : me("CSCGasCollisions"),
      gasDensity(2.1416e-03),
      deCut(1.e05),
      eion(14.95),
      ework(34.0),
      clusterExtent(0.001),
      theGammaBins(N_GAMMA, 0.),
      theEnergyBins(N_ENERGY, 0.),
      theCollisionTable(N_ENTRIES, 0.),
      theCrossGap(nullptr),
      theParticleDataTable(nullptr),
      saveGasCollisions_(false),
      dumpGasCollisions_(false) {
  dumpGasCollisions_ = pset.getUntrackedParameter<bool>("dumpGasCollisions");

  edm::LogInfo(me) << "Constructing a " << me << ":";
  edm::LogInfo(me) << "gas density = " << gasDensity << " g/cm3";
  edm::LogInfo(me) << "max eloss per collision allowed = " << deCut / 1000.
                   << " keV (for higher elosses, hits should have been simulated.)";
  edm::LogInfo(me) << "ionization threshold = " << eion << " eV";
  edm::LogInfo(me) << "effective work function = " << ework << " eV";
  edm::LogInfo(me) << "cluster extent = " << clusterExtent * 1.e04 << " micrometres";
  edm::LogInfo(me) << "dump gas collision and simhit information? " << dumpGasCollisions();
  edm::LogInfo(me) << "save gas collision information? -NOT YET IMPLEMENTED- " << saveGasCollisions();

  readCollisionTable();
}

CSCGasCollisions::~CSCGasCollisions() {
  edm::LogInfo(me) << "Destructing a " << me;
  delete theCrossGap;
}

void CSCGasCollisions::readCollisionTable() {
  // I'd prefer to allow comments in the data file which means
  // complications of reading line-by-line and then item-by-item.

  // Is float OK? Or do I need double?

  // Do I need any sort of error trapping?

  // We use the default CMSSW data file path
  // We use the default file name 'collisions.dat'

  // This can be reset in .orcarc by SimpleConfigurable
  //    Muon:Endcap:CollisionsFile

  // TODO make configurable
  string colliFile = "SimMuon/CSCDigitizer/data/collisions.dat";
  edm::FileInPath f1{colliFile};
  edm::LogInfo(me) << ": reading " << f1.fullPath();

  ifstream fin{f1.fullPath()};

  if (fin.fail()) {
    string errorMessage = "Cannot open input file " + f1.fullPath();
    edm::LogError("CSCGasCollisions") << errorMessage;
    throw cms::Exception(errorMessage);
  }

  fin.clear();             // Clear eof read status
  fin.seekg(0, ios::beg);  // Position at start of file

  // @@ We had better have the right sizes everywhere or all
  // hell will break loose. There's no trapping.
  LogTrace(me) << "Reading gamma bins";
  int j = 0;
  for (int i = 0; i < N_GAMMA; ++i) {
    fin >> theGammaBins[i];
    LogTrace(me) << ++j << "   " << theGammaBins[i];
  }

  j = 0;
  LogTrace(me) << "Reading energy bins \n";
  for (int i = 0; i < N_ENERGY; ++i) {
    fin >> theEnergyBins[i];
    LogTrace(me) << ++j << "   " << theEnergyBins[i];
  }

  j = 0;
  LogTrace(me) << "Reading collisions table \n";

  for (int i = 0; i < N_ENTRIES; ++i) {
    fin >> theCollisionTable[i];
    LogTrace(me) << ++j << "   " << theCollisionTable[i];
  }

  fin.close();
}

void CSCGasCollisions::setParticleDataTable(const ParticleDataTable *pdt) { theParticleDataTable = pdt; }

void CSCGasCollisions::simulate(const PSimHit &simhit,
                                std::vector<LocalPoint> &positions,
                                std::vector<int> &electrons,
                                CLHEP::HepRandomEngine *engine) {
  const float epsilonL = 0.01;  // Shortness of simhit 'length'
  //  const float max_gap_z = 1.5;                     // Gas gaps are 0.5
  //  or 1.0 cm

  // Note that what I call the 'gap' may in fact be the 'length' of a PSimHit
  // which does not start and end on the gap edges. This confuses the
  // nomenclature at least.

  double mom = simhit.pabs();  // in GeV/c - see MuonSensitiveDetector.cc
  //  int iam       = simhit.particleType();           // PDG type
  delete theCrossGap;  // before building new one
  assert(theParticleDataTable != nullptr);
  ParticleData const *particle = theParticleDataTable->particle(simhit.particleType());
  double mass = 0.105658;  // assume a muon
  if (particle == nullptr) {
    edm::LogError("CSCGasCollisions") << "Cannot find particle of type " << simhit.particleType() << " in the PDT";
  } else {
    mass = particle->mass();
  }

  theCrossGap = new CSCCrossGap(mass, mom, simhit.exitPoint() - simhit.entryPoint());
  float gapSize = theCrossGap->length();

  // Test the simhit 'length' (beware of angular effects)
  //  if ( gapSize <= epsilonL || gapSize > max_gap_z ) {
  if (gapSize <= epsilonL) {
    edm::LogVerbatim("CSCDigitizer") << "[CSCGasCollisions] WARNING! simhit entry and exit are too close - "
                                        "skipping simhit:"
                                     << "\n entry = " << simhit.entryPoint() << ": exit  = " << simhit.exitPoint()
                                     << "\n particle type = " << simhit.particleType()
                                     << " : momentum = " << simhit.pabs()
                                     << " GeV/c : energy loss = " << simhit.energyLoss() * 1.E06 << " keV"
                                     << ", gapSize = " << gapSize << " cm (< epsilonL = " << epsilonL << " cm)";
    return;  //@@ Just skip this PSimHit
  }

  // Interpolate the table for current gamma value
  // Extract collisions binned by energy loss values, for this gamma
  std::vector<float> collisions(N_ENERGY);
  double loggam = theCrossGap->logGamma();
  fillCollisionsForThisGamma(static_cast<float>(loggam), collisions);

  double anmin = exp(collisions[N_ENERGY - 1]);
  double anmax = exp(collisions[0]);
  double amu = anmax - anmin;

  LogTrace(me) << "collisions extremes = " << collisions[N_ENERGY - 1] << ", " << collisions[0] << "\n"
               << "anmin = " << anmin << ", anmax = " << anmax << "\n"
               << "amu = " << amu << "\n";

  float dedx = 0.;        // total energy loss
  double sum_steps = 0.;  // total distance across gap (along simhit direction)
  int n_steps = 0;        // no. of steps/primary collisions
  int n_try = 0;          // no. of tries to generate steps
  double step = -1.;      // Sentinel for start

  LocalPoint layerLocalPoint(simhit.entryPoint());

  // step/primary collision loop
  while (sum_steps < gapSize) {
    ++n_try;
    if (n_try > MAX_STEPS) {
      int maxst = MAX_STEPS;
      edm::LogVerbatim("CSCDigitizer") << "[CSCGasCollisions] WARNING! n_try = " << n_try
                                       << " is > MAX_STEPS = " << maxst << " - skipping simhit:"
                                       << "\n entry = " << simhit.entryPoint() << ": exit  = " << simhit.exitPoint()
                                       << "\n particle type = " << simhit.particleType()
                                       << " : momentum = " << simhit.pabs()
                                       << " GeV/c : energy loss = " << simhit.energyLoss() * 1.E06 << " keV"
                                       << "\n gapSize = " << gapSize << " cm, last step = " << step
                                       << " cm, sum_steps = " << sum_steps << " cm, n_steps = " << n_steps;
      break;
    }
    step = generateStep(amu, engine);
    if (sum_steps + step > gapSize)
      break;  // this step goes too far

    float eloss = generateEnergyLoss(amu, anmin, anmax, collisions, engine);

    // Is the eloss too large? (then GEANT should have produced hits!)
    if (eloss > deCut) {
      edm::LogVerbatim("CSCDigitizer") << "[CSCGasCollisions] WARNING! eloss = " << eloss << " eV is too large (> "
                                       << deCut << " eV) - trying another collision";
      continue;  // to generate another collision/step
    }

    dedx += eloss;      // the energy lost from the ionizing particle
    sum_steps += step;  // the position of the ionizing particle
    ++n_steps;          // the number of primary collisions

    if (n_steps > MAX_STEPS) {  // Extra-careful trap for bizarreness
      edm::LogVerbatim("CSCDigitizer") << "[CSCGasCollisions] WARNING! " << n_steps
                                       << " is too many steps -  skipping simhit:"
                                       << "\n entry = " << simhit.entryPoint() << ": exit  = " << simhit.exitPoint()
                                       << "\n particle type = " << simhit.particleType()
                                       << " : momentum = " << simhit.pabs()
                                       << " GeV/c : energy loss = " << simhit.energyLoss() * 1.E06 << " keV"
                                       << "\ngapSize=" << gapSize << " cm, last step=" << step
                                       << " cm, sum_steps=" << sum_steps << " cm";
      break;
    }
    LogTrace(me) << "sum_steps = " << sum_steps << " cm , dedx = " << dedx << " eV";

    // Generate ionization.
    // eion is the minimum energy at which ionization can occur in the gas
    if (eloss > eion) {
      layerLocalPoint += step * theCrossGap->unitVector();  // local point where the collision occurs
      ionize(eloss, layerLocalPoint);
    } else {
      LogTrace(me) << "Energy available = " << eloss << " eV is too low for ionization (< eion = " << eion << " eV)";
    }

  }  // step/collision loop

  if (dumpGasCollisions())
    writeSummary(n_try, n_steps, sum_steps, dedx, simhit);

  // Return values in two container arguments
  positions = theCrossGap->ionClusters();
  electrons = theCrossGap->electrons();
  return;
}

double CSCGasCollisions::generateStep(double avCollisions, CLHEP::HepRandomEngine *engine) const {
  // Generate a m.f.p.  (1/avCollisions = cm/collision)
  double step = (CLHEP::RandExponential::shoot(engine)) / avCollisions;

  // Without using CLHEP: approx random exponential by...
  //    double da = double(rand())/double(RAND_MAX);
  //    double step = -log(1.-da)/avCollisions;

  LogTrace(me) << " step = " << step << " cm";
  // Next line only used to fill a container of 'step's for later diagnostic
  // dumps
  if (dumpGasCollisions())
    theCrossGap->addStep(step);
  return step;
}

float CSCGasCollisions::generateEnergyLoss(double avCollisions,
                                           double anmin,
                                           double anmax,
                                           const std::vector<float> &collisions,
                                           CLHEP::HepRandomEngine *engine) const {
  // Generate a no. of collisions between collisions[0] and [N_ENERGY-1]
  float lnColl = log(CLHEP::RandFlat::shoot(engine, anmin, anmax));

  // Without using CLHEP: approx random between anmin and anmax
  //    double ra = double(rand())/double(RAND_MAX)*avCollisions;
  //    cout << "ra = " << ra << std::endl;
  //    float lnColl = static_cast<float>( log( ra ) );

  // Find energy loss for that number
  float lnE = lnEnergyLoss(lnColl, collisions);
  float eloss = exp(lnE);
  // Compensate if gamma was actually below 1.1
  if (theCrossGap->gamma() < 1.1)
    eloss = eloss * 0.173554 / theCrossGap->beta2();
  LogTrace(me) << "eloss = " << eloss << " eV";
  // Next line only used to fill container of eloss's for later diagnostic dumps
  if (dumpGasCollisions())
    theCrossGap->addEloss(eloss);
  return eloss;
}

void CSCGasCollisions::ionize(double energyAvailable, LocalPoint startHere) const {
  while (energyAvailable > eion) {
    LogTrace(me) << "     NEW CLUSTER " << theCrossGap->noOfClusters() + 1 << " AT " << startHere;
    LocalPoint newCluster(startHere);
    theCrossGap->addCluster(newCluster);

    //@@ I consider NOT subtracting eion before calculating range to be a bug.
    //@@ But this changes tuning of the algorithm so leave it until after the
    // big rush to 7_5_0
    //@@  energyAvailable -= eion;

    // Sauli CERN 77-09: delta e range with E in MeV (Sauli references Kobetich
    // & Katz 1968 but that has nothing to do with this expression! He seems to
    // have  made a mistake.) I found the Sauli-quoted relationship in R.
    // Glocker, Z. Naturforsch. Ba, 129 (1948): delta e range R = aE^n with
    // a=710, n=1.72 for E in MeV and R in mg/cm^2 applicable over the range E =
    // 0.001 to 0.3 MeV.

    // Take HALF that range. //@@ Why? Why not...
    double range = 0.5 * (0.71 / gasDensity) * pow(energyAvailable * 1.E-6, 1.72);
    LogTrace(me) << " range = " << range << " cm";
    if (range < clusterExtent) {
      // short-range delta e
      // How many electrons can we make? Now use *average* energy for ionization
      // (not *minimum*)
      int nelec = static_cast<int>(energyAvailable / ework);
      LogTrace(me) << "short-range delta energy in = " << energyAvailable << " eV";
      // energyAvailable -= nelec*(energyAvailable/ework);
      energyAvailable -= nelec * ework;
      // If still above eion (minimum, not average) add one more e
      if (energyAvailable > eion) {
        ++nelec;
        energyAvailable -= eion;
      }
      LogTrace(me) << "short-range delta energy out = " << energyAvailable << " eV, nelec = " << nelec;
      theCrossGap->addElectrons(nelec);
      break;

    } else {
      // long-range delta e
      LogTrace(me) << "long-range delta \n"
                   << "no. of electrons in cluster now = " << theCrossGap->noOfElectrons();
      theCrossGap->addElectrons(1);  // Position is at startHere still

      bool new_range = false;
      while (!new_range && (energyAvailable > ework)) {
        energyAvailable -= ework;
        while (energyAvailable > eion) {
          double range2 = 0.5 * 0.71 / gasDensity * pow(1.E-6 * energyAvailable, 1.72);
          double drange = range - range2;
          LogTrace(me) << "  energy left = " << energyAvailable << " eV, range2 = " << range2
                       << " cm, drange = " << drange << " cm";
          if (drange < clusterExtent) {
            theCrossGap->addElectronToBack();  // increment last element
          } else {
            startHere += drange * theCrossGap->unitVector();  // update delta e start position
            range = range2;                                   // update range
            new_range = true;                                 // Test range again
            LogTrace(me) << "reset range to range2 = " << range << " from startHere = " << startHere << "  and iterate";
          }
          break;  // out of inner while energyAvailable>eion

        }  // inner while energyAvailable>eion

      }  // while !new_range && energyAvailable>ework

      // energyAvailable now less than ework, but still may be over eion...add
      // an e
      if (energyAvailable > eion) {
        energyAvailable -= ework;          // yes, it may go negative
        theCrossGap->addElectronToBack();  // add one more e
      }

    }  // if range

  }  // outer while energyAvailable>eion
}

void CSCGasCollisions::writeSummary(int n_try, int n_steps, double sum_steps, float dedx, const PSimHit &simhit) const {
  // Switched from std::cout to LogVerbatim, Mar 2015

  std::vector<LocalPoint> ion_clusters = theCrossGap->ionClusters();
  std::vector<int> electrons = theCrossGap->electrons();
  std::vector<float> elosses = theCrossGap->eLossPerStep();
  std::vector<double> steps = theCrossGap->stepLengths();

  edm::LogVerbatim("CSCDigitizer") << "------------------"
                                   << "\nAFTER CROSSING GAP";
  /*
  edm::LogVerbatim("CSCDigitizer") << "no. of steps tried             = " <<
  n_try
                                 << "\nno. of steps from theCrossGap  = " <<
  theCrossGap->noOfSteps()
                                 << "\nsize of steps vector           = " <<
  steps.size();

  edm::LogVerbatim("CSCDigitizer") << "no. of collisions (steps)      = " <<
  n_steps
                                 << "\nsize of elosses vector         = " <<
  elosses.size()
                                 << "\nsize of ion clusters vector    = " <<
  ion_clusters.size()
                                 << "\nsize of electrons vector       = " <<
  electrons.size();
  */

  size_t nsteps = n_steps;          // force ridiculous type conversion
  size_t mstep = steps.size() - 1;  // final step gets filled but is outside gas
                                    // gap - unless we reach MAX_STEPS
  if ((nsteps != MAX_STEPS) && (nsteps != mstep)) {
    edm::LogVerbatim("CSCDigitizer") << "[CSCGasCollisions] WARNING! no. of steps = " << nsteps
                                     << " .ne. steps.size()-1 = " << mstep;
  }
  size_t meloss = elosses.size();

  if (nsteps != meloss) {
    edm::LogVerbatim("CSCDigitizer") << "[CSCGasCollisions] WARNING! no. of steps = " << nsteps
                                     << " .ne. no. of elosses = " << meloss;
  } else {
    edm::LogVerbatim("CSCDigitizer") << "[CSCGasCollisions] # / length of step / energy loss per collision:";
    for (size_t i = 0; i != nsteps; ++i) {
      edm::LogVerbatim("CSCDigitizer") << i + 1 << " / S: " << steps[i] << " / E: " << elosses[i];
    }
  }

  size_t mclus = ion_clusters.size();
  size_t melec = electrons.size();

  if (mclus != melec) {
    edm::LogVerbatim("CSCDigitizer") << "[CSCGasCollisions] WARNING! size of cluster vector = " << mclus
                                     << " .ne. size of electrons vector = " << melec;
  } else {
    edm::LogVerbatim("CSCDigitizer") << "[CSCGasCollisions] # / postion of "
                                        "cluster / electrons per cluster: ";
    for (size_t i = 0; i != mclus; ++i) {
      edm::LogVerbatim("CSCDigitizer") << i + 1 << " / I: " << ion_clusters[i] << " / E: " << electrons[i];
    }
  }

  int n_ic = count_if(elosses.begin(), elosses.end(), [&](auto c) { return c > this->eion; });

  edm::LogVerbatim("CSCDigitizer") << "[CSCGasCollisions] total no. of collision steps = " << n_steps;
  edm::LogVerbatim("CSCDigitizer") << "[CSCGasCollisions] total sum of steps = " << sum_steps << " cm";
  if (nsteps > 0)
    edm::LogVerbatim("CSCDigitizer") << "[CSCGasCollisions] average step length = " << sum_steps / float(nsteps)
                                     << " cm";
  edm::LogVerbatim("CSCDigitizer") << "[CSCGasCollisions] total energy loss across gap = " << dedx
                                   << " eV = " << dedx / 1000. << " keV";
  edm::LogVerbatim("CSCDigitizer") << "[CSCGasCollisions] no. of primary ionizing collisions across gap = "
                                      "no. with eloss > eion = "
                                   << eion << " eV = " << n_ic;
  if (nsteps > 0)
    edm::LogVerbatim("CSCDigitizer") << "[CSCGasCollisions] average energy loss/collision = " << dedx / float(nsteps)
                                     << " eV";

  std::vector<int>::const_iterator bigger = find(electrons.begin(), electrons.end(), 0);
  if (bigger != electrons.end()) {
    edm::LogVerbatim("CSCDigitizer") << "[CSCGasCollisions] TROUBLE! There is a cluster with 0 electrons.";
  }

  int n_e = accumulate(electrons.begin(), electrons.end(), 0);

  edm::LogVerbatim("CSCDigitizer") << "[CSCGasCollisions] SUMMARY: simhit"
                                   << "\n entry = " << simhit.entryPoint() << ": exit  = " << simhit.exitPoint()
                                   << "\n particle type = " << simhit.particleType()
                                   << " : momentum = " << simhit.pabs()
                                   << " GeV/c : energy loss = " << simhit.energyLoss() * 1.E06 << " keV";

  if (nsteps > 0) {
    edm::LogVerbatim("CSCDigitizer") << "[CSCGasCollisions] SUMMARY: ionization"
                                     << " : steps= " << nsteps << " : sum(steps)= " << sum_steps
                                     << " cm : <step>= " << sum_steps / float(nsteps) << " cm"
                                     << " : ionizing= " << n_ic << " : ionclus= " << mclus << " : total e= " << n_e
                                     << " : <dedx/step>= " << dedx / float(nsteps)
                                     << " eV : <e/ionclus>= " << float(n_e) / float(mclus)
                                     << " : dedx= " << dedx / 1000. << " keV";
  } else {
    edm::LogVerbatim("CSCDigitizer") << "[CSCGasCollisons] ERROR? no collision steps!";
  }

  // Turn off output file -  used for initial development
  //  if ( saveGasCollisions() ) {
  //    ofstream of0("osteplen.dat",ios::app);
  //    std::copy( steps.begin(), steps.end(),
  //    std::ostream_iterator<float>(of0,"\n"));

  //    ofstream of1("olperc.dat",ios::app);
  //    std::copy( elosses.begin(), elosses.end(),
  //    std::ostream_iterator<float>(of1,"\n"));

  //   ofstream of2("oclpos.dat",ios::app);
  //   std::copy( ion_clusters.begin(), ion_clusters.end(),
  //   std::ostream_iterator<LocalPoint>(of2,"\n"));

  //   ofstream of3("oepercl.dat",ios::app);
  //   std::copy( electrons.begin(), electrons.end(),
  //   std::ostream_iterator<int>(of3,"\n"));
  // }
}

float CSCGasCollisions::lnEnergyLoss(float lnCollisions, const std::vector<float> &collisions) const {
  float lnE = -1.;

  // Find collision[] bin in which lnCollisions falls
  std::vector<float>::const_iterator it = find(collisions.begin(), collisions.end(), lnCollisions);

  if (it != collisions.end()) {
    // found the value
    std::vector<float>::difference_type ihi = it - collisions.begin();
    LogTrace(me) << ": using one energy bin " << ihi << " = " << theEnergyBins[ihi]
                 << " for lnCollisions = " << lnCollisions;
    lnE = theEnergyBins[ihi];
  } else {
    // interpolate the value
    std::vector<float>::const_iterator loside =
        find_if(collisions.begin(), collisions.end(), [&lnCollisions](auto c) { return c < lnCollisions; });
    std::vector<float>::difference_type ilo = loside - collisions.begin();
    if (ilo > 0) {
      LogTrace(me) << ": using energy bin " << ilo - 1 << " and " << ilo;
      lnE = theEnergyBins[ilo - 1] + (lnCollisions - collisions[ilo - 1]) *
                                         (theEnergyBins[ilo] - theEnergyBins[ilo - 1]) /
                                         (collisions[ilo] - collisions[ilo - 1]);
    } else {
      LogTrace(me) << ": using one energy bin 0 = " << theEnergyBins[0] << " for lnCollisions = " << lnCollisions;
      lnE = theEnergyBins[0];  //@@ WHAT ELSE TO DO?
    }
  }

  return lnE;
}

void CSCGasCollisions::fillCollisionsForThisGamma(float logGamma, std::vector<float> &collisions) const {
  std::vector<float>::const_iterator bigger =
      find_if(theGammaBins.begin(), theGammaBins.end(), [&logGamma](auto c) { return c > logGamma; });

  if (bigger == theGammaBins.end()) {
    // use highest bin
    LogTrace(me) << ": using highest gamma bin"
                 << " for logGamma = " << logGamma;
    for (int i = 0; i < N_ENERGY; ++i)
      collisions[i] = theCollisionTable[i * N_GAMMA];
  } else {
    // use bigger and its lower neighbour
    std::vector<float>::difference_type ihi = bigger - theGammaBins.begin();
    if (ihi > 0) {
      double dlg2 = *bigger--;  // and decrement after deref
      // LogTrace(me) << ": using gamma bins "
      //                   << ihi-1 << " and " << ihi;
      double dlg1 = *bigger;  // now the preceding element
      double dlg = (logGamma - dlg1) / (dlg2 - dlg1);
      double omdlg = 1. - dlg;
      for (int i = 0; i < N_ENERGY; ++i)
        collisions[i] = theCollisionTable[i * N_GAMMA + ihi - 1] * omdlg + theCollisionTable[i * N_GAMMA + ihi] * dlg;
    } else {
      // bigger has no lower neighbour
      LogTrace(me) << ": using lowest gamma bin"
                   << " for logGamma = " << logGamma;

      for (int i = 0; i < N_ENERGY; ++i)
        collisions[i] = theCollisionTable[i * N_GAMMA];
    }
  }
}