How can I get information of secondaries generated at the particular step?

All secondaries generated along a track was accumulated in G4SteppingManager and there is no direct way of accessing to the secondaries produced by a particular step. From your UserSteppingAction, you can get all such information by accessing appropriately to G4SteppingManager. List of processes which not only limited but also involved to the step can be listed as well. Further information beyond this tip can be found in the implementation of G4SteppingVerbose class.

Here is the sample implementation of G4UserSteppingAction

Source file (T01SteppingAction.cc) 

#include "T01SteppingAction.hh"
#include "G4SteppingManager.hh"
#include "G4Track.hh"
#include "G4Step.hh"
#include "G4ios.hh"
#include "G4UnitsTable.hh"

void T01SteppingAction::UserSteppingAction(const G4Step * theStep)
{
  G4cout << "Step is limited by "
   << theStep->GetPostStepPoint()->GetProcessDefinedStep()->GetProcessName()
   << G4endl;
  G4cout << "Processes involved to the step" << G4endl;
  G4StepStatus stepStatus = fpSteppingManager->GetfStepStatus();

  if(stepStatus==fAtRestDoItProc)
  {
    G4ProcessVector* procAtRest = fpSteppingManager->GetfAtRestDoItVector();
    G4SelectedAtRestDoItVector* selProcAtRest
     = fpSteppingManager->GetfSelectedAtRestDoItVector();
    size_t MAXofAtRestLoops = fpSteppingManager->GetMAXofAtRestLoops();
    for(size_t i1=0;i1<MAXofAtRestLoops;i1++)
    {
      if((*selProcAtRest)[MAXofAtRestLoops-i1-1]==2)
      { G4cout << "  At rest : " << (*procAtRest)[i1]->GetProcessName() << " (forced)" << G4endl; }
      else if((*selProcAtRest)[MAXofAtRestLoops-i1-1]==1)
      { G4cout << "  At rest : " << (*procAtRest)[i1]->GetProcessName() << G4endl; }
    }
  }

  if(stepStatus!=fExclusivelyForcedProc && stepStatus!=fAtRestDoItProc)
  {
    G4ProcessVector* procAlong = fpSteppingManager->GetfAlongStepDoItVector();
    size_t MAXofAlongStepLoops = fpSteppingManager->GetMAXofAlongStepLoops();
    for(size_t i2=0;i2<MAXofAlongStepLoops;i2++)
    {
      if((*procAlong)[i2]!=0)
      G4cout << "  Along step : " << (*procAlong)[i2]->GetProcessName() << G4endl;
    }
  }

  if(stepStatus!=fAtRestDoItProc)
  {
    G4ProcessVector* procPost = fpSteppingManager->GetfPostStepDoItVector();
    G4SelectedPostStepDoItVector* selProcPost
     = fpSteppingManager->GetfSelectedPostStepDoItVector();
    size_t MAXofPostStepLoops = fpSteppingManager->GetMAXofPostStepLoops();
    for(size_t i3=0;i3<MAXofPostStepLoops;i3++)
    {
      if((*selProcPost)[MAXofPostStepLoops-i3-1]==2)
      { G4cout << "  Post step : " << (*procPost)[i3]->GetProcessName() << " (forced)" << G4endl; }
      else if((*selProcPost)[MAXofPostStepLoops-i3-1]==1)
      { G4cout << "  Post step : " << (*procPost)[i3]->GetProcessName() << G4endl; }
    }
  }

  G4int nSecAtRest = fpSteppingManager->GetfN2ndariesAtRestDoIt();
  G4int nSecAlong  = fpSteppingManager->GetfN2ndariesAlongStepDoIt();
  G4int nSecPost   = fpSteppingManager->GetfN2ndariesPostStepDoIt();
  G4int nSecTotal  = nSecAtRest+nSecAlong+nSecPost;
  G4TrackVector* secVec = fpSteppingManager->GetfSecondary();

  if(nSecTotal>0)
  {
    G4cout << "  :----- List of 2ndaries - " << std::setw(3) << nSecTotal
           << " (Rest=" << std::setw(2) << nSecAtRest
           << ",Along=" << std::setw(2) << nSecAlong
           << ",Post="  << std::setw(2) << nSecPost << ")" << G4endl;

    for(size_t lp1=(*secVec).size()-nSecTotal; lp1<(*secVec).size(); lp1++)
    {
      G4cout << "    : "
             << G4BestUnit((*secVec)[lp1]->GetPosition(), "Length") << " "
             << std::setw( 9) << G4BestUnit((*secVec)[lp1]->GetKineticEnergy() , "Energy") << " "
             << std::setw(18) << (*secVec)[lp1]->GetDefinition()->GetParticleName()
             << " generated by " << (*secVec)[lp1]->GetCreatorProcess()->GetProcessName() << G4endl;
    }
  }
}

An example output of this class is the following. Please note that the following list was snipped out from lengthy output.

sample output from T01SteppingAction.cc 
Step is limited by Transportation
Processes involved to the step
  Along step : Transportation
  Along step : msc
  Along step : eIoni 
  Post step : Transportation
  Post step : msc
  Post step : eIoni (forced)

Step is limited by eIoni
Processes involved to the step
  Along step : Transportation
  Along step : msc
  Along step : eIoni
  Post step : Transportation
  Post step : msc
  Post step : eIoni (forced)
  :----- List of 2ndaries -   1 (Rest= 0,Along= 0,Post= 1)
    : -0.019113 6.51462 0.000138179 cm     1.95542 keV   e-     generated by eIoni

Step is limited by eBrem
Processes involved to the step
  Along step : Transportation
  Along step : msc
  Along step : eIoni
  Post step : Transportation
  Post step : msc
  Post step : eBrem (forced)
  :----- List of 2ndaries -   1 (Rest= 0,Along= 0,Post= 1)
    : -0.288498 25.3216 0.000807671 cm      63.393 MeV   gamma     generated by eBrem

Step is limited by conv
Processes involved to the step
  Along step : Transportation
  Post step : Transportation
  Post step : conv (forced)
  :----- List of 2ndaries -   2 (Rest= 0,Along= 0,Post= 2)
    : -0.881643 51.3114 -0.000172928 cm     11.2898 MeV   e-     generated by conv
    : -0.881643 51.3114 -0.000172928 cm     51.0812 MeV   e+     generated by conv

Step is limited by annihil
Processes involved to the step
  At rest : annihil (forced)
  :----- List of 2ndaries -   2 (Rest= 2,Along= 0,Post= 0)
    : -1.01094 51.7613 -0.00290133 cm     510.999 keV   gamma     generated by annihil
    : -1.01094 51.7613 -0.00290133 cm     510.999 keV   gamma     generated by annihil

We are aware of the common requirement of doing this in sensitive detector. It will be available in near future release.

Makoto Asai