You do not need to stop the tracking ( altough you can (*)). You should use a sensitive detector and register the energy of the incoming particle in its first step inside the sensitive volume ( the first hit). 

The step object is one of the parameters of the ProcessHits method of the G4VSensitiveDetector class, where you should code for the SV information:

G4bool WhateverApplicationSD::ProcessHits(G4Step*aStep, G4TouchableHistory*)...

( in which WhateverApplicationSD class is public of G4VSensitiveDetector)

Inside this method you should access the track from the step object:

G4Track *aTrack = aStep->GetTrack() ;

Then you should get the  KineticEnergy of the track if the process is "Transportation" and if the volume remains the same in the next step. See the code lines below:

G4StepPoint* preStepPoint = aStep->GetPreStepPoint();

const G4VProcess* CurrentProcess=preStepPoint->GetProcessDefinedStep();

if (CurrentProcess != 0) {

const G4String & StepProcessName = CurrentProcess->GetProcessName();

G4String volumePos = aTrack->GetNextVolume()->GetName();

if(StepProcessName== "Transportation" && volumePos == thisVolume) {

// processing hit when entering the volume

G4double kineticEnergy = aStep->GetTrack()->GetKineticEnergy();

}

}

I hope this was useful.

(*) in order to kill the track you just have to type ( inside ProcessHits): 

aTrack->SetTrackStatus(fStopAndKill);

Thank you for your answer.

I think that stepGetTrack()->GetKineticEnergy() returns the kinetic energy at the end of the current track. So, I guess that it is why you are encouraging me to get the it only if the process is a simple transportation and if the volume remains the same in the next step?

How to ensure this condition? Is it not safer to use an object derived from UserTrackInformation to do that?

Furthermore, I fear that scattering induces multiple counts of a same particle and that was why I prefer kill the track after this measurement, but the UserTrackInformation should help me to avoid this possible bias, except if there is an other way?

Thanks again!

"I think that stepGetTrack()->GetKineticEnergy() returns the kinetic energy at the end of the current track. So, I guess that it is why you are encouraging me to get the it only if the process is a simple transportation and if the volume remains the same in the next step?"

No, that is not the end of that track. the GetKineticEnergy() method  returns the kin. energy for the current state of the track. The transportation and volume constrains whose code I wrote in my answer are the ones needed to assure that you are getting the kin. energy information when the particle has just entered the SV: Transportation is the process that occurs when a track crosses a boundary between 2 volumes: it only occurs when the track goes in or out of a volume. If the next volume is the same it means that the track has entered the sensitive volume( it enters the volume through a step that places it right at the volume boundary and then it will interact again, inside that volume). This should answer your first 3 questions, because this code should ensure the condition you require. 

As for the last question (as I wrote before) if you want to kill the track you can just code:

aTrack->SetTrackStatus(fStopAndKill);  ( inside ProcessHits):

and the track will be killed  inside the SV, after the first step.

All right, thank you very much!

Patrícia Serrano Gonçalves

Greetings Patrícia,

I followed your advice and used the code you recommended to Arnaud Chapon to measure the entrance spectrum of particles (in this case protons).

Unfortunately, the kinE(MeV) value that I'm getting is not the one fulfilling the process (Transportation) and Volume if conditions, but the one of the process right after it.

For example, I shot a 100MeV proton to a water cube and looking at the verbose, indeed I can see that the first step is Transportation, in "Cube" (the volume I am interested in) and 100 MeV as kinE. The next process in this case is hIoni, still Cube, kinE of 94.4 MeV, but this 94.4 MeV is the value that is being printed out and added to my histogram, instead of the 100 MeV.

This is the code that I used:

if(step->GetTrack()->GetDefinition()==G4Proton::Proton()){

if (CurrentProcess != 0) {

const G4String & StepProcessName = CurrentProcess->GetProcessName();

G4String volumePos = aTrack->GetNextVolume()->GetName();

if(StepProcessName== "Transportation" && volumePos == "Cube") {

// processing hit when entering the volume

G4double kineticEnergy = step->GetTrack()->GetKineticEnergy();

CubeHit* kinEn = (*fHitsCollection)[0];

kinEn->AddKineticEnergy(kineticEnergy);

}

}

}

Could you please tell me what am I doing wrong?

I am also attaching a screenshot of the Verbose to make this a bit clearer.

Thank you very much in advance!