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Forum: Hadronic Processes
Re: Question Energy deposited by neutrons in plastic scintillator (Estela Suarez)
Date: 01 Oct, 2009
From: Estela Suarez <Estela Suarez>

Hi again,

I have found that using the QGSP_BERT_HP physics list, the output of the neutron  simulation looks as it should look like. So I guess that the problem was due to a not proper treatment of the low energy (E<15 MeV) neutron cross sections in my own physics list (taken from examples/advanced/Tiara), where I was using:
         G4LElastic* theElasticModel1
         G4LENeutronInelastic* theInelasticModel
         G4LFission* theFissionModel 
         G4LCapture* theCaptureModel

The point is that I am not only interested in low energy neutrons, but also in X-ray polarized photons. My previous User Physics List was leading with the Compton polarized effect through a ConstrucElectromagnetic() function that contained the following statements:
      pmanager->AddDiscreteProcess(new G4LowEnergyPolarizedCompton());
      pmanager->AddDiscreteProcess(new G4LowEnergyGammaConversion());
      pmanager->AddDiscreteProcess(new G4LowEnergyPhotoElectric());

I am not sure what QGSP_BERT_HP is doing with the electromagnetic processes, but it is certainly not taking care of the photon polarization.

Is it possible to define my own electromagnetic physics using at the same time the hadronic physics from QGSP_BERT_HP?

I have already tried using HadronPhysicsQGSP_BERT_HP.hh, but the neutron output is not correct. The neutrons never experience elasticScattering, and only inelastic scattering at large energies is taking place. I think that this is due to the following sentences, extracted from HadronPhysicsQGSP_BERT_HP.cc:

00061 theNeutrons=new G4NeutronBuilder;

00062 theNeutrons->RegisterMe(theQGSPNeutron=new

G4QGSPNeutronBuilder(QuasiElastic));

00063 theNeutrons->RegisterMe(theLEPNeutron=new G4LEPNeutronBuilder);

00064 theLEPNeutron->SetMinEnergy(19.9*MeV);

00065 theLEPNeutron->SetMinInelasticEnergy(9.5*GeV);

00066 theLEPNeutron->SetMaxInelasticEnergy(25*GeV); 00067 00068 theNeutrons->RegisterMe(theBertiniNeutron=new G4BertiniNeutronBuilder);

00069 theBertiniNeutron->SetMinEnergy(19.9*MeV);

00070 theBertiniNeutron->SetMaxEnergy(9.9*GeV);

The thresholds are just too high for the energies with whom I am dealing (below 15MeV). But I do not know how to modify these settings from my UserPhysicsList.

Please, could anybody tell me how to do one of the following things:

- Define polarized low energy electromagnetic processes (in particular Polarized Compton Scattering) in QGSP_BERT_HP physics list

- Introduce properly the QGSP_BERT_HP hadron physics in my User physics List.

I could send you my physics list file if that would help.

Best regards, Estela.

On Tue, 29 Sep 2009 10:23:21 GMT, Estela Suarez wrote:

> Hello,
> 
> I am simulating neutrons in a plastic scintillator material composed by
> 50% Carbon and 50% Hydrogen. The incoming neutron spectrum is close to a
> decreasing exponential between 1MeV and 10MeV. (see figure
> specNeut_input.pdf)
> 
> I am having some problems to understand the spectrum of the energy
> deposited in my detector.
> 
> - Since the cross section for neutron scattering in H is about twice as
> large as in C for incoming energies between 1MeV and 3MeV, I would
> expect that there are about 2 times more interactions with protons than
> with C12 atoms in my scintillator (for this energy range). What I
> observe is a factor 4.
> 
> - The energy deposited when the neutron interacts with a C12 atom (via
> HadronElastic, followed by a ionIonization) is usually less than 20%
> that of the incoming photon, what I think is right. But there is a gap
> in the deposited energy between 200 and 300 keV where no events appear
> (see the red line in figure specNeut_components.pdf). I would expect the
> spectrum of the deposited energy to be a continuous line, exponentially
> decreasing as the incoming spectrum is.
> 
> - Concerning the deposited energy when the neutron interacts with a
> proton (via HadronElastic followed by hIonization), the energy
> transfered to the proton is in most of the cases all the energy from the
> neutron (see the blue line in figure specNeut_components.pdf). I would
> expect it to be a uniform distribution between 0 and Eini (see Krane
> 1988, "Introductory Nuclear Physics").
> 
> I do not quite understand what would explain this spectral shape. I am
> not including yet the Birks' effect in the simulation. I am only
> obtaining the energy deposited by each particle using fStep ->
> GetTotalEnergyDeposit() in my SteppingVerbose.
> 
> I would be very thankful if somebody could give me some hint.
> 
> Best regards, Estela.
> 
>    Attachment:
>       http://hypernews.slac.stanford.edu/HyperNews/geant4/get/AUX/2009/09/29/03.15-94667-specNeut_input.pdf
>       http://hypernews.slac.stanford.edu/HyperNews/geant4/get/AUX/2009/09/29/03.15-63313-specNeut_components.pdf
> 

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1 Agree: Re: Energy deposited by neutrons in plastic scintillator   (Vladimir Ivanchenko - 01 Oct, 2009)
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