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Question Neutron detector efficiency change between 9.0 and 9.2p02 

Forum: Hadronic Processes
Date: 23 Nov, 2009
From: Manoel Couder <Manoel Couder>

Hello everybody,

We do not understand the difference between the results from a G4.9.0 and G4.9.2 version of a simulation of twenty 3He counters embedded in Polyethylene arranged on two concentric rings. Our code is exactly the same for both versions.

Initial conditions :

* We use QGSP_BIC_HP
* a monoenergetic neutron beam
* isotropic source

The overall number of 3He(n,p) reactions in the 3He counters is the same for both simulations within 1%.

The problem is that the reactions in G4.9.2 are occurring with higher probability in the outer ring.

The attached plot 100keV_vec000_ratio_492_490.eps shows the ratio of the count rate in each 3He counter between version 4.9.2 and version 4.9.0. As one can see, the number of events in counter numbers 9-20 has increased while it decreased for counter 1-8.

After reviewing the release notes of G4.9.1 and G4.9.2 we were only able to find those changes:

1) Update from G4NDL3.11. to G4NDL3.13

We have tested the simulation with each version of G4 with the different data sets and see no difference.

2) G4NeutronInelasticCrossSection.cc, line 77

This has been changed ( see thread #915 ) but can that explain our observation?

3) "Fixed bug in G4NeutronHPChannel for treatment of Z in elements." from : http://geant4.web.cern.ch/geant4/support/ReleaseNotes4.9.1.html

Unfortunately we do not know what exactly the bug here was or what has been corrected.

4) Wrong AME masses, Thread # 981

It appears, that the masses for light nuclei were not correct. As thread #981 states, that should have been corrected and the release notes of 4.9.0.p01 state " + Fixed values of masses for light ions."

The question is, if that effects the number of recorded 3He(n,p)3He reactions and if that is a problem with version 4.9.0.

Additionally we performed an experiment to validate our simulation. The results for the ratio provided by version 4.9.0 match our experimental data almost exactly while the results from version 4.9.2 do not agree. The attached ratiotest.png plot shows the good agreement between the G4.9.0 simulation and our "Feb/June" experiment.

Our suspicion is that the thermal scattering of neutrons in Polyethylene is treated differently.

We would appreciate any information on what exactly has been changed in the neutron treatment and why these changes were required.

Thanks in advance, Manoel Couder for Sascha Falahat

   Attachment:
      http://hypernews.slac.stanford.edu/HyperNews/geant4/get/AUX/2009/11/23/10.45-98115-ratiotest.pdf
      http://hypernews.slac.stanford.edu/HyperNews/geant4/get/AUX/2009/11/23/10.45-44678-_vec000_ratio_492_490.pdf

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