Designing a neutron detector

I was simulating a neutron detector using the plastic scintillator ‘G4_PLASTIC_SC_VINYLTOLUENE’. I have used the physics list from the Hadr04 example in geant4. I have used GPS for defining the monoenergetic neutrons of energy 3.02 MeV. The problems is, after the run, the simulation is not giving any energy deposition for neutrons in the plastic scintillator. I have no idea where the error is made or is it the problem of the scintillating material. Hence I used different materials to check the energy deposition such as water BC537 etc, but still there is no energy deposition for the neutrons. I am attaching the relevant files of the simulation along with this post. (3.2 KB) (3.8 KB) (1.0 KB) (1.5 KB) (9.7 KB) (3.6 KB) (5.6 KB) (3.1 KB)

I do not see where you have registered any electromagnetic processes. The energy deposited in the scintillator will ultimately come from proton recoils that ionize the material, emit X-rays that are absorbed, etc. A neutron at that energy wil not directly deposit energy in the scintillator.

Perhaps try a reference physics list like QGSP_BERT_HP.

Or complete your PhysicsList from Hadr06

Thanks a lot John and Maire regarding your comments. I modified my simulation using the Physics list from Hadr06. I put 2.5 MeV neutron, using GPS macro and the resulting spectrum obtained using a thick sodium iodide(NaI) scintillator is quite surprising. I am adding the relevant physics lists and the spectrum below. (4.3 KB) (3.1 KB) (3.3 KB) (3.3 KB) (6.2 KB)

Also this is my source macro
run2.mac.txt (492 Bytes).

The spectrum looks as follows

Eventhough the neutron energy is 2.5 MeV, I can see a long high energy tail. My best guess is that it can be arising from the gamma energy deposition. But I am not sure about the origin of it. Is it the gamma arising from the Na(n,gamma) and I(n,gamma) reactions? From where Geant 4 is taking the reaction cross sections for such reactions? Or these gammas are arising from any other mechanism?
Also I can see some distinct peaks arising in the energy region 0 to 2.5 MeV. I am wondering what those can be as well.

Thank you in advance for your time and interest

The energies above 2.5 MeV may be due to neutron capture, since the Q-values for capture are ~6.8 MeV for I-127 and ~6.9 MeV for Na-23. Adding in the incident neutron energy, depending on the type and amount of scattering before capture and you can get up to ~9.4 MeV. There may be other processes too, e.g., I-127(n, alpha), etc., but I haven’t looked them all up.

I’m not sure what is responsible for energies above 9.4 MeV. There are several threads in the forum on the correct choice of HP neutron parameters. For some combinations, energy is not conserved for a given reaction but only on average. You might want to check that.

Thanks a lot for the suggestion, John. I will look into it

Here, a run of Hadr06, and the plot of energy deposited.

gokul.mac.txt (367 Bytes)
gokul.out.txt (3.9 KB)

With Hadr03, you can see the nuclear reactions involved and their Q.

gokul03.mac.txt (398 Bytes)
gokul03.out.txt (2.6 KB)

Hi Maire,
Thanks a lot for your response. I have tried the macro file you provided in my geant 4 installation. But what I find is little different from what you got. I am using GEANT4 version 11.1.2 and the spectrum I obtained for the macro you used is as follows.

I can see the energy deposition up to ~ 15MeV in my simulation and the spectrum shape is also little bit different. Did you used the same macro you provided to create the spectrum you got? If then, it is quite confusing why both the simulations are showing differences.

i do not know what physics lists you and Michel are using in Hadr06. Are they the same?

@John_McFee Hello Sir! I wrote all the necessary program to find out the time resolution of my scintillator detector. But I am not getting the arrival time distribution so that I can find out the mean arrival time of optical photon. Can you help me?

Hello @shapeandsymmetry. This is a different subject than the rest of this thread and should be posted as a new question. When you do, please include the code you are using, the version of Geant4 and what the specific problem is. (For instance “I am not getting the arrival time distribution” is too vague to allow anyone to determine what might be the problem. Are there error messages in the output? What output were you expecting?..).

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Thanks John, I have found the origin of the difference ! (4.6 KB)

Thanks a lot Maire and John. I was working on the simulation these days and thanks a lot for helping me to sort out the problem. I have started using the QGSP_BERT_HP physics list and now it is working fine. Thanks a lot for your valuable suggestions and comments .

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