Unexpected Photon Spectra from Neutron-Silicon Interactions in PGNAA

Dear FLUKA experts,
I am working on a basic PGNAA problem where a 14.1 MeV neutron beam is directed at Silicon, assigned to the C4 region. I am measuring the photons generated from the neutron-Silicon interaction using a NaI detector in the C100 region. However, the resulting spectra do not match the expected standard, and I am unsure where the issue lies. I am seeking guidance from experts to help identify what I might be doing wrong. Below is the output I am currently obtaining.


The input files I am using are attached here
pgnaa.flair (7.7 KB)
pgnaa.inp (9.0 KB)
Thanks

Dear Sajid,

I simplified your model to a neutron beam and a simple thin Silicon target and checked the spectra afterwards. Thus, I avoid spectra contamination with other lines and minimize the energy loss.

As you see from this comparison,


it contains most of the peaks (left image source: Acquisition of prompt gamma-ray spectra induced by 14 MeV neutrons and comparison with Monte Carlo simulations)

Further, I will revert to the original complexity to understand how it changes deposition spectra in the NaI detector

Best wishes,
Volodymyr

Thank you @vorodin
But how can we get the right spectra in our original input. For simple geometry it works but not for mine.
Thanks

In the full simulation, you will see the modified original spectra by all possible gamma lines from other materials in the geometry, via USRBDX. Change these scoring cards to be from C4 to C100 region, and you will see something like this:


where the highest Si line is still visible.

Your DETECT instead will show the energy deposition spectra in the NaI detector for these gammas and other secondary particles crossing the requested region

Volodymyr

Thank you for @vorodin for your considerations. But what if I only want to consider the spectra from only C4 region and not from others? Is it possible? Moreover will it work for other materials too? Also can you please share the modified input files?
Thanks alot

@vorodin I have tried with your instructions but I am not getting the same plot as you are getting. what I am getting is attached here with the input files
pgnaa
pgnaa.flair (7.9 KB)
pgnaa.inp (9.0 KB)

I am sorry for sending to many messages in a short period. I can now get the spectra but the peaks are not at the standard energy values. For example here in the case of Hydrogen the standard peak is at 2.223MeV but I am getting at 4.45MeV. The peaks are not at their correct energy position. I even tested for other materials but getting similar results @vasilis is it the problem in the flair/fluka or I am doing something wrong @vorodin .
I had attached the updated files


pgnaa.flair (8.2 KB)
pgnaa.inp (9.2 KB)

Dear @sajidwajid480,

Here are the input files, I defined variable “simple” which assigns most of the regions to Vacuum and sets Si thickness to 1 mm:
It results in this picture for USRBDX22 and DETECT17:

pgnaa_origin.flair (10.6 KB)
pgnaa_origin.inp (9.8 KB)

Cheers,
Volodymyr

Hydrogen, itself only, is quite a bad neutron absorber ~1 barn capture cross-section for thermal neutrons, and much less for keV-MeV energies.

You can observe a better 2.223MeV peak with water which helps to thermalize incoming neutrons even more:

Best wishes,
Volodymyr

Thank you for your input, but I don’t understand the reasoning behind assigning vacuum material to the majority of the geometry regions. Won’t this affect the results when we conduct/compare the simulated results with real-time experimental ones?

Thanks again

As I said in the previous posts:

Thus, I avoid spectra contamination with other (gamma) lines and minimize the energy loss.

It helps to see that FLUKA works correctly, and shows what peaks you need to look for

In the full simulation, you will see the modified original spectra by all possible gamma lines from other materials in the geometry, via USRBDX (and DETECT cards)

Further, I will revert to the original (simulation) complexity to understand how it changes deposition spectra in the NaI detector

You should do it step by step, checking what kind of background signal it brings to your NaI detector

Volodymyr

I think I wasn’t able to clear my problem. Actually I’m looking for a capture neutron spectra where a themal neutron wilo be captured by a material, say Si, then the gammas emitted by the capture neutron interaction with material will be recorded by the NaI. For mcnp I got a similar spectra


Where the top left graph is for capture neutron whereas the below this one is inelastic. What we’re getting is also inelastic but I’m more interested in capture neutron plot.
That’s what I want to get using fluka.
Thanks

In FLUKA you will obtain spectra from a combination of both processes, capture and inelastic. You can separate them by looking into reaction fragments using mgdraw subroutine:

Commenting out the “simple” variable in the input which I shared with you, will revert the setup to the original complexity

Here, it’s clearly dominated by the background in NaI, but you still can see deposition peaks around 8 and 10 MeV from here:

Volodymyr