Unexpected Result from Pulse-Height Spectra of NM64

Dear FLUKA experts,

I am trying to generate a pulse-height spectra of the NM64 neutron counter. I’m using the galactic cosmic ray induced neutron flux at ground level as my source. Since the main detect media is the Boron-10, I am expecting to see that the main contribution of the signal to be alpha and Lithium. However, I got this huge resonance in the 0.5 MeV to 20 MeV range which is rather weird. Did I do something wrong with my simulation setup? If not, can someone can educate me more on what’s going on here or point me in the right direction so I can try to understand?

The attached are my files for the simulation.

GND_BF3.inp (2.1 KB)
GCR_N.txt (6.4 KB) GND_BF3_01001.out (140.9 KB)

Thanks in advance.


Hi @KillMartin,
happy new year, to yourself and any reader.
Those resonances above 2 MeV are not weird, rather exactly the expected alpha and 7Li signatures. With your geometry, you mainly get thermal neutron captures on 10B, yielding the alpha-7Li pair with a total kinetic energy of 2.3 MeV (in most of the cases, together with a 480 KeV photon from 7Li* de-excitation flying away) or 2.8 MeV (in the less frequent cases of 7Li ground state direct production). These are the first two peaks you get, as a result of the alpha and 7Li energy deposition in the gas. The following ones correspond to multiple capture events in the course of the same primary neutron history.

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Hi @ceruttif,

Happy new year to you as well. Thank you for your explanation. I was able to make some deduction on this with more simulations I did over the past few weeks. Your explanation perfectly summarizes it.

However, I do have some confusion on comprehending the last sentence of your explanation “The following ones correspond to multiple capture events in the course of the same primary neutron history.”. What do you mean by this?

Thank you again for your help.


I mean that a single high energy neutron, as sampled from your original spectrum, can eventually generate several thermal neutrons that may be captured in the gas on 10B, as a result of the same initial neutron history. With each capture giving a well identified energy deposition contribution of 2.3 (2.8) MeV, such a multiple capture occurrence is responsible for the peaks at n * 2.3 (+ m * 2.8) MeV, with n,m positive integers, in the pulse-height spectrum.

Ah, thank you. That makes sense since the result is normalized to the primary neutron number.