Missing peaks in Energy Spectrum

Dear Experts,

I am trying to simulate BN layer for thermal neutron. As per the 10B+n reaction, there should be a peaks appearing at 1.015 MeV (7Li)+ 1.77 MeV (alpha) and 0.840 MeV (Li*) + 1.47MeV (alpha*) and maybe two other peaks between 2MeV and 3MeV. But following spectrum is the one I get from simulating. I can not understand why the other peaks are not appearing in the spectrum. I would be grateful, if anyone can help with this.

hBN_15May21.flair (4.8 KB)

Dear Nadeera,

Note that the BEAM card in your input file assigns your neutron beam an energy of 1 MeV. These neutrons do not have the chance to thermalize in your geometry, so the conditions to meet your expectations (valid for thermal neutron energies) are not satisfied.

If you reduce the beam energy to thermal (or put a more elaborate/realistic surrounding material geometry to allow for neutron thermalization) the picture will change substantially and steer towards your expectation.

If you want to see the two peaks, respectively at 2.31 (=0.84+1.47) MeV and 2.785 (=1.015+1.77) MeV, you should define 10B as an isotopically pure material, a la

MATERIAL          5.                2.37                           10.BORON-10

The choice of the name BORON-10 is intentional and purposeful: FLUKA will look up this material name in its low-energy neutron cross section library, find it, and automatically associate to your material the desired 10B cross sections. Doing so (while using DEFAULTS=PRECISIOn) has the beneficial (and necessary in your problem) effect of switching on explicit and correlated secondary generation of the alpha and 7Li. This will lead to your expected two peaks in the DETECT card spectrum. Of course, you can combine your explicit BORON-10 into a COMPOUND with BORON-11 to replicate the isotopic composition of natural boron accordingly.

Please note also that there is an error in your geometry (open it in Flair and it spots it immediately in the Geometry tab): your two RPPs are overlapping. It should suffice to set Zmin=0.01 in your RPP called “sipm”.

Hope this helps,


Many thanks for the explanation and guidelines.