# Neutron yield and gamma yield calculation

Hello all,
I’m making the simulation with a simple model in FLUKA. A proton beam and Be-target was set, and a PE moderator was outside of the Be-target. Now I want to get the total neutron yield, neutron yield from (n,2n)reaction and the total gamma yield in the system. However, I can just find the neutron yield from (p,n) reaction from the .out file. Thus, How can I calculate the neutron yield from (n,2n) reaction and the gamma yield in the system. Be22M.inp (4.4 KB)

Hello,

At the end of the output file there is a table with the average number of secondaries generated in inelastic interactions per beam particle, resolved by particle species. You can look up the total photon and neutron yield (in your whole geometry) in this table. Note that this table is not resolved by geometry region or reaction channel.

Likewise, there’s a table showing the average number of secondaries generated by low-energy (<=20 MeV) neutron per primary.

In FLUKA’s multigroup treatment of low-energy neutron interactions, secondary neutron production is modelled in terms of a group-dependent non-absorption probability: if (n,n), (n,2n), (n,3n), etc are possible, 1 or more neutrons can be emitted. This description respects the average number of emitted neutrons, but there is no unique correspondence with the actual probabilities for (n,n), (n,2n), (n,3n), etc: it is lost as a result of the inclusive nature of the evaluated nuclear datafiles. Thus, I would refrain from directly extracting the yield of the individual (n,2n) channel from the simulation

In your case, an alternative to estimate the (n,2n) yield is the following. You could consider scoring the neutron energy fluence \Phi(E) with USRTRACK, multiply it by the (n,2n) macroscopic cross section \mathcal{N}\sigma(E), where \mathcal{N} is the number of atoms per unit volume and \sigma(E) is the cross section for the (n,2n) process from e.g. ENDF-B-VIII.0 (easily via the Janis code/website, it will be in mb --> convert to cm^2), and integrate numerically over E:

N_e = \int_0^{E_\textrm{max}} \textrm{d}E \; \Phi(E)\; \mathcal{N} \sigma(E).

If you do not specify a volume in USRTRACK, your energy fluence will be in units of cm/GeV, and N_e will be a dimensionless number giving you the average number of (n,2n) events. To get the neutron yield from the (n,2n) channel you would still need to multiply by a factor 2. By the way, it looks like Janis allows you to fold \sigma(E) with a user-provided spectrum (I’ve never tried though).

Cheers,

Cesc

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