I am trying to simulate neutron spectrum from an Americium-Beryllium source. I am using an alpha source distributed over a cylindrical volume (filled with Am-Be material). I have also used LAM-BIAS card to bias nuclear inelastic interactions by He-4. I have tried to score resultant neutron spectrum using USRBDX and USRTRACK scorers. Following is the resultant neutron spectrum as USRBDX output:
Similar results were obtained from USRTRACK card as well.
My doubt is the appearance of flat regions in the intervals ~ 3 to 6 MeV and ~ 7 to 10 MeV as can be seen in the above plot. I was expecting some peaks to appear at around 3.3 MeV, 5 MeV and 7.7 MeV which are clearly not visible here.
Am I missing some aspects of a correct alpha particle transport simulation, or this result is expected? Kindly help me understand this.
I have used SOURCE routine for primary alpha generation and following are all the files used for the simulation. a_source.txt (579 Bytes) AmBe_Source.flair (5.1 KB) AmBe_Source.inp (2.9 KB) source.f (20.5 KB)
For an AmBe source simulation, the recommended way is to directly input the neutron spectrum by means of the source user routine.
The explicit neutron generation you ask for, suffers from the unsuitability of the \alpha interaction model at these so low interaction energies.
The three ranges you observe correspond to the neutron production together with:
a 12C nucleus in the ground state (high energy range)
a 12C nucleus in the first excited level emitting a de-excitation gamma (intermediate range), and
three \alpha particles from the 12C fragmentation (low energy range).
You see a flat profile rather than a peak because of the assumed neutron isotropic emission in the centre-of-mass frame, moving the peak as a function of the emission angle and therefore giving such a resulting shape.
Nevertheless, as pointed out at the beginning, please consider to start directly with the neutron spectrum featured by the AmBe source.
