Dear FLUKA users and expects,
I’m trying to score neutron fluence energy distribution (spectrum) born on 27Al(p, x) reaction on aluminum attenuator of a cyclotron target assembly.
I would like to know the energy distribution of the neutrons born in inelastic hadron reactions in the attenuator and than bombarding the molybdenum target sample (which contains 98Mo isotope and can undergo (n, \gamma) reaction).
There are two problems:
1. Energy of the protons
The proton beam initial energy is 18 MeV distributed according to the Gaussian law with \sigma_E \approx 0.12 MeV. It also has an annual shape (3 cm in diameter), but than the beam is collimated with the target assembly (ABSORB reg.) which has inner diameter of 0.6 cm. The beam starts to run towards the attenuator (ALFOIL3 reg.) through air and loses some kinetic energy. The first data I have to know what’s the precise proton energies just before hitting the attenuator. I use USRBDX card to score the quantity, but the result happens to be not what I’d expected. The shape of the curve looks like the Gaussian, but it is not symmetrical and has slightly distorted shape. Please, have a look at the image below:
While scoring after leaving the attenuator (boundary crossing between ALFOIL3 and AIR2) gives me pretty nice shape and symmetry:
What might cause this strange shape of the E_p curve?
2. Neutron spectrum
The most important goal of the simulation is getting the energy spectrum of the neutrons born in the inelastic nuclear reaction on 27Al and moving towards the molybdenum sample (POCKET reg.). I also use the USRBDX card to find it. I’m interested in energies of the neutrons crossing the boundary between the attenuator and air (AIR2 reg.) and also those which are moving thought air (AIR2) and Mo-sample (POCKET). Let’s focus on the first sort of them. You can see the result on the image below:
As you can see the distribution has large uncertainties and does not follow an expected shape (the outlier about 0.7 MeV). I ran 3E7 primaries for 5 cycles using 10 CPU threads (Intel i7 9th gen) and it took about 8 hours. Is the number of protons enough? I suspect no(?), than how can I increase FoM and speed the calculation up? (not including increasing the number of primaries)
Probably I have to use a more powerful machine to run it (?). May I use a variance reduction technique (and which one?)? Won’t it affect the investigation of the physical processes?
The spectrum of the neutrons hitting the Mo-sample is much more worse:
What else can I do to improve the results and get a nice proper neutrons and protons? Physics, scoring, etc …
Sincerely,
Ihor
Input files
sim_v2.flair (15.2 KB)
sim_v2.inp (12.9 KB)
Versions
FLUKA: 4-5.1
Flair: 3.4-4
OS: Ubuntu 24.04 LTS