# Neutron thermalization spectrum

Hello all!

This is my first message at Fluka forum. There is my question.
I’m injecting needle-like beam of neutrons (monochromatic line E=0.1 eV, no energy spreading) to layer (with thickness of 2 cm) of built-in polyethylene (C2H4x) material. And I use 2 scorings on surface (USRBDX): initial spectrum falling on polyethylene layer and spectrum coming from layer. There I expected to observe some thermalization effect on hydrogen, and it is. But there is spreading to the higher energy region too, up to 0.5 eV. Is there physical meaning, what is the mechanism for rising energy?
I know that ultra-cold neutrons is heating in collisions with surrounding material and could gain some energy. But theirs energy much lower <0.2 meV.
Poly_therm.pdf (20.3 KB)

Thank you!

Hi @dilin
the “heating effect” as you mention is not only for ultra-cold neutrons, but it extends up to the sub-eV range, which for this reason is called thermal region .
On elastic collisions of thermal neutrons the code has to take into account the thermal motion of the nuclei of the medium (in fluka typically with T=296 ^oK ) following a Maxwellian distribution of energies with kT\approx0.025 eV.
Therefore in every collision the neutron can either lose or gain energy, and it will arrive to a thermal equilibrium with the medium. Thus the neutrons will follow also a Maxwell Boltzmann distribution.
If you make the energy probability distribution plot you will see that the tail can arrive to much higher energies than the most probable energy E= {1\over 2} kT.

In the FLUKA group wise approach the thermal treatment is using as well the up-scattering matrix containing the transfer probability to a group of higher energy

Dear Vasilis, thank you for answer!