An increase of NEUTRON scored by USRBIN in Polyethylene

Dear experts,

I am using a round neutron source incident into a cylinder of Polyethylene, they both have a radius of 5cm.

When I scored NEUTRON with USRBIN, I found that the number of neutron inside the PE increased first and then decreased, which is shown below:

And the 1D projections of NEUTRON, BEAMPART and THNEU-EQ scored by USRBIN are shown below:

So how to understand the increase of NEUTRON? Is it because of the generation of thermal neutrons? And which quantity can indicate the attenuation of the neutron beam?


Dear @Jungle,

Looking at your plot

I can see two processes: the propagation along the beam axis and the later dispersion of the beam.

As your neutron beam propagates inside the target, it will interact causing the backscattering of the neutrons and the production of secondary neutrons. The consequence of this, is that the neutron density increases with z inside the target. While the beam continues to propagate, the particles that compose it, loose energy and cannot propagate any longer, therefore causing a decrease of the particle density.

It is also important not to forget that after any interaction the neutrons will not propagate in the same direction but will be scattered. Therefore, neutrons that have interacted close to the lateral edge of the target can escape from it. This results in a decrease of the neutron density with the distance from the beam axis.

Last not least, don’t forget that neutrons that escape the target don’t propagate anymore along its length.

Dear @amario ,

Thank you for your reply.
When you say “secondary neutrons”, you mean the neutrons that are scattered by the material, right?
So the hot spot is filled with scattered neutrons from the surroundings; while in the outer part, the flux decreases because some of the scattered neutrons escape the material?

With “secondary neutrons” both to neutrons from the primary beam and those “extracted” in nuclear reaction, even if it doesn’t make much sense to distinguish between the two.

This explains the "radial behavior.

Thanks for your answer! It solved my confusion.


Dear @ Jungle

As you know dealing with neutrons needs to know at what energy it have. Is it thermal, Epi-thermal or fast. Or it is a spectrum energy like in case of Am-Be neutron source? or it is mono energetic neutrons.
Every case must be studied with each element in the target material.
Here we have target consists of carbon and hydrogen. And the open reaction channels depending on
the abundance of isotopes of H and C
and according to the excitation function of the expected open reaction channels