Neutron energy spectrum and Neutron flux

Dear expert,

I’m doing a neutron shielding study.
And I want to know how to obtain the neutron energy spectrum produced by proton collisions and the neutron flux at the position of interest?How should the detection card be set up?

Dear @Junjie_Zhang,
Are you referring to the simulation you already discussed in this thread?
Do you want to know the neutron energy spectrum when they cross into the detection area?

Dear amario,

The previous problem is a shielding problem with neutrons as point sources.
What I want to know is how to calculate the neutron energy spectrum generated by a proton source bombarding a target and how to calculate the flux of secondary neutrons at a specified position.

For example, see the following figure.

The proton source is on the left, the target is in the middle, and the shield is on the right. How to obtain neutron flux on both sides of the shield and secondary neutron spectra produced by proton bombardment.

Dear @Junjie_Zhang,
What you are looking for in this case is the USRBDX card. Look at the manual and at this lecture from the course to understand how it works.
It is very important, to separate forward and backward escaping particle populations, that you define the regions before and after the shielding as two different regions, otherwise the two populations will be merged and you’ll get a single overall result.

Dear amario,

I have seen this course before. I have calculated a simple simulation. Is this simulation correct? I have the following questions.
123.flair (1.8 KB)
123.inp (1.2 KB)

What is the physical significance of this USRBDX card design? Or what can I get from this card?

What does area mean in USRBDX?

What does the y-coordinate mean?

Dear @Junjie_Zhang,
I’ll try to address all your questions.

Well, the scope of this forum is not to validate the simulations of the users, but to provide the users with support for problems and questions they may have either technically or physicals matters related to Fluka. Plus, most importantly, as we don’t know what it the actual aim of your simulation, it would be difficult for anyone to say whether the simulation is “correct”.

I’m quoting the slides from the course: “USRBDX scores average d 2$\Phi$ / dE d$\Omega$ (double differential fluence or current) of a given type or family of particles on a given surface”. This means that, once you have selected a surface between two regions, the estimator can provide you the fluence (or current) through that surface, in one or two ways. The fluence (or current) can be provided as function of the energy of the particle crossing the surface and as function of the angle at which the surface is crossed.

The results of the USRBDX estimator are provided in units of cm^-2 GeV^-1 sr^-1 per incident primary. Therefore, the code needs to know what is the area of the surface in order to properly normalize the results. Indeed, on slide 9 of the aforementioned lecture, it is shown where the number you’re referring to comes from. It is the area of a cylindrical surface of radius 5 cm: A = pi * R^2.
If the area is not provided, then the user has to properly take it into account at the result post-processing level.

This comes from a lethargy plot (as explained on slide 10) and the energy binning, it gives you the actual number of particles crossing the surface (provided that all the other normalizations have been properly taken into account).