Numbers of energy intervals for scoring are a little weird

Scoring
1

Dear fluka experts,

I have set the Emin、Emax、andEbins in the USRBDX card, but the output is not what I set. There are two kind of energy boundaries in the sum.lis file, and there are 310 energy intervals in the tab.lis file while I set 100 in the USRBDX card. Why is this happening?

50+50.inp (6.4 KB)
sum.lis&tab.lis.zip (135.1 KB)

Li

2

Hi Li!

This can be explained with FLUKA’s special treatment for low energy neutrons (E < 20 MeV). In this energy range FLUKA uses a group wise treatment of the neutrons based on evaluated data.

See: https://flukafiles.web.cern.ch/flukafiles/manual/10.1.html

To be able to score low energy neutrons correctly the same group energy bins has to be used, which FLUKA does automatically for you.

The number and with of bins you get are calculated as the following:
First the bins from the USRBDX card are calculated, the this list is cut in half at 20 MeV, and the lower energy part is replaced with the relevant part of the neutron energy groups.

Cheers,
David

3

I want to sample the output neutron spectrum，but the result is not the same as I expected, there is a big difference.I’m afraid it’s related to the neutron energy gap.
I’ll upload my file. If you have time, please check it for me.Thank you in advance.
input&output.zip (61.4 KB)

Li

4

Hi Li!

The drop at 20 MeV you see is due to that your source routine interprets the 3rd column of the spectrum.dat file as probability of the of the bin, while the FLUKA output if in the units of [1/GeV/primary].

If I plot the flu_43.tab.lis, with the value option: Y*DX (Multiplying the value of the bin, with the width of the bin), I get the unit of [1/primary], and this plot matches with the spectrum specified in your source data file.

Cheers,
David

5

Should I replace the 3rd column of the spectrcum.dat with the “cumul. flux (part/cmq/pr)” data in the output_22_sum.lis file? If so, what bothers me is that I didn’t find the same output format as the. tab.lis file with the unit of part/cmq/pr. Is there any way for me to get output in this format？

Li

6

Hi Li,

Three things:

  1. For the sampling at the end, you need the probability of the bin. If you specify [1/GeV] in the spectrum.dat file, you will need to modify source1.f, to get the probability, by multiplying this value, with the width of the bin.

  2. Your input spectrum doesn’t have to follow the FLUKA neutron energy groups, since FLUKA will only see a given neutron energy after the sampling, and it will correctly assign it to the appropriate group.

  3. If I understand correctly, your input spectrum if already a result of a FLUKA simulation, thus the units are [1/GeV/primary]. So if you change the line in the source1.f, where you calculate the cumulative probability to:
    ENECUM(I) = ENEPRO(I) * ( ENEPOI2(I)-ENEPOI1(I) ) + ENECUM(I-1)
    Then it should match your input spectrum (after normalization)

Cheers,
David

7

Dear David,

Your understanding is very correct and my doubts and problems have been solved. Thank you again.

Li