RE: [fluka-discuss]: Problems with Energy Binning Control and Discontinuity Across High/Low Neutron Energy Boundary

From: Anton Lechner <>
Date: Sun, 5 Feb 2017 11:17:58 +0000

Dear Benjamin,

Thanks for the input file.

Here are a few answers/remarks:
1) If you score neutron spectra with USRBDX and if you set set the high-energy boundary of your scoring to a value >20 MeV, then FLUKA generates two histograms, one below and one above 20 MeV. The bin size above 20 MeV can be controlled with the USRBDX card (see following post for more details how the bin sizes are calculated in this case: The discontinuity at 20 MeV you see in your output is likely because of bad statistics. The binning is very fine below 20 MeV and the scores close to 20 MeV likely have large errors (several bins have a zero entry). In your case I would bias the number photo-nuclear interactions (via the LAM-BIAS card), otherwise it will take a very long time to converge. I made a test run with biasing and the discontinuity seems to disappear (I didn't run very high statistics, but still better than in the file you sent to the list). Please let me know if, with high statistics, you still see any discontinuity. In addition, I would adjust the high energy binning to have smaller bins, which are more comparable to the sub-20 MeV binning.

2) The SI1MEVNE scoring does not score neutron fluence. It gives you the equivalent fluence of 1 MeV neutrons which you need in order to produce the same damage in Si as the damage induced by various particle types of different energy in your simulation. Since different particle types contribute to this scoring, you will get two histograms which can overlap: one histogram accounting for the contribution of sub-20 MeV neutrons (the bin sizes following again the neutron groups) and a second one accounting for all other particle types at all energies (including neutrons above 20MeV). The second histogram follows the binning definition on the USRBDX card. The large difference of the two histograms within the overlapping energy range is due to non-neutron contributions (in your case high-energy electrons!).

3) You don't have the DEFAULTS card in your input file, hence the physics default settings follow NEW-DEFA. This implies quite high production and transport cuts for some particles. I would use DEFAULTS with SDUM=PRECISIO instead.

I hope this helps.

From: Benjamin Smithers []
Sent: 03 February 2017 18:46
To: Anton Lechner
Cc:; Bruce Andrew Schumm;
Subject: Re: [fluka-discuss]: Problems with Energy Binning Control and Discontinuity Across High/Low Neutron Energy Boundary

Dear Anton,

Indeed I did, my apologies! I have attached the correct input file to this email.

Thank you,

On Thu, Feb 2, 2017 at 11:01 PM, Anton Lechner <<>> wrote:
Dear Benjamin,

I suppose you produced the output with another input file than the one attached? Could you be so kind and send me the input file which contains the scorings you attached to the Email.


From:<> [<>] on behalf of Benjamin Smithers [<>]
Sent: 03 February 2017 01:25
Cc: Bruce Andrew Schumm;<>
Subject: [fluka-discuss]: Problems with Energy Binning Control and Discontinuity Across High/Low Neutron Energy Boundary

Dear FLUKA Experts,

We're simulating the neutron flux that arises when a 10GeV electron beam strikes a Tungsten target.

The target geometry and requested energy binning (USRBDX) is shown in the attached .inp file.

We are looking at both the raw (Neutrons_tab.lis) and 1-MeV equivalent (1MeVNeut_tab.lis) neutron fluxes. For the output we believe we are requesting 20 energy bins between the beam energy and 1 KeV and we are having trouble interpreting the results.

In the 1 MeV equivalent output the requested course binning overlaps with the fine low energy neutron groups output. Expressed in flux/cmq/primary/GeV, the output seems to differ between the coarse and fine binning.

Although our binning requests in the input file for raw neutrons and 1 MeV equivalent neutrons are identical, for the raw neutron binning there is no overlap in the coarse and fine energy range. Again, though, expressed in flux/cmq/primary/GeV there appears to be a discontinuity at the high-energy low-energy boundary of 20 MeV.

We are perplexed by this behavior, and wonder if there is a problem in our input card or our interpretation of the output files. Any hints that you can provide will be greatly appreciated.

Benjamin Smithers
Bruce Schumm
Santa Cruz Institute for Particle Physics

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Received on Sun Feb 05 2017 - 15:25:35 CET

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