Re: [fluka-discuss]: [fluka-discuss] neutron fluence spectrum

From: George Kharashvili <>
Date: Wed, 19 Dec 2018 19:25:46 +0000

Dear Angel,

You should not expect USRBDX and USRTRACK to produce identical results in this case. With USRBDX you are scoring fluence at the boundary of the 5-cm concrete region, while with USRTRACK you are scoring average fluence throughout the region. The results would be identical if neutrons did not interact (so vacuum, or a very thin region) and you had the same spectrum entering and exiting the region.

For issues specific to neutrons, such as the multigroup treatment and the corresponding scoring, please review the Low energy neutron lecture from one of the FLUKA courses.
I would again encourage you to read the discussion on the subject of plotting neutron spectrum from this forum
It explains why it makes sense to use isolethargic neutron spectrum - dPhi(E)/d(log E) vs. log E.

Best regards,
From: Angel <>
Sent: Monday, December 17, 2018 5:45:03 PM
To: George Kharashvili
Cc: FLUKA Discussion List
Subject: Re: [fluka-discuss]: [fluka-discuss] neutron fluence spectrum

Hello George,

Thank you very much for your kind reply! I am probably missing something
since I get different results by using the two cards. Is this due to poor
statistics or my misunderstanding on how to use the cards? I have attached
my .inp file. In order to compare the outputs of the two cards I compare
the results outputted, respectively, in bins 32 and 33.

Best regards,

> Hello Angel,
> You can use either in this case. The difference is that USRBDX scores
> double differential (energy and angle) particle distributions across a
> boundary surface which gives
> results per cm2, per GeV, per steradian, per primary, while USRTRACK
> calculates differential fluence as a function of energy by scoring
> track-length in a given region and gives results per cm2, per GeV, per
> primary. The scoring lecture from FLUKA beginners courses provides summary
> descriptions of the two
> One thing to keep in mind when scoring low energy (<20 MeV) neutron
> spectra: the transport of low energy neutrons in FLUKA is performed
> through a multigroup treatment, for which the neutron energy is not
> exactly defined but belongs to one of the 260 predefined intervals of the
> cross section library. Because of this, whether you use USRBDX or USRTRACK
> to score energy spectrum, energy binning is forced to match the 260 energy
> intervals regardless of what was requested by the user.
> For very helpful information regarding scoring neutron spectra I would
> also recommend reading the replies to the following thread:
> Best regards,
> George
> ----- Original Message -----
> From: "Angel" <>
> To: "FLUKA Discussion List" <>
> Sent: Saturday, December 8, 2018 6:17:31 PM
> Subject: [fluka-discuss]: [fluka-discuss] neutron fluence spectrum
> Dear FLUKA experts and users,
> I need to obtain the neutron fluence spectrum at different depths inside a
> concrete wall. The manual points that I can use the USRBDX or USRTRACK
> card for the purpose. What is the difference between the two cards? Using
> the first one I can choose to score the neutron fluence on a surface
> passing in both directions or just in one. The second gives me neutron
> fluence in a given volume. Which one would you advise me to use? Are these
> cards the right ones to use?
> Best regards,
> Angel

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Received on Wed Dec 19 2018 - 21:39:47 CET

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