RE: thermal neutron capture in Hydrogen

From: Anton Lechner <>
Date: Thu, 24 Jan 2013 10:17:08 +0000

2013 11:17:17 +0100 (CET)
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First of all, do not use NEW-DEFA as DEFAULTS setting but PRECISIO.

Note that photons (with energy ~0.002223 MeV) produced in 1-H(n,gamma)2-H interactions may still interact within your target, and therefore your USRBDX scoring of photons leaving the target is not the most suitable way to exactly count such neutron interactions. If you are interested to count individual photons produced in 1-H(n,gamma)2-H, I would rather use the MGDRAW routine, in particular the USDRAW entry. If activated via the USERDUMP card (see manual), USDRAW is called after physical interactions; secondaries like photons will then be available in the GENSTK common. The ICODE value for your neutron interaction is 300. One can then for example loop over stack entries to retrieve particle type, energy and weight, e.g.:
       DO 10 INP = 1, NP
           WRITE(50,*) 'Particle=',KPART(INP),' Energy=',TKI(INP),
      & ' Weight=',WEI(INP)
In other words, if a 1-H(n,gamma)2-H interaction occurs you will obtain a ~0.002223 MeV photon on the stack after such a neutron capture.
You may find more information about MGDRAW (and USDRAW in particular) in the "Programming User Routines" lecture of the last advanced FLUKA course (TRIUMF2012).

Concerning the scoring of residual nuclei, your approach is the correct one, i.e. you will get the number of 2-H directly in the RESNUCLE output (obviously per primary). One important point is the following (thanks to Francesco for reminding me this): the residuals from low-energy neutron interactions, such as the considered 1-H(n,gamma)2-H, are only created for scoring purposes if one issues the RESNUCLE card. This means, no information about the residuals is available in USDRAW.

Just as a general comment, I would like to remind you that many of your neutrons will just undergo elastic interactions in the target and eventually leave the target without being captured (at these neutron energies the capture cross section of 1-H being about two orders of magnitude smaller than the cross section for elastic scattering). You can account for neutrons leaving your target in the BXDRAW entry of MGDRAW.

Cheers, Anton

From: [] on behalf of []
Sent: 14 December 2012 13:24
Subject: thermal neutron capture in Hydrogen

Dear fluka experts
We are trying to find out the reaction rate of the thermal neutron capture
reaction 1H(n,gamma)2D. We have taken a cylinder (rad = 1.784cm, H = 10cm)
filled with hydrogen at a density 1g/cm3. We have used the following
tallies (by taking all the integral values, surface=1, volume=1):
1) The USRTRACK to find out the integral thermal neutron fluence (Phi) in
the volume of the cylinder. This output is used to calculate reaction rate
(N*sigma*Phi) by taking sigma = 0.33 barns. (of course we got N*sigma*Phi=
2) The RESNUCLEI to find out the production of 2D.
3) The USRBDX to find the out the number of capture gammas (2.2MeV)
crossing the cylinder.
But none of these three values match with each other. We are attaching the
input and output files. Please suggest any correction in the input file.
Received on Thu Jan 24 2013 - 19:02:43 CET

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