Dear Sizwe,
With the following scoring cards (from det1 -> target, target -> det2):
*...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+....
USRBDX 99. NEUTRON -21. det1 TARGET nflux-i
USRBDX 1E-14 1. &
USRBDX 99. NEUTRON -21. TARGET det2 nflux-f
USRBDX 1E-14 1. &
indeed one gets 0.99 and 1.03 (Neutron/primary) for nflux-i and f, respectively. The overall increase in neutron fluence is expectable since you are shooting a monoenergetic beam of fast neutrons into polyethylene, resulting into various secondaries being generated (including neutrons). Some of the secondary neutrons will add up to the neutrons exiting the target, hence your fluence increment.
However, note that you do get a reduction of the neutron fluence at 14 MeV as the primary neutrons traverse the target. As you add up more layers to your setup the amount of higher energy neutrons will be further reduced in favor of lower energy neutrons. The degree of variation will depend on the material properties and geometry of the layers.
Hope this helps,
Ricardo
//Ricardo dos Santos Augusto
TRIUMF-EH&S
Office: +1 604 222 10 47 (6375)
4004 Wesbrook Mall,
Vancouver, BC V6T 2A3, Canada//
________________________________________
De : owner-fluka-discuss_at_mi.infn.it [owner-fluka-discuss_at_mi.infn.it] de la part de SIZWE SCELO MHLONGO [3698012_at_myuwc.ac.za]
Envoyé : mardi 26 mai 2020 18:56
À : fluka-discuss_at_fluka.org
Objet : [fluka-discuss]: Scoring primaries
Dear fluka experts and community,
I am doing calculations with the aim to find a suitable neutron moderator material thickness. In my input (attached) I have a beam of 14 MeV neutrons going through a polyethylene cylinder (h = 2 cm, r = 8.5 cm). I made two scoring regions - one is for scoring beam particles before the beam interacts with the target and the other is for scoring the beam particles after going through the target region (I chose usrbdx as estimator). I noticed that, although I requested 100 000 histories in my runs, the fluence scored before the beam interacts with the target is less than the requested primaries;
Furthermore, the moderated spectrum (when polyethylene and water are used) tends to have a high fluence of thermal neutrons - even higher than the requested primaries. What could be the cause of this? Thanks.
Best regards,
Sizwe
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Received on Wed May 27 2020 - 03:15:54 CEST