RE: secondary neutrons missing from usrrnc.f

From: Vasilis Vlachoudis <>
Date: Mon, 3 Sep 2012 12:31:19 +0000

Dear Shirley,
unfortunately this is not a bug but a feature. Due to the group-wise treatment
of the low-energy neutrons the meaning of a single interaction is almost lost,
with the direct consequence on the information of the residual nuclei.
Therefore FLUKA (and only for the low-energy neutrons with group-wise xs)
is sampling totally uncorrelated the residual nucleus with the event and
the deposited energy (only if a residual scoring is requested).



From: [] on behalf of Shirley Li []
Sent: 31 August 2012 22:17
Subject: secondary neutrons missing from usrrnc.f

Dear fluka users,
I'm trying to simulate neutron yield from a muon beam passing through
water. I modified mgdraw.f and count neutron yield from every
interaction. The number is consistent with published results to about
10%. Then, I tried to compare neutron yield to residual nuclei, i.e.,
every time some neutrons get generated, there should be residual
nucleus produced at the same time. So I modified usrrnc to look at the
reactions/neutron yield. However, some low energy neutron interactions
look different in the outputs from mgdraw and usrrnc. In both
subroutines, I look at interaction by

do ip =3D 1, NP
    WRITE ( unit, *) ip, KPART(ip), TKI(ip)

do ipheav =3D 1, NPHEAV
    WRITE ( unit, *) ipheav, -KHEAVY(ipheav), TKHEAV(ipheav)

There are a lot of reactions reading from mgdraw.f output is a parent
low energy neutron generates one neutron. However, this reaction
seeing from usrrnc output is one parent neutron generates a 16O (since
it's in water) with NO child neutron. This also happens to some
similar reactions where basically it's low energy neutron bouncing off
nuclei and losing energy with no other real secondaries. However, the
neutron doesn't appear in usrrnc.f secondary but it does appear in
mgdraw.f secondary. Does anyone know why this is?


Shirley Li
Received on Mon Sep 03 2012 - 23:05:08 CEST

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