Re: Low energy - High energy neutron spectrum mismatch with USRTRACK

From: Alberto Fasso' <fasso_at_SLAC.Stanford.EDU>
Date: Thu, 26 Feb 2009 08:43:28 -0800 (PST)

Dear Roy,

Anna has already found the reason for the large mismatch between
the low and the high energy range.
Here I would like only to stress that the correct way to plot a spectrum
is differential in energy (n/cm2/GeV), exactly as it is output by
FLUKA, and not n/cm2 as you are trying to do by multiplying each
bin value by its energy width.
Multiplying by the bin width doesn't give you a spectrum, but a histogram
where the physical information of each bin is its area and not its
height: so you cannot get any insight by looking at the bin heights.
The bin heights are irrelevant, since they depend on the bin widths,
which are arbitrary.


On Wed, 25 Feb 2009, Roy Keyes wrote:

> Hello Fluka users,
> I am trying to measure the neutron fluence energy spectrum of
> annihilation neutrons produced by antiprotons incident on a copper
> target. I am using USRTRACK to score the neutrons, but when I calculate
> and plot the "low" and "high" energy parts of the spectrum together,
> there is a large mismatch in the spectrum at 20 MeV (see attached plot).
> I am using the 260 group low energy neutron transport library and am
> scoring up to 0.2 GeV. To obtain the fluence energy spectrum, I am
> multiplying each bin by its energy width in GeV.
> I am not sure if my 0.0564 cm thick target qualifies as "very thin" and
> would lead to the artifacts mentioned in section 10.1.1 of the Fluka manual.
> If anyone has any idea why I am seeing such a disjointed spectrum, I
> would greatly appreciate hearing back. I suspect I am making an obvious
> mistake somewhere.
> Thanks,
> Roy

Alberto Fasso`
SLAC-RP, MS 48, 2575 Sand Hill Road, Menlo Park CA 94025
Phone: (1 650) 926 4762   Fax: (1 650) 926 3569
Received on Thu Feb 26 2009 - 18:12:55 CET

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