From: Alberto Fasso' (fasso@SLAC.Stanford.EDU)
Date: Wed Jul 25 2007 - 11:09:55 CEST
On Tue, 24 Jul 2007, Feracci Vincent wrote:
> I have a few questions about low energy neutrons :
> I am measuring low-energy neutron fluence (< 19,6 MeV) with USRTRACK and I
> would like to know if it is possible to change the energy boundaries of the
> measures as they are fixed by the boundaries of the 72 energy groups used to
> simulate low energy neutrons.
no, it is not possible.
> Physically I would also like to understand some details on how does the
> multigroup model work : is the energy of a low-energy neutron set to a
> specific value according to the group he is currently in (i.e : under 19,6
> MeV the energy of a neutron can only take 72 different values) or has the
> neutron a value that can change inside a specific group (with the
> cross-section values being fixed in each energy group) ?
In general, a low-energy neutron does not have an energy value, but only
a group number. There are only two exceptions:
1) when a neutron enters into a material for which pointwise energy transport
has been requested (at the moment there are only two materials for which
this is possible: 1-H (above 10 eV - note that the manual is out of date
and still says 10 keV)) and 6-Li. In this case, the energy of the
neutron is sampled randomly within the energy group.
2) when a neutron of energy < 19.6 MeV is produced in a reaction which is
not part of the multigroup treatment (e.g. evaporation in a high-energy
collision, a (p,n) reaction etc.). In this case, the energy of the
neutron is kept until the moment when the neutron starts to be
transported. At that time, the neutron is assigned a group number and
the energy value is lost.
> If it is the latter case is it then possible to change the energy boundaries
> (and increase the number of steps) of detectors such as USRTRACK in these
> energy levels?
No: USRTRACK "sees" the neutron at transport time, when only the group
number is known.
> If this is impossible due to the use of the 72-group ENEA library, is it
> possible to use another multigroup library with FLUKA (which would have more
> energy groups) ?
At this moment, no. A new library with many more energy groups is in
preparation and should be available before the end of the year.
-- Alberto FassÚ SLAC-RP, MS 48, 2575 Sand Hill Road, Menlo Park CA 94025 Phone: (1 650) 926 4762 Fax: (1 650) 926 3569 firstname.lastname@example.org
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