Re: Low energy Neutrons

From: Alberto Fasso' (fasso@SLAC.Stanford.EDU)
Date: Wed Jul 25 2007 - 11:09:55 CEST

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    Hi Vincent,

    On Tue, 24 Jul 2007, Feracci Vincent wrote:

    > Hi,
    > 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        this is possible: 1-H (above 10 eV - note that the manual is out of date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

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