Re: 1 MeV fluences?

From: Alfredo Ferrari <>
Date: Thu, 27 Nov 2008 19:26:47 +0100

Dear Ian

I don't remember where I took the (low energy) neutron data, but it was
one of the best publications in the field (obviously I can find it looking
in my office) based on the latest ENDF/B evluations of the time.
For all particles we should use different values for 28-Si, 29-Si and
30-Si: the reason why it is done only for neutrons and only at low
energies is twofold

a) separate data exist for neutrons below 20 MeV and are very precise,
    nothing like this exists for the other particles as far as I know
    (note that data for neutrons above 20 MeV are always the same, see b))
b) physics can be dramatically different for neutrons at low energy going
    from one isotope to another, while at the energies where protons/pions
    etc matter it is changing rather smoothly moving from one isotope
    to the next one. Furthermore at those energies/particles our knowledge
    is not at comparable in accuracy to the one for neutrons below 20 MeV

As a final comment, given the relatively low abundance of 29-Si and 30-Si
the impact is not dramatic.

If you wish/believe the data could be improved (they are relatively old
and better data could be around, but for neutrons using the same set for
the three isotopes is an avoidable approximation), please help!

  On Thu, 27 Nov 2008, Ian Dawson wrote:

> Dear Fluka experts,
> I see that there is now the option to score directly 1MeV neutron
> equivalent fluences, which I think is a useful feature, that I would
> like to use. But I'm not sure I understand the conversion damage
> values for neutrons found in "sidan.dat". There are 3 data sets
> corresponding to Si28, Si29 and Si30, which are presumably combined
> with some weighted average according to isotopic composition of
> Silicon. However, when I look at "official" Si damage values
> (produced by ROSE/RD48), which can be seen in:
> members/gunnar/Si-dfuncs.html , they simply have one neutron data
> set. Is this the weighted average? I notice you've taken the pion
> damage values from the ROSE source, so would like to know why the
> neutrons are treated differently?
> Thanks, Ian

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Received on Tue Dec 02 2008 - 15:46:43 CET

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