Re: Large uncertainty in low energy neutron simulation

From: Stefan Roesler <sroesler@mail.cern.ch>
Date: Thu Jun 12 2008 - 20:01:18 CEST

Hi Keith

Are you sure that you are not at the limit of getting any particles into
your gas?

Probably at the very low energies most particles are absorbed in the
chamber-wall and only very very few make an energy deposition in the gas,
which could explain the large fluctuations. I suggest that you also score
fluence in both gas and wall and check the concerned neutron cross
sections to verify this suspicion.

Apart from that, you are probably aware of the limitations of the
multi-group transport for this kind of application (see my pevious
posting on this list regarding low-E neutron transport), including kerma
approximation, three scattering angles etc.?

Best regards
Stefan

On Wed, 11 Jun 2008, keep wrote:

> Dear all,
>
>
>
> I am simulating the response of a tissue equivalent chamber in neutron
> field.
>
>
>
> I setup a parallel beam with mono-energetic neutron, and a chamber at 10 cm
> away from source.
>
> The incident neutron energy is changed from 10 MeV to 0.1 eV,
> and I use USRBIN to tally the "energy deposition" in the chamber gas which
> is about 0.1 cm3.
>
>
>
> When the incident neutron energy is above 100 eV, the USRBIN results have
> errors about 3 % for 2x109 primary particles.
>
> When the neutron energy is 10 eV or below 10 eV, the USRBIN results have
> large error up to 60 %.
>
>
>
>
>
> Could anyone tell me what cause the large error? How can I solve it??
>
>
>
>
>
> Thanks in advance,
>
>
>
> Keith Wang
>
>

-- 
___________________________________
Stefan Roesler
CERN, SC/RP
CH-1211 Geneva 23
Switzerland
Phone:  +41-22-7679891
Fax:    +41-22-7669639
E-mail: Stefan.Roesler@cern.ch
Received on Fri Jun 13 10:35:54 2008

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