Re: Biasing question

From: Francesco Cerutti (
Date: Sat Mar 08 2008 - 10:05:03 CET

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    Dear Frances,

    > My question is therefore, are these high results in any way 'real' (in
    > that you would expect to measure doses of this magnitude outside the
    > shield wall occasionally) or are they artefacts of the biasing ? If it
    > is my choice of biasing which is leading to these fluctuations, can
    > anyone suggest how I might improve it to obtain 'better' results outside
    > the shield wall ?
    > I should also point out that the errors on the gamma dose rates outside
    > the shielding are typically very high ( > 90 %) which suggests there is
    > scope for improvement. Results for neutrons have lower errors but are
    > still subject to fairly large fluctuations.

    Leading particle biasing (activated by EMF-BIAS with SDUM=LPBEMF) is a
    technique aiming to reduce CPU time (with a dramatic - essential - gain in
    some cases) carrying at the same time weight fluctuations which increase the
    variance, but your spikes outside the shielding to my mind are mainly due
    to low statistics there, not to weight fluctuation.
    In order to get acceptable errors and a more regular and reliable
    pattern, you might i. [brute force!] increase significantly the total
    number of run primaries (up to the order of 1e8-1e9) - the idea is that
    more histories with LPB activated are more convenient than less histories
    without LPB - ii. increase the bin volume (if you don't really need there
    bins of 2cmx1cmx1cm) and iii. set up suitable importance biasing as
    yourself envisaged in one of your previous messages.

    More information on biasing is available at

    Hope this helps


    Francesco Cerutti
    CH-1211 Geneva 23
    tel. ++41 22 7678962
    fax ++41 22 7668854

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