RE: How do i improve statistics (produce more neutrons)

From: Alberto Fasso' <fasso_at_SLAC.Stanford.EDU>
Date: Fri, 4 Sep 2009 08:33:12 -0700 (PDT)

Dear Lawrence,

of course it is your choice. But my suggestion is that you should consider
doing separate calculations for neutron dose and photon dose, with different
settings. As you yourself have found out, it takes a lot of time to
get sufficient neutron statistics with your present cutoffs. On the
other hand, with those cutoffs you could achieve a good photon statistics
in a much shorter time.
But, as I have said, it is up to you to chose the best strategy

Kind regards,


On Fri, 4 Sep 2009, Lawrence Mhatiwa wrote:

> Dear Alberto and all Fluka users
> Thanks Alberto .I think i forgot to mention that i am supposed to compare
> also the Neutron to photon dose at the isocenter of the Linac.So if i use
> energy cut offs of 7MeV for photons and 10MeV for neutron i have a filling
> that my comparison will not truly represent neutron/photon dose at the
> isocenter on the actual LINAC.
> For now iam going to increase the baising of the photon hadronic interaction
> length from the present 0.02 to 0.01
> Please help
> Lawrence Mhatiwa
> Medical Physics Department
> University Of Free State
> South Africa
>> Date: Thu=2C 3 Sep 2009 10:07:55 -0700
>> From: fasso_at_SLAC.Stanford.EDU
>> To:
>> CC:
>> Subject: Re: How do i improve statistics (produce more neutrons)
>> Dear Lawrence
>> first of all you should set a much higher cutoff for electrons and
>> photons. In your present input=2C the cutoff is set at 1 MeV TOTAL energy
>> for electrons (i.e. about 500 keV kinetic energy) and 10 keV for
>> photons. The lowest neutron production threshold in the materials
>> you are using is a little higher than 7 MeV: therefore tracking particles
>> below that energy is a complete waste of time. I suggest that you set
>> the photon cutoff at 7 MeV and the electron cutoff a bit higher (10 MeV).
>> That should already improve the efficiency a lot.
>> If you find that it is not sufficient=2C you can increase the biasing of
>> the photon hadronic interaction length (WHAT(2) of LAM-BIAS) from the
>> present 0.02 to 0.01. But not much more=2C I think.
>> Best regards
>> Alberto
>> On Thu=2C 3 Sep 2009=2C Lawrence Mhatiwa wrote:
>>> --_64ce674f-922d-4c25-9d79-0ff7a6eadf85_
>>> Content-Type: multipart/alternative=3B
>>> boundary=3D"_91c33713-57ec-4927-9629-c276ca06bd77_"
>>> --_91c33713-57ec-4927-9629-c276ca06bd77_
>>> Content-Type: text/plain=3B charset=3D"iso-8859-1"
>>> Content-Transfer-Encoding: quoted-printable
>>> Dear Fluka Users
>>> I am characterising photoneutrons produced by a 15MeV Linear Accelerato=
> r. I
>>> am using USRTRACK to score neutron fluence spectra at various locations
>>> inside and outside the Bunker as shown in my input file.I can score neu=
> trons
>>> at these locations but my problem is that iam running 5.8 Billlion(20 M=
> il/cycle)
>>> histories and it takes one week to finish 4 cycles but there is no impr=
> ovement
>>> on the statistics of the spectra scored on locations outside the bunker=
> .
>>> Could someone please help me on how I can improve (generate more neutro=
> ns)
>>> on the statistics of my scores in a reasonably shorter time.I have atta=
> ched
>>> my input and project files to this mail. Also attached is the log-log n=
> eutron
>>> fluence spectrum behind the door outside the bunker=3D2C you can seee t=
> hat
>>> the statistics is still very poor.
>>> Please help
>>> Lawrence Mhatiwa
>>> Medical Physics Department
>>> University Of Free State

Alberto Fasso`
SLAC-RP, MS 48, 2575 Sand Hill Road, Menlo Park CA 94025
Phone: (1 650) 926 4762   Fax: (1 650) 926 3569
Received on Sat Sep 05 2009 - 11:46:58 CEST

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