Re: Secondary protons in shielding

Hi Joel,

your FLUKA results are fully consistent with the artefact you introduced
in choosing a beam spot exactly matching your detector surface
(0.4cmx0.5cm), whereas in reality the beam size is MUCH larger (15cmx15cm
according to the paper you quote). This way in your simulation the
detectors will never see the secondary protons - responsible for the dose
rise as you say - coming from larger radii.

Furthermore, I do not understand the choice of 68153689 as number of
primaries. I see that this corresponds to 100mGy according to the expected
dose of 1 proton, but for FLUKA this number has no particular meaning
(not more than 75d6 or 50d6, provided that the statistics is large
enough). FLUKA will always give you the dose per primary proton, to be
multiplied by the beam intensity at postprocessing level (by you).
Remember that if you are now going to increase the beam spot over the
whole target transverse size (10cmx10cm according to your model; obviously
there is no point of going beyond if the target had actually these
dimensions), you should then normalize taking into account the beam
current (integrated over the irradiation time) through that surface.

Ciao

Francesco

**************************************************
Francesco Cerutti
CERN-EN/STI
CH-1211 Geneva 23
Switzerland
tel. ++41 22 7678962
fax ++41 22 7668854

On Mon, 23 Mar 2009, Joel DeWitt wrote:

> fluka-discuss:
>
> As part of a shielding baseline I have run 1 GeV protons on Al, Cu,
> and polyethylene targets of various depths. The attached plot shows
> the results of such simulations. As you can see, FLUKA and experiment
> disagree both quantitatively and qualitatively. Background: We used
> Al2O3 OSLDs to measure dose in these experiments since the LET of the
> protons is too low to see with plastic detectors.
>
> Consider also that in Radiat Environ Biophys 46, 107 (2007) a similar
> experiment was performed and compared to results of the PHITS code.
> Here they agree very well. The increase in dose--both in our
> experimental results and theirs--is attributed to secondary protons
> emitted from the target material.
>
> Why FLUKA is not showing this phenomenon is puzzling.
>
> Any feedback will be appreciated.
>
> Joel DeWitt
> Oklahoma State University
> USA
Received on Tue Mar 24 2009 - 09:05:11 CET