Re: [fluka-discuss]: Particle fluence & PART-THR in NEW-DEFA

From: Francesco Cerutti <Francesco.Cerutti_at_cern.ch>
Date: Thu, 26 Feb 2015 16:14:36 +0100

Dear Mina,

by definition of threshold, the spectrum below threshold is meaningless.
What you see there is a combination of different effects. First, the
ranging out mentioned by Anna: a proton reaching the 10 MeV threshold is
granted a last step bringing it to rest. In differential fluence scoring,
this is in principle attributed to a 5 MeV proton (half of the energy at
the beginning of the step). However, in case this last step crosses region
boundaries, you will see in the spectrum a corresponding number of
contributions at different energies, assuming a constant energy loss over
the step (hence the crudity of ranging out, which nevertheless assures a
good estimation of the residual range, i.e. of the last step length).
Things are then smeared by Landau fluctuations, making not all protons to
reach the threshold and the ranging out treatment exactly at 10 MeV.
On top of that, you have evaporation protons generated below threshold by
nuclear reactions and then ranged out. As well as, in case of hydrogen
presence, recoil protons generated below threshold by low energy neutrons.

In conclusion, if you are really interested in that part of the spectrum
(I guess not) and yours is not just curiosity about its puzzling shape,
you have to lower the threshold accordingly.

Kind regards

Francesco

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

On Tue, 24 Feb 2015, Mina Nozar wrote:

> Dear Anna,
>
> Thank you for the explanation. I now understand that the protons are ranged
> out; however, I don't understand the shape of the spectrum. I would have
> expected a gradual drop off of the protons with decrease in energy but what I
> see is a drop at 10 MeV, then a rise, a bit of a plateau, then a sharp drop
> off at 1 MeV.
>
> What is responsible for the observed distribution?
>
> Thank you and best wishes,
> Mina
>
> On 15-02-21 10:28 AM, Anna Ferrari wrote:
>> Dear Mina,
>>
>> in FLUKA setting a threshold for heavy charged particles does not mean
>> that you stop the transport, but that you stop the very refined transport:
>> below the threshold the transport is more crude, but whatever the
>> threshold is heavy charged particles are ranged out to rest.
>> An exception is if the energy cutoff has been set by the user > 100 MeV.
>> As you know, the situation is different for electrons/positrons: ranging
>> out is not done, mainly because it is difficult to define with precision a
>> residual range (you can see
>> http://www.fluka.org/web_archive/earchive/prova/1305.html )
>>
>> Greetings!
>> Anna
>>
>> Am Thu, 19 Feb 2015 12:56:05 -0800 schrieb Mina Nozar <nozarm_at_triumf.ca>:
>> > Hello everyone,
>> >
>> > I just did a quick study, looking at the energy/power deposition in an
>> > Al flange, enclosing an Indium seal. The flange sits upstream of a Be
>> > target (at a 45 deg angle and a distance of 45 cm from the target). The
>> > target is bombarded by 480 MeV proton beam.
>> >
>> > I chose NEW_DEFA as the defaults (so a PART-THR of 10 MeV for protons).
>> > I did not lower this threshold using PART-THR card. When I look at the
>> > particle fluences in the Al flange (beam particles, protons, and
>> > neutrons), I see protons down to 1 MeV energies and I am wondering why.
>> > It is like seeing a second set of data from 10 to 100 MeV.
>> >
>> > Please see the image enclosed.
>> >
>> > Thank you very much for helping me understand what is going on.
>> >
>> > Best wishes,
>> > Mina
>> >
>> >
>> >
>> >
>>
>> ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
>> Dr. Anna Ferrari
>> Institute of Radiation Physics
>> Helmholtz-Zentrum Dresden-Rossendorf e.V.
>> Tel. +49 351 260 2872
>> a.ferrari_at_hzdr.de
>> http://www.hzdr.de
>>
>> Vorstand: Prof. Dr. Dr. h. c. Roland Sauerbrey, Prof. Dr. Dr. h. c. Peter
>> Joehnk
>> Vereinsregister: VR 1693 beim Amtsgericht Dresden
>>
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Received on Thu Feb 26 2015 - 17:31:23 CET

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