[fluka-discuss]: RE: Cos angle low energy

From: Anton Lechner <Anton.Lechner_at_cern.ch>
Date: Thu, 15 Nov 2018 17:08:09 +0000

Dear Thomas,

Before answering your question why you see the beam impact orthogonal on the absorber, I would like to point out a few important changes you should make:

You are using PRECISIO as defaults (which is good), however the proton transport thresholds are still too high (100 keV) considering your primary energy (75 keV). The protons are still produced but the code simply ranges them out in order to distribute the sub-threshold energy. You have to decrease the transport threshold using the PART-THR card (to 1 keV which is the lowest recommended value). At the same time you should also lower the production and transport thresholds for electrons using the DELTARAY and EMFCUT cards (also to 1 keV). See the manual for details how to use these cards.

Secondly, you should rather use the BEAMAXES cards to define the beam direction. Note that the sampling of the transverse beam distribution (as defined on the BEAM card) is done in the beam reference frame defined by the BEAMAXES cards. The direction cosines on the BEAMPOS card do not change the beam reference frame - see also Note 2 under BEAMPOS card in the manual. This means the beam profile sampling is still done in the xy plane and not in the plane orthogonal to your beam direction.

Your protons have a very short range (less than 1 um CSDA range in Cu), so they stay very close to the surface. It is advisable that you switch on single scattering using the MULSOPT card.

Finally, why do you still see the beam orthogonal? In fact, it is simply because of your binning. Your bin size in x and y direction is 0.00333 cm. However, 75 keV protons only have a CSDA range of 6E-5 cm, which is much smaller than your xy bin size. So even for very grazing angles, a proton impacting on the copper block will in most cases stay within the same xy bin. This gives the impression that the impact is orthogonal wrt to the copper surface, but this is not the case.

Cheers, Anton





________________________________
From: Thomas Grandsaert [Thomas.Grandsaert_at_esss.se]
Sent: 14 November 2018 13:38
To: Anton Lechner; fluka-discuss
Subject: RE: Cos angle low energy

Thank you for your altruism! Here is the input file.

T

From: Anton Lechner <Anton.Lechner_at_cern.ch>
Sent: den 13 november 2018 14:11
To: Thomas Grandsaert <Thomas.Grandsaert_at_esss.se>; fluka-discuss <fluka-discuss_at_fluka.org>
Subject: RE: Cos angle low energy

Dear Thomas,

Could you please send the input file? Without the input it is hard to guess what you are doing (e.g. what particle type? are you using the BEAMAXES card? etc.)

Cheers, Anton

________________________________
From: owner-fluka-discuss_at_mi.infn.it<mailto:owner-fluka-discuss_at_mi.infn.it> [owner-fluka-discuss_at_mi.infn.it] on behalf of Thomas Grandsaert [Thomas.Grandsaert_at_esss.se]
Sent: 13 November 2018 11:50
To: fluka-discuss
Subject: [fluka-discuss]: Cos angle low energy
Hi Fluka experts,

I am trying to simulate a grazing angle of approximately 11 degrees on a copper surface at an energy of 75 KeV. When I look at the energy deposition, it shows what appears to be an orthogonal beam distribution (in both X/Y) planes.

I tried reducing the step size, however it did not affect the distribution. Could someone please enlighten me how to most accurately simulate the beam at low angles in this energy regime? Thank you in advance.

Cheers,
T

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Received on Thu Nov 15 2018 - 19:37:33 CET

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