RE: [fluka-discuss]: Issues with the LOW-MAT card(B

From: Vasilis Vlachoudis(B <Vasilis.Vlachoudis_at_cern.ch>
Date: Mon, 12 Sep 2016 06:22:03 +0000

Thanks Mattias,

for pointing out this problem. As you already figured out in private communication
flair when using a standard FLUKA name as a material, it is
trying to find the first match from the neutron database and display it.
However it was not updating the values (what(2,3,4) when a match was found.
If you change them then it should be updated it correctly.

Now I've modified it, so when it finds a match it will update the
corresponding whats. You can access the corrected version from the svn repository

Cheers
Vasilis

________________________________________
From: owner-fluka-discuss_at_mi.infn.it [owner-fluka-discuss_at_mi.infn.it] on behalf of Mattias Lantz [Mattias.Lantz_at_physics.uu.se]
Sent: Friday, September 09, 2016 15:37
To: fluka
Subject: [fluka-discuss]: Issues with the LOW-MAT card

Dear FLUKA experts,

The use and non-use of the LOW-MAT card is a relatively common subject
in the forum, and in spite of repeated recommendations of only using it
in specific cases it seems to be a source for confusion and user mistakes.

Based on some of my own mistakes I decided to make a few systematic
checks of a number of input cards in order to see if something changes
when inserting them although they are already implicit from the selected
default option. These tests leave me very confused about the LOW-MAT
card. Either there is a mistake or mismatch in the code, or, more
likely, I am making some serious mistakes, so please help me out.

* Attached is an input file for 30 MeV protons on a thick Beryllium
plate in vacuum (some disabled input cards have been tested in different
combinations).
* Attached are also 3 plots of energy spectra for the neutrons produced,
one for neutrons emitted at 30 degrees, and two (in linear and
logarithmic scales) for the total neutron production (i.e. 0-180 deg).
* USRBDX cards are used for scoring neutrons at different angles 1 m
from the Beryllium plate.
* No DEFAULT card is activated, which implies that NEW-DEFAULT is used.
Thus there is no need for the LOW-NEUT card (it is implicit from
NEW-DEFAULTS), and according to the notes in the FLUKA manual there is
no reason for a LOW-MAT card for Beryllium.

1. If I run the file as it is then I get a certain energy distribution
(in the plots this is called "Default" and is indicated with red
histograms).
2. If I activate the LOW-MAT card for Beryllium 296 K I expect no
change. The card is unnecessary and my basic assumption is that this
should give the same result as the "Default" setting. In the plots this
is indicated with blue histograms. As seen there is a drastic difference.
3: If I instead activate the LOW-MAT card for Beryllium 87 K I would
expect some difference from the 286 K case. There is indeed a
difference, but the 87 K case is also indentical with the default setting.

The conclusion I draw, with strong hesitation, is that a default setting
without LOW-MAT card will use cross sections for 87 K. Thus a LOW-MAT
card is always necessary in order to simulate experimental setups at
room temperature. I have tried this also for higher energies and for
heavier target nuclei and get similar results. Other cards, like
PART-THR or the PHYSICS option for coalescence, are not included here,
but when included the pattern is similar.

Besides this conclusion (which I find it hard to believe in), the
physical features of the neutron distributions do not seem to make
sense. The 286 K case has neutrons down to thermal energies, my
gut-feeling tells me there can not be such efficient moderation from the
Beryllium block itself, and there should not be many neutrons produced
with such low energies. Furthermore there should not be so drastic
differences between these two temperatures. What is it with the LOW-MAT
cards that affect the neutron production so drastically?

I must be missing something (or a number of things...), but what?

Best wishes,
Mattias Lantz


--
Mattias Lantz - Researcher, PhD
$B%i%s%D(B $B%^%F%#%"%9(B
Department of Physics and Astronomy
Division of Applied Nuclear Physics
Uppsala University, Box 516
SE - 751 20, Uppsala, Sweden
phone:  +46-(0)18-471-3754
cell:   +46-(0)730-454-384
fax:    +46-(0)18-471-5999
email:  mattias.lantz_at_physics.uu.se
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Received on Mon Sep 12 2016 - 09:55:44 CEST

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