# RE: Thermal and epithermal neutrons--more questions

From: Tang, Alfred (atang@phys.ksu.edu)
Date: Thu May 31 2007 - 00:46:29 CEST

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Dear Stefan and other experts,

I am interested to calculate the muon-induced productions such as neutrons and residual nuclei. I ran two simulations--one with the just the free atom model (which I call "the free case") with the following material definition for hydrogen:

MATERIAL 1.0 1.00794 0.0000837 3.0 0.0 HYDROGEN

I ran another simulation using the LOW_MAT card to simulate chemical binding (which I call "the low-mat case"):

MATERIAL 1.0 1.00794 0.0000837 3.0 0.0 HYDROGEN
LOW-MAT 3.0 1.0 -4.0 293.0 0.0 0.0 HYDROGEN
MATERIAL 1.0 1.00794 0.0000837 4.0 0.0 H-WATER
LOW-MAT 4.0 1.0 -2.0 293.0 0.0 0.0 HYDROGEN
MATERIAL 1.0 1.00794 0.0000837 5.0 0.0 H-OIL
LOW-MAT 5.0 1.0 -3.0 293.0 0.0 0.0 HYDROGEN

Of course I use consistent material numbers for different kinds of hydrogen in the rest of both input files. The average muon energy is about 280GeV. What I have found is that (1) neutron track length in the low-mat case is 5 times higher than the free case and that (2) the residual nuclei production in the low mat case is about 40 times lower than the free case. I do not have any feeling for the difference in the residual nuclei production in relation to use of the LOW_MAT card. But the factor of 40 difference seems very high to me. I also know that the neutron track length in the low-mat case is supposed to be only about 25-30% higher than the free case for neutron energy below 2eV. Since epithermal and thermal neutrons constitute only a relatively small part of the neutron spectrum, the factor of 5 difference seems very high too. Here is my question: Does the LOW-MAT card replace the high energy neutron scattering cross section with the thermal neutron cross secti!
on indiscriminantly without respect to neutron energy? So far my guess is that the low-mat numbers seem wrong. Am I right in assuming that? Or did I make a mistake in my material definitions?
Regards,
Alfred

*****************************************************
Alfred Tang
Physics Department
Kansas State University
13 Cardwell Hall
Manhattan, KS 66506

(785) 532-1699

-----Original Message-----
From: owner-fluka-discuss@mi.infn.it on behalf of Stefan Roesler
Sent: Thu 5/17/2007 3:15 PM
To: Tang, Alfred
Cc: fluka-discuss@fluka.org
Subject: Re: Thermal and epithermal neutrons

Dear Alfred,

There are low-energy neutron cross section data sets available in FLUKA
which take into account molecular bindings. The cross section data set has
to be associated with the material by the card LOW-MAT.

The following cross section data set list has been copied from the Manual,
Chapter 10. You would have to specify the data set giving the Identifiers
in the LOW-MAT card.

Material Temp. Origin RN Name Identifiers Gam
H H2O bound nat. Hydrogen (1) 293K JEF-2.2 N HYDROGEN 1 -2 293 Y
H CH2 bound nat. Hydrogen (1) 293K JEF-2.2 N HYDROGEN 1 -3 293 Y
H Bound nat. Hydrogen (1) 293K JEF-2.2 N HYDROGEN 1 -4 293 Y
H Free natural Hydrogen (1) 293K JEF-2.2 N HYDROGEN 1 -5 293 Y
1H H2O bound Hydrogen 1 293K JEF-2.2 N HYDROG-1 1 +1 293 Y
1H CH2 bound Hydrogen 1 293K JEF-2.2 N HYDROG-1 1 +11 293 Y
1H bound Hydrogen 1 293K JEF-2.2 N HYDROG-1 1 +21 293 Y
1H Free Hydrogen 1 293K JEF-2.2 N HYDROG-1 1 +31 293 Y
H H2O bound nat. Hydrogen (1) 87K JEF-2.2 N HYDROGEN 1 -2 87 Y

Hope this helps,

Best regards
Stefan

On Thu, 17 May 2007, Tang, Alfred wrote:

> Dear FLUKA experts,
> I know that FLUKA has a very good reputation in neutron simulations down
> to the thermal neutron regime (<1eV). In the case of complex materials
> such as hydrogen rich compounds, my understanding is that the chemical
> bindings of the molecules affect neutron transport because of the
> rotation and vibration of the molecules. MCNPX deals with chemical
> binding below 2eV (epithermal neutron regime) by letting the
> S(\alpha,\beta) treatment take over the free atom model. It seems that
> FLUKA does not ask for information about the chemical formula of the
> molecule nor specify any S(\alpha,\beta) treatment. Shall I assume that
> FLUKA also uses a free atom model above 2eV? My real question is "How
> does FLUKA simulate neutrons below 2eV?"
> Regards,
> Alfred
>
>
> **************************************
> Alfred Tang
> Physics Department
> Kansas State University
> 13 Cardwell Hall
> Manhattan, KS 66506
>
> (785) 532-1699
>
> http://www.phys.ksu.edu/personal/atang
>
>
>

--
___________________________________
Stefan Roesler
CERN, SC/RP
CH-1211 Geneva 23
Switzerland
Phone:  +41-22-7679891
Fax:    +41-22-7669639
E-mail: Stefan.Roesler@cern.ch


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