Re: request to identify the error

Dear Patil,

there is no error on your part: the photoneutron angular distribution for
photons in the MeV range is indeed peaked at 90 degrees, in the direction
of the photon electric field. The form of the distribution is
A + B sin^2(theta).
However, in FLUKA as in all other Monte Carlo codes I know, the interaction
is simulated in an approximate way as if it was evaporation or Fermi
break up, i.e., an isotropic distribution. For heavy nuclei, this choice
is due to the difficulty to implement a collective nuclear effect (the Giant
Resonance) as a Monte Carlo model. But for Beryllium, where the interaction
is mostly with a single, loosely bound neutron, I admit that one could be
more accurate. Sorry!
Concerning the energy distribution, in the case of Beryllium it is well
known analytically, since the interaction is a two-body one and the angle of
emission is directly correlated to the neutron energy (see A. Hanson, in
Fast Neutron Physics, ed. J. Marion and J. Fowler, Interscience, New York
1960, pp. 3-48). Also in this case, I am afraid, the spectrum calculated
by FLUKA is not the correct one.
I hope we will take care of this anomaly in the near future.

The only attempt to reproduce the actual Giant Resonance angular
distribution in a Monte Carlo code that I am aware of, is that of my friend
Mario Santana in his thesis:
http://msantana.web.cern.ch/msantana/Thesis.html
However, it is a purely phenomenological approach, not based on a physical
model.

This is certainly a field where there is space for improvements. The
implementation of photonuclear reactions in FLUKA was done originally to
allow shielding calculations for electron accelerators: and for this purpose
it is not essential to describe the initial angular distribution with
great accuracy, since the "memory" of them is soon lost after a few
interactions. But now, since our users are becoming more demanding and,
as you do, expect good accuracy even at the single interaction level,
we need to do some work about it.

Alberto

On Mon, 22 Jun 2009, Mr. Bhushankumar Jagnnath Patil. wrote:

> Dear Fluka Experts,
> I am trying to simulate the experiment given in the paper entitled
> "The Angular Distribution of the Photo neutrons from Beryllium." B.
> Hamermesh et. al, Phy. Rev., Vol 76(5), pp 611, (1949).
> In his experiment the incident gamma energy was used 2.76 MeV on the
> beryllium target and measured the photneutrons at various angles. For
> the detection of neutrons he has used the Bf3 pulse ion chamber
> embedded in paraffin cylinder. The distance between the source-target
> and target-detector was kept by him 10 cm and 66 cm respectively.
> The results obtained in this paper shows peak at 90 degree. I have
> tried to simulate the same experiment by keeping the same parameters in
> FLUKA. I have checked the neutron flux at 0, 45, 90 degree. In addition I
> have also checked the double differential neutron yield w.r.t. kinetic
> energy and angles. But the simulated results are not similar to the
> experimental one. In simulated case it shows nearly isotropic distribution
> of neutrons. I am attaching herewith my input file for your ready
> reference.
> The link to the reference paper is as follows
> http://prola.aps.org/abstract/PR/v76/i5/p611_1
>
> Thanking you.
>
> Patil B. J.
>
> Mr. Bhushankumar Jagnnath Patil.
> Microtron Accelerator Lab.
> Department of Physics,
> University of Pune,
> Ganeshkhind, Pune 411007
> Ph. No. Off. 020-25692678 Ext. 421
> Mob. 9823968377
> Webpage http://physics.unipune.ernet.in/~bjp
> Alternate e-mail amolbhushan_at_gmail.com
>
>

-- 
Alberto Fasso`
SLAC-RP, MS 48, 2575 Sand Hill Road, Menlo Park CA 94025
Phone: (1 650) 926 4762   Fax: (1 650) 926 3569
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