RE: [fluka-discuss]: Thermal neutron fluence using USRBDX scoring

From: Vasilis Vlachoudis <>
Date: Tue, 21 Apr 2015 12:08:53 +0000

Hi George,

are you sure on your setup (geometry, dimensions, support materials,
beam direction,incoming neutron energy etc.) vs the experimental one?

Maybe I am wrong but all isotopes of Ge have a higher capture (~1b)
cross section than H (0.3b), but not so much to justify a 20%
reduction with your present setup.

You are correct, the thermal neutrons they do spread as a gas everywhere
however they are attenuated with the distance 1/r2 and by absorption in H.
If you run more stats (kill the EMF to increase the simulating speed) you
will see on your USRBIN that they do reach the Cd volume


From: Georgios Tsiledakis
Sent: 20 April 2015 17:04
To: Vasilis Vlachoudis;
Subject: RE: [fluka-discuss]: Thermal neutron fluence using USRBDX scoring

Dear Vasilis,

Thank you a lot for the suggested corrections. It was an unfortunate stupidity of mine not to pay enough attention at my input.
I have applied all of yours - as it can be seen at the attached inputs.
I made 8 runs of 1 M events each.
Thus, for 8M primaries I am getting:

  **** NavickNoGe2_33.bnx ****

      from reg. 3 to 4 ,
      one way scoring,
      fluence scoring scoring)

     Tot. resp. (Part/cmq/pr) 3.1791675E-05 +/- 10.09971 %

  **** NavickGe2_33.bnx ****
    Tot. resp. (Part/cmq/pr) 3.2917935E-05 +/- 9.496796 %

or looking at the n-flux at the Co target:

  **** NavickNoGe2_27.trk ****
     Tot. response (p/cmq/pr) 1.6087519E-07 +/- 7.758788 %

  **** NavickGe2_27.trk ****
     Tot. response (p/cmq/pr) 1.6074226E-07 +/- 10.33780 %

What is the expected behaviour as you pointed out?

Experimentally, with a similar set up as the one that is represented in the geometry of this fluka input, we measure neutron flux 20% less. And we were thinking that this is because of the presence of Germanium cylinder which captures many neutrons.

Is this true?

Why the beam primary particles (thermal neutrons) cannot reach the cobalt target after entering onto H20 cylinder? I was expecting that being thermal they should behave like a gas spreading everywhere in the volume....

Thank you very much in advance

Best regards


From: Vasilis Vlachoudis
Sent: 20 April 2015 15:10
To: Georgios Tsiledakis;
Subject: RE: [fluka-discuss]: Thermal neutron fluence using USRBDX scoring

Hi George, Sunil,

The difference is not caused by the S(a,b), thresholds or physics cards, but
rather by the wrong scoring quantity in your input.
Your plot trigger me with the high energy events, while you are starting
with a pure thermal neutron beam.
You are scoring ALL-NEUT instead of NEUTRON therefore all neutral
particles including gammas, and of course with Ge you will have more
gammas generated than without.
Replacing the scoring to NEUTRON I get the expected behaviour.

Other errors in the input
Double definition of the DEFAULTS with PRECISION and EM-CASCADE
the second is not necessary.

Unnecessary cards:
- Many of the LOW-MAT can me deleted
- EMF, EMFCUT (as a thermal neutron problem do you care for the gammas?)
- USERWEIG (do you use comscw routine?)


From: [] on behalf of Georgios Tsiledakis []
Sent: 20 April 2015 11:04
Subject: RE: [fluka-discuss]: Thermal neutron fluence using USRBDX scoring

Dear Sunil,

I have tried what you suggested but in vein...
I changed the LOW-MAT:

I attach the input files as well, where

The results are still similar like before, looking for example the neutron flux (***_26_sum.lis files)

n-flux in Co target without Ge:
Tot. response (p/cmq/pr) 1.2499129E-07 +/- 8.447819 %

With Ge:
Tot. response (p/cmq/pr) 1.5400029E-07 +/- 12.29173 %

Also from the beam particles dont look to have the behaviour of the thermal neutron gas inside the water....

Is the thermal neutron scattering process somewhat not active in my input?

Or the mistake it in LOW-NEUT?

Thank you very much in advance

Best regards


From: Sunil C []
Sent: 20 April 2015 09:28
To: Georgios Tsiledakis
Subject: Re: [fluka-discuss]: Thermal neutron fluence using USRBDX scoring

Dear Georgios

Please try with Low mat for H2O bound natural hydrogen to invoke the S(alpha,beta) treatment and let us know. You are currently using free gas natural hydrogen at 296K.
Cheers, Sunil

On Thu, Apr 16, 2015 at 2:47 PM, Georgios Tsiledakis <<>> wrote:
Dear Sunil,

Thank you very much for your answer...
I have tested what you proposed and its true that the beam particles dont hit the Cobalt target...
But since they are thermal neutrons, no matter how they are emitting from the beam, inside Water they should create a kind of thermal neutron gas travelling in all directions and finally hit the thin target...
My logic was that the presence of Germanium cylinder inside the Water volume should absorb many neutrons, resulting at a measured neutron fluence in the target smaller than the one without the presence of Germanium...
What I measured with high statistics was just the one way scoring of neutrons from Water towards Cobalt. Surprisingly, the results are the opposite I was expecting...
Is any mistake considering my DEFAULTs settings or any more appropriate PHYSIC card that could be used?

Thank you very much in advance

Best regards

From: Sunil C [<>]
Sent: 13 April 2015 07:35
To: Georgios Tsiledakis
Subject: Re: [fluka-discuss]: Thermal neutron fluence using USRBDX scoring

Dear Georgios

I see that your beam is directed towards the positive Z, while your Co is somewhere in the -Y half. Thus the beam is not going anywhere near your cobalt. If you plot a USRBIN data containing the beam particles, you can see that this is indeed the case. I have attached such a plot. The difference in your 2 plots or the numbers you report do not seem to be very much and I suspect is due to statistical fluctuations. With 14-18% uncertainty, I will be very cautious with any conclusion.

Change your beam direction to point towards the -Y region and try.

Cheers Sunil

On Fri, Apr 10, 2015 at 4:19 PM, Georgios Tsiledakis <<>> wrote:
Dear fluka experts,

I have a cylinder of H2O and a beam of thermal neutrons that hits on it...
Inside the water cylinder there is a small thin box made of Cobalt (target) which
is next to a Germanium cylinder...
Using USRBDX scoring -one way- , I would like to demonstrate that the flux of neutrons from H20
that hit the Cobalt target is smaller with the presence of the germanium cylinder.

But the results as you can see at the attached picture are the opposite...

Why is that?

Is any other way to conclude concerning the effect of Ge in the neutron flux on the Co target?

Thanks a lot in advance

Best regards


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Received on Tue Apr 21 2015 - 15:48:39 CEST

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