Re: [fluka-discuss]: Probelm regarding the neutron fluence and moderation of neutrons in graphite

From: Talanov Vadim <vadim.talanov_at_psi.ch>
Date: Mon, 10 Jul 2017 11:42:14 +0200

Dear Tom,

Just to quantify what has been already explained by Vasilis:

You can compare the slowing down power for C (0.087) to the one of H2O
(1.39) (Table 13.2, Handbook of Spallation Research by Filges and
Goldenbaum),

Kind regards,
Vadim

On 10.07.2017 09:52, Vasilis Vlachoudis wrote:
> This is not a problem, is the correct behavior that one should expect
> from neutron physics.
> Hydrogen rich material are very good as moderators, thanks to the high
> average logarithmic
> energy loss per collision, the neutron needs only a few collisions to
> thermalize
> which can be achieved with a few cm of Water, PMMA etc...
> While in contrary in C it needs a lot more collisions, therefore much
> more material.
> Briefly you don't have enough C in your geometry to see the
> thermalization of neutrons,
> they simply escape before they are thermalized.
>
> V.
>
>
>
> ------------------------------------------------------------------------
> *From:* owner-fluka-discuss_at_mi.infn.it [owner-fluka-discuss_at_mi.infn.it]
> on behalf of Tom Jonas Janzen [tom.jonas.janzen_at_hs-furtwangen.de]
> *Sent:* Sunday, July 09, 2017 13:01
> *To:* fluka-discuss_at_fluka.org
> *Subject:* RE: [fluka-discuss]: Probelm regarding the neutron fluence
> and moderation of neutrons in graphite
>
> Dear Vasilis,
>
> at least i can try. The detectors of my simulation are cylinders with a
> thickness of 1cm (10 cylinders). With the help of those cylinders i am
> able to track the neutron fluence inside of the cylinders depending on
> the energy of the neutrons and the travelled distance of the neutrons.
>
> All my simulation results for the materials PMMA, HDPE and Water (all
> moderator mterials) are almost identically. The neutron fluence hits the
> maximum at the energyspectrum of 2E-11 GeV, which is the energy level of
> neutrons. This means, that the moderation of neutrons is good with those
> materials and as the results the neutron fluence is rising in the energy
> spectrum of thermal neutrons.
>
> Only the results of graphite are different. The neutron fluence hits
> it‘s maximum at the starting energy of my neutron source (0,003 GeV) and
> then only decreases. No further maximums and especially no detectable
> neutron fluence inside the thermal energy spectrum. Also the plots of my
> _tab.lis files are normaly standardized in the range of 1E-12 to 1E-9
> GeV on the x-axis. But in the case of graphit i had to decrease the
> range to
>
> 1E-8 to 1E-2 GeV due to the lack of detectable neutron fluence in lower
> energy ranges (e.g. 1E-11 GeV).
>
> So i expected from the simulation with graphite results, which are
> comparable to the results of PMMA, HDPE and Water. I hoped to see a
> increase of the neutron fluence on the energy level of 1E-11 GeV.
> Therefore i questioned myself, if graphite only moderates fast neutrons?
> Is there another cross section for graphite, which allows me to simulate
> neutron fluence in lower energy ranges, especially at 1E-11 GeV (thermal
> level)? Why is there no detectable neutron fluence in the energy
> spectrum of 1E-12 to 1E-9 GeV?
>
> I hope i could explain my problem a bit better for you!
>
> Again thank you for your time effort!
>
> Cheers,
>
> Tom
>

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Received on Mon Jul 10 2017 - 12:36:43 CEST

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