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 - 11:06:27 CEST