Dear FLUKA experts,
in my current simulations I investigate the moderating influence of water in a terbium (TbCl3 x 6H2O) sample qualitatively. I read experimentally measured spectrum data with a 2 column .txt file (attached, first column energy bins, second column emission probability, i.e. number of neutrons in each bin divided over total flux) and I use the source routine (source file attached). The rectangular shaped sample is rotated 45° around the y-axis (relative to the incident neutron beam, which hits the target into positive z-direction perpendicular).
The incoming neutron spectrum is analyzed with a USRTRACK (rectangular vacuum region of same area than the sample) and a USRBDX (between the rectangular shaped vacuum region and the rectangular shaped sample) detector. The reduced sample flux is determined with another USRTRACK card just for the sample region. The original input spectrum actually has 3 parts (thermal, epithermal and fast). The fast neutron regime spans from 0.06 MeV to 20 MeV. With an integral flux (over all 3 neutron groups) of 1.15E+08 neutrons one should obtain a number of fast neutrons of 1.13E08. Therefore each energy bin in this region (actually not exactly down to 0.06 MeV as the FLUKA internal low-energy neutron group boundary is slightly lower) is multiplied by its width in units of energy and the total number of neutrons, i.e. total flux (experimentally, 1.15E+08). The BEAMPART scoring results in ~ 1.02E+08 and the NEUTRON scoring inside the sample results in 1.12E+08 neutrons/cm2/s. I have also attached the output files of the USRTRACK and USRBDX scoring.
I do not understand how the NEUTRON scoring can yield me more neutrons than were simulated (BEAMPART). The incident neutron spectrum has to be attenuated due to absorption and scattering processes. A neutron self-shielding factor for the fast neutron group could be determined by dividing the fast neutron flux in the sample by the incident fast flflux. The shielding factor for fast neutrons in the Tb sample was analytically determined to be ~ 0.90. It is interesting to recognize that one obtains a value of 0.91 by dividing the incident flux of 1.02E08 over the sample flux 1.12E08 neutrons/cm2/s (but it should be vice versa!).
Since the "effective" width of the target projected on the x-axis is reduced due to rotation of 45° around y-axis, the neutron beam also has to be smaller in width (to prevent that some neutrons even do not enter the sample). I have sampled the neutron source as a plane source with x and y being sampled. X is sampled in a way that all neutrons hit the sample.
Does anyone have an idea where the problem is or what might be wrong with my input? If it is a problem related to statistics, what number of primaries would I have to run as a minimum?
Thanks in advance and kind regards,
Niklas Ophoven
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Forschungszentrum Juelich GmbH
52425 Juelich
Sitz der Gesellschaft: Juelich
Eingetragen im Handelsregister des Amtsgerichts Dueren Nr. HR B 3498
Vorsitzender des Aufsichtsrats: MinDir Volker Rieke
Geschaeftsfuehrung: Prof. Dr.-Ing. Wolfgang Marquardt (Vorsitzender),
Karsten Beneke (stellv. Vorsitzender), Dr. Astrid Lambrecht,
Prof. Dr. Frauke Melchior
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Received on Tue Sep 07 2021 - 19:38:49 CEST