Re: [fluka-discuss]: A puzzled neutron shielding problem

From: <>
Date: Sat, 25 Jun 2016 08:31:13 +0800 (GMT+08:00)

Dear Tao Yang,

I am not really a FLUKA expert but I will try to answer yoour question (if I am wrong, please correct me). In my opinion, the results of USRBDX are not accurate because you are counting some neutrons several times.

Indeed, it is possible form one neutron first to go from BPE to Pb, then to bounce back to BPE, then to go again into Pb, etc.... In this case, the same neutron would be counted twice in in the USRBDX card.

You could calculate the fluence usingTWO USRBDX card for each surface: for example, one for the neutrons from BPE to Pb and one for the neutrons from Pb to BPE; the total fluence will be the difference between the two cards. I am not sure if using the two-way scoring this can be done with just one USRBDX card, but this can be easily checked.

Best Regards

-----Original Messages-----
From: "YANG Tao" <>
Sent Time: Thursday, June 23, 2016
Subject: [fluka-discuss]: A puzzled neutron shielding problem

Hello, everyone!

I simulated the neutron shielding effect by three layers as shown in fig1, the first layer is 4-cm-thick iron, the second layer is 4-cm-thick borated polyethylene(BPE), and the last one is 2.5-cm-thick lead. Neutrons of 1 MeV injects on the iron layer from left to light. Now I set three scoring cards(USRBDX, one-way current) at the interface of iron to BPE, BPE to lead and lead to the right vacuum to obtain the neutron fluence. However, I obtain the strange results as shown in fig2, it seems that the neutron number becomes greater after they penetrate the BPE layer since the red line is higher than the black line. But when I check the corresponding sum.lis files, the Tot. resp. is respectively 0.7257115, 0.3018902, 0.1735702 , which are reasonable for the neutron counting decreases as they penetrate the layers. However, how to explain the inconformity of fig2 and the total response (neutron counting) in sum.lis files?

Could someone help me in this question please?
Thank you.
With the best regards,

Tao Yang



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