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From: <emilio_at_impcas.ac.cn>

Date: Sat, 25 Jun 2016 19:47:25 +0800 (GMT+08:00)

Dear Tao Yang,

you are right, you cannot know if a neutron that bounces back will go back into Pb or not. However, let's consider the two cases:

1) BPE -> Pb; Pb -> BPE; BPE -> Pb; Pb-> outside Pb (NOT back into BPE)

Here if take the difference between the two USRBDX card the net neutron current is 1

2) BPE -> Pb; Pb -> BPE; neutron absorbed in BPE (or escaped in the vacuum)

Here the difference between the two cards is zero, and I would say that in this case also the neutron current between the two surfaces should be considered zero, since the neutron entered in Pb but then it went back. Do you agree?

In general, I would consider the neutron curent between the two materials as the difference between the neutrons coming out (from BPE to Pb) and the neutrons coming in (from Pb to BPE); this means that in both cases the difference between the USRBDX cards gives you the right results. Does it sound reasonable to you? As I said, I am not sure if this can be achieved simply using the two-way fluence (hence, using only one USRBDX card), but this can be tested very easily.

Best Regards

Emilio

-----Original Messages-----

From: "YANG Tao" <yangt_at_ihep.ac.cn>

Sent Time: Saturday, June 25, 2016

To: emilio_at_impcas.ac.cn

Cc: fluka-discuss_at_fluka.org

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

Dear emilio,

Thanks for your kind help. In the first mail, you suggested that I could calculate the fluence using two USRBDX cards 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 think it cannot be known whether or not that neutrons from Pb to BPE can completely bounce again back BPE to Pb, so I think the direct subtract of the two USRBDX cards can not be considered as the net one-way neutron fluence.Maybe my understanding is wrong, so please contact me if I misunderstand it.

Best Regards,

Tao Yang.

-----原始邮件-----

发件人:emilio_at_impcas.ac.cn

发送时间: 2016年6月25日 星期六

收件人: "YANG Tao" <yangt_at_ihep.ac.cn>

抄送:fluka-discuss_at_fluka.org

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

Dear Tao Yang,

I'm afraid i didn't explain myself properly. The problem is exactly that you are using an one-way estimator for the fluence. Indeed, consider the case

1) Neutron goes from BPE to Pb (BPE -> Pb, counted once)

2) Due to elastinc collision, it bounces back Pb -> BPE (not counted, since with WHAT(1) set on "one-way" you are not considering the neutrons going from Pb to BPE )

3) It bounces again back BPE -> Pb (counted twice)

4) ....

This can be easily verified using two USRBDX cards, as I described (I guess also using the two-way current estimator should take care of the problem, but I am not sure if the incoming current, in that case, is considered negative or positive).

Best Regards

Emilio

-----Original Messages-----

From: "YANG Tao" <yangt_at_ihep.ac.cn>

Sent Time: Saturday, June 25, 2016

To:emilio_at_impcas.ac.cn

Cc:fluka-discuss_at_fluka.org

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

Dear emilio,

Thanks for your reply! What(1) setting of USRBDX is one-way current, not two-way form, so I think it would never to occur as you pointed out ("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.").

Best regards,

Tao Yang

-----原始邮件-----

发件人:emilio_at_impcas.ac.cn

发送时间: 2016年6月25日 星期六

收件人: "YANG Tao" <yangt_at_ihep.ac.cn>

抄送:fluka-discuss_at_fluka.org

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

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

Emilio

-----Original Messages-----<! br> From: "YANG Tao" <yangt_at_ihep.ac.cn>

Sent Time: Thursday, June 23, 2016

To:fluka-discuss_at_fluka.org

Cc:

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 pe! netrate 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

CAS

Date: Sat, 25 Jun 2016 19:47:25 +0800 (GMT+08:00)

Dear Tao Yang,

you are right, you cannot know if a neutron that bounces back will go back into Pb or not. However, let's consider the two cases:

1) BPE -> Pb; Pb -> BPE; BPE -> Pb; Pb-> outside Pb (NOT back into BPE)

Here if take the difference between the two USRBDX card the net neutron current is 1

2) BPE -> Pb; Pb -> BPE; neutron absorbed in BPE (or escaped in the vacuum)

Here the difference between the two cards is zero, and I would say that in this case also the neutron current between the two surfaces should be considered zero, since the neutron entered in Pb but then it went back. Do you agree?

In general, I would consider the neutron curent between the two materials as the difference between the neutrons coming out (from BPE to Pb) and the neutrons coming in (from Pb to BPE); this means that in both cases the difference between the USRBDX cards gives you the right results. Does it sound reasonable to you? As I said, I am not sure if this can be achieved simply using the two-way fluence (hence, using only one USRBDX card), but this can be tested very easily.

Best Regards

Emilio

-----Original Messages-----

From: "YANG Tao" <yangt_at_ihep.ac.cn>

Sent Time: Saturday, June 25, 2016

To: emilio_at_impcas.ac.cn

Cc: fluka-discuss_at_fluka.org

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

Dear emilio,

Thanks for your kind help. In the first mail, you suggested that I could calculate the fluence using two USRBDX cards 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 think it cannot be known whether or not that neutrons from Pb to BPE can completely bounce again back BPE to Pb, so I think the direct subtract of the two USRBDX cards can not be considered as the net one-way neutron fluence.Maybe my understanding is wrong, so please contact me if I misunderstand it.

Best Regards,

Tao Yang.

-----原始邮件-----

发件人:emilio_at_impcas.ac.cn

发送时间: 2016年6月25日 星期六

收件人: "YANG Tao" <yangt_at_ihep.ac.cn>

抄送:fluka-discuss_at_fluka.org

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

Dear Tao Yang,

I'm afraid i didn't explain myself properly. The problem is exactly that you are using an one-way estimator for the fluence. Indeed, consider the case

1) Neutron goes from BPE to Pb (BPE -> Pb, counted once)

2) Due to elastinc collision, it bounces back Pb -> BPE (not counted, since with WHAT(1) set on "one-way" you are not considering the neutrons going from Pb to BPE )

3) It bounces again back BPE -> Pb (counted twice)

4) ....

This can be easily verified using two USRBDX cards, as I described (I guess also using the two-way current estimator should take care of the problem, but I am not sure if the incoming current, in that case, is considered negative or positive).

Best Regards

Emilio

-----Original Messages-----

From: "YANG Tao" <yangt_at_ihep.ac.cn>

Sent Time: Saturday, June 25, 2016

To:emilio_at_impcas.ac.cn

Cc:fluka-discuss_at_fluka.org

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

Dear emilio,

Thanks for your reply! What(1) setting of USRBDX is one-way current, not two-way form, so I think it would never to occur as you pointed out ("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.").

Best regards,

Tao Yang

-----原始邮件-----

发件人:emilio_at_impcas.ac.cn

发送时间: 2016年6月25日 星期六

收件人: "YANG Tao" <yangt_at_ihep.ac.cn>

抄送:fluka-discuss_at_fluka.org

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

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

Emilio

-----Original Messages-----<! br> From: "YANG Tao" <yangt_at_ihep.ac.cn>

Sent Time: Thursday, June 23, 2016

To:fluka-discuss_at_fluka.org

Cc:

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 pe! netrate 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

CAS

-- 杨涛 中科院高能所东莞分部（东莞中子科学中心）加速器技术部 地址：东莞市大朗镇中子源路1号中国散裂中子源A2栋606室 E-mail：yangt_at_ihep.ac.cn 电话：0769-38944239 邮编：523803 -- 杨涛 中科院高能所东莞分部（东莞中子科学中心）加速器技术部 地址：东莞市大朗镇中子源路1号中国散裂中子源A2栋606室 E-mail：yangt_at_ihep.ac.cn 电话：0769-38944239 邮编：523803 __________________________________________________________________________ You can manage unsubscription from this mailing list at https://www.fluka.org/fluka.php?id=acc_infoReceived on Sat Jun 25 2016 - 15:19:38 CEST

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