Re: problem with neutron shielding with borated polyethylene

Dear Alberto,

Thanks for your help.

Actually in my original design I let the beam hit lead, and with the
borated polyethylene surrounding the lead, but the 5% one and 30% one
make little difference. The plots is appended.

I tried to directly inject slow neutron (0.5eV) on the borated
polyethylene, but the 5% one and 30% one again make little difference.
I really doubt I make something wrong in my input file, but I can't
find where is wrong. Would you please help me find out where's wrong?

Thanks again,
En-Min

引述 Alberto Fasso' <fasso_at_slac.stanford.edu>:

> Dear En-Min,
>
> borated polyethylene is a good shielding for thermal and epithermal neutrons,
> but if you inject 2 GeV electrons on borated polyethylene, you don't get
> immediately thermal neutrons. You get gammas, which produce neutrons
> of about 2 MeV (and the number of neutrons produced is not the same if
> the polyethylene is 5% borated or 30% borated!) These neutrons are
> partly escaping, and partly slowed down until some of them are
> becoming thermal
> and are captured by the boron. It is a complex situation: therefore,
> it is not at all obvious that the total effect would be to reduce
> the neutron
> fluence. It depends also on the size of the polyethylene shield.
>
> Such a complex situation is difficult to assess with simple considerations:
> and that is the reason you are using Monte Carlo!
> Try some modifications on your problem: for instance let the
> electron beam hit
> a target instead of the polyethylene, so that the neutron production happens
> outside the polyethylene, and the latter acts only as a shield and not
> also as a neutron-producing target.
>
> Alberto
>
>
> On Mon, 19 Dec 2011, b97901029_at_ntu.edu.tw wrote:
>
>> Dear FLUKA experts,
>>
>> I found some problems when I was trying to shield neutron
>> generated from electron beam by 5% borated polyethylene and 30%
>> borated polyethylene. The electron beam is injected in the borated
>> polyethylene. I had plotted the spread of neutron fluence, but much to
>> my surprise, the spread of neutron in 30% borated polyethylene is
>> larger than that in 5% borated polyethylene. Does that make sense? I
>> think 30% borated polyethylene should have better shielding effect and
>> can't explain what's wrong with the result. I would really appreciate
>> if someone can tell me what's wrong with the result.
>>
>> Attached are the input files and the neutron fluence plots.
>>
>> Thanks in advance,
>> En-Min
>

• application/x-fluka attachment: neutron_boron_5per.inp

• application/x-fluka attachment: neutron_boron_30per.inp