well, I remember that to shield the radiation coming from the core in the Italian plant of Montalto di Castro , we used a special concrete with some boron and lead : the presence of the lead has been due because , according to calculations , the most responsible contribution to the dose rate outside was the gammas coming from the neutron capture in the iron ( actually steel ) of the vessel , as I already mentioned . I also remember that the concrete has been successfully tested in the Casaccia lab ( Rome ) Triga reactor , but this is an other story.....
----Messaggio originale----
Da: Zafar.Yasin_at_cern.ch
Data: 20/07/2015 21.04
A: "Alberto Fasso"<Alberto.Fasso_at_cern.ch>, "fluka-discuss_at_fluka.org"<fluka-discuss_at_fluka.org>
Cc: "杨博"<b-yang12_at_mails.tsinghua.edu.cn>
Ogg: RE: [fluka-discuss]: A optimaized ratio between iron and polyethylene
-->
Dear Bo Yang, Alberto, P. Neuhold, and fluka experts,
In addition to useful comments by Alberto, I want to add one thing more and if I am wrong please do correct me.
According to our calculations and knowledge, the best shielding materials for neutrons are mixtures of iron and
borated polyethylene instead of alone iron or polyethylene. Using a combination of iron and polyethylene can even
reduces flux up to 20-50 times or even more than using alone iron or polyethylene.
Selection of iron or polyethylene depends upon the energy of neutrons, neutron shielding depends on the energy
of the neutrons. For high energy neutrons , greater than a few MeV and higher, iron is the best shielding
material. Actually, iron slows down fast neutrons to thermal energies. Polyethylene is the best for slow or thermal neutrons.
So, an optimization is first use iron, to slow down neutrons and then borated polyethylene to absorb neutrons. An optimasation
can be 50 % iron and 50 % polythelene and according to our simulations, 60-70 % iron and 30-40 % borated polyethylene.
Now in the present case energy of proton is 200 MeV. For this type of shielding there are two rough suggestions;
1. Use around 70-100 cm iron and then 70-100 cm borated polyethylene and dose will be withing the limits.
2. Use slabs of iron and borated polyethylene, one can select, 20 cm iron+20 cm Polythelene+20 cm iron +20 Cm Polythelen +
..............................and go up to 150-200 cm where you get doses within the limits.
And the compare these two cases with single shields of iron and polythelene and you will see a large variation.
Now question arises, where is concrete, or which is better borated polyethylene or concrete? I don't have numerical
values for these and I am looking for this but MC simulations are not finishing as I am running on pc with a single processor.
If any one can comment which is more better, but I think my simulations will show that polyethylene is more better for neutron
shielding than concrete. I am talking about only neutron shielding, no doubt concrete has other benefits and shielding walls are
always made of concrete and there is another discussion why shielding is used.
Best regards,
zafar Yasin
________________________________________
From: owner-fluka-discuss_at_mi.infn.it [owner-fluka-discuss_at_mi.infn.it] on behalf of Alberto Fasso [fasso_at_mail.cern.ch]
Sent: 20 July 2015 18:22
To: fluka-discuss_at_fluka.org
Cc: 杨博
Subject: Re: [fluka-discuss]: A optimaized ratio between iron and polyethylene
Dear Bo Yang,
First of all, you want to optimize shielding in the lateral direction or
in the forward direction, or both? These are two very different problems.
Then, yours is an interesting question, but I am not aware of any existing
optimization. You could try it yourself: I am sure that your results would
be very appreciated by many people.
Some information has been given by J.D. Cossairt in his radiation Course
(Fermilab Report TM-1834, p. 160), but for concrete instead of
polyethylene:
The first excited state of 56 Fe, the dominant isotope in natural iron
(92% abundance), is at 847 keV. Due to the inefficiency of the
transfer of energy by means of elastic scattering, neutrons having
kinetic energies above 847 keV in a given spectrum will be
slowed by inelastic scattering to E n ≈ 847 keV only to build up at
energies just below this value. Amplifying this effect, when one
considers the dose equivalent external to such shields, is the fact
that the quality factor for neutrons as a function of energy also has its
maximum value at about 500 keV (see Fig. 1.3). Thus, pure iron
shields are rather ineffective in attenuating neutrons in this energy
region.
.....................................................................
In general, an iron shield "capped" or "backed" by ... a concrete
shield will be an efficient use of space. It has been determined that
about 60 cm of concrete is the most efficient thickness to use for
this purpose.
Just three pieces of advice:
1) as warned by P. Neuhold, be careful of the high energy Fe capture gammas
2) be careful of the "window" in the iron cross section explained above
3) remember that the density of iron can vary a lot: from 7.0 to 7.8 g/cm3
Alberto
On Mon, 20 Jul 2015, 杨博 wrote:
>
> Hi,every Fluka user
>
> Recently I want to use FLUKA to design a compact shielding for a 200MeV proton loss point. Due to existence
> of high-energy neutron, I try to use iron plus polyethylene as a shielding. But I wonder if there is a
> optimized ratio between iron thickness and polyethylene thickness to effectively shield the high-energy
> neutron. If anybody can give me some suggestion or reference, thanks very much.
>
> Bo yang
>
>
>
>
>
>
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Received on Tue Jul 21 2015 - 13:29:10 CEST