Re: [fluka-discuss]: Re: Trying to understand an observation in photon dose rates (or photon fluence)

From: Santana, Mario <msantana_at_SLAC.Stanford.EDU>
Date: Thu, 26 Sep 2013 23:20:34 -0700

Hi Mina,

Just some thoughtsŠ

You know that lead is not the best shielding for high energy neutrons.
They will bounce through the structure and make it to the other end of
your shielding. A light material will reduce the energy of the neutrons
very effectively at each collision. Moreover, it has a fairly large photo
neutron cross-section. Also lead is toxic, and when exposed to radiation
it will become 'mixed waste', so you may want to minimize its use. This is
why it is often used close to the source, to absorb most of the
electromagnetic cascades, and then concrete is used in the accelerator
enclosure to 'take care of the neutrons'.

Polyethylene is a good option for neutrons (specially if it is borated),
but as you indicate it may degrade (and also catch fire). Also it is less
dense than concrete so it does not absorb photons as well as concrete. An
advantage over concrete is that its residual activity is mainly
short-lived.

About heavy concrete, it is a hybrid solution that seems to work
successfully when shielding space is scarce, but it is expensive and you
have to make sure that your heavy concrete is sufficiently homogeneous.
Also some suppliers don't provide the composition, so you may have a hard
time to simulate its effects.

Mario

On 9/26/13 6:21 PM, "Mina Nozar" <nozarm_at_triumf.ca> wrote:

>Just wanted to update everyone with my finding on this. Because the
>beam energy (75 MeV) is way above the neutron production (gamma, n)
>threshold in lead (6.73 MeV), there are lots of neutrons generated in
>the lead. These neutrons in turn produce other gammas through (n,
>gamma) reactions. So the effectiveness of lead in shielding gammas stop
>at some thickness (for energies higher than the gamma, n threshold).
>The way I got around this was to layer the shielding design (Pb + PE +
>Pb + PE ...). This design works in attenuating photons and neutrons so
>that the total dose rate is below the required limit.
>
>However, I am wondering about the radiation resistance properties of
>Polyethylene. The first (innermost) layer of PE is seeing Dose rates on
>the order of 1E7 mSv/h or 10 kSv/h, so very high. Looking through
>literature, I am seeing Total dose limits of 1E6 Gy (or Sv) - from CERN
>98-01. This gives 100 hours of operation before PE is damaged.
>
>So I am thinking the Pb/PE layering is not a viable option. Does anyone
>know of PE being used in high radiation fields? Does anyone have
>experience in this area?
>
>Another option I have investigated is to use high density concrete which
>seems to do the trick. So we might go with that. I just want to make
>sure I am not missing something before writing off the Pb/PE layering
>option.
>
>Any help/advice is appreciated.
>Best wishes,
>Mina
>
>On 13-07-19 06:19 PM, Mina Nozar wrote:
>> Hello everyone,
>>
>> We are working on optimizing the shielding for a 100 kW (75 MeV, 1.3 mA)
>> electron beam dump. BD is basically a chunk of Al with a slanted plane
>> in the middle to distribute the incident beam (square in shape 4x4 cm).
>> The BD is surrounded with lead and concrete, lead and polyethylene (PE),
>> or lead and borated PE(BPE) depending on the study. The BD is 111 cm
>> long, 20 cm wide, and 12.5 cm high.
>>
>> In order to compare thickness of PE, BPE, or concrete required in
>> shielding neutrons and finding the minimum thickness of lead required to
>> bring down the dose rates from photons to meet different limits, I have
>> been looking at dose rate distributions from neutrons and photons
>> separately.
>>
>> I have set the production and transport thresholds for e+/e- to 100 keV
>> and 10 keV for photons.
>>
>> I originally thought if I extracted TVL from the attenuation of photons
>> in lead and neutrons in concrete (PE/BPE), I could find the required
>> thickness of each material to reduce the dose rates down to the limits.
>> However, I have observed a 'build-up' behaviour of photons in lead that
>> I don't understand (and which is affecting my conclusions). I would
>> like to find out whether this is a by product of geometry effects,
>> thresholds set, etc. or if this is a real build-up effect I am
>> observing, suggesting that past some thickness, the effectiveness of
>> lead to shield photons goes down.
>>
>> I am attaching plan and side views of the geometry around the BD and two
>> figures showing 1-D distributions of the dose rates (from neutrons,
>> photons, and total), as a function of width and height of the BD. The
>> slices I have made are 10 cm wide around the beam line and center of the
>> beam dump. The arrows point to the areas showing the 'build up'
>> feature which continues through the lead/concrete boundary.
>>
>> I am showing results of the lead and concrete study only but I observe
>> the same behaviour for lead and PE.
>>
>> Thank you very much for any insight you might have on this. I
>>
>> Best wishes,
>> Mina
>
Received on Fri Sep 27 2013 - 09:08:15 CEST

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