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

From: Mina Nozar <nozarm_at_triumf.ca>
Date: Thu, 26 Sep 2013 18:21:55 -0700

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

Any help/advice is appreciated.
Best wishes,

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 - 04:44:03 CEST

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