Portable shield for 3x3" NaI

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Lposter
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Joined: 13 Jan 2018, 08:01
Location: Oslo
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Portable shield for 3x3" NaI

Post by Lposter » 06 Aug 2018, 18:35

I made a Shield recently for a 3x3" NaI which was intended to be "portable" (at least less than baggage limits on flights).

The old lead was to be cast in a form made from steel ventilation ducting. An outer and inner tube were set in a baking tin full of plaster of paris to make the mould. This was then left to dry for weeks and baked for two days at 90 degrees C to remove moisture.

We cooked up 30 kilos of lead on an old gas cooker using an Ikea saucepan. There are plenty of stories of casting lead and moisture and even With all the drying, the plaster had retained some water and caused the lead to "boil" and bubble when poured. We ran away and came back when it had stopped. The Bubbles caused some cavities and these had to be machined away.

The Shield has an outer dimesion of 150 mm, the inner cavity has a diamter of 100 millimeters. The shield thickness all round is 25 mm. The inner liner is 5 millimeter thick plexiglass tubing which support a 25 millimeter thick lead plug or cap. The detector is supported by a machined aluminium piece at the back. The entire thing is clad in 0.75 millimeter thick steel.

I attach some photos - I have some "body work" to do on it and will remove the jubille clips as they were only to hold the steel clading in place till the adhesive set.

Total weight is a shade less than 20 kg and is fairly practical to carry at least for a short while.

Take home message from me - be super careful casting large amounts of lead and if possible stay away from anything that might have moisture in it, even if you have baked the hell out of it.

P
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Lars Poster, Oslo

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Steven Sesselmann
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Joined: 27 Apr 2015, 11:40
Location: Sydney
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Re: Portable shield for 3x3" NaI

Post by Steven Sesselmann » 09 Aug 2018, 16:35

Lars,

Nice job on the casting and machining, it's not easy to machine lead, it tends to hook onto the cutter. I would suggest welding up a form from steel pipes and filling it with lead. Steel is actually not bad as an inner liner as it attenuates the Pb x-rays without emitting any xrf in our region of interest. Steel is easier to get hold of and easier to work with than both copper and tin (pewter).
Steven Sesselmann | Sydney | Australia | gammaspectacular.com | groundpotential.org | rephopper.com | beejewel.com.au |

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