Polyethylene Blocks and Lead Shielding for Neutrons

[MikeD99]

I want to activate small amounts of elements with small amounts of Neutrons from an Amstat Nuclespot and Beryllium sheet neutron source. Using a custom-built-by-Steven neutron detector consisting of an SI19N He3 tube and one of his Schmitt trigger boards in a nice case that he made for me, I am pretty sure that I am detecting around 75 neutrons per second and about 3,500 to 4,000 neutrons per minute with an Arduino ESP32 board using SW that I wrote. I followed a few of the setup procedures that I found on the Internet from Carl Willis and his NIM Bin and on Fusor.net to detect about 1 NPM without a source and dialed in the voltage to eliminate the Geiger effect of gamma ray both with and without a source in order to be kind of sure that I really am counting neutrons. In addition to the Po/Be source, I have placed next to it an Am241 stick/mast from a Pyrotronics F3/5A with a Beryllium ring around it. I think the Am241 stick has about 40 uCi of the original Pyrotronics smoke detector's total of 80.

I have posted questions about detecting Au198 decay in another post. But in a reply to my other post there was some concern expressed that I'd need shielding from the neutrons I'm releasing. That is what this post is about.

I have 40 polyethylene moderating blocks surrounding the two neutron sources and the SI19N tube. Should I add some lead sheet on the outside of that? I don't have much lead sheet (I think it's 1/16" thick and about 3 sq ft) and I was thinking of putting it on the side of the poly blocks between me and the neutron sources.

Does that sound like a viable way to mitigate my neutron exposure when I am in my hobby room? I usually bake my activation targets about 4 days at a time. I don't store the alpha sources and the Beryllium together.

These are the polyethylene blocks that I bought:
https://www.ebay.com/itm/324151416075

Some pictures of my setup about which I am asking advice:








My precious GOLD BAR (2 gm) -- Will the plastic wrapper affect the activation in any way?


The neutron detection/activation set up. I place the gold bar on top of the sources and cover the whole thing three blocks deep:


The Chinese Geiger Counter board with piggyback ESP32 Wemos Lolin32 Arduino that I programmed:

[Sesselmann]

Mike,

That's a pretty impressive pile of HDPE, and your setup looks good to me allthough I have never personally attempted this.

I checked carl Willis video again and he claimed that his source was making 1000 n/s but he might have meant per minute as 1000/s sound quite high...not sure.

In any case I sincerely hope you can detect the gamma from Au and look forward to your report.

Steven

[NuclearPhoenix]

Mike,

what an interesting project you have there. Never seen somebody build themselves an alpha-beryllium source at home, but it's nice to see it working just fine. I think the neutron flux is probably what you expect, though. It obviously highly depends on your alpha activity, but I think 75 n/s sounds realistic.

What results are you getting after just 4 days with this kind of weak neutron source?

Also, I would guess your polyethylene blocks will get activated slowly too. Are you seeing any of that?

[MikeD99]

Matthias,
I checked one of the poly blocks that was always right up against the Po/Be source with a Geiger counter and it didn't read any differently than one that had never been included in my poly castle. So I guess I don't have enough neutrons to activate polyethylene.

I am again trying Gold foil, only this time in a more exacting procedure.

Mike D.

[NuclearPhoenix]

Very interesting topic. I'd make sure there are no (big) gaps between you and the neutron source.

Also, what I learned recently is that the neutrons will in fact activate the polyethylene, but that will decay instantly while releasing some very high-energy gammas. That's also why the gamma dose rate will be higher behind the blocks than without them.
So I would definitely do some additional shielding with lead behind the moderating blocks somewhere around where you are most of the time at least. It's probably not too much activity to worry about anyways, but you might as well do that if you have lead lying around.

Do not put any dense materials (lead) in front of the blocks inside your shielding because that won't help much at all and just scatter around the neutrons.