Help with thickness of shielding needed (neutron absorption)?

This area is for discussions involving any fusion related radiation metrology issues. Neutrons are the key signature of fusion, but other radiations are of interest to the amateur fusioneer as well.
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Jeremy Richard
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Help with thickness of shielding needed (neutron absorption)?

Post by Jeremy Richard » Sat Jul 20, 2019 2:58 am

Myself and my group are planning a fusor, and are trying to look into how much shielding we'd need.

We want to run up to about 48kV, and maybe around 40mA. I'm not entirely sure how many neutrons per second we'd be getting with that.

But let's say the fusor is 30cm in diameter, and we have a neutron flux at it's surface of about 36 neutrons/cm^2 s. Of course I know with distance that would drop off quite a lot. But... if we are using borated polyethylene, how thick would it need to be to stop most of the neutrons/be able to handle higher fluxes when we get to it?

I've found different formulas online but the answers I get range from 0.7mm to 90cm, neither makes a whole lot of sense. If someone could give some guidance on how to calculate how many neutrons get through a specified thickness of shielding, it would be much appreciated.

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Richard Hull
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Re: Help with thickness of shielding needed (neutron absorption)?

Post by Richard Hull » Sat Jul 20, 2019 4:37 am

As noted in all the FAQs on shielding, it is the x-radiation that is the most daunting. At 48KV you can get by with as little as 1/8" thick lead plate. Neutrons will not be an issue as long as you are producing less than 1.5 millions neutrons per second isotropic and stay back 8 feet from the fusor vessel. The lead shielding should be enough. 1/4" lead would be overkill if you are worried. Lead sheeting is sold by Rotometals.com in various thicknesses

Borax will absorb thermal neutrons. I have mentioned a custom shadow shield in the FAQs and told of its possible construction and use.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

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