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hifructosetrashjuice t1_ivtekb4 wrote

Reply to comment by 2science in Why do neutrons tend to interact more readily with light nuclei rather than heavy ones (i.e., why is water a better neutron shield than lead)? by Calgaris_Rex

liquid hydrogen or liquid methane is great moderator when you need very cold neutrons. remember you can cool down neutrons only down to energy of thermal motion - liquid hydrogen is pretty cold at 20K

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Yaver_Mbizi t1_ivuk2vl wrote

And if you want ultracold neutrons, superfluid isotopically-pure Helium-4 is by far the best, with deuterium ice in second place.

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mfb- t1_ivx1xin wrote

> isotopically-pure Helium-4

Does that need any special purification effort? Helium-3 is already a tiny fraction of the helium we extract.

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chugalug_donna t1_iw19dy4 wrote

You can distill them apart, it's part of how dilution refrigerators work

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mfb- t1_iw19xaz wrote

I know it's possible, but I'm asking if it's necessary for this application.

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Yaver_Mbizi t1_iwcnub0 wrote

Sorry for taking so long to reply - I first wanted to get the numbers right and then kinda forgot about this comment.

> Does that need any special purification effort?

Yes. Specifically something called "superleak" or "superfluid helium filter" is used. It's basically a filter with material so tightly compressed that only a superfluid can go between the particles. As helium-3 reaches superfluidity at a much lower temperature level than helium-4, the isotopes can be thusly separated.

>Helium-3 is already a tiny fraction of the helium we extract.

Yep, but the less of it, the better. The natural content is about 10^(-6), whereas by using a superleak you can get something around 10^(-11), which is much better.

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hifructosetrashjuice t1_ivtf9aw wrote

also you need to keep your material chemically simple, because when interacting with neutron proton is kicked out of molecule. this leaves water, because all products of that can react back forming water again, same goes for hydrogen. this does not work for oil, for example, that is if you wanted to put oil in nuclear reactor for some reason

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