Viewing a single comment thread. View all comments

Yaver_Mbizi t1_ivuk2vl wrote

Reply to comment by hifructosetrashjuice 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

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

5

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.

2

chugalug_donna t1_iw19dy4 wrote

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

2

mfb- t1_iw19xaz wrote

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

2

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.

2