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Suppose we have two isotopes of the same element. For example, isotope A could have a nuclear spin I = 0, isotope B could have nuclear spin I = 3/2. Let us say we have a compound where this element is participating, and in this compound we have a mixture of the two isotopes A, B. Up to this point, this is a rather universal case: with the exception of isotopically purified samples, all compounds contain mixtures of isotopes for all of their elements.

Let us now cool down a sample of our compound, below liquid He temperatures, and, for the purposes of this question, let us say we can cool it down arbitrarily low temperatures. Now the different isotopes A, B will start to behave differently, presenting different physical and chemical properties, if only because their quantum eigenstates are different, and differently populated at low temperature. This is on top of their different masses, of course.

The question is:

How could one separate this mixture of isotopes A, B, presenting different nuclear spins, at sub-Kelvin temperatures?

In particular, if more than one purification procedure is possible, can one define one such procedures which is based on the different magnetic properties of isotope A and B?

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    $\begingroup$ Have you seen eg the behaviour of helium isotopes at low temperatures, including the “superfluid” behaviour? This should be described quite thoroughly on the web. The difference arises from boson vs fermion spin-statistics. Fermions cannot occupy the same quantum state (Pauli exclusion principle) but bosons can. $\endgroup$
    – orthocresol
    Apr 7 at 8:29
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    $\begingroup$ Also, please don’t directly crosspost questions, unless you differentiate the question sufficiently such that you have a different focus on each site. If you want, you can draw attention to this question on Physics.SE chat. Or you can just post there and delete this, if you think you might get a better response there. $\endgroup$
    – orthocresol
    Apr 7 at 8:31
  • $\begingroup$ Separation of isotopes near 0 K is very difficult. The helium case is a rare exception. $\endgroup$
    – Poutnik
    Apr 7 at 17:00
  • $\begingroup$ @Poutnik Hello! I am investigating this based on a lead from the link below, where the Harvard grad student stopped research. My hypothesis is that UF6 might be one of the exceptions you are referring to. However, not much research was done on this subject. But the difference in spin and the resulting difference in atomic structure is very interesting. $\endgroup$ Apr 7 at 17:41
  • $\begingroup$ aaas.org/resources/… $\endgroup$ Apr 7 at 17:41

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