Short-lived beta-ray emitters, B-12, sustaining nuclear spin polarizat
ion were introduced into superfluid helium. The nuclear polarization o
f B-12 was observed via measurement of beta-ray asymmetry. It was foun
d that the nuclear polarization was preserved throughout the lifetime
of B-12 (20.3 ms). This suggests that the ''snowball'', an aggregation
of helium atoms produced around an alien ion, constitutes a suitable
milieu for freezing-out the nuclear spin of the core ion and that most
likely the solidification takes place at the interior of the aggregat
ion.