Preservation of nuclear spin polarization of ¹²B in superfluid helium

Beams of unstable nuclei, ⁸Li and spin-polarized ¹²B, were introduced into superfluid helium and "snowballs", aggregates of helium atoms surrounding the singly charged impurity ions, were formed through electrostriction. β-rays from the unstable nuclei encapsulated in the snowballs were detected after the snowballs were transported under an electrostatic field. It was found that approximately 20 % of the implanted ions formed snowballs. The spin polarization of ¹²B in snowballs was measured by β-NMR method. The first decisive evidence was obtained that the polarization was maintained throughout the lifetime of ¹²B (τ = 29 ms). The snowballs can be versatile environments for spectroscopic studies of unstable nuclei.