Superfluid helium and cryogenic noble gases as stopping media for ion catchers

Many scientific disciplines make use of radioactive ions in the form of a low energy ion beam or a cold ion or atom cloud. Since radioactive isotopes are often best produced at very high energy, techniques to transform high-energy ions efficiently and quickly into low-energy ones are essential. In this context, the
usefulness of superfluid helium and cryogenic noble gases was investigated.
The extreme purity of cryogenic noble gases allowed to demonstrate for the
first time that the maximum efficiency in this kind of systems is determined by
the chance of survival of ions during slowing down. This principal limit, a few
tens of percent, is high enough to make the method of practical use. Experiments
in which high-density cryogenic helium was ionized by a proton beam show the
importance of a large electric field; a field that quickly pulls ions and electrons
apart and thus prevents neutralization. We demonstrated that at high fields, the
maximum efficiency is maintained at ionization densities many times larger than
achieved up to now with other systems.
Two mechanisms play a role in the extraction of ions out of superfluid helium:
thermal excitation, which is strongly temperature dependent, and an as yet unknown temperature independent mechanism. The transition between these two lies at a temperature of about 1.3 Kelvin. The combined efficiency for ion survival in and extraction out of superfluid helium lies between 1 and 10 percent; high enough for practical applications.