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.