  We investigate neutralization processes (especially thermal surface
neutralization), which are required for the magneto-optical trapping of
radioactive atoms. A variety of neutralization methods are first summarized:
neutral beam injection for fusion reactors, neutral atom implantation in
semiconductor processing, and the production of radioactive neutral atoms in
accelerators. We focus on thermal surface neutralization, which produces
neutral atoms in the thermal energy range for laser cooling. The experiments
were carried out with yttrium, gadolinium, and zirconium foils to neutralize
francium and rubidium ions for magneto-optical trapping. The results reconfirm
that yttrium foil is a good neutralizer (i.e., it has a neutral release
efficiency $>65\%$). In addition, the release fraction when using yttrium foil
exceeds 75\% at 1350 K, which is greater than the release fraction for the
other foils. This reconfirmation is important because few previous studies have
focused on thermal surface neutralization. Moreover, the results show that the
neutralization efficiency is strongly influenced by the experimental process
itself.
