This is conventional technology as far as I can tell, but interesting:
http://rfdesign.com/military_defense_electronics/nuclear-accelerated_beta_radiation/
"Nuclear-accelerated generator harvests beta radiation
Oct 1, 2007 12:00 PM
David Weber, owner and founder of Executive Engineering, is
developing a technology, the nuclear accelerated generator (NAG),
which has been considered for powering miniature autonomous security
devices for an undisclosed application. The main breakthrough
represented by the technology is the direct conversion of nuclear
energy into low-power electrical energy. This enables the technology
to extract useful power from extremely small quantities of obtainable
radioactive isotopes, including those in regular production by the
U.S. Department of Energy.
In general, NAG technology is intended for use with isotopes that
emit beta-minus radiation (electrons), which is crucial for the
process by which the technology converts nuclear energy into
electricity. Isotopes that are theoretically compatible with the
technology include strontium-90 (Sr-90), nickle-63 (Ni-63), and
promethium-147 (Pm-147). Among these materials, the latter is the
most desirable. One reason for this, Weber stated, is that it decays
into a stable element (inert promethium) within 30 years. . . ."
