Monty40xi wrote:I guess the question is, can you get more energy from harnessing the electrical charge than you can from using the particle for heat, as in the RTG?
I think it depends on the specific isotopes involved and the efficiency of the thermoelectric conversion. A 10 mCi beta emitter will directly produce 3.7x108
/s = 59 pA. If the beta emitter is Ni-63, then each electron is carrying 67 keV. If you extracted all the energy from the electrons, then this would be 4 microwatts. A 10 mCi source of Ni-63 should mass 0.16 mg, if I've done my math correctly. So, Ni-63 could generate a maximum of 24.5 mW/g. Pu-238 has a decay rate of 17300 mCi/g, so a 10 mCi source contains 0.578 mg Pu, or 0.656 mg PuO2
. Because the decay mode for this isotope spews out 5592 keV alpha particles, the maximum available heat from the conversion of kinetic energy would be 331 microwatts or 505 mW/g. The efficiency of the heat to electricity conversion is ~5%, so the Pu-238 would make about 25 mW of electricity/g PuO2
. Assuming the beta decay to electricity efficiency is ~50% (seems reasonable since the radiation is already current, the electrodes could be shaped to capture a significant fraction of the emitted particles, and the emitted electrons can be used to generate secondary electrons in a gas as the voltage drops), then Ni-63 would generate about 12.3 mW of electricity/g Ni-63. These are pretty close to each other, which gives me some confidence that there would be an isotope that would generate more electricity per gram through beta decay than the thermoelectric conversion of Pu-238.
A quick "reasonable-ness" check - the 10 mCi Ni-63 actually generates about 10 nA of current in a gas filled chamber (knocking additional electrons off of the gas as it slows down). Assuming the additional electrons have ~10eV of energy, this means the Ni-63 source would generate ~0.1 microwatts. The same decay rate of Pu-238 generates 16.4 microwatts, so the RTG has a 40-fold increase in electricity production on a gram basis.