= Solution
<Alpha decay> releases energy in charged particles with short stopping ranges in matter, so much of their <kinetic energy> becomes local <heat>. An appropriate <radioisotope thermoelectric generator> can use a predictable <radioactive decay> <heat> source without requiring sunlight or a sustained <nuclear chain reaction>. Low penetrating photon and <neutron> output is desirable, but being an alpha emitter does not by itself establish the complete radiation spectrum.
\b[The isotope's <half-life> must suit the mission.] A short <half-life> gives high initial specific activity but a rapid decline in useful thermal <power>; a much longer one gives a steadier output but less activity per atom. Selection also depends on recoverable decay energy, specific <power>, daughter emissions, chemical and thermal stability, compatibility with the <power>-conversion system, availability and cost. Practical containment and radiological acceptability remain separate system-level requirements.
A <thermoelectric generator> converts a <temperature> difference into electrical output through the <Seebeck effect>; not all decay <heat> becomes electricity. NASA's operational account describes this heat-to-electricity distinction and the need to match source lifetime to long space missions.<Radioisotope Power Systems FAQ by National Aeronautics and Space Administration>[What is an RTG, and the discussion of isotope-selection and mission-duration criteria]{ref}
For the assumed three-<half-life> mission, the thermal <power> falls by a factor of eight. That large decline is the important disadvantage of this particular <half-life> even though the initial specific thermal <power> is high.
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