thorium-228) emit a significant level of gamma radiation, resulting in dose rates that are higher than either ²⁴⁴Cm or ²³⁸Pu heat sources of comparable size. This leaves ²³⁸Pu and ²⁴⁴Cm as the only isotopes worthy of further consideration.³

Table D.2 compares the characteristics of ²³⁸Pu and ²⁴⁴Cm. Both produce gamma radiation (although the amount produced is much smaller than the amount from isotopes that produce gamma radiation as a primary emission). As shown, ²⁴⁴Cm produces much more gamma radiation than ²³⁸Pu. Also, the fast neutron radiation level from ²⁴⁴Cm is nearly 450 times that of ²³⁸Pu. These high gamma and neutron radiation levels would require shielding during handling and use of the ²⁴⁴Cm heat sources to protect personnel and sensitive components. The shield weights would most likely be too heavy for deep-space applications.

Nearly all of the gamma dose from ²³⁸Pu is attributable to the decay chain of the ²³⁶Pu isotope impurity in the fuel, which is limited to very small amounts by ²³⁸Pu fuel quality specifications.

The power density (watts/cubic centimeter) and specific power (watts/gram) of radioisotope fuel is directly proportional to the energy absorbed per disintegration and is inversely proportional to half-life. (As shown in Table D.2, ²⁴⁴Cm has a higher specific power and power density than ²³⁸Pu, because the former has a shorter half-life, but the selection of RPSs powered by ²³⁸Pu to power many important missions has demonstrated that its specific power and power density are acceptable.) Higher power density leads to smaller volume heat sources for comparable power levels and higher specific power leads to lighter weight heat sources. Both characteristics are highly significant for space power heat sources. For radioisotope fuels with comparable half-lives, a beta emitting heat source will be larger and heavier than an alpha emitter.

The radioisotope fuel must be used in a fuel form that has a high melting point and remains stable during credible launch accidents and accidental reentries into Earth’s atmosphere. The fuel form must also be noncorrosive and chemically compatible with its containment material (metallic cladding) over the operating lifetime of the power system. It is desirable that the fuel form have a low solubility rate in the human body and the natural environment. Daughter products and the decay process must not affect the integrity of the fuel form. All of the alpha emitting isotopes listed in