Curium is one of the most radioactive isolable elements. Its two most common isotopes 242Cm and 244Cm are strong alpha emitters (energy 6 MeV); they have relatively short half-lives of 162.8 days and 18.1 years, and produce as much as 120 W/g and 3 W/g of thermal energy, respectively. Therefore, curium can be used in its common oxide form in radioisotope thermoelectric generators like those in spacecraft. This application has been studied for the 244Cm isotope, while 242Cm was abandoned due to its prohibitive price of around 2000 USD/g. 243Cm with a ~30 year half-life and good energy yield of ~1.6 W/g could make a suitable fuel, but it produces significant amounts of harmful gamma and beta radiation from radioactive decay products. Though as an α-emitter, 244Cm requires a much thinner radiation protection shielding, it has a high spontaneous fission rate, and thus the neutron and gamma radiation rate are relatively strong. As compared to a competing thermoelectric generator isotope such as 238Pu, 244Cm emits a 500-fold greater fluence of neutrons, and its higher gamma emission requires a shield that is 20 times thicker—about 2 inches of lead for a 1 kW source, as compared to 0.1 in for 238Pu. Therefore, this application of curium is currently considered impractical.