Apollo 15 Mission
Science Experiments - Alpha Particle Spectrometer
The Alpha Particle Spectrometer Experiment measured radon emission from the lunar surface.
The Alpha Particle Spectrometer was flown on Apollo 15 and 16. It was designed to map regions on the Moon of high U abundance (or high outgassing rates) by mapping variations in the abundance of Radon-222 and Polonium-210 in orbit. The isotope Uranium-238 undergoes radioactive decay to a series of lighter isotopes. One of the steps in this chain involves Radon-222, which is a gas. Rn-222 has a radioactive half-life of 3.8 days, which is long enough to allow some of it to diffuse through the lunar regolith and enter the lunar atmosphere. A later step in this same decay chain involves Polonium-210. Both Rn-222 and Po-210 undergo radioactive decay by releasing alpha particles (an alpha particle is the same as a Helium-4 nucleus). The experiment worked by detecting the alpha particles released when these isotopes decayed radioactively.
Both Rn-222 and Po-210 were successfully detected in lunar orbit. On Apollo 15, an enhanced level of Rn-222 emission was observed in the vicinity of Aristarchus crater. Also on Apollo 15, enhanced levels of Po-210 were observed between Mare Crisium and the crater Van de Graaf on the Moon's farside. On Apollo 16, similar enhancements of Po-210 were observed over the Grimaldi basin and over Mare Fecunditatis and Mare Smythii. More generally, levels of Po-210 were observed to be higher near the edges of lunar mare than elsewhere on the Moon.