Particle track record of the luna missions

Measurements are reported of particle-track densities in 100–200µ crystalline grains taken from one level of the soil column returned from the lunar highlands between Mare Fecunditatis and Mare Crisium by Luna 20 and from two levels in that from Mare Fecunditatis by Luna 16. Ninety-three percent of the grains from Luna 16 have very high densities, > 10⁸ cm^–2 and the lower-track density grains are all in the deeper soil level. In contrast, most Luna 20 grains show densities < 10⁸ cm^–2. Track density gradients and exposure times have been measured for six Luna 16 grains with a wide spread in absolute track densities. The more extensive track counts in crystals strengthens our earlier conclusion that the Luna 16 soil has received long irradiations very close to the surface. Two possible histories are that the highly irradiated soil blanket at the Luna 16 site is either well mixed and thin, or else has accumulated by transport from surrounding higher regions. The single sample of doubtful depth from Luna 20 shows a much lesser near-surface irradiation, giving results similar to those on the Apollo 12 core and the 54–80 depth sample from the Apollo 15 deep core.Paper dedicated to Professor Harold C. Urey on the occasion of his 80th birthday on 29 April, 1973.