Particle track record of the Luna missions.

Measurements are reported of particle-track densities in 100 to 200-micron 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 the soil column from Mare Fecunditatis by Luna 16. Ninety-three percent of the grains from Luna 16 have very high densities, greater than 10 to the 8th power per cu cm and the lower-track density grains are all in the deeper soil level. In contrast, most Luna 20 grains show densities less than 10 to the 8th power per cu cm. 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 an 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.