Cosmic ray intensity over long time scales

In this report we review recent work on variations of cosmic ray intensity on time scales longer than, or which may have occurred before, the modern era of direct observations of cosmic rays. The possibility of discovering such variations comes mainly from the fact that cosmic‐ray nuclei above a modest energy make nuclear interactions leading to characteristic “cosmogenic” nuclear debris in targets like meteorites, lunar rocks, and planetary atmospheres, which are exposed to them. High Z cosmic rays also make etchable tracks in glassy and crystalline targets. Cosmogenic nuclei which happen to be chemically or isotopically distinct from the targets are detected as radioactive (half lives less than) about 10 9 years) or stable nuclides, especially rare gases, in meteorites, lunar and terrestrial materials.