Cosmogenic nuclides in recently fallen meteorites: Evidence for galactic cosmic ray variations during the period 1967–1978

Abstract

Cosmogenic radionuclides were measured on 48 fragments of 24 meteorites which fell between 1967 and 1978. Nondestructive gamma counting techniques were used to obtain data on 7Be, 46Sc, 48V, 51Cr, 54Mn, 56Co, 57Co, 58Co and 60Co on at least some of the samples. Sodium 22 and 26Al measurements are reported on all 48 samples. In addition, new rare gas data and exposure ages are reported for the meteorites Guibga, Gorlovka, Dhajala, Louisville, Acapulco, Jilin, Kabo, Alta‐Ameen, and Canon City. The cosmogenic radioisotope and rare gas data are interpreted in terms of a time dependent modulation of galactic cosmic rays spanning one full 11 year sun spot cycle. Special attention is given to the data on 22Na, 46Sc, 54Mn, and 48V with either 26Al or 22Ne/21Ne used to provide a shielding correction. The shielding normalized data using the 26Al method appear to correlate well with calculated production rates scaled against the Deep River neutron monitor. The data for the four isotopes are consistent with a production rate variation of a factor of 2.5–3 between solar maximum and solar minimum for sun spot cycle 20. These data demonstrate that the production rates of cosmic ray‐produced nuclides in meteorites vary considerably according to modulation by the 11‐year solar cycle and support the concept that variations of solar‐modulated, cosmic ray flux of similar magnitude have occurred over much longer time periods.