A model of possible variations of the galactic cosmic ray intensity over the recent billion years

Abstract

Based on the analysis of published data on exposure ages of iron meteorites determined with the ⁴⁰K/K method (T_K) and ages calculated using short-lived cosmogenic radionuclides (with the half-life T_1/2 < 1 Myr) in combination with stable cosmogenic isotopes of noble gases (T_RS), the following results have been obtained. (1) The distribution of T_RS ages (106 values) has an exponential shape, similar to that for ordinary chondrites, but different from the distribution of T_K ages (80 values). The difference is most likely due to small amounts of data for meteorites with low T_K ages (less than ~200–300 Myr). The latter can be ascribed to the difficulty of measurement of small concentrations of cosmogenic potassium isotopes. This circumstance makes the selection of meteorites with ⁴⁰K/K ages nonrepresentative and casts doubt on the correctness of conclusions about the variations of the intensity of galactic cosmic rays (GCR) based on the analysis of distribution of these ages. (2) The magnitude of the known effect (systematic overestimation of T_K ages in comparison with T_RS ages) has been refined. The value k = T_K/T_RS = 1.51 ± 0.03 is acquired for the whole population of data. We have shown the inefficiency of the explanation of this effect on account of an exponential change in the GCR intensity (I_T) with time (T) according to the relation I_T = I₀exp(–γT) over the whole range of ages of iron meteorites. (3) In order to explain the overestimation of T_K ages in comparison with T_RS ages, a model has been proposed, according to which the GCR intensity has exponentially increased in the interval of 0–1500 Myr governed by the relation: I_T = I_{T = 1500} (1 + αexp(–βT)). For one of the variants of this model, the GCR intensity has exponentially increased by a factor of two only over the recent ~300 Myr, remaining approximately constant for the rest of the time. The data acquired with the use of this model indicate that the measured T_K ages are close to the actual time that the meteorites existed in space; the data are in agreement with the observed exponential distribution of T_RS ages.