On the cosmogenic radionuclides in meteorites

Abstract

Different fluxes and spectra of galactic cosmic rays (GCR) and solar cosmic rays result in a wide variety of radioactive and stable-nuclides production rates. The records of cosmic-ray interactions, have been studied in terrestrial archives, in meteorites, cosmic dust and lunar samples. We present here some results obtained in our laboratory on the study of cosmogenic radionuclides produced in meteorites which fell in the last century. The laboratory is situated in the underground station of Monte dei Cappuccini, Torino, at a depth of 70 m water equivalent to reduce the cosmic-ray background level. To measure γ-activity in meteorites we have set up a highly efficient and selective Ge-NaI(Tl) γ-ray spectrometer surrounded by a passive shield. The solar modulation affects the intensity of GCR and therefore the isotopes production rates in meteorites. The 11 γ solar cycle is well reproduced by the variations of ²²Na (T ½ = 2.6 y) in meteorites which fell during the past three solar cycles. The ⁴⁴Ti (T ½ = 66.6 y) activity measured in eight chondrites which fell in the last 110 γ is qualitatively consistent with the century-scale solar-activity variation, called the Gleissberg cycle. The increase is roughly four times higher, than expected (about 5%) for a GCR flux calculated by a model in which the solar-activity modulation is determined solely by the sunspot number index R. This result suggests a much weakened modulation of GCR during the prolonged Gleissberg minimum during which highly ordered magnetic fields set up in the heliosphere.