Experimental set-up for gamma-activity measurements of astromaterials

The most important proxies of past solar activity are radionuclides produced by nuclear interactions of cosmic rays in the Earth’s atmosphere, in meteorites and in planetary surfaces. In fact, the production of these radioisotopes depends on the cosmic ray flux and energy spectrum and vary in the heliosphere because of solar modulation. Therefore the study of cosmogenic isotopes in terrestrial archives and in meteorites, which fell at different times, gives information on solar activity in the past. Because of its half life, ⁴⁴Ti (T½ = 59.2 years) in meteorites has been proposed as an ideal index for the study of centennial scale solar activity variations. The main experimental difficulties are related to the very low (∼1 dpm/kg) ⁴⁴Ti γ activity in meteorites and to the strong interference by ²¹⁴Bi γ’s from the decay chain of the naturally occurring ²³⁸U. On the basis of the decay scheme of ⁴⁴Ti, we have set up a low background spectrometer, whose high specificity derives from a selective coincidence between the principal HPGe detector and the active shield of NaI(Tl). Using this spectrometer we have reconstructed the solar activity over the past 235 years by measuring ⁴⁴Ti and ²⁶Al in meteorite falls; in particular we have shown that the intensity of cosmic rays has linearly decreased during this period, in agreement with some models proposed for the past solar activity. In order to improve the Ge-NaI coincidence spectrometer, we have recently developed a multiparametric acquisition system, allowing better selectivity in the choice of the coincidence windows, crucial for the ⁴⁴Ti detection. A background measurement of the apparatus, taken for about a month, shows efficient elimination of the ²¹⁴Bi γ signal in coincidence mode. Moreover the coincidence spectrum of Dhajala meteorite (fall, 1976) shows a - - good ⁴⁴Ti γ peak.