부산대학교 도서관

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, 44 Ti (T ½ = 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) 44 Ti γ activity in meteorites and to the strong interference by 214 Bi γ’s from the decay chain of the naturally occurring 238 U. On the basis of the decay scheme of 44 Ti, 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 44 Ti and 26 Al 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 44 Ti detection. A background measurement of the apparatus, taken for about a month, shows efficient elimination of the 214 Bi γ signal in coincidence mode. Moreover the coincidence spectrum of Dhajala meteorite (fall, 1976) shows a good 44 Ti γ peak.