44Ti (half-life 59.2 years), produced by spallation reactions of Galactic Cosmic Rays (GCR) in meteorites, provides information about the GCR flux in the past and allows estimation of the modulation effects of the Sun between about 1 to 3 AU on the centennial time scale. We have measured 44Ti in 18 meteorites that fell since 1760 on an efficient and selective hyperpure Ge+NaI(Tl) coincidence γ-ray spectrometer system set up in the Laboratory of Monte dei Cappuccini in Torino. Although the errors in the measured 44Ti activity are large, particularly for old meteorite falls, the results can be understood in terms of a cyclic variation with a period of about 100 years and with amplitude of ∼20% (peak to trough) and a linear trend of decreasing 44Ti-activity. The results imply a decrease of GCR flux, of about 1% per decade, over the past 250 years, superimposed on the Gleissberg cycle. In order to confirm the very low activity of 44Ti in meteorites fallen before 1850, we have recently set up a new spectrometer with higher efficiency and resolution. The background is thus reduced by a factor of ∼2 in the 44Ti-region. Measurements on some meteorites on the new spectrometer confirm our previous results. Study of very old falls to confirm the long-term trend is now in progress.
Heliospheric modulation of cosmic rays based on 44Ti produced in stony meteorites over the past 250 years
TARICCO, Carla;COLOMBETTI, Paolo;
2005-01-01
Abstract
44Ti (half-life 59.2 years), produced by spallation reactions of Galactic Cosmic Rays (GCR) in meteorites, provides information about the GCR flux in the past and allows estimation of the modulation effects of the Sun between about 1 to 3 AU on the centennial time scale. We have measured 44Ti in 18 meteorites that fell since 1760 on an efficient and selective hyperpure Ge+NaI(Tl) coincidence γ-ray spectrometer system set up in the Laboratory of Monte dei Cappuccini in Torino. Although the errors in the measured 44Ti activity are large, particularly for old meteorite falls, the results can be understood in terms of a cyclic variation with a period of about 100 years and with amplitude of ∼20% (peak to trough) and a linear trend of decreasing 44Ti-activity. The results imply a decrease of GCR flux, of about 1% per decade, over the past 250 years, superimposed on the Gleissberg cycle. In order to confirm the very low activity of 44Ti in meteorites fallen before 1850, we have recently set up a new spectrometer with higher efficiency and resolution. The background is thus reduced by a factor of ∼2 in the 44Ti-region. Measurements on some meteorites on the new spectrometer confirm our previous results. Study of very old falls to confirm the long-term trend is now in progress.
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