Century scale solar variability imprinted in the ⁴⁴Ti activity in meteorites

Abstract

The measurements of γ-activity of the ⁴⁴Ti (T_½ = 66.6 years) produced by spallation reaction of galactic cosmic rays (GCR) in Alfianello, Olivenza, Rio Negro, Dhajala and Torino meteorites, which fell in the time interval 1883 AD (Alfianello) - 1988 AD (Torino), show a century scale modulation, connected to long-term solar-wind flux variations in the interplanetary space. The variation of the ⁴⁴Ti activity with the time of fall of meteorites is qualitatively consistent with the Gleissberg solar cycle, but the amplitude is three-four times higher than expected (about 5%) for GCR flux modulated by solar activity, as determined solely by the sunspot number. The cosmogenic ⁴⁴Ti is a suitable radioisotope for this investigation, but its activity in meteorites is very low. We performed these measurements in the underground low level counting station of Monte dei Cappuccini in Torino, by means of a big hyperpure Ge crystal (≈ 2 kg) in selective coincidence with a heavy NaI (Tl) scintillation detector (≈ 28 kg). This system allows a reliable and non destructive measurement of ⁴⁴Ti (⁴⁴Sc) in meteorites of 200-1200 g in weight. The background is about 1 count per day in the γ -peak at 1157 keV of ⁴⁴Sc in equilibrium with its parent ⁴⁴Ti. The high stability of the performance allows long-lasting runs (≈10⁷ s) in order to attain results with a standard deviation up to about 10%.