Physics
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30r.489b&link_type=abstract
Meteoritics, vol. 30, no. 5, page 489
Physics
1
Activity, Chondrites, Cosmic Rays, Galactic, Isotopes, Cosmogenic, Meteorites, Alfianello, Dhajala, Lancon, Mbale, Monze, Olivenza, Rio Negro, Torino, Solar Modulation
Scientific paper
The solar modulation affects the intensity of galactic cosmic rays, particularly below 1 GeV and therefore the isotopes production rates in meteorites varies in antiphase with the solar activity. To ascertain the existence of a century scale variation, connected to the Gleissberg cycle of solar activity, we have measured the activity of the cosmogenic radioisotope 44Ti (T1/2=66.6 yr) in chondrites as a function of their time of fall. This radioisotope is produced in meteorites by spallation reactions of galactic cosmic rays on heavy elements such as Fe and Ni. It has a very low production rate in meteorites and therefore it is difficult to measure with good accuracy. 44Ti can be measured by g-spectroscopy of the 1157 keV gamma-ray of its short lived daughter 44Sc (T1/2=3.93 h). To measure small levels of 44Ti activity and to avoid interference (at 1155.2 keV) due to ubiquitous, naturally occurring 214Bi, a member of 238U series, we have designed and set up a highly sensitive and selective Ge-NaI (Tl) gamma-ray spectrometer [1,2], operating in the underground station of Monte dei Cappuccini in Torino at 70 m of water equivalent depth. We measured the gamma-activity of 44Ti (44Sc) in eight chondrites which fell in the last 110 years: Alfianello (1883), Lancon (1897), Olivenza (1924), Rio Negro (1934), Monze (1950), Dhajala (1976), Torino (1988) and Mbale (1992). The 44Ti activity was corrected for the target element concentrations and shielding depth within meteorites, calculated from cosmic ray track density and exposure ages. The activity of Alfianello and Lancon meteorites which fell at the end of past century is about equal to the activities of Dhajala, Torino and Mbale meteorites which fell recently. The activity of Olivenza, Rio Negro and Monze meteorites which fell in the time interval 1924-1950 is about 20% higher. This century scale modulation of the 44Ti activity is qualitatively consistent with the century scale solar activity variation, due to the Gleissberg cycle, but the increase is roughly four times more than expected (about 5%) and calculated on the basis of isotope production models [3,4] for a GCR flux modulated by the solar activity, as determined simply by sunspot numbers [5]. It can be noticed that the higher activity of 44Ti, related to the Gleissberg minimum at the turn of this century, is expected between 1925-1955. The phase difference between the sunspot minimum and the 44Ti maximum arises because of the integrating nature of the radioisotope production. These results show that during a sequence of low amplitude 11-yr (Schwabe) solar cycles, at the Gleissberg minimum, a higher flux of galactic cosmic rays can penetrate into the inner heliosphere, reflecting the regularity and orderlines of the heliospheric magnetic field during such epochs. References: [1] Bonino G. et al. (1992) Nuovo Cimento, 15C, 99-104. [2] Bonino G. (1993) Nuovo Cimento, 16C, 29-33. [3] Bhandari N. (1981) Proc. Ind. Acad. Sci., 90, 359. [4] Bhandari N. et al. (1993) GCA, 57, 2361. [5] Bonino G. et al., in preparation.
Bhandari Narendra
Bonino Gianfranco
Castagnoli Cini G.
Taricco Carla
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