[Previous] | [Session 61] | [Next]
G. Bonino, G. Cini Castagnoli, C. Taricco (Dipartimento di Fisica Generale, Università di Torino, Italy and Istituto di Cosmogeofisica, CNR, Torino), N. Bhandari, K.M. Suthar (Physical Research Laboratory, Ahmedabad, India), M. Killgore (Southwest Meteorite Laboratory, Payson, Arizona, USA)
The Portales Valley (H6) meteorite shower fell on 13 June 1998 near the town of Portales, New Mexico. About 50 fragments, ranging between 12 g to 34 kg, covered a strew field of about 10 km x 2 km. The different fragments consist of large silicate-rich regions crosscut by unusual thick veins of metallic Fe-Ni of different sizes and abundances. Galactic cosmic ray (GCR) particles interact with meteoroids in the interplanetary space, thereby producing a large variety of both radioactive and stable nuclides and radiation damage in minerals (tracks) produced by heavy nuclei. As Portales Valley is a large shower, various fragments located at different depth within the meteoroid provide an opportunity to study cosmogenic effects also as a function of depth and for different target element abundance. Measured depth profiles are available only for few meteorites. Furthermore a fresh fall, like Portales Valley, allows to extend this analysis to several nuclides. We measured the gamma-activity in a fragment weighing about 600 g shortly after fall. Twelve cosmogenic radioisotopes with half-lives ranging between 16.1 days to 7.2 x 105 years have been measured with high precision. Measurements in other 3 fragments were done successively. These measurements were performed in the underground Laboratory in Torino by a highly sensitive and selective Ge-NaI(Tl) gamma-spectrometer. The Na-22(T1/2=2.6y)/Al-26(T1/2=7.2 105y) ratio, which is nearly independent of meteoroid composition and shielding, gives about the same value of 1.5 for the 4 fragments of Portales Valley , very similar to those of Fermo and Torino (measured in our Laboratory) which fell in 1996 and 1988 respectively, near to the minima (GCR maxima) of the Schwabe cycle of solar activity. In addition we will present results of the nuclear tracks density measureded at PRL and of the chemical analysis and we will discuss these results in relation to the different cosmogenic effects in a meteoroid. Acknowledgments-This work was supported by MURST, Co-fin97 and CNR