One particularly interesting set of secondaries contains radioactive nuclei with beta-decay halflives comparable to the time that cosmic rays spend travelling through interstellar space. The figure above shows mass histograms obtained from CRIS for four elements which include such clock isotopes: Be-10 (1.6 million year halflife), Al-26 (0.87 Myr), Cl-36 (0.30 Myr), and Mn-54 (0.8 Myr estimated). The abundances of all of these radioactive species are small compared to those of neighboring isotopes (off scale in the plots) because significant fractions have been lost by decay during the ~10 Myr the cosmic rays have spent in the Galaxy before arriving at Earth. Even so, small quantities of these nuclides are clearly observed and can be used to investigate the confinement time of cosmic rays in the Galaxy and the distribution of matter with which cosmic ray particles interact to produce secondary nuclei.
The data shown here were collected over a period of 19 weeks between late August 1997 when ACE was launched and early January 1998 and include only about one third of the events presently available for studying the clock isotopes. As a result of the large geometrical factor of the CRIS instrument (~250 cm^2-sr) this data set (~55 Be-10 events, ~120 Al-26, ~25 Cl-36, and ~130 Mn-54 in the plots above) already has statistical accuracy comparable to that of the best previous measurements of cosmic ray clocks. With additional data collection and analysis it will be possible to greatly improve the precision of cosmic ray confinement time determinations and begin to develop more realistic models of cosmic ray confinement in the Galaxy.
....contributed by Mark Wiedenbeck, JPL/Caltech
Last modified 13 February 1998, mrt
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