Astronomy and Astrophysics – Astrophysics
Scientific paper
Jan 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995phdt.........9g&link_type=abstract
Thesis (PH.D.)--WASHINGTON UNIVERSITY, 1995.Source: Dissertation Abstracts International, Volume: 56-06, Section: B, page: 3241
Astronomy and Astrophysics
Astrophysics
2
Cosmic Rays
Scientific paper
The earth is constantly awash in high energy nuclei. These nuclei arrive here after spending millions of years traversing the interstellar medium (ISM). The elemental and isotopic composition of these cosmic rays promise to tell us about the astrophysical processes that created their sources and accelerated them. To obtain the source composition of the cosmic rays it is important to correct for their fragmentation in the ISM, which is mainly composed of hydrogen. The propagation models used to do this correction are strongly dependent on fragmentation probabilities, especially for ultra-heavy cosmic rays with Z > 30, which have a very high likelihood for interaction. This thesis is an attempt to measure some of these fragmentation probabilities, also known as cross sections. I will describe the results of an experiment in which a beam of 10.6 GeV/nucleon gold was directed at targets of rm CH_2, C, Al, Cu, Sn and Pb. Our experimental array was able to measure the charge of the largest fragment from the incident gold nucleus if the fragment was within approximately 30 charge units of gold. These charge-changing fragmentation cross sections for ultra-heavy nuclei were measured at an energy much higher than previously studied. The total charge-changing cross sections for any charge change of the Au nuclei were found to be higher than those measured at energies of <=1 GeV/nucleon. The partial charge-changing cross sections for fragmentation into elements with charges close to Au differed significantly from those measured at lower energies: they are larger or equal for large charge changes and smaller for small charge changes. The hydrogen partial cross sections show an additional change from a roughly exponential form with charge change to a power law relationship. These changes in the partial cross sections are large enough to seriously affect calculations of the effects of interstellar propagation on the abundances of ultra-heavy cosmic rays. Both partial and total cross sections were fit with semi-empirical expressions. Proton electromagnetic dissociation of the gold nuclei was also explored.
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