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More on Dr. Thompson's Thesis Work
David's Ph.D. thesis was on atmospheric gamma radiation. It has long been known
that the Earth is bombarded by huge numbers of charged particle cosmic
rays, mostly protons, electrons, and alpha particles. It was also known
that these cosmic rays interact with the atoms in the Earth's atmosphere to
produce even more particles, including pi mesons, mu mesons, and gamma
rays. In fact, even at the highest altitudes that can be reached by
scientific balloons (about 130,000 feet), the secondary gamma rays
outnumber the cosmic gamma rays that can tell us about high-energy
phenomena in astrophysical sources. When he started working at Goddard, the
gamma-ray group was building the first imaging gamma-ray satellite
telescope, SAS-2.
David's advisor, Carl Fichtel, realized that some of these many cosmic rays
hitting the atmosphere would produce gamma rays that scattered upward or
sideways and could potentially interfere with the satellite once it was
launched. His thesis was a combined experimental and modeling effort to
understand the atmospheric gamma radiation in three dimensions. David
refurbished and flew a small gamma-ray telescope on a balloon, looking
sideways and downward to measure the scattered gamma radiation. He also
developed a Monte Carlo model of the showers of interactions the cosmic
rays made in the atmosphere, tracing the particles in a 3-D way in order to
compare with the observations.
He found that the earth's atmosphere is a bright source of gamma rays, and
the horizon is particularly bright. This has been an important
consideration in all succeeding gamma-ray telescopes, which have to avoid
looking at the earth's horizon or at least not trying to do astronomy when
the horizon interferes with the celestial observations. The Monte Carlo
calculation also suggested that the Moon could be a source of gamma rays,
through the same sort of cosmic ray collisions that take place in the
atmosphere. It was not until EGRET flew that that prediction was
confirmed.
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