Department of Physics , Washington University in St. Louis
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Noble gases are the natural places to study nuclear effects in nature. Their low natural abundance means that their isotope ratios are easily changed by even low yield nuclear reactions. Professor Hohenberg developed a new type of mass spectrometer that defines the state of the art noble gas mass spectrometry (Hohenberg, 1980). Combining nearly perfect ion optics with the ultimate sensitivity afforded by ion-counting detection and the unique chemical properties of noble gases resulted in an instrument that can measure samples containing as few as several thousand noble gas atoms. Focused laser heating allows the noble gases contained in individual micron-sized mineral grains to be routinely studied. Applied to a variety of problems these techniques have opened up new research in exciting new directions:
129I is now extinct in nature. It's short half-life, 15.6 million yr, means that 129I has long since decayed away in a solar system that is 4.6 billion years old. 129I is made only in supernovae where freshly processed material is ejected molecular clouds. These clouds coalesced to form the solar nebula and eventually solid material while live 129I did exist in the early solar system. Its decay product, 129Xe, still exists in iodine host sites in primitive meteoritic material. Measurement of the ratio of radiogenic 129Xe to stable 127I provides a sensitive and precise chronometer for early solar system history. Comparisons of the times of mineral formation in different objects can be made with precision, easily resolving time differences as small as 100,000 years in the formation of objects 4.6 billion years ago (Brazzle, et al., 1999).
Individual mineral grains in meteorites contain unique records of their exposure to energetic particles. Neutrons and protons with energies in excess of a few MeV interact with Al and Mg targets to produce 21Ne by spallation. These grains therefore contain records of energetic particle irradiation. They can be studied in the laboratory by laser extraction noble and gas mass spectrometry to reveal the history of energetic particle exposure prior to meteorite formation 4.6 billion years age. Some of these particles show very large amounts of spallation-produced 21Ne, larger than likely from contemporary sources of energetic particles, probably due to irradiation by the sun as it went through as active (T-Tauri) phase prior to settling onto the main sequence (Caffee, et al., 1987).
Ages of lunar craters, and other features, are most accurately determined by the Kr-Kr exposure age method. Sub-surface lunar material is initially shielded from cosmic ray spallation reactions but, once brought to the surface by an impact, production of stable krypton isotopes and radioactive 81Kr begins. With production rates monitored by radioactive 81Kr and the integrated effects determined by the stable spallation products, surface exposure ages can be obtained without prior knowledge of specific production rates. This method established the age of lunar craters North Ray and South Ray as 50 and 2 million years, respectively, and Tycho as 100 million years (Drozd, et al., 1974a).
Fission reactors occurred in nature several billion years ago when the concentration of 235U was still high enough to provide viable reactor fuel. The properties of these natural reactors can be deduced through the study of the fission Xe the produce (Drozd, et al., 1974b; Meshik, et al., 1999). Likewise, our discovery of fission products of 244Pu, and radiogenic 129I by the measurement of xenon isotopic spectra in lunar material greatly restrict viable models for lunar origin.
Measurement of the (( half-life of 130Te and 128Te from xenon isotopes in old native Te. This measurement of 7.7 x 1024 years for 128Te is the longest half-life ever determined experimentally (and constrains the electron neutrino mass to a few eV (Bernatowicz, et al., 1992; 1993). This laboratory is part of the science team for the NASA Discovery Class Mission, Genesis, specifically with the development of collector material and the analysis of returned solar wind samples.
Charles Hohenberg received his bachelor's degree in Physics from Princeton University and his PhD degree in Physics from the University of California, Berkeley. He has served on numerous national committees as well as international advisory boards. His keen interest in the basic simplicity and order of nature that physics reveals, and his talent for developing new instrumentation, have brought him to many exciting and diverse scientific frontiers.