Unraveling the details of beta decay.

Many radioactive atoms decay by emitting beta particles, or electrons, thereby transforming themselves into new elements. For each atomic isotope, these beta particles emerge from nuclei with a characteristic distribution, or spectrum, of energies. Theorist Steven E. Koonin of the California Institute of Technology in Pasadena has now shown that electrons surrounding an atomic nucleus have a discernible but hitherto overlooked effect on a given radioactive isotope's beta-decay spectrum. Reporting in the Dec. 12 NATURE, he notes that this newly identified quantum-mechanical effect would typically produce a sequence of minute oscillations that slightly distort a low-energy beta-particle spectrum.

The existence of this quantum effect and the "fine structure" it produces in a spectrum may have important consequences in the search for heavy neutrinos (SN:4/27/91, p.260). Each beta decay produces not only an electron but also an invisible neutrino, and researchers have long relied on measurements of beta-decay spectra for information about the accompanying neutrinos. Koonin cautions that the presence of fine structure could change the interpretation of certain experiments designed to search for heavy neutrinos.

This characteristic spectral fingerprint may also serve as a means of gleaning information about the chemical, or electronic, environment surrounding an atomic nucleus, Koonin suggests. For example, he says, by precisely measuring the shape and size of the fine-structure spectrum resulting from tritium (a radioactive hydrogen isotope) embedded in a crystal, researchers could map the location of hydrogen in solids.