A Search for Bound-state Beta Decay of {}193Ir in the Galactic Cosmic Rays

Bound-state beta decay ( b decay) is an uncommon beta decay mode in which the electron is emitted into a bound atomic state rather than into the continuum. Because of the exclusion principle, the final atomic state must be initially empty, so isotopes which can only decay by b decay are stable as neutral atoms. However, when completely ionized as in the galactic cosmic rays, these isotopes can decay. The isotope 193 Ir has been predicted to undergo b decay. 193 Ir is the most abundant isotope of iridium in the Solar System. The results from the Trek detector suggest a depletion of iridium in the galactic cosmic rays. Such a signal would be consistent with the b decay of this isotope. This would be only the third example of b decay ever observed and the first observation in a natural system.