Environmental fine structure in low-energy β-particle spectra

SOME recent measurements of beta-decay spectra show anomalous line shapes which have been interpreted^1-5 as providing evidence for a neutrino with a mass of about 17 keV; other similar studies, however, find no anomalies^6-13. Regardless of the reason for these discrepancies, such studies require a detailed understanding (at the level of accuracy of one part per thousand) of the spectral shape and the detector response expected in the absence of any exotic phenomena. Influences on the beta decay process from outside the nucleus are of great importance in this regard, and the role of atomic screening corrections^14-16 and atomic and molecular effects on the spectrum near the end point¹⁷ have been considered previously. These produce only smooth, monotonic distortions of the spectrum. Here I note the existence of an effect, previously overlooked, that results in an oscillatory structure in the beta spectrum. The effect is of sufficient magnitude to be relevant to searches for heavy neutrinos that involve tritium beta decay, and it may also provide a basis for studies of molecular or crystal structure, in a manner analogous to extended X-ray absorption fine structure (EXAFS).