Detection of environmental fine structure in the low-energy β-decay spectrum of 187Re

Determining whether neutrinos have mass is an important test of grand unified theories. Neutrino masses can in principle be measured from small distortions' in the spectra of electrons emitted in low-energy beta-decay processes. Detailed knowledge of the beta-particles' spectral shape and the detector response is therefore required in order to distinguish(1-5) real effects from instrumental artefacts. The interaction between the emitted beta-partide and its local environment is predicted(6) to produce oscillations in the beta-particle spectrum, known as beta environmental fine structure; the effect is analogous to extended X-ray absorption fine structure(7) (EXAFS), which provides the basis for spectroscopic surface studies of local molecular structure. But the low energy resolution and high operating temperatures of traditional radiation detectors have precluded observation of beta environmental fine structure. Cryogenic microcalorimeters, operated as particle detectors, offer a means of overcoming these problems, as they can reach energy resolutions up to ten times higher than traditional detectors(8). Here we report the detection of beta environmental fine structure in the beta-decay spectrum of Re-187, using a cryogenic microcalorimeter. Our results, which are in good agreement with recent theoretical predictions(9), may facilitate studies of molecular or crystalline structures in a manner similar to EXAFS.