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 β-decay processes. Detailed knowledge of the β-particles' spectral shape and the detector response is therefore required in order to distinguish real effects from instrumental artefacts. The interaction between the emitted β-particle and its local environment is predicted to produce oscillations in the β- particle spectrum, known as β environmental fine structure; the effect is analogous to extended X-ray absorption fine structure (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 β 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. Here we report the detection of β environmental fine structure in the β-decay spectrum of 187Re, using a cryogenic microcalorimeter. Our results, which are in good agreement with recent theoretical predictions, may facilitate studies of molecular or crystalline structures in a manner similar to EXAFS.
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