A composite bolometer as a charged-particle spectrometer

The measurement of radioactivity by direct conversion of nuclear radiation into a temperature rise of a calorimeter is as old as nuclear physics itself. As part of a general programme aiming at a determination of the mass of the electron neutrino, we have designed an improved version of a He-cooled composite diamond bolometer with a monolithic germanium thermistor, developed at the Laboratoire de Physique Stellaire et Planetaire (LPSP)¹. Our approach, based on an idea by De Rujula², is to study the shape, near the upper end-point of the internal bremsstrahlung spectrum in electron-capture β decay. The best nucleus for a precise measurement seems to be ¹⁶³Ho, for which we have determined³ the Q_EC value to be 2.83±0.05 keV. A particularly interesting possibility is to use total absorption spectrometry⁴ (calorimetry), in which the radioactive holmium forms part of the sensitive volume of the detector. With 5-6-MeV α particles impinging on the diamond wafer of the bolometer, a full-width-at-half-maximum (FWHM) of 36 keV was obtained at a temperature of 1.3 K. The theoretical resolution at 100 mK is a few electron-volts, so this new detection technique should give greatly enhanced energy resolution compared with present solid-state conductors based on charge carrier collection.