Volume 247, number 2,3 PHYSICS LETTERS B 13 September 1990 A cryogenic tellurium detector for rare events and gamma rays A. Alessandrello 1, C. Brofferio, D. Camin, O. Cremonesi, E. Fiorini, A. Giuliani and G. Pessina Dipartimento di Fisica dell'Universith di Milano and Sezione di Milano delI'INFN, 1-20133 Milan, Italy Received 11 June 1990 In view of a future experiment on double beta decay we have constructed and operated for the first time a bolometric detector made with an ultrapure tellurium crystal of about 2.1 gram. With this detector we have been able to measure high energy gamma rays from various calibration sources with an energy resolution of 2% in the region of neutrinoless double beta decay. The thermal properties of tellurium at very low temperature are determined for the first time in view of the construction of larger detectors. Low temperature bolometric detection of single particles has been proposed for measurements of X- rays and neutrino mass [ 1,2 ] and for detection of rare processes like electron and double beta decay [ 1 ]. The operating principle of these detectors is based on the consideration that the heat capacity of a very cold pure dielectric and diamagnetic crystal is roughly proportional to the cube of the ratio between the op- erating temperature T and the Debye temperature Tt~. This capacity can therefore be made so small that even the tiny energy delivered by an incident particle in the form of heat can produce a sizeable increase of the temperature which can be measured by the change of the resistance of a suitable thermistor in thermal contact with the crystal itself. Exciting and very promising results have been recently obtained in the measurement of X-rays with small detectors [3 ] ~ and, conversely, in the development of large mass de- tectors to search for rare events. In the latter case, many results have been obtained with the construc- tion of large [ 3,5-7 ] bolometers of low-Z materials like sapphire, which present the advantage of a large Debye temperature and could be in principle very promising for searches on cosmic dark matter. Our group has developed the complementary approach of high-Z detectors suitable to investigate rare pro- Present address: Laboratori Nazionali del Gran Sasso, As- sergi, L'Aquila, Italy. ~ In a recent measurement the NASA-Wisconsin Collaboration has obtained a resolution of 7.3 eV on the 5.9 keV line of 55Fe [41. cesses like double beta decay and to measure high en- ergy gamma rays. In neutrinoless double beta decay [ 8 ], a very powerful tool to search for lepton number nonconservation, two electrons and no neutrinos are simultaneously emitted. Since the nuclear recoil en- ergy is negligible, the spectrum of the sum of the two electron energies should present a peak correspond- ing to the transition energy [ 8 ]. In order to minimize the background of spurious counting in this very rare process it has been suggested [ 9 ] to use a double beta decay active material acting at the same time as source and detector of the decay. A series of experiments carried out with germanium semiconductors starting in 1967 [ 10] have in fact recently produced lower limits on the half-life of 76Ge exceeding 1024 yr [ 11,12 ]. Recent theoretical calculations have, how- ever, indicated a suppression of the nuclear matrix elements for this process which seems to disappear [8] for nuclei of large atomic number, which are therefore more promising candidates for double beta decay. From a more practical point of view, high-Z bolometers are in addition excellent detectors of high energy gamma rays since the total photon cross sec- tion, and particularly the photoelectric one, increases strongly with atomic number. Our group has recently developed bolometers made with large germanium crystals and thermally mea- sured for the first time high energy gamma rays [ 13 ]. Despite the drawback of a Debye temperature for germanium lower by a factor of two with respect to materials like sapphire or silicon which are normally 442 0370-2693/90/$ 03.50 © 1990 - Elsevier Science Publishers B.V. ( North-Holland )