${}^7$Be electron capture decay to ${}^7$Li has been investigated by means of a cryogenic $\mu$ calorimeter. A cryogenic particle detector, due to its low energy threshold and good energy resolution, allows us to detect for the first time the complete ${}^7$Be calorimetric energy spectrum with a peak width of 24 eV full width at half maximum at 112 eV. The observed decrease in the rate of the absorber activity corresponds to the expected ${}^7$Be half-life of 53 days. This kind of detector has an important application in the environment of a lithium radiochemical solar neutrino experiment which is in progress in Moscow. A discussion about the possible processes which take place inside the BeO absorber during the energy thermalization is reported.

• Received 8 July 1997

DOI:https://doi.org/10.1103/PhysRevC.57.2017