First results of the RAP experiment (acoustic detection of particles) in the low temperature regime

The results on cosmic rays detected by the gravitational wave antenna NAUTILUS have motivated an experiment (RAP) based on a suspended cylindrical bar, which is made of the same aluminum alloy as NAUTILUS and is exposed to a high energy electron beam. Mechanical vibrations originate from the local thermal expansion caused by warming up due to the energy lost by particles crossing the material. The aim of the experiment is to measure the amplitude of the fundamental longitudinal vibration at different temperatures. We report on the results obtained down to a temperature of about 4 K for an Al 5056 bar, which agree at the level of 10% with the predictions of the model describing the underlying physical process. Very preliminary results for a Niobium bar at temperatures below and above the transition temperature are also reported.