Measurement of the nuclear recoil thermal relative efficiency factor with an undoped sapphire scintillating bolometer

In previous work of the ROSEBUD (Rare Objects SEarch with Bolometers UndergrounD) collaboration, undoped sapphire has shown high light yield at very low temperature and low particle discrimination energy threshold which convert sapphire scintillating bolometers in very suitable detectors for dark matter searches. Measuring the nuclear recoil thermal efficiency factor relative to gamma events is required in order to correctly estimate the sensitivity for Weakly Interacting Massive Particles (WIMPs). Here we report on the results of a dedicated experiment with an external Po-210 source which aims at the estimate of the relative thermal efficiency factor of nuclear recoil versus gamma events depositing the same energy in an undoped sapphire scintillating bolometer. A value slightly larger than one has been derived and its implications in the search for WIMPs are briefly outlined. We also present the results of an analysis of Pu-236 calibration data obtained with the same scintillating bolometer, deriving an estimate of the relative thermal efficiency factor for nuclear recoil versus alpha events.