Thermal relative efficiency factor for recoiling 206Pb nuclei in a sapphire bolometer

Scintillating bolometers are very suitable detectors for dark matter searches. They offer a wide absorber choice allowing to tune up different masses and couplings of WIMPs, background rejection capability and high efficiency in the conversion into heat of nuclear recoil energy releases. In a previous work we have shown that undoped sapphire can be an excellent dark matter detector since, due to its high light yield at very low temperature, very low particle discrimination energy thresholds are achievable. In this Letter, we report the results of an experiment devoted to the estimate of its thermal relative efficiency factor (REF) for nuclear recoil versus gamma events which is a relevant parameter in WIMP search experiments. Taking advantage of the discrimination capability of scintillating bolometers, we have registered the energy spectrum of 206Pb recoiling nuclei from a 210Po alpha-source. Ensuing from the analysis we obtain an REF=1.044±0.008stat−0.008syst+0.056 close to but slightly greater than unity.