Study of parylene-coated NaI(Tl) at low temperatures for bolometric applications

NaI(Tl) is a widely-used scintillator at room temperature, and it is particularly interesting as a target for dark matter searches. Its hygroscopic character however makes it unsuitable for many applications, in particular for bolometric particle detection at very low temperature. Despite that, a NaI scintillating bolometer would provide unique features for dark matter detection, like β / γ background rejection through particle discrimination and thermal quenching factors for nuclear with respect to electron recoils close to one. With the long-term goal of developing a scintillating NaI bolometer, we have tested NaI(Tl) crystals coated by vapor-deposited poly-p-xylylene (parylene) and studied their optical and mechanical behavior in the mK range. We present X-ray excited scintillation spectra of a parylene-coated NaI(Tl) sample at 1.5, 4 and 77 K, and measurements of the light output as function of the temperature over the 1.5–300 K range. At 1.5 K the wavelength of maximum emission is observed at 325 nm. Thermoluminescence peaks are found at around 60, 95 and 150 K. Tests of mechanical and optical resistance to thermal cycles of 45 g parylene-coated NaI(Tl) cylinders are also presented, and the adequacy and effectiveness of this coating technique is discussed.