Simultaneous measurement of phonons and ionization using silicon crystal acoustic detectors

We have configured a double-sided silicon crystal acoustic detector (SiCAD) [B.A. Young et al., Nucl. Instr. and Meth. A 311 (1992) 195] for simultaneous measurement of both phonons and ionization. This detector operates at ∼ 370 mK and consists of a Ti transition edge sensor (TES) on one side, and a similar pattern of metal acting as an electrode on the other side of a 300 μm thick high-purity, monocrystalline Si wafer. Distinguishing electron-recoil from nuclear-recoil events is possible due to the different partitioning of energy into phonons and ionization. This effect provides a powerful background rejection technique for neutrino physics experiments and weakly interacting, particulate dark matter searches where the events of interest are nuclear-recoils. In addition, the phonon sensor is position sensitive; it measures the depth of an event and can be used to reject background events occurring near the surface. We present the results of experiments which demonstrate the discrimination capability and position sensitivity of the detector for energy depositions above ∼ 3 keV.