Position Dependence in the CDMS II ZIP Detectors

The Ge and Si detectors developed by the Cryogenic Dark Matter Search (CDMS) II experiment rely on the simultaneous detection of athermal phonons and ionization produced by interactions in the crystal. The athermal phonons not only provide the total energy deposited during an interaction, but also carry information about the position of the interaction. We describe extracting this information from the pulse shapes in the four phonon sensors arrays. We present the result of measurements made on a Si detector from the first CDMS II production batch. We also investigate ways of using the event position information to extract further information about the phonon signal and about the detector itself. The ZIP detectors [1, 2, 3] developed by the CDMS collaboration are designed to search for dark matter in the form of Weakly Interacting Massive Particles [4]. We record both the ionization signal and the athermal phonon signal obtained by the Transition-Edge Sensors (TES) in the electrothermal feedback (ETF) mode. In this paper we describe two ways of extracting the event position information, and we use this information to study the phonon signal. We present data collected with a Si ZIP detector exposed to a 12-hole collimator 109Cd source covering one surface of the detector and a single-hole collimator 241Am source. Figure 1 (left) shows a drawing of the setup. The ZIP detectors have 4 phonon sensors (labelled A through D), each covering a quadrant of the Si or Ge crystal. One approach to extracting the event position is to use amplitudes of the four signals, since the largest signal occurs in the primary quadrant (in which the event occured). We define partitions px C D A B A B C D and py A D B C A B C D , where A B C D are the pulse amplitudes in the four sensors. Figure 1 (right) shows the event distribution on the detector surface generated by applying this procedure to the Si data. This plot is not a realistic position map it has a rectangular shape instead of the expected circular one. This is because a part of the phonon signal is lost for events that happen toward the edge of the crystal. Hence, the py-px plot folds on itself and the edges −4 −3 −2 −1 0 1 2 3 4 −4 −3 −2 −1 0 1 2 3 4