Studies on Point-Contact Germanium Detector Bulk-Surface Discrimination at Near-Threshold Energies using Pulser-Generated Pulse Shapes

Jia-Shian Wang^1*, Hau-Bin Li¹, Manoj K. Singh¹, Henry T. Wong¹

¹Institute of Physics, Academia Sinica, Taipei, Taiwan

* Presenter:Jia-Shian Wang, email:jswang2022@gate.sinica.edu.tw

The p-type point-contact Germanium detectors (PPCGe) [1] have been widely adopted in searches of low energy events such as neutrinos and dark matter, e.g. TEXONO [2], CoGeNT [3] and CDEX-1 [4], etc. This is due to their enhanced capabilities of background rejection, sensitivity at energies as low as the sub-keV range and particularly fine energy resolution. Nonetheless, the PPCGe is subject to irregular behaviour caused by surface effects for events near the passivated surface. The signal pulses of such events will exhibit slower rise time and lower amplitude compared to that of proper signals originating from the detector bulk volume. Surface events can, therefore, be distinguished from bulk events via simple pulse-rise-time comparison. In the sub-keV region, however, the rise time spectra of bulk and surface pulses overlap, and become hardly distinguishable. Thus, it is vital to develop techniques that improve the measurements of bulk-surface leakage at lower energies.

In previous studies [5, 6], the bulk-surface leakage was measured mainly using radioactive sources. The main limitation of the approach is that only sources of well-known spectral shapes can be used for calibration. Alternatively, one can produce artificial pulses imitating different signal pulses by using an external pulse generator (pulser). In this study, we will demonstrate that pulser-generated pulses can indeed be rendered to mimic both bulk and surface pulses. By confirming that the pulse shapes of pulser-generated pulses and signal pulses match at higher energies, the pulser-generated pulses of reduced amplitude should then, in principle, be representative of the low energy signal pulses. In this case, the pulser can be employed as a controlled input that is able to reproduce pulses of both types at arbitrary energy. Our aim is then to develop an effective pulse shape discrimination technique based on these pulser-generated pulses to improve the bulk-surface leakage measurement at near-threshold energies.


Reference:
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Keywords: Germanium Detector, Pulse Shape Discrimination, Ultra-Low Energy