CoGeNT: A Search for Low-Mass Dark Matter using p-type Point Contact Germanium Detectors

TL;DR

CoGeNT demonstrates the viability of p-type point-contact germanium detectors for probing low-mass dark matter candidates around $m_ ext{\chi}\sim 10~\mathrm{GeV}/c^2$ by achieving a low energy threshold and efficient surface-background rejection. The study presents a detailed apparatus description, calibration, and a comprehensive background model that accounts for neutrons, cosmogenic isotopes, radon, and cryostat radioactivity, concluding that known backgrounds cannot fully explain the observed low-energy excess or the previously reported modulation. A robust surface-event discrimination strategy is developed and quantified, enabling an irreducible bulk spectrum to be extracted, which is then used to place WIMP-like interpretation regions and limits in the context of other experiments. The results confirm the strengths of PPC technology for long-term modulation searches and motivate the planned C-4 expansion to enhance sensitivity and mass scale, while informing design choices to mitigate dominant resistor-related backgrounds.

Abstract

CoGeNT employs p-type point-contact (PPC) germanium detectors to search for Weakly Interacting Massive Particles (WIMPs). By virtue of its low energy threshold and ability to reject surface backgrounds, this type of device allows an emphasis on low-mass dark matter candidates (wimp mass of about 10 GeV/c2). We report on the characteristics of the PPC detector presently taking data at the Soudan Underground Laboratory, elaborating on aspects of shielding, data acquisition, instrumental stability, data analysis, and background estimation. A detailed background model is used to investigate the low energy excess of events previously reported, and to assess the possibility of temporal modulations in the low-energy event rate. Extensive simulations of all presently known backgrounds do not provide a viable background explanation for the excess of low-energy events in the CoGeNT data, or the previously observed temporal variation in the event rate. Also reported on for the first time is a determination of the surface (slow pulse rise time) event contamination in the data as a function of energy. We conclude that the CoGeNT detector technology is well suited to search for the annual modulation signature expected from dark matter particle interactions in the region of WIMP mass and coupling favored by the DAMA/LIBRA results

Figures (31)

• Figure 1: Partially disassembled shield of the CoGeNT detector at SUL, showing the cylindrical OFHC end cap and innermost 5 cm of ancient 0.02 Bq $^{210}$Pb/kg lead, characteristically oxidized following etching. The preamplifier is visible at the top right (black box). A minimum of 7 cm of lead thickness shields the detector from the naturally occuring radioactivity in the preamplifier's electronic components.
• Figure 2: Layout of the complete shield for the CoGeNT detector. The outermost component is a layer of recycled HDPE, used to moderate neutrons. Next towards the interior, a 1 inch thick layer of borated polyethylene captures moderated neutrons. Three layers of lead are indicated by the three different inner shaded regions. The outermost lead is composed of stock bricks, not chemically etched, the middle layer is chemically etched and cleaned, and the innermost layer consists of ultra-low background ancient lead. An automated liquid nitrogen transfer system refills the detector Dewar every 48 hours, maintaining the germanium crystal at a near constant temperature. See text for a full description of these components.
• Figure 3: Schematic of the data acquisition system for the CoGeNT detector at SUL (see text).
• Figure 4: Example digitized traces from the six CoGeNT DAQ read-out channels, corresponding to an event with energy $\sim2.5$ keVee. Preamplifier traces are DC-offset at the Phillips Scientific 777 amplifier to allow for rise time measurements of pulses in the range 0-12 keVee, following offline wavelet denoising Aal11 (not yet applied to these traces).
• Figure 5: Neutron scattering measurements of the low-energy quenching factor for nuclear recoils in germanium, compared to Lindhard theory predictions. CoGeNT adopts the expression relating ionization and recoil energy $E_{i}$(keVee) $= 0.2 \times E_{r}^{1.12}$(keVr), valid for the range 0.2 keVr $<E_{r}<$ 10 keVr, and essentially indistinguishable from the Lindhard case plotted.