Abstract
Light, MeV-scale dark matter (DM) is an exciting DM candidate that is undetectable by current experiments. A germanium (Ge) detector utilizing internal charge amplification for the charge carriers created by the ionization of impurities is a promising new technology with experimental sensitivity for detecting MeV-scale DM. We analyze the physics mechanisms of the signal formation, charge creation, charge internal amplification, and the projected sensitivity for directly detecting MeV-scale DM particles. We present a design for a novel Ge detector at helium temperature (sim 4 K) enabling ionization of impurities from DM impacts. With large localized E-fields, the ionized excitations can be accelerated to kinetic energies larger than the Ge bandgap at which point they can create additional electron–hole pairs, producing intrinsic amplification to achieve an ultra-low energy threshold of sim 0.1 eV for detecting low-mass DM particles in the MeV scale. Correspondingly, such a Ge detector with 1 kg-year exposure will have high sensitivity to a DM-nucleon cross section of sim 5 times 10^{-45} cm^{2} at a DM mass of sim 10 MeV/c^{2} and a DM-electron cross section of sim 5 times 10^{-46} cm^{2} at a DM mass of sim 1 MeV/c^2.
Highlights
We describe a Ge detector utilizing internal amplification of charge carriers created by the ionization of impurities below
Since 2010, we have developed methods [62,63,64,65] to improve the quality of large size Ge crystals, demonstrating our ability to control the parameters for growth of low-dislocation (3000– 7000 etch pits/cm2), large diameter (∼ 12 cm), and highpurity Ge single crystals (∼ 1010/cm3) for fabricating into detectors
To fabricate a Ge detector with internal amplification, one must: (1) use a Ge crystal that guarantees a uniform distribution of impurities to provide a homogeneous electric field near the anode; (2) create a wide shallow junction layer under the strips so that the electric field near the strips is defined by junction dimensions; and (3) guarantee reliable cooling of the crystal, since the critical electric field and amplification factor depend on the free path of charge carriers, which in turn depend on the temperature
Summary
Many experiments have conducted searches for WIMPs using various targets [6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25]. Current direct detection experiments search for nuclear recoils with the lowest accessible nuclear recoil energy being around 1 keV [7,8,17,20] This corresponds to DM with masses greater than 6 GeV/c2. We describe the design of a novel Ge detector that develops ionization amplification technology for Ge in which very large localized E-fields are used to accelerate ionized excitations produced by particle interaction to kinetic energies larger than the Ge bandgap at which point they can create additional electron–hole (e–h) pairs, producing internal amplification This amplified charge signal could be readout with standard high impedance JFET or HEMT [41] based charge amplifiers. Purposeful doping of the Ge could lower the ionization threshold by ∼ × 10 (∼ 0.1 eV), making the detector sensitive to 100 keV DM via electronic recoils
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