N. Wang

A 20 mK temperature sensor

The authors report tests of a 20-mK temperature sensor made of neutron-transmutation-doped (NTD) germanium, which they have developed for use as a phonon detector in a dark matter search. Current-voltage, responsivity, and AC measurements have been made, and the minimum detectable energy has been estimated. They have found that NTD germanium thermistors around 20 mK have resistances that are a strong function of temperature and have sufficient sensitivity to eventually reach a base line RMS energy fluctuation of 6 eV at 25 mK. The authors consider that further work is needed to understand the extreme sensitivity of the thermistors to bias power. >

Performance of a 60 gram cryogenic germanium detector

A 60-g particle detector which utilizes the ionization and the phonons produced by a particle interaction has been developed. Six NDT (neutron transmutation doped) Ge thermistors are attached to a pure germanium crystal which has implanted contacts for drifting charge. The authors have operated the detectors at 30 mK and have studied its response to irradiation by 18- and 60-keV photons from an /sup 241/Am source. An analysis of the resolution of the detector which addresses the noise of the front-end electronics and signals from extraneous sources such as microphonics is presented. The simultaneous measurement of phonons and ionization has been demonstrated to have a resolution of 4 keV. This joint detection should prove to be very valuable in distinguishing between candidate dark matter interaction events and competing background processes. >

A cryogenic detector with simultaneous phonon and ionization measurement for background rejection

We report on the performance of a 60 g Ge detector that measures both ionization and phonons at cryogenic (∼ 25 mK) temperatures. This simultaneous measurements is a powerful new tool in experiments where the primary signal is a nuclear recoil (e.g., WIMP dark matter) that must be distinguished from a background of photons or charged particles that produce electron recoils. We have measured nuclear recoils produced by neutrons from a 252Cf source, and can distinguish them from photons at energies as low as 2 keV in the ionization measurement. An important effect that can limit the efficiency of the background rejection technique is incomplete charge collection. We have tested our detector at ionization drift fields as low as 5 mV/cm and find that the phonon energy measured for each event depends on the amount of charge collected. From this we deduce details about charge trapping mechanisms.

Development of a cryogenic dark matter detector

The continuing development of a cryogenic dark matter detector is described. 60-keV gamma pulses with submicrosecond risetimes have been observed in neutron transmission doped (NTD) germanium thermistors, consistent with the hot electron model. The authors have also developed a eutectic bonding method for attaching thermistors to a larger crystal, and they discuss preliminary studies of the phonon transmission qualities of this technique. They report on the measurement of ionization created by particle interactions in a pure germanium crystal with applied bias fields as low as 1 V/cm. >

Simultaneous Detection of Phonons and Ionization in a 60 g Germanium Detector

We have demonstrated simultaneous high resolution measurement of ionization and phonons created by particle interactions in a 60g Ge crystal operated at 21mK[1]. The detector is a 4cm diameter and lcm thick disk of p-type germanium with net dopant concentration ≲ 2 1011cm−3. Phonons are detected with neutron transmutation doped (NTD) germanium sensors which are sensitive both to the temperature of the crystal and high energy phonons[2]. The ionization from the event is collected by means of a 0.5V bias applied across identical implanted (p+) ohmic contacts on both faces of the disk. The schematics of the electronics used are shown in Figure la. Signals are sensed using cold JFETs mounted in the cryostat. An AC biasing technique is used to avoid 1/f noise.

Phonon Lifetime and Collection in Germanium Particle Detectors at 20 mK

We are developing low temperature particle detectors in which the ionization and the phonons created by a particle interaction are simultaneously measured. We describe in an accompanying paper[1] such a detector which has a mass of 60g and is operated typically at 21 mK.

Simultaneous measurement of thermal and ionization signals in a 60 g cryogenic germanium detector

The authors present new results from a cryogenic detector instrumented to measure both the phonon and ionization signals in a 60 g Ge disk operated at 30 mK. Significant improvements in the electronics noise, threshold, and energy resolution of the device have been made. The baseline full width at half maximum (FWHM) corresponding to total system noise, is 1.55 keV in the phonon channel and 1.59 keV in the ionization channel. At 60 keV the FWHM is 1.9 keV and 1.7 keV in the phonon and ionization channels, respectively. A discrimination between electron recoils and nuclear recoils at low energy has been demonstrated.<<ETX>>