Paul N. Luke

Unipolar charge sensing with coplanar electrodes-application to semiconductor detectors

A novel method to perform preferential sensing of single-polarity charge carriers in ionization detectors is presented. It achieves the same function as Frisch grids commonly employed in gas ion chambers but uses a coplanar electrode configuration suitable for semiconductor detectors. Through the use of this method, good energy resolution can be obtained from room-temperature compound semiconductor detectors despite their poor hole-collection characteristics. Experiments using a CdZnTe detector demonstrate the effectiveness of this technique. Schemes to correct for electron trapping and to obtain position information are also described. >

Single‐polarity charge sensing in ionization detectors using coplanar electrodes

A new method to preferentially sense the collection of single‐polarity charge carriers in ionization detectors is presented. It uses coplanar electrodes to closely emulate the function of Frisch grids commonly employed in gas and liquid ionization detectors. The coplanar electrode structure allows for easy implementation on semiconductor detectors. This method can be used to obtain good energy resolution from detectors in which only one polarity type of carriers is efficiently collected. Experimental results using a CdZnTe detector demonstrate the effectiveness of this method.

Voltage-assisted calorimetric ionization detector

A new approach to ionizing radiation detection is proposed. The amount of ionization produced in a detector medium is measured by the heat generated as the charged carriers are drifted across the device under an applied voltage. The amount of energy generated can be orders of magnitude larger than that deposited by the radiation itself. A dramatic increase in detector mass can be achieved compared to simple calorimetric particle detectors for equivalent energy thresholds. It is possible to obtain a sensitivity level sufficient for single‐carrier detection. The principle of operation has been demonstrated with an experimental device operated at a temperature of 1.8 K, and improved performance is expected at lower temperatures.

The MAJORANA DEMONSTRATOR Neutrinoless Double-Beta Decay Experiment

The MAJORANA DEMONSTRATOR will search for the neutrinoless double-beta decay of the isotope Ge with a mixed array of enriched and natural germanium detectors. The observation of this rare decay would indicate that the neutrino is its own antiparticle, demonstrate that lepton number is not conserved, and provide information on the absolute mass scale of the neutrino. The DEMONSTRATOR is being assembled at the 4850-foot level of the Sanford Underground Research Facility in Lead, South Dakota. The array will be situated in a low-background environment and surrounded by passive and active shielding. Here we describe the science goals of the DEMONSTRATOR and the details of its design.

Characterization of large cadmium zinc telluride crystals grown by traveling heater method

The focus of this paper is to evaluate thick, 20×20×10 and 10×10×10mm3, cadmium zinc telluride (CZT), Cd0.9Zn0.1Te, crystals grown using the traveling heater method (THM). The phenomenal spectral performance and small size and low concentration of Te inclusions/precipitates of these crystals indicate that the THM is suitable for the mass production of CZT radiation detectors that can be used in a variety of applications. Our result also proves that with careful material selection using IR and high-quality fabrication processes, the theoretical energy resolution limit can be achieved.

Electron trapping nonuniformity in high-pressure-Bridgman-grown CdZnTe

Gamma-ray spectroscopy is a valuable tool of science and technology. Many applications for this tool are in need of a detector technology capable of achieving excellent energy resolution and efficient detection while operating at room temperature. Detectors based on the material cadmium zinc telluride (CdZnTe) could potentially meet this need if certain material deficiencies are addressed. The coplanar-grid as well as other electron-only detection techniques are effective in overcoming some of the material problems of CdZnTe and, consequently, have led to efficient gamma-ray detectors with good energy resolution while operating at room temperature. At the present time, the performance of these detectors is limited mainly by the degree of uniformity in electron generation and transport. Despite recent progress in the growth of CdZnTe material, small variations in these properties remain a barrier to the widespread success of such detectors. Alpha-particle response characterization of CdZnTe crystals fabric...

Performance of CdZnTe coplanar-grid gamma-ray detectors

CdZnTe crystals grown using the high-pressure Bridgman method exhibit many properties that are desirable for radiation detector fabrication, such as high resistivity, stable operation, relative ease of processing, and the availability of large volume crystals. However, as is common with other compound semi-conductor materials, currently available CdZnTe crystals have poor charge transport characteristics. This seriously the spectral performance of detectors, especially in gamma-ray detection. The coplanar-grid detection technique was recently developed to address such charge collection problems. This technique was first demonstrated using a 5 mm cube CdZnTe detector, and a dramatic improvement in spectral response has been achieved. These early results verified the effectiveness of this technique and suggested that large-volume gamma-ray detectors with high energy resolution can be realized. To further the development of such detectors, it is important to understand the various factors that affect detector performance. The purpose of this paper is to examine the effects of material properties on the spectral performance of CdZnTe coplanar-grid detectors. Theoretical spectral response is to show the level of performance that can be achieved given the typical carrier mobility-lifetime ({mu}{tau}) properties of present-day materials. Nonuniformity in the charge transport properties of the material, which could limit the energy resolution of the detectors, has been studied experimentally and some of the results are presented here.

Evaluation of THM-Grown CdZnTe Material for Large-Volume Gamma-Ray Detector Applications

Over 25 1-cm3 CdZnTe crystals produced using the Traveling Heater Method at Redlen Technologies have been characterized. The charge carrier mobility and lifetime, and charge carrier transport uniformity of each crystal were measured using alpha particles. Some of the crystals were made into coplanar-grid detectors and their performance characterized using 662 keV gamma rays. The average electron mobility-lifetime product for these crystals was found to be a factor of about five times greater than that measured from crystals obtained over the last decade from two other commercial crystal growers. The coplanar-grid detectors produced from the material typically achieved an energy resolution at 662 keV near 2% FWHM when operated at room temperature. This is comparable to the best coplanar-grid detectors commercially produced today.

Amorphous Ge bipolar blocking contacts on Ge detectors

The authors report on the performance of high-purity Ge radiation detectors with amorphous Ge (a-Ge) contacts fabricated using RF sputtering techniques. Electrical contacts formed using sputtered a-Ge films on high-purity Ge crystals were found to exhibit good blocking behavior in both polarities with low leakage currents. The a-Ge contacts have thin dead layers associated with them and can be used in place of lithium-diffused, ion-implanted or Schottky barrier contacts on Ge radiation detectors. Multielectrode detectors can be fabricated with very simple processing steps using these contacts.<<ETX>>

Low capacitance large volume shaped-field germanium detector

A large-volume (150 cm/sup 3/) germanium detector with a full-depletion capacitance of only approximately=1 pf has been fabricated. The effect of impurity space charge was utilized to obtain an appropriate electric field distribution in the detector so that carriers are collected on a small-area electrode. Detectors based on this principle are capable of very-low-noise operation and have immediate applications in experiments for the direct detection of dark matter. Detector pulse shapes and carrier-trapping effects were also examined for possible applications involving higher energy radiations. >