Mark Amman

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...

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.

DETECTION AND IMAGING OF THE CRAB NEBULA WITH THE NUCLEAR COMPTON TELESCOPE

The Nuclear Compton Telescope (NCT) is a balloon-borne Compton telescope designed for the study of astrophysical sources in the soft gamma-ray regime (200 keV–20 MeV). NCT’s 10 high-purity germanium crossedstrip detectors measure the deposited energies and three-dimensional positions of gamma-ray interactions in the sensitive volume, and this information is used to restrict the initial photon to a circle on the sky using the Compton scatter technique. Thus NCT is able to perform spectroscopy, imaging, and polarization analysis on soft gamma-ray sources. NCT is one of the next generation of Compton telescopes—the so-called compact Compton telescopes (CCTs)—which can achieve effective areas comparable to the Imaging Compton Telescope’s with an instrument that is a fraction of the size. The Crab Nebula was the primary target for the second flight of the NCT instrument, which occurred on 2009 May 17 and 18 in Fort Sumner, New Mexico. Analysis of 29.3 ks of data from the flight reveals an image of the Crab at a significance of 4σ . This is the first reported detection of an astrophysical source by a CCT.

Coplanar-grid detector with single-electrode readout

The coplanar-grid technique provides substantial spectral performance improvement over that of conventional detector designs and electronics when applied to gamma-ray detectors based on compound semiconductors. The technique realizes this improvement by measuring the difference between the induced charge signals from two interdigitated coplanar-grid electrodes. By adjusting the relative gain between the two grid signals prior to subtraction, the difference signal can be made less sensitive to the poor carrier transport properties of the detector material and thus improve the spectral response of the detector. In this paper, we discuss a variation of the coplanar-grid method in which the signal from only one grid electrode is read out. The signal response is optimized by changing the relative areas of the two grid electrodes and the bias applied across the detector. In this scheme, only one preamplifier is needed and signal subtraction is not necessary. This eliminates the electronic noise contribution from the additional preamplifier used in the normal coplanar-grid implementation, and conventional single- amplifier detector electronics can be used. Experimental results using CdZnTe detectors are presented.

Spectral Response of THM Grown CdZnTe Crystals

The spectral response of several crystals grown by the Traveling Heater Method (THM) were investigated. An energy resolution of 0.98% for a Pseudo Frisch-Grid of 4 times 4 times 9 mm3 and 2.1% FWHM for a coplanar-grid of size 11 times 11 times 5 mm3 were measured using 137Cs-662 keV. In addition a 4% FWHM at 122 keV has also been measured on 20 times 20 X 5 mm3 monolithic pixellated devices. The material shows great potential toward producing large-volume detectors with spectral performance that meets the requirement for high-resolution gamma-ray spectroscopy.

The Gamma-Ray Imager/Polarimeter for Solar Flares (GRIPS)

The balloon-borne Gamma-Ray Imager/Polarimeter for Solar flares (GRIPS) instrument will provide a near-optimal combination of high-resolution imaging, spectroscopy, and polarimetry of solar-flare gamma-ray/hard X-ray emissions from ~20 keV to >~10 MeV. GRIPS will address questions raised by recent solar flare observations regarding particle acceleration and energy release, such as: What causes the spatial separation between energetic electrons producing hard X-rays and energetic ions producing gamma-ray lines? How anisotropic are the relativistic electrons, and why can they dominate in the corona? How do the compositions of accelerated and ambient material vary with space and time, and why? The spectrometer/polarimeter consists of sixteen 3D position-sensitive germanium detectors (3D-GeDs), where each energy deposition is individually recorded with an energy resolution of a few keV FWHM and a spatial resolution of <0.1 mm3. Imaging is accomplished by a single multi-pitch rotating modulator (MPRM), a 2.5-cm thick tungstenalloy slit/slat grid with pitches that range quasi-continuously from 1 to 13 mm. The MPRM is situated 8 meters from the spectrometer to provide excellent image quality and unparalleled angular resolution at gamma-ray energies (12.5 arcsec FWHM), sufficient to separate 2.2 MeV footpoint sources for almost all flares. Polarimetry is accomplished by analyzing the anisotropy of reconstructed Compton scattering in the 3D-GeDs (i.e., as an active scatterer), with an estimated minimum detectable polarization of a few percent at 150–650 keV in an X-class flare. GRIPS is scheduled for a continental-US engineering test flight in fall 2013, followed by long or ultra-long duration balloon flights in Antarctica.

The Majorana Project

Building a Ovββ experiment with the ability to probe neutrino mass in the inverted hierarchy region requires the combination of a large detector mass sensitive to Ovββ, on the order of 1-tonne, and unprecedented background levels, on the order of or less than 1 count per year in the Ovβ β signal region. The MAJORANA Collaboration proposes a design based on using high-purity enriched 76Ge crystals deployed in ultralow background electroformed Cu cryostats and using modern analysis techniques that should be capable of reaching the required sensitivity while also being scalable to a 1-tonne size. To demonstrate feasibility, the collaboration plans to construct a prototype system, the MAJORANA DEMONSTRATOR, consisting of 30 kg of 86% enriched 76Ge detectors and 30 kg of natural or isotope-76-depleted Ge detectors. We plan to deploy and evaluate two different Ge detector technologies, one based on a p-type configuration and the other on n-type.

Noise in CdZnTe detectors

Noise in CdZnTe devices with different electrode configurations was investigated. Measurements on devices with guard-ring electrode structures showed that surface leakage current does not produce any significant noise. The parallel white noise component of the devices appeared to be generated by the bulk current alone, even though the surface current was substantially higher. This implies that reducing the surface leakage current of a CdZnTe detector may not necessarily result in a significant improvement in noise performance. The noise generated by the bulk current is also observed to be below full shot noise. This partial suppression of shot noise may be the result of Coulomb interaction between carriers or carrier trapping.

Coplanar-grid CdZnTe detector with three-dimensional position sensitivity

A 3-dimensional position-sensitive coplanar-grid detector design for use with compound semiconductors is described. This detector design maintains the advantage of a coplanar-grid detector in which good energy resolution can be obtained from materials with poor charge transport. Position readout in two dimensions is accomplished using proximity-sensing electrodes adjacent to the electron-collecting grid electrode of the detector. Additionally, depth information is obtained by taking the ratio of the amplitudes of the collecting grid signal and the cathode signal. Experimental results from a prototype CdZnTe detector are presented.