S. M. Schindler

Charge Loss between contacts of CdZnTe pixel detectors

The surface of Cd_(1−x)Zn_xTe (CZT) material has high resistivity but is not a perfect dielectric. Even a small surface conductivity can affect the electric field distribution, and therefore, the charge collection efficiency of a CZT pixel detector. The paper describes studies of this phenomenon for several contact configurations made on a single CZT detector. We have determined the maximum inter-contact separation at which the surface inter-pixel charge loss can be neglected.

COSMIC RAY ORIGIN IN OB ASSOCIATIONS AND PREFERENTIAL ACCELERATION OF REFRACTORY ELEMENTS: EVIDENCE FROM ABUNDANCES OF ELEMENTS 26Fe THROUGH 34Se

We report abundances of elements from _(26)Fe to _(34)Se in the cosmic radiation measured during fifty days of exposure of the Trans-Iron Galactic Element Recorder (TIGER) balloon-borne instrument. These observations add support to the concept that the bulk of cosmic ray acceleration takes place in OB associations, and they further support cosmic ray acceleration models in which elements present in interstellar grains are accelerated preferentially compared with those found in interstellar gas.

Development of the High-Energy Focusing Telescope (HEFT) balloon experiment

The High Energy Focusing Telescope (HEFT) is a balloon-borne experiment employing focusing optics in the hard X-ray/soft gamma-ray band (20 - 100 keV) for sensitive observations of astrophysical sources. The primary scientific objectives include imaging and spectroscopy of 44Ti emission in young supernova remnants, sensitive hard X-ray observations of obscured Active Galactic Nuclei, and spectroscopic observations of accreting high-magnetic field pulsars. Over the last four years, we have developed grazing-incidence depth-graded multilayer optics and high spectral resolution solid stat Cadmium Zinc Telluride pixel detectors in order to assemble a balloon-borne experiment with sensitivity and imaging capability superior to previous satellite missions operating in this band. In this paper, we describe the instrument design, and present recent laboratory demonstrations of the optics and detector technologies.

Numerical modelling of charge-sharing in CdZnTe pixel detectors

In this paper, we describe our study of charge-sharing events in CdZnTe detectors being developed for the HEFT telescope. We specify the detector design, and discuss an experiment we have performed to investigate charge sharing between pixels. We have also developed a numerical model to study the charge transport in the detector. It emulates the physical processes of charge transport within the CdZnTe crystal, especially the process of drift. We discuss this numerical model of the detector in detail. With our numerical model, we are able to reproduce the general features of the charge-sharing events. We have found that the amount of charge loss is very sensitive to the surface /spl mu//spl tau/, the product of charge mobility and trapping time, of CdZnTe; we present estimates of (/spl mu//spl tau/)/sub surface/ from our model. Further work will focus on more detailed analysis of diffusion, in order to gain a complete understanding of these charge-sharing events in CdZnTe pixel detectors.

Measurement of the Abundance of Radioactive Be-10 and Other Light Isotopes in Cosmic Radiation Up to 2 GeV /Nucleon with the Balloon-Borne Instrument Isomax

The Isotope Magnet Experiment (ISOMAX), a balloon-borne superconducting magnet spectrometer, was designed to measure the isotopic composition of the light isotopes (3 ≤ Z ≤ 8) of cosmic radiation up to 4 GeV nucleon^(-1) with a mass resolution of better than 0.25 amu by using the velocity versus rigidity technique. To achieve this stringent mass resolution, ISOMAX was composed of three major detector systems: a magnetic rigidity spectrometer with a precision drift chamber tracker in conjunction with a three-layer time-of-flight system, and two silica-aerogel Cerenkov counters for velocity determination. A special emphasis of the ISOMAX program was the accurate measurement of radioactive ^(10)Be with respect to its stable neighbor isotope ^9Be, which provides important constraints on the age of cosmic rays in the Galaxy. ISOMAX had its first balloon flight on 1998 August 4–5 from Lynn Lake, Manitoba, Canada. Thirteen hours of data were recorded during this flight at a residual atmosphere of less than 5 g cm^(-2). The isotopic ratio at the top of the atmosphere for 10Be/9Be was measured to be 0:195 ± 0:036 (statistical) ± 0:039 (systematic) between 0.26 and 1.03 GeV nucleon^(-1) and 0:317 ± 0:109 (statistical) ± 0:042 (systematic) between 1.13 and 2.03 GeV nucleon^(-1). This is the first measurement of its kind above 1 GeV nucleon^(-1). ISOMAX results tend to be higher than predictions from current propagation models. In addition to the beryllium results, we report the isotopic ratios of neighboring lithium and boron in the energy range of the time-of-flight system (up to ~1 GeV nucleon^(-1)). The lithium and boron ratios agree well with existing data and model predictions at similar energies.

Effects of bulk and surface conductivity on the performance of CdZnTe pixel detectors

We studied the effects of bulk and surface conductivity on the performance of high-resistivity CdZnTe (CZT) pixel detectors with Pt contacts. We emphasize the difference in mechanisms of the bulk and surface conductivity as indicated by their different temperature behaviors. In addition, the existence of a thin (10-100 /spl Aring/) oxide layer on the surface of CZT, formed during the fabrication process, affects both bulk and surface leakage currents. We demonstrate that the measured I-V dependencies of bulk current can be explained by considering the CZT detector as a metal-semiconductor-metal system with two back-to-back Schottky-barrier contacts. The high-surface leakage current is apparently due to the presence of a low-resistivity surface layer that has characteristics that differ considerably from those of the bulk material. This surface layer has a profound effect on the charge-collection efficiency in detectors with multicontact geometry; some fraction of the electric field lines that originated on the cathode intersects the surface areas between the pixel contacts where the charge produced by an ionizing particle gets trapped. To overcome this effect, we place a grid of thin electrodes between the pixel contacts. When the grid is negatively biased, the strong electric field in the gaps between the pixels forces the electrons landing on the surface to move toward the contacts, preventing the charge loss. We have investigated these effects by using CZT pixel detectors indium bump-bonded to a custom-built VLSI readout chip.

The Cosmic-Ray 3He/4He Ratio from 200 MeV per Nucleon−1 to 3.7 GeV per Nucleon−1

The abundances of cosmic-ray helium isotopes between 0.2 and 3.7 GeV nucleon^(-1) were measured by nthe Isotope Matter Antimatter Experiment (IMAX) during a flight from Lynn Lake, Manitoba, Canada non 1992 July 16-17. The IMAX balloon-borne magnetic spectrometer realized a direct measurement of nthe charge, the velocity, and the rigidity of cosmic rays using plastic scintillators, a high-resolution time-of-flight system, and two silica-aerogel Cerenkov counters in conjunction with a drift chamber/multiwire proportional chamber tracking system. About 75,000 helium isotopes are identified by their mass using the velocity versus magnetic rigidity technique. The measured ^3He/^4He ratios are corrected to the top of the atmosphere, and a comparison with previous data is given. The observed isotopic composition is found to be generally consistent with the predictions of a standard leaky box model of cosmic-ray transport in the Galaxy.

The Effect of Cathode Bias (Field Effect) on the Surface Leakage Current of CdZnTe Detectors

Surface resistivity is an important parameter of multi-electrode CZT detectors such as coplanar-grid, strip, or pixel detectors. Low surface resistivity results in a high leakage current and affects the charge collection efficiency in the areas near contacts. Thus, it is always desirable to have the surface resistivity of the detector as high as possible. In the past the most significant efforts were concentrated to develop passivation techniques for CZT detectors. However, as we found, the field-effect caused by a bias applied on the cathode can significantly reduce the surface resistivity even though the detector surface was carefully passivated. In this paper we illustrate that the field-effect is a common feature of the nCZT multi-electrode detectors, and discuss how to take advantage of this effect to improve the surface resistivity of CZT detectors.

AN IMAGING OBSERVATION OF SN 1987A AT GAMMA-RAY ENERGIES

The Caltech imaging γ-ray telescope was launched by balloon from Alice Springs, NT, Australia for observations nof SN 1987A during the period 1987 November 18.60-18.87 UT. The preliminary results presented nhere are derived from 8200 s of instrument live time on the supernova and 2500 s on the Crab Nebula and npulsar at a float altitude of 36 km. We have obtained the first images of the SN 1987A region at γ-ray energies nconfirming that the bulk of the γ-ray emission comes from the supernova and not from LMC X-1. We ncompare our flux measurements to recent predictions concerning the distribution of ^(56)Co in the supernova nejecta and find the data to be consistent with models invoking moderate mixing of core material into the nenvelope.

Hard x-ray optics for the HEFT balloon-borne payload: prototype design and status

We report on the current status and performance of prototype hard x-ray optics we are producing for use on the high energy focusing telescope (HEFT) experiment. The baseline substrates are thermally formed glass mirrors that are overcoated with multilayers to provide good performance throughout the 20-80 keV bandpass. Progress made in the thermal forming process as well as in the multilayer performance has allowed production of optics that meet or exceed all HEFT requirements. We present metrology on the substrates and result from x-ray characterization. A novel mounting scheme for the individual telescope shells is currently being tested. If successful the mounting technique will produce a monolithic, extremely stiff and robust optic.