Stephen M. Schindler

A Measurement of the Cosmic-Ray Antiproton Flux and a Search for Antihelium

A balloon-borne instrument has measured the cosmic-ray antiproton flux between 130 and 320 MeV and searched for antihelium between 130 and 370 MeV per nuclear. These particles were selected from the background of normal-matter cosmic rays by combining a selective trigger with a detailed spark chamber visualization of each recorded event. Antiprotons are identified by their characteristic annihilation radiation. Residue from background processes meeting the selection criteria is small. The observed 14 antiprotons yield a measured differential flux of 1.7±0.5X 10^(-4) antiprotons m^(-2) sr(-1) s^(-1)i Mev^(-1) at the top of the atmosphere. The corresponding antiproton/pro-ton ratio is 2.2±0.6X10^(-4), only slightly smaller than the ratio observed by other experiments at higher energies. Thus the antiprotons have a spectral shape similar to the protons, at least down to about 100 MeV. The expected flux of these particles can be calculated under the assumption that they were created by collisions of high-energy cosmic rays with the interstellar gas. Calculations using the standard leaky box model for propagation in the Galaxy predict a flux two orders of magnitude smaller than that observed. A small low-energy flux is predicted due to a kinematic suppression of the production of low-energy antiprotons. The discrepancy between calculations and experiment may be evidence that cosmic-ray protons have passed through substantially more than 5 g cm^(-2) of material during their lifetime. In addition, the combined results from this experiment and previous ones may be evidence for stochastic, energy-changing processes in interstellar space which act upon the secondary antiprotons after their creation. The search for cosmic-ray antihelium sets a 95% confidence level upper limit on the He /He ratio of 2.2 X 10^(-5).

The Absolute Flux of Protons and Helium at the Top of the Atmosphere Using IMAX

The cosmic-ray proton and helium spectra from 0.2 GeV nucleon^(-1) to about 200 GeV nucleon^(-1) have been measured with the balloon-borne experiment Isotope Matter-Antimatter Experiment (IMAX) launched from Lynn Lake, Manitoba, Canada, in 1992. IMAX was designed to search for antiprotons and light isotopes using a superconducting magnet spectrometer together with scintillators, a time-of-flight system, and Cherenkov detectors. Using redundant detectors, an extensive examination of the instrument efficiency was carried out. We present here the absolute spectra of protons and helium corrected to the top of the atmosphere and to interstellar space. If demodulated with a solar modulation parameter of Φ = 750 MV, the measured interstellar spectra between 20 and 200 GV can be represented by a power law in rigidity, with (1.42 ± 0.21) × 10^4R^(-2.71±0.04) (m^2 GV s sr)^(-1) for protons and (3.15 ± 1.03) × 10^3R^(-2.79±0.08) (m^2 GV s sr)^(-1) for helium.

Properties of Pt Schottky Type Contacts on High-Resistivity CdZnTe Detectors

In this paper, we present studies of the I- V characteristics of CdZnTe (CZT) detectors with Pt contacts fabricated from high-resistivity single crystals grown by the high-pressure Bridgman process. We have analyzed the experimental I- V curves using a model that approximates the CZT detector as a system consisting of a reversed Schottky contact, in series with the bulk resistance. Least-square fit to the experimental data yields 0.78- 0.79 eV for the Pt-CZT Schottky barrier height, and <20V for the voltage required to deplete a 2mm thick CZT detector. We demonstrate that, at high bias, the thermionic current over the Schottky barrier, the height of which is reduced due to an interfacial layer between the contact and CZT material, controls the leakage current of the detectors. In many cases, the dark current is not determined by the resistivity of the bulk material, but rather the properties of the contacts; namely, by the interfacial layer between the contact and CZT material.

Coded-aperture imaging of the Galactic center region at gamma-ray energies

The first coded-aperture images of the Galactic center region at energies above 30 keV have revealed two strong y-ray sources. One source has been identified with the X-ray source 1E 1740.7-2942, located 0°.8 away from the nucleus. If this source is at the distance of the Galactic center, it is one of the most luminous objects in the galaxy at energies from 35 to 200 keV. The second source is consistent in location with the X-ray source GX 354 + 0 (MXB 1728- 34). In addition, y-ray flux from the location of GX 1 + 4 was marginally detected at a level consistent with other post-1980 measurements. No significant hard X-ray or γ-ray flux was detected from the direction of the Galactic nucleus (Sgr A*), or from the direction of the recently discovered γ-ray source GRS 1758-258.

Characterization of the HEFT CdZnTe pixel detectors

We have developed large format CdZnTe pixel detectors optimized for astrophysical applications. The detectors, designed for the High Energy Focusing Telescope (HEFT) balloon experiment, each consists of an array of 24x44 pixels, on a 498 μm pitch. Each of the anode segments on a CdZnTe sensor is bonded to a custom, low-noise application-specific integrated circuit (ASIC)optimized to achieve low threshold and good energy resolution. We have studied detectors fabricated by two different bonding methods and corresponding anode plane designs---the first detector has a steering electrode grid, and is bonded to the ASIC with indium bumps; the second detector has no grid but a narrower gap between anode contacts, and is bonded to the ASIC with conductive epoxy bumps and gold stud bumps in series. In this paper, we present results from detailed X-ray testing of the HEFT pixel detectors. This includes measurements of the energy resolution for both single-pixel and split-pixel events, and characterization of the effects of charge trapping, electrode biases and temperature on the spectral performance. Detectors from the two bonding methods are contrasted.

GRIP-2: A sensitive balloon-borne imaging gamma-ray telescope

This paper describes the balloon-borne coded-aperture telescope, GRIP-2, which is designed for imaging observations of astrophysical sources in the 30 keV-2 MeV band. GRIP-2 employs the rotating hexagonal coded-mask imaging technique pioneered by the Caltech GRIP-1 [Cook et al., IEEE Trans. Nucl. Sci. NS-31 (1984) 771] instrument. GRIP-2 has an angular resolution of 33′ over a 15° FWHM field of view. A large (3850 cm^2 geometric area) NaI(Tl)/CsI(Na) phoswich detector provides excellent source detection sensitivity. We describe here the instrument design, performance, and preliminary results from the first two balloon flights.

First test results from a high-resolution CdZnTe pixel detector with VLSI readout

We are developing a CdZnTe pixel detector with a custom low- noise analog VLSI readout for use in the High-Energy Focusing Telescope balloon experiment, as well as for future space astronomy applications. The goal of the program is to achieve good energy resolution (< 1 keV FWHM at 60 keV) and low threshold in a sensor with approximately 500 micrometers pixels. We have fabricated several prototype detector assemblies with 2 mm thick, 680 by 650 micrometers pitch CdZnTe pixel sensors indium bump bonded a VLSI readout chip developed at Caltech. Each readout circuit in the 8 X 8 prototype is matched to the detector pixel size, and contains a preamplifier, shaping amplifiers, and a peak stretcher/discriminator. In the first 8 X 8 prototype, we have demonstrated the low-noise preamplifier by routing the output signals off-chip for shaping and pulse-height analysis. Pulse height spectra obtained using a 241Am source, collimated to illuminate a single pixel, show excellent energy resolution of 1.1 keV FWHM for the 60 keV line at room temperature. Line profiles are approximately Gaussian and dominated by electronic noise, however a small low energy tail is evident for the 60 keV line. We obtained slightly improved resolution of 0.9 keV FWHM at 60 keV by cooling the detector to 5 degree(s)C, near the expected balloon- flight operating temperature. Pulse height spectra obtained with the collimated source positioned between pixels show the effect of signal sharing for events occurring near the boundary. We are able to model the observed spectra using a Monte-Carlo simulation that includes the effects of photon interaction, charge transport and diffusion, pixel and collimator geometry, and electronic noise. By using the model to simulate the detector response to uncollimated radiation (including the effect of finite trigger threshold for reconstruction of the total energy of multi-pixel events), we find the energy resolution to be degraded by only 10% for full-face illumination, compared to the collimated case. The small value of the degradation is due directly to the low readout noise and amplifier threshold.

An Observation of the Galactic Center Hard X-Ray Source, 1E 1740.7-2942, with the Caltech Coded-Aperture Telescope

The Galactic center region hard X-ray source 1E 1740.7-2942 has been observed with the Caltech Gamma-Ray Imaging Payload (GRIP) from Alice Springs, Australia, on 1988 April 12 and on 1989 April 3 and 4. We report here results from the 1989 measurements based on 14 hr of observation of the Galactic center region. The observations showed lE 1740.7 -2942 to be in its normal state, having a spectrum between 35 and 200 keV characterized by a power law with an exponent of -2.2 ± 0.3 and flux at 100 keV of (7.0 ± 0.7) x 10^(-s) cm^(-2) s^(-1) kev^(-1). No flux was detected above 200 keV. A search for time variability in the spectrum of lE 1740.7-2942 on one hour time scales showed no evidence for variability.

Recent results of Gamma-ray imaging observations of the Galactic center and Crab/A0535+26 regions

Imaging observations at hard X-ray and γ-ray energies of the Galactic center and Crab/A0535+26 regions are presented. The measurements were made in a series of high altitude balloon flights of a coded aperture γ-ray telescope developed by Caltech. The first coded aperture images of the Galactic center region above 35 keV reveal a strong γ-ray source located 0.7 ± 0.1° away from the Galactic nucleus, consistent with the known X-ray source 1E1740.7-2942. In addition a second source consistent in position with GX354+0 has been observed. Flux estimates and upper limits from two other known sources are also discussed. Coded aperture observations of the Crab and A0535+26 are presented, demonstrating the ability of coded aperture instruments to observe multiple time variable sources simultaneously.

Recent Cosmic-Ray Antiproton Measurements and Astrophysical Implications

Cosmic-ray antiprotons have been detected by a new balloon-borne experiment which covers the energy range between 130 and 320 MeV. Fourteen detected events yield a measured flux of 1.7±0.5 x 10^(-4) antiprotons m^(-2) sr^(-1) s(-1) MeV^(-1). The corresponding antiproton/proton ratio is 2.2± 0.6 x 10^(-4), only slightly smaller than the ratio observed by other experiments at higher energies. The measured flux is significantly larger than predicted, and some cosmic-ray models which could explain this result are discussed.