T.J. O'Neill

Development of the TIGRE Compton telescope for intermediate-energy gamma-ray astronomy

Gamma-ray observations in the low and medium energy range (0.1-100 MeV) with sufficiently sensitive telescopes will provide unique insights into many outstanding high-energy astrophysics questions. The University of California, Riverside (UCR) Tracking and Imaging Gamma-Ray Telescope (TIGRE) Compton gamma-ray telescope uses multilayers of silicon strip detectors to, for the first time, track the Compton electron and CsI(Tl)-photodiode detectors to measure the scattered photon energy. By combining the Compton telescopes inherent imaging capability with improved background discrimination, a larger field-of-view and improved spectral and spatial resolutions, a significant improvement in sensitivity over Compton Gamma-Ray Observatory (CGRO) and INTEGRAL can be achieved. The well-type calorimeter design also enhances the instrument as a gamma-ray polarimeter. The development and flight of a robust Compton telescope represents a unique opportunity to continue the momentum of recent discoveries in low and medium energy gamma-ray astrophysics with CGRO and an absolutely essential step to an extended satellite mission by 2010.

Compton recoil electron tracking with silicon strip detectors

The application of silicon strip detectors to Compton gamma-ray astronomy telescopes is described. The silicon Compton recoil telescope tracks Compton recoil electrons in silicon strip converters to provide a unique direction for Compton-scattered gamma rays above 1 MeV. With strip detectors of modest positional and energy resolutions of 1 mm full width at half maximum (FWHM) and 3% at 662 keV, respectively, true imaging can be achieved to provide an order of magnitude improvement in sensitivity to 1.6*10/sup -6/ gamma /cm/sup 2/-s at 2 MeV. The results of extensive Monte Carlo calculations of recoil electrons traversing multiple layers of 200- mu m silicon wafers are presented. Multiple Coulomb scattering of the recoil electron in the silicon wafer of the Compton interaction and the next adjacent wafer is the basic limitation to determining the electrons initial direction.<<ETX>>

Calibration and performance of the UCR double Compton gamma ray telescope

Results of the field calibration and performance of the UCR double Compton gamma-ray telescope are presented. The telescope is a balloon-borne instrument with an upper array of 16 plastic scintillator bars and a lower one of 16 NaI(Tl) bars. The telescope is sensitive to celestial gamma rays from 1 to 30 MeV. The data were collected on Feb. 14, 1988 prior to the launch in Alice Springs, Australia to observe SN 1987A. Radioactive sources were used to calibrate the energy deposits in the scintillators. Each bar was analyzed laterally using pulse height or timing to obtain the positions of the gamma ray interactions. Double scatter events from a /sup 24/Na source simulating a celestial source were studied to obtain the general performance of the telescope and to develop imaging techniques, later used with the flight data. An angular resolution of 11 degrees FWHM (full width at half maximum) and energy resolutions of 13% FWHM at 1.37 MeV and 10% FWHM at 2.75 MeV were found. The efficiency of the telescope is 3.5*10/sup -3/ at an energy of 1.37 MeV and zenith angle of 31 degrees . The magnetometer calibration gives the orientation of the detector with respect to the Earth to an accuracy of 0.5 degrees . >

Monte Carlo Simulation of a New Gamma Ray Telescope

A new Monte Carlo code has been written to simulate the response of the new University of California double scatter gamma ray telescope. This package of modular software routines, written in VAX FORTRAN 77 simulates the detection of 0.1 to 35 MeV gamma rays. The new telescope is flown from high altitude balloons to measure medium energy gamma radiation from astronomical sources. This paper presents (1) the basic physics methods in the code (2) and the predicted response functions of the telescope. Gamma ray processes include Compton scattering, pair production and photoelectric absorption in plastic scintillator, NaI(Tl) and aluminum. Electron transport processes include ionization energy loss, multiple scattering, production of bremsstrahlung photons and positron annihilation.

The TIGRE desktop prototype results for 511 and 900 keV gamma rays

A small desktop prototype of the Tracking and Imaging Gamma-Ray Experiment (TIGRE) has been assembled and tested at 511 keV and 900 keV. TIGRE was designed to observe cosmic gamma ray sources at energies of 0.3 to 100 MeV. Its major feature is its use of multi-layer silicon strip detectors to track Compton recoil electrons and positron-electron pairs. Our small prototype consists of 7 double sided silicon strip detectors 3.2 cm/spl times/3.2 cm/spl times/300 micron with 1 mm pitch in both the x and y directions. The direction and energy of the Compton scattered gamma ray is measured with small CsI(Tl) photodiode detectors. Knowing the energy and momentum of the scattered electron and scattered photon allows us to determine the incident direction uniquely. In the small prototype 36 CsI(Tl) crystals of 1 cm/spl times/1 cm/spl times/1.7 cm were used. Non-tracked events, those interacting in only a single silicon plane, can only be determined to within the Compton scatter ring. The silicon strips were calibrated using the 60 keV photons from Am/sup 241/ and the Landau peak obtained from a Sr/sup 90/ beta source. The energy resolution of the silicon was measured to be 8 keV (1/spl sigma/) at 60 keV and 7.8% FWHM for CsI at 900 keV. Total energy resolutions at 511 and 900 keV were measured to be 11% and 8.9% FWHM respectively. An important requirement of TIGRE will be its ability to separate the upward moving gamma rays produced by cosmic ray interactions in the atmosphere from the downward moving gamma rays. For tracked events this is done by defining a direction of motion (DOM) parameter for the electron by its energy deposition and multiple scattering in the silicon layers. Measurements at 511 and 900 keV show that the DOM parameter is correctly predicted at 70% and 75% for tracked events which constitute 9% and 20% of the data. Monte Carlo simulations show similar results and show the percentage increasing to 98% at 6 MeV in which nearly all of the events are tracked. >

Observations of Nuclear Reactors on Satellites with a Balloon-Borne Gamma-Ray Telescope

Gamma rays at energies of 0.3 to 8 megaelectron volts (MeV) were detected on 15 April 1988 from four nuclear-powered satellites including Cosmos 1900 and Cosmos 1932 as they flew over a double Compton gamma-ray telescope. The observations occurred as the telescope, flown from a balloon at an altitude of 35 kilometers from Alice Springs, Australia, searched for celestial gamma-ray sources. The four transient signals were detected in 30 hours of data. Their time profiles show maxima with durations of (21 � 1) and (27 � 1) seconds (half-width at half maximum) for the lower two satellites and (85 � 5) and (113 � 7) seconds for the remaining two. Their durations place the origin of the two shorter signals at orbital radii of 260+40-60 and 260 � 60 km above the earth and the two longer at 800+100-300 and 800+250-300 kilometers. Their luminosities for energies >0.3 MeV are then (6.1 � 1.5) x 1015, (3.9 � 1.0) x 1015, (1.10 � 0.28) x 1016, and (1.30 � 0.32) x 1016 photons per second. The imaging of the strongest signal indicates a southeastern direction passing nearly overhead. The energy spectrum can be fit to an exponential with index 2.4 � 1.4. These transient events add to the already large backgrounds for celestial gamma ray sources.

Atmospheric gamma rays at geomagnetic latitudes of −29° and +43°

We present results of atmospheric gamma ray measurements obtained during two balloon flights from Alice Springs, Australia (λ = −29°), and Fort Sumner, New Mexico, United States of America (λ = 43°) at geomagnetic cutoff rigidities of 8.5 GV and 4.3 GV, respectively. The fluxes, in the energy range of 1–15 MeV, are derived as functions of zenith angle, residual depth, and latitudinal rigidity. We find while the downward moving gamma ray flux at the float level (4.8 g cm−2) is not a strong function of rigidity the upward flux at λ = −29° is, on average, by factors of 2 to 4 lower than at λ = 43°. The energy spectra of the downward moving gamma rays at various altitudes are harder than the upward moving gamma rays. The spectral indices for both upward and downward fluxes at λ = −29° are lower than at λ = 43°.

Polarization measurements with an imaging gamma-ray telescope

The proposed Tracking and Imaging Gamma-Ray Experiment (TIGRE), operating in the 0.3–100 MeV energy interval, will be an efficient polarimeter with a modulation factor of ∼50% at 0.5 MeV. The polarization detection parameters of TIGRE were estimated using a Monte Carlo simulation modified to include the polarization dependence of the Klein-Nishina formula. Using Compton scattering of low energy photons and approximately 3π acceptance angle after scattering, TIGRE will be able to measure strong sources with 20% fractional polarization at 3 σ significance in a typical balloon-borne exposure and ≤5% during a 4-week satellite observation.

Tracking and imaging gamma-ray experiment (TIGRE) for 300-keV to 100-MeV gamma-ray astronomy

The Tracking and Imaging Gamma-Ray Experiment (TIGRE) uses multilayers of silicon strip detectors both as a gamma-ray converter and to track Compton recoil electrons and positron-electron pairs. The silicon strip detectors also measure the energy losses of these particles. For Compton events, the direction and energy of the Compton scattered gamma ray are measured with arrays of small CsI(TI)-photodiode detectors so that an unique direction and energy can be found for each incident gamma ray. The incident photon direction for pair events is found from the initial pair particle directions. TIGRE is the first Compton telescope with a direct imaging capability. With a large (pi) -steradian field-of-view, it is sensitive to gamma rays from 0.3 to 100 MeV with a typical energy resolution of 3% (FWHM) and a 1-(sigma) angular resolution of 40 arc-minutes at 2 MeV. A small balloon prototype instrument is being constructed that has a high absolute detection efficiency of 8% over the full energy range and a sensitivity of 10 milliCrabs for an exposure of 500,000 s. TIGREs innovative design also uses the polarization dependence of the Klein-Nishina formula for gamma-ray source polarization measurements. The telescope will be described in detail and new results from measurements at 0.5 MeV and Monte Carlo calculations from 1 to 100 MeV will be presented.

All-sky X-ray and gamma-ray monitor (AXGAM)

A wide field-of-view, arcsecond imaging, high energy resolution X-ray and low energy gamma ray detector is proposed for a future space mission. It is specifically designed to detect and find counterparts at other wavelengths for gamma ray bursts (GRBs). Detection of GRBs require wide field-of-view (/spl pi/ to 2/spl pi/ field-of-view) and high sensitivity. This will be achieved by using high quantum efficiency CdZnTe pixel detectors, low energy threshold (few keV) to observe larger flux levels that may be possible at lower energies and large effective area (625 to 1,000 cm/sup 2/) per coded aperture imaging module. Counterpart searches can only be done with ultra high angular resolution (10 to 30 arcsecond FWHM) which gives 1 to 5 arcsecond position determination especially for strong GRBs. A few arcsecond resolution error box is expected to contain only one counterpart observed at another wavelength. This will be achieved by using ultra high spatial resolution pixel detectors (50/spl times/50 to 100/spl times/100 micron) and a similar resolution coded aperture to achieve the required angular resolution. AXGAM also has two other important advantages over similar detectors: (1) excellent low energy response (>1 keV) and (2) high energy resolution (<6%@5.9 keV, <3%@14 keV, <4%@122 keV). The low energy range may provide important new information on their cause and the high energy resolution is expected to help in the observation and identification of emission and absorption lines in the GRB spectrum. The effective energy range is planned to be 2 to 200 keV which is exceptionally wide for such a detector. AXCAM will be built in the form of a Bucky Ball using a coded aperture mask in a semi geodesic dome arrangement placed over a 2D high resolution CdZnTe pixel detector array using newly developed p-i-n detector technology. The p-i-n structure decreases the electron and hole trapping effect and increases energy resolution significantly. The major scientific goals of the proposed mission in addition to continuously monitoring gamma-ray bursts, is to observe AGN, transient phenomena, isolated and binary pulsars, and solar flares. A space deployed AXGAM detector is expected to observe several hundred gamma ray bursts per year.