O.T. Tumer

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 . >

Gamma Ray Detection with Long NaI(Tl) Scintillator Bars

Extensive position, energy and timing resolution measurements have been made for a l00×5×5 cm3 NaI(Tl) scintillator bar. The arrival times and the pulse heights of signals from two photomultiplier tubes, one at each end of the bar, are used to provide the energy loss, linear position and time-of-flight for gamma ray interactions in the bar over the energy range from 0.5 to 20 MeV. The scintillator bar has an approximately exponential roll-off with a mean attenuation coefficient of 0.015 cm-1. At 0.662 MeV, resolutions (FWHM) of 5 cm, 9.4% and 10 ns were obtained for position, energy and timing, respectively. The timing resolution is 6 ns at 1.25 MeV. At 6.13 MeV, a spatial resolution of 2.2 cm was obtained. This bar is a prototype element of a Compton scatter telescope that uses both plastic and NaI(TI) scintillator bars. The technique is being used for medium energy gamma ray astronomy from a balloon platform. The telescope will have 3.5° (FWHM) angle and 6% energy resolutions at 6 MeV.

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.

The UCR gamma ray telescope data acquisition system

A description is given of an electronics system based on the DEC Falcon SBC-11/23+, which has been designed and built to support a balloon-borne double Compton gamma-ray telescope. The system provides support for commands, data acquisition, data routing and compression, and photomultiplier tube gain control. The software consists of a number of interrupt-driven routines of differing priorities to handle each system task. This includes two circular buffers for onboard processing and bit encoding before transmission of the information to the ground computer. Acquisition of gamma-ray events at rates above the 200-Hz telemetry constraint is easily achieved. >

Direct linear algebraic deconvolution imaging of Compton telescope data

A general direct linear algebraic deconvolution (DLAD) method for imaging Compton gamma‐ray telescope event data is described. This method gives an image of the gamma ray source distribution that is linearly related to the binned event data. When a physically realistic positivity constraint for the image field is imposed on the solution it strongly stabilizes the resulting image permitting pixel sizes at or below the instrument spatial resolution. Two algorithms for imposing the positivity constraint are investigated and compared; the Non‐negative Least Squares (NNLS) method of Lawson and Hanson (1974) and the Inequality Constrainted Generalized Least Squares (ICLS) method of Werner (1990). The latter is applied here to CGRO COMPTEL event data for Viewing Period ♯1 centered on the Crab Nebula. Preliminary flux images and their statistical significant are presented.