R. S. White

A large double scatter telescope for gamma rays and neutrons

Abstract A large aperture, area and solid angle double scatter telescope for measurements of the flux, energy, angle and time distributions of gamma rays and neutrons from a balloon is described. It is sensitive to gamma rays from 0.5 to 30 MeV and to neutrons from 2 to 100 MeV. A gamma ray is identified by Compton scatters in each of two large liquid scintillator tanks of cross section 1 m × 1 m spaced 1 m apart. For better angular resolution each tank is divided into 28 cells and each is viewed by its own photomultiplier. A neutron is identified by an elastic scatter from a proton in the first tank and a scatter by a proton or an inelastic scatter from carbon in the second tank. Measurements of the Compton electron recoil energies in each scintillator give estimates of the incident gamma ray energy and restrict its incident angle to a cone. Measurements of the proton recoil energy in the first scintillator and the time of flight between scintillators give the neutrons energy and incident angle to a cone. Time of flight separates the gamma rays from the neutrons and determines their direction through the telescope. For point sources the gamma ray detection efficiency is a maximum of 3% at 4 MeV and drops to 1% at 0.5 MeV and 0.4% at 30 MeV. The corresponding geometrical factors are 300, 100 and 40 cm 2 , respectively. The greatest efficiency occurs at 10° and falls by a factor of 3 at 60° for a 5 MeV gamma ray. For a neutron point source, its peak efficiency is 8% at 6 MeV and falls rapidly to 1% at 3 MeV and slowly to 0.4% at 100 MeV. The geometrical factors are 800, 100 and 40 cm 2 , respectively. Its efficiency is maximum for a 45° scatter and drops slowly for smaller and larger angles. For the large cells the gamma ray and neutron energy resolutions are both about 20% hwhm for the useful energies, and the cone angle resolutions are about 10° hwhm. Energy and angle resolution for the smaller cells are somewhat better. The detector has a high background rejection particularly because of the high hydrogen to carbon ratio of the scintillator of 2.0, the nanosecond time resolution and the 10° hwhm angular resolution.

The TIGRE gamma-ray telescope

TIGRE is an advanced telescope for gamma-ray astronomy with a few arcmin resolution. From 0.3 to 10 MeV it is a Compton telescope. Above 1 MeV, its multi-layers of double sided silicon strip detectors allow for Compton recoil electron tracking and the unique determination for incident photon direction. From 10 to 100 MeV the tracking feature is utilized for gamma-ray pair event reconstruction. Here we present TIGRE energy resolutions, background simulations and the development of the electronics readout system.

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>>

Large Area Double Scattering Telescope for Balloon-Borne Studies of Neutrons and Gamma Rays

A large area (1 m × 1 m) double scattering telescope1 for balloon-borne research will be described here. It measures the flux, energy and direction of 2-100 MeV neutrons and 0.5-30 MeV gamma rays. These measurements are made using time-of-flight and pulse height analysis techniques with two large tanks of mineral oil liquid scintillator. Results from Monte Carlo calculations of the efficiency, energy resolution and angular resolution are presented and the electronics implementation for the processing of 80 photomultiplier tubes signals will be discussed. The detector weighs 800 kg with a large part of this weight being the liquid scintillator (320 kg). It will be flown at 3 mbars for flight durations up to 40 hours. The first flight is planned for Spring, 1975.

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 . >

On the influence of the magnetization of a model solar wind on a laboratory magnetosphere

The interaction of a magnetized plasma beam with a stationary dipole field, analogous to the interaction of the solar wind with the Earths magnetosphere, is explored in a laboratory experiment. Experimental parameters are chosen to scale qualitatively similar to the parameters in the Earths magnetosphere. We find that the magnetization of the laboratory “solar wind,” generated by injecting a plasma across a preexisting magnetic field, requires a certain minimum magnetic field strength. Differences between the resulting magnetospheres for northward and southward “solar wind” or “interplanetary” magnetic fields (IMF) are demonstrated by global pictures and by magnetic field measurements above the north polar region. These measurements show patterns of the variation of the transverse field component which are similar to those found by satellite measurements above the Earth. This indicates the presence of similar field-aligned current systems. We demonstrate particularly the presence (for northward IMF) and absence (for southward IMF) of the pattern attributed to the “NBZ” (northward Bz) current system.

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.