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NASA NTRS1972

The response of covered silicon detectors to monoenergetic gamma rays

Measurements were made of the efficiency in detecting gamma rays of a 0.3-mm, a 3-mm, and a 5-mm silicon detector covered with different absorbers. Calibrated sources covering the range from 279 KeV to 2.75 MeV were used. The need for the absorbers in order to obtain meaningful results, and their contribution to detector response at electron biases from 50 to 200 KeV, are discussed in detail. It is shown that the results are independent of the atomic number of the absorber. In addition, the role of the absorber in increasing the efficiency with increasing photon energy for low bias setting is demonstrated for the 0.3-mm crystal. Qualitative explanations are given for the shapes of all curves of efficiency versus energy at each bias.

Reier, M.↗
NASA NTRS1972

The response of covered silicon detectors to monoenergetic gamma rays.

Measurements have been made of the efficiency in detecting gamma rays of a 0.3-mm-, 3-mm-, and 5-mm-thick silicon detector covered with different absorbers. Calibrated sources over the range from 279 keV to 2.75 MeV were used. The need for the absorbers to obtain meaningful results and their contribution to the response of the detectors at electron biases from 50 to 200 keV are discussed in detail. It is shown that the results are virtually independent of the atomic number of the absorber. In addition, the role of the absorber in increasing the efficiency with increasing photon energy for low bias settings is demonstrated for the 0.3-mm crystal. Qualitative explanations are given for the shapes of all curves of efficiency versus energy at each bias.

Reier, M.↗
NASA NTRS1991

Advanced Ge detectors for gamma-ray astronomy

Externally segmented coaxial detectors are fabricated for high efficiency in detecting gamma rays from cosmic sources with good sensitivities. The external background is reduced by enclosing the Ge detector array inside a thick active shield. The outer electrode of the coaxial detectors is subdivided into five segments, and internal beta activity is rejected by operating the segmented detector in a multisegment mode. The multisegment mode requires that events be detected in two or more segments before they are recorded. The full-energy-peak (FEP) efficiency of the unit is tested as a function of the incident gamma-ray energy and of the discriminator threshold of the segments. Measurements of beta-rejection and FEP efficiency are compared with Monte Carlo calculations, and good agreement is noted.

Varnell, Larry S.↗
NASA NTRS1995

Statistical properties of the time histories of cosmic gamma-ray bursts detected by the BATSE experiment of the Compton gamma-ray observatory

The main scientific objectives of the project were: (1) Calculation of average time history for different subsets of BATSE gamma-ray bursts; (2) Comparison of averaged parameters and averaged time history for different Burst And Transient Source Experiments (BASTE) Gamma Ray Bursts (GRB's) sets; (3) Comparison of results obtained with BATSE data with those obtained with APEX experiment at PHOBOS mission; and (4) Use the results of (1)-(3) to compare current models of gamma-ray bursts sources.

Sagdeev, Roald↗
NASA NTRS1976

Preliminary Results from the European Space Agency's COS-B satellite for gamma-ray astronomy

The COS-B satellite carries a single experiment, capable of detecting gamma rays with energies greater than 30 MeV. Its objectives are to study the spatial, energy, and time characteristics of high energy radiation of galactic and extragalactic origin. The capability to search for gamma ray pulsations is enhanced by the inclusion in the payload of a proportional counter sensitive to X-rays of 2-12 Kev. The experiment was calibrated using particle accelerators. The results of these measurements are presented and the performance of the system in orbit is discussed.

Swanenburg, B.↗
NASA NTRS1988

Nonpotential magnetic fields at sites of gamma ray flares

The relation between the degree of nonpotentiality of photospheric magnetic fields and the occurrence of gamma ray flares is examined. The parameter delta phi (magnetic shear) and the strength of the magnetic field intensity are used as measures of the degree of nonpotentiality, where delta phi is defined as the angular difference between the observed direction of the transverse component of the photospheric field and the direction of the potential field prescribed by the distribution of measured photospheric flux. An analysis of the great flare of April 24 to 25, 1984 is presented as an example of this technique to quantify the nonpotential characteristics of the pre-flare magnetic field. For this flare, which produced a large gamma ray event, strong shear and high field strengths prevailed over an extended length of the magnetic neutral line where the flare occurred. Moreover, the flare began near the area of strongest measured shear (89 to 90 deg). Four other flaring regions were analyzed; one of these produced a moderate gamma ray event while the other three did not produce detectable gamma rays. For all four regions the flares were located in the area where the field was not nonpotential, regardless of the class of flare. The fields of the gamma ray flares were compared with those associated with the flares without gamma rays, and little distinction was found in the degree of magnetic shear. The major difference is seen in the extent of the sheared field: for gamma ray events, the field is sheared over a longer length of the neutral line.

Hagyard, M. J.↗
NASA NTRS2011

High-Energy Radiation from Thunderstorms with ADELE: TGFs, Steps, and Glows

The biggest challenge in the study of high-energy processes in thunderstorms is getting a detector to the vicinity of the electrically active regions of a storm. The Airborne Detector for Energetic Lightning Emissions (ADELE) has been used to detect gamma rays from aircraft above storms and from a storm-chasing van on the ground. In August 2009, ADELE flew above Florida storms in a Gulfstream V jet, detecting the first terrestrial gamma-ray flash (TGF) seen from a plane and continuous glows of high-energy emission above thunderclouds. The presence of these glows suggests that a gradual process of relativistic runaway and feedback may help limit the total amount of charging in thunderstorms, in contrast to the traditional view that only lightning discharges compete with the charging process. The upper limits on TGF emission from intracloud and cloud-to-ground lightning from the ADELE flights demonstrated conclusively that a TGF of the sort seen from space is not associated with most lightning and not necessary to trigger it. In August 2010, observations from a van detected stepped-leader x-ray emission from at least four lightning strikes in ten days of operations. This mode of operation is therefore promising for future observations of the stepping process, although a more varied suite of instrumentation, in particular a flash-distance detector, would be useful. We will report on these results and on future possibilities for ADELE campaigns.

Smith, David M.↗
NASA NTRS1990

A study of the sensitivity of an imaging telescope (GRITS) for high energy gamma-ray astronomy

When a gamma-ray telescope is placed in Earth orbit, it is bombarded by a flux of cosmic protons much greater than the flux of interesting gammas. These protons can interact in the telescope's thermal shielding to produce detectable gamma rays, most of which are vetoed. Since the proton flux is so high, the unvetoed gamma rays constitute a significant background relative to some weak sources. This background increases the observing time required to pinpoint some sources and entirely obscures other sources. Although recent telescopes have been designed to minimize this background, its strength and spectral characteristics were not previously calculated in detail. Monte Carlo calculations are presented which characterize the strength, spectrum and other features of the cosmic proton background using FLUKA, a hadronic cascade program. Several gamma-ray telescopes, including SAS-2, EGRET and the Gamma Ray Imaging Telescope System (GRITS), are analyzed, and their proton-induced backgrounds are characterized. In all cases, the backgrounds are either shown to be low relative to interesting signals or suggestions are made which would reduce the background sufficiently to leave the telescope unimpaired. In addition, several limiting cases are examined for comparison to previous estimates and calibration measurements.

Yearian, Mason R.↗
NASA NTRS1974

Neutral point detection by satellites

The concept of a neutral point depends on the physical phenomena described. The regions with B less than about 1 gamma detected by Schindler and Ness may be interpreted as neutral regions for the ion-tearing process. The assumption of the presence of a multiple neutral point structure (with temporal variations) is still the most promising interpretation of the Explorer 34 data. Alternatives suggested by Russell lead to difficulties. Nevertheless, the final answer can come only from multiple satellite systems. A 1-day displacement of the day count in the data discussed by Schindler and Ness is corrected.

Schindler, K.↗
NASA NTRS2017

Measuring Surface Bulk Elemental Composition on Venus

The extreme surface environment (462 C, 93 bars pressure) of Venus makes subsurface measurements of its bulk elemental composition extremely challenging. Instruments landed on the surface of Venus must be enclosed in a pressure vessel. The high surface temperatures also require a thermal control system to keep the instrumentation temperatures within their operational range for as long as possible. Since Venus surface probes can currently operate for only a few hours, it is crucial that the lander instrumentation be able to make statistically significant measurements in a short time. An instrument is described that can achieve such a measurement over a volume of thousands of cubic centimeters of material by using high energy penetrating neutron and gamma radiation. The instrument consists of a Pulsed Neutron Generator (PNG) and a Gamma-Ray Spectrometer (GRS). The PNG emits isotropic pulses of 14.1 MeV neutrons that penetrate the pressure vessel walls, the dense atmosphere and the surface rock. The neutrons induce nuclear reactions in the rock to produce gamma rays with energies specific to the element and nuclear process involved. Thus the energies of the detected gamma rays identify the elements present and their intensities provide the abundance of each element. The GRS spectra are analyzed to determine the Venus elemental composition from the spectral signature of individual major, minor, and trace radioactive elements. As a test of such an instrument, a Schlumberger Litho Scanner oil well logging tool was used in a series of experiments at NASA's Goddard Space Flight Center. The Litho Scanner tool was mounted above large (1.8 m x 1.8 m x.9 m) granite and basalt monuments and made a series of one-hour elemental composition measurements in a planar geometry more similar to a planetary lander measurement. Initial analysis of the results shows good agreement with target elemental assays

Venus↗
NASA NTRS1983

Characteristics of gamma-ray line flares

Observations of solar gamma rays by the Solar Maximum Mission (SMM) demonstrate that energetic protons and ions are rapidly accelerated during the impulsive phase. To understand the acceleration mechanisms for these particles, the characteristics of the gamma ray line flares observed by SMM were studied. Some very intense hard X-ray flares without detectable gamma ray lines were also investigated. Gamma ray line flares are distinguished from other flares by: (1) intense hard X-ray and microwave emissions; (2) delay of high energy hard X-rays; (3) emission of type 2 and/or type 4 radio bursts; and (4) flat hard X-ray spectra (average power law index: 3.1). The majority of the gamma ray line flares shared all these characteristics, and the remainder shared at least three of them. Positive correlations were found between durations of spike bursts and spatial sizes of flare loops as well as between delay times and durations of spike bursts.

Bai, T.↗
NASA NTRS1985

Characteristics of gamma-ray line flares

Observations of solar gamma rays by the Solar Maximum Mission (SMM) demonstrate that energetic protons and ions are rapidly accelerated during the impulsive phase. To understand the acceleration mechanisms for these particles, the characteristics of the gamma ray line flares observed by SMM were studied. Some very intense hard X-ray flares without detectable gamma ray lines were also investigated. Gamma ray line flares are distinguished from other flares by: (1) intense hard X-ray and microwave emissions; (2) delay of high energy hard X-rays, (3) emission of type 2 and/or type 4 radio bursts; and (4) flat hard X-ray spectra (average power law index: 3.1). The majority of the gamma ray line flares shared all these characteristics, and the remainder shared at least three of them. Positive correlations were found between durations of spike bursts and spatial sizes of flare loops as well as between delay times and durations of spike bursts.

Bai, T.↗
NASA NTRS1992

EGRET detection of high energy gamma rays from the gamma-ray burst of 3 May 1991

On May 3, 1991, the Energetic Gamma Ray Experiment Telescope on the Compton Gamma Ray Observatory detected a gamma-ray burst both in the energy measurement subsystem and independently in the spark chamber assembly. Six individual photons were detected in the spark chamber, allowing a determination of the burst arrival direction which was l(II) = 171.9 deg +/- 1.3 deg, b(II) = 5.3 deg +/- 1.1 deg. Three energy spectra were measured from 1 to 200 MeV; they were measured during the first second after the Burst and Transient Sources Experiment trigger, the next two seconds, and the subsequent four seconds. The first two spectra exhibit a similar differential spectra index of about -2.2 with no apparent high-energy cut-off. By the time of the third spectrum, an additional soft component is evident.

Schneid, E. J.↗
NASA NTRS1994

The first energetic gamma-ray experiment telescope (EGRET) source catalog

In this catalog the results related to high-energy gamma-ray sources obtained from the Energetic Gamma Ray Experiment Telescope (EGRET) on the Compton Gamma Ray Observatory are summarized for the period from 1991 April 22 to 1992 November 17, called phase 1. This phase of the Compton Observatory mission was devoted to an all-sky survey. Tables are included for the following EGRET results: solar flares with detected gamma radiation; pulsars; sources with absolute value of b is less than 10 deg, gamma-ray bursts; normal galaxies; clusters of galaxies; positive detections of radio-loud quasars and BL Lac objects; marginal detections of radio-loud quasars and BL Lac objects; upper limits for radio-loud quasars and BL Lac objects; upper limits for Seyfert galaxies and selected radio-quiet quasars, and other sources with absolute value of b is greater than or equal to 10 deg the Galactic plane. There is also a table relating the dates of the observations to the Compton Observatory viewing period numbers to assist in referencing the observations.

Fichtel, C. E.↗
NASA NTRS1993

Detection of gamma-ray emission from the quasar PKS 0208-512

High-energy gamma-ray emission has been detected from the quasar PKS 0208-512 in the energy range above 30 MeV by the Energetic Gamma Ray Experiment Telescope (EGRET) on the Compton Gamma Ray Observatory. This region of sky was observed in five different viewing periods, and evidence of time variability in the gamma-ray emission by more than a factor of 3 was found. At the maximum intensity between 1991 September 19 and October 3, the flux density above 100 MeV was (9.1 +/- 0.4) x 10 exp -7 gamma/sq cm per sec. The photon spectrum during this period may be expressed as a power law with an exponent of - 1.69 +/- 0.05 between 30 MeV and 4 GeV. This is the hardest quasar spectrum observed by EGRET up to the present time.

Bertsch, D. L.↗
NASA NTRS1993

Na-22 decay gamma rays from classical novae

NASA Grant NAG 5-1565 has provided support for a program of theoretical research in nuclear astrophysics and related areas, focusing upon the possibility of detecting gamma rays from nearby novae. Particular attention has been given to the evaluation of the theoretical expectations for gamma ray emission from four possible sources: (1) the positron decays of the unstable CNO and fluorine isotopes that are transported to the surface regions of the envelope in the earliest stages of the outbursts; (2) Be-7 decay gamma rays, (3) Na-22 decay gamma rays released in the later stages of the outbursts; and (4) Al-26 decay gamma rays from novae and their possible contribution to Galactic emission. The critical questions of (1) the frequency of occurrence of ONeMg-enriched novae; (2) the expected Galactic distribution of the novae that produce 26Al; and (3) the nature of the observed soft X-ray emission from classical novae, have also been addressed. Considerable progress in research has been achieved on many of these fronts. Brief summaries of the results of several research projects are presented.

Truran, James W.↗
NASA NTRS2018

Development of a Silicon Tracker for the All-Sky Medium Energy Gamma-ray Observatory Prototype

The gamma-ray sky from several hundred keV to a hundred MeV has remained largely unexplored due to the challenging nature of detecting gamma rays in this regime. At lower energies, Compton scattering is the dominant interaction process whereas at higher energies pair production dominates, with a crossover at a few MeV. Thus, an instrument designed to work in this energy range must be optimized for both Compton and pair-production events. AMEGO, the All-sky Medium Energy Gamma-ray Observatory, a Probe-class mission in consideration for the 2020 decadal survey, is designed to operate at energies from 200 keV to > 10 GeV with over an order of magnitude increase in sensitivity and with superior angular and energy resolution compared to previous instruments. AMEGO comprises four major subsystems: a plastic anticoincidence detector for rejecting cosmic-ray events, a silicon tracker for tracking pair-production products and tracking and measuring the energies of Compton-scattered electrons, a cadmium-zinc-telluride (CZT) calorimeter for measuring the energy and location of Compton scattered photons, and a CsI calorimeter for measuring the energy of the pair-production products at high energies. A prototype instrument, known as ComPair, is under development at NASA’s Goddard Space Flight Center and the US Naval Research Laboratory. In this contribution, we provide details on the development of the silicon tracker subsystem.

Griffin, Sean↗
NASA NTRS1987

Solar flare nuclear gamma rays and interplanetary proton events

We compared flare gamma ray line (GRL) events and solar energetic proton (SEP) events for the period from Feb. 1980 - Jan. 1985 and substantiated earlier results showing a lack of correlation between gamma-ray-producing ions and interplanetary protons. This poor correlation results primarily from several large SEP events that originated in flares without detectable gamma ray emission. The converse case of GRL events unassociated with SEP events is rare. We present evidence which suggests that the ratio of trapped to escaping protons in GRL/SEP flares depends on the spatial scale size of the flare. We affirm the result of Bai and Dennis (1985) that GRL flares are generally accompanied (75 percent) by metric Type 2 bursts.

Cliver, E. W.↗