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Observations of the Large Magellanic Cloud (LMC), particularly in the radio and high-energy gamma-ray range, offer the possibility of understanding the distribution of cosmic rays in a galaxy other than our own and determining the role that these quantities play in galactic dynamic balance. The LMC has the advantage of being our nearest galactic neighbor and of being observed at a not too large an angle to the normal to the plane. The cosmic-ray energy density distribution for the LMC is calculated quantitatively based on the concept of dynamic balance and a scale of coupling between the cosmic rays and matter in a range allowed by present observations. Based on the very slowly varying ratio of the cosmic-ray electron to nucleon energy density ratio with relevant local galactic conditions and the close similarities to our galaxy, the cosmic-ray nucleon density distribution in the LMC is also determined from the cosmic-ray electron density distribution deduced from synchrotron radiation measurements in a manner consistent with dynamic balance. It is seen that within uncertainties there is quantitative agreement between the two for a cosmic-ray, matter coupling scale of about 2 1/2 kiloparsecs both in terms of magnitude and distribution, thus supporting both the concept of dynamic balance and the galactic origin of the bulk of cosmic rays. Future gamma-ray astronomy measurement above ~ 102 MeV will be able to provide a test of this cosmic-ray density distribution for the LMC.

As gamma-ray astronomy moves from the discovery to the exploratory phase, the promise of gamma-ray astrophysics noted by theorists in the late 1940s and 1950s is beginning to be realized. In the future, satellites should carry instruments that will have over an order of magnitude greater sensitivity than those flown thus far, and, for at least some portions of the gamma-ray energy range, these detectors will also have substantially improved energy and angular resolution. The information to be obtained from these experiments should greatly enhance our knowledge of several astrophysical phenomena including the very energetic and nuclear processes associated with compact objects, astrophysical nucleosynthesis, solar particle acceleration, the chemical composition of the planets and other bodies of the solar system, the structure of our galaxy, the origin and dynamic pressure effects of the cosmic rays, high energy particles and energetic processes in other galaxies especially active ones, and the degree of matter-antimatter symmetry of the universe. The gamma-ray results of the forthcoming programs such as Gamma-I, the Gamma Ray Observatory, the gamma-ray burst network, Solar Polar, and very high energy gamma-ray telescopes on the ground will almost certainly provide justification for more sophisticated telescopes. These advanced instruments might be placed on the Space Platform currently under study by N.A.S.A.

The EGRET telescope, one of the instruments to be carried on NASA's Gamma Ray Observatory, is designed to explore the high-energy portion (20 - 24,000 MeV) of the electromagnetic spectrum. A calibration source of ?-rays over this wide energy range has been used at SLAC, together with an appropriate set of intensity-monitoring instruments, to conduct a calibration of the EGRET telescope over this energy range for the full telescope aperture. The calibration method and an overview of the results are described.

γ-ray astronomy is the study of the most energetic photons originating in our Galaxy and beyond, and therefore, provides the most direct means of studying the largest transfers of energy occurring in astrophysical processes. The first certain detection of celestialγ-rays came from a satellite experiment flown on OSO-III (Kraushaaret al., 1972); more recently two second generation spark chamberγ-ray telescopes, flown on the SAS-2 (Fichtelet al., 1975) and COS-B (Bennettet al., 1974) satellites, are now obtaining more detailed results on the high energy celestial radiation causingγ-ray astronomy to move from the discovery phase to the exploratory phase. The most striking feature of the celestial sphere when viewed in the frequency range ofγ-rays is the emission from the galactic plane, which is particularly intense in the galactic longitudinal region from 300° to 50°. The longitudinal and latitudinal distributions are generally correlated with galactic structural features and when studied in detail suggest a non-uniform distribution of cosmic rays in the galaxy. Several pointγ-ray sources have now been observed, including four radio pulsars. This last result is particularly striking since only one radio pulsar has been seen at either optical or X-ray frequencies. Nuclearγ-ray lines have been seen from the Sun during a large solar flare and future satellite experiments are planned to search forγ-ray lines from supernovae and their remnants. A general apparently diffuse flux ofγ-rays has also been seen whose energy spectrum has interesting implications; however, in view of the possible contribution of point sources and the observation of galactic features such as Gould's belt, its interpretation must awaitγ-ray experiments with finer spatial and energy resolution, as well as greater sensitivity. Instruments with much greater sensitivity and improved energy and angular resolution are now available and will greatly enhance our understanding of high energy processes in astrophysics, especially in view of the high penetrating power ofγ-rays, which for example permit them to reach the solar system from the far side of the galaxy essentially unattenuated.

From the earliest satellite measurements, it was clear that the high energy gamma ray sky was dominated by radiation from the galactic plane. The theoretical analyses of the time, and now, suggest that the majority of this radiation is diffuse, coming from cosmic ray interactions with the galactic matter and photons. The gamma ray measurements, therefore, provide the opportunity to gain significant additional insight into the galactic cosmic ray density distribution, the level of coupling of the cosmic rays and the matter, and, as it has developed, a better estimate of the molecular hydrogen density normalization. A cosmic ray density variation, at least broadly coupled to the galactic matter distribution, seems now to be supported by most of the analyses. This conclusion is consistent with the cosmic radiation being galactic and suggests the importance of future more sensitive gamma ray observations for the study of the detailed relationship between the cosmic rays and the galactic matter. An analysis of the radiation from high galactic latitudes suggest that, when the galactic radiation is subtracted, there is a residual isotropic radiation, which is believed to be extragalactic in origin. The diffuse gamma radiation from three local normal galaxies, the Small and Large Magellanic Clouds and M31, should be visible to at least one of the gamma ray telescopes on the Gamma Ray Observatory, and the first two should be seen by GAMMA-1. It is expected that the intensity and structure of both the Magellanic Clouds can be examined sufficiently well to study the cosmic ray density and thereby to determine the relevant scale of coupling for the cosmic rays and diffuse matter. Further, because of the apparently markedly different distributions of molecular and atomic hydrogen, particularly in the Small Magellanic Cloud, and the differences between the galaxies, an independent measure of the normalization of the diffuse molecular hydrogen should be possible for these two galaxies.

Gamma-ray astronomy which includes the spectral region from above approximately 100 keV to greater than or equal to 1000 GeV permits investigation of the most energetic photons originating in our galaxy and beyond and provides the most direct means of studying the largest transfers of energy occurring in astrophysical processes. Of all the electromagnetic spectrum, high-energy gamma-ray astronomy measures most directly the presence and dynamic effects of the energetic charged cosmic ray particles, element synthesis, and particle acceleration. Further, gamma rays suffer negligible absorption or scatterings as they travel in straight paths; hence, they may survive billions of years and still reveal their source. The high energy processes in stellar objects (including our Sun), the dynamics of the cosmic-ray gas, the formation of clouds and nebulae, galactic evolution and even certain aspects of cosmology and the origin of the universe may be explored by gamma-ray observations.

Within our Galaxy, cosmic rays can reveal their presence in interstellar space and probably in source regions by their interactions with interstellar matter which lead to γ-rays with a very characteristic energy spectrum. From the study of the intensity of the high energy γ radiation as a function of galactic longitude, it is already clear that cosmic rays are almost certainly not uniformly distributed in the Galaxy and are not concentrated in the centre of the Galaxy. The galactic cosmic rays appear to be tied to galactic structural features, presumably by the galactic magnetic fields which are in turn held by the matter in the arm segments and the clouds. On the extra-galactic scale, it is now possible to say that cosmic rays are probably not at the density seen near the Earth. The diffuse celestial γ-ray spectrum that is observed presents the interesting possibility of cosmological studies and possible evidence for a residual universal cosmic ray density, which is much lower than the present galactic cosmic-ray density.

The high energy γ-ray telescope selected for definition studies on the Gamma Ray Observatory provides a substantial improvement in observational capability over earlier instruments. It will have about 20 times more sensitivity, cover a much broader energy range, have considerably better energy resolution and provide a significantly improved angular resolution. The design and performance are described.

Composition and energy spectra of solar cosmic rays - november 12, 1960 - solar flare, hydrogen, helium, and nuclei

Elemental abundances of low energy cosmic rays in July 1964 observed by sounding rocket, noting components

The primary cosmic-rayα-particle energy spectrum has been investigated, using photographic emulsions flown from Minneapolis on July 30th, 1957. The total flux observed was (151±9)particles/cm2 sr s. The differential energy spectrum shows a broad maximum between 400 and 600 MeV/nucleon, and appears different in shape from the spectrum observed at solar minimum. No particles were observed to have kinetic energies below 200 MeV/nucleon, but a substantial flux was observed between 200 and 300 MeV/nucleon. From the centered dipole approximation to the geomagnetic field, one would expect a cut-off energy at this latitude of 292 MeV/nucleon. The results presented here are in disagreement with this figure, but do not allow a choice to be made between cut-off energies predicted from other models.

Systems description of SAS-B gamma ray telescope with multilayer digitized spark chamber for gamma rays with energy exceeding 20 MeV

Energy spectra of cosmic ray hydrogen and helium nuclei determined from nuclear emulsions flown in high altitude balloon

A series of high-altitude balloon flights designed to look at possible point sources of cosmic gamma radiation have been made beginning in September 1966. The heart of the detector system used in these investigations consists of a 32-level magnetic-core digitized spark chamber, with 256 wires per level and thin (0.02 radiation length) high-Z pair-producing plates between each level. This spark chamber is surrounded by a bell-shaped plastic scintillator dome and is triggered by a coincidence between a thin central scintillator and a Cerenkov counter in anticoincidence with the dome. This device is flown in an oriented balloon gondola. Results related to several potential γ-ray sources have been obtained in a series of three flights. Upper limits for the flux of γ rays above 30 MeV of (2–4) × 10−5 (cm2 s)−1 have been established for Sco X-1, Sgr SR-1, Sgr SR-2, Sgr SR-3, the Kepler Supernovae (Oph XR-1), the galactic center, the moon, and the quiet sun. Somewhat less severe limits have been set for Centaurus A and Taurus A.

Numerical values have been calculated for the scattering constant for multiply charged particles in photographic emulsion. The constant has been calculated both with and without cut-off being applied for correction for single scattering, and the results are presented in graphical form.

Balloon-borne nuclear emulsion detection of finite fluxes of low energy cosmic ray heavy nuclei and interstellar cosmic radiation propagation

Cosmic ray propagation, discussing interstellar matter effect on composition and diffusion effect necessary to study radiation models

The heavy component of the primary cosmic radiation was studied at geomagnetic latitude 55° N at a time of solar maximum. The total flux was found to be down by a factor of about two, compared to the results obtained during solar minimum, but the charge spectrum was found to be the same, to within experimental errors. The extrapolated fluxes at the top of the atmosphere were found to be: (3.08±0.64) part./m2 sr s for 3⩽Z⩽5; (8.26±0.82) part./m2 sr s for 6⩽Z⩽9; and (2.96±0.49) part./m2 sr s for 10⩽Z. When this work was combined with that of other experimentalists obtained at different latitudes and compared to the data obtained at solar minimum, the change in the integral spectrum was found to be in fair agreement with the predictions of an electric deceleration modulating mechanism.

Solar wind modulation of low energy galactic cosmic radiation, estimating primary source spectra, proton and helium fluxes

A magnetic core digitized spark chamber suitable for use on balloons and ultimately on satellites has been developed. The assembled spark chamber consists of 32 active spark modules and forms the heart of a gamma-ray telescope. The individual modules consist of two orthogonal wire grids which form the spark chamber electrodes. Inclusion of a terminating resistor on each wire localizes the effect of the discharge, resulting in good multiple spark efficiency at low overvoltages. The lower voltages result in better spatial resolution with fewer wires sharing the spark. In the construction, considerable care was given to strength and long term operation under satellite conditions. The on-board data-handling system consists of a buffered memory readout, a housekeeping multiplexer, a tape recorder and a telemetry encoder and transmitter.

The significance of high energy (≳ 30 MeV) γ-Ray astronomy and its relationship to cosmic rays and many of the high energy processes of the universe has been realized for over a decade. Through the last six to eight years, searches for point sources, mostly from balloon experiments, but also Explorers 11 and OSO-3, have been unsuccessful in clearly establishing the existence of any point source. Recently on OSO-3, Clark et al. (1968) have obtained positive evidence for a celestial γ-Ray flux which is anisotropic with a higher intensity in the direction of the galactic center region. In this talk, I wish to summarize our balloon results relating to both of these questions and indicate what we hope to accomplish with our new large γ-Ray detector over the coming months.

A second generation gamma-ray telescope has been developed for further experiments in balloon-borne astronomy. This instrument is composed of 32 wire-grid spark chamber modules, each 50 × 50 cm2, utilizing two-coordinate ferrite core readout. The detector is flown on a 27 million cubic foot research balloon in a search for discrete gamma-ray sources and supernovae explosions.

Relativistic cosmic rays primordial chemical composition above atmosphere from abundance data obtained with satellite-borne nuclear emulsion detector

Type IV radio emission is thought to be the most important observable characteristic of solar flares producing high-energy particles. ft is shown that solar cosmic-ray particles arrived at the earth on September 27, 1960, after a small flare accompanied by a type IV burst of ahort duration. Apart from the magnitude, the characteristics of this event do not differ essentially from those of larger events.

Energetic helium and heavy nuclei (Z ≥ 3) were detected in the July 18, 1961, solar particle event. This result brings to four the number of solar cosmic-ray bursts in which heavy nuclei have been seen. The particles were detected in nuclear emulsions flown on a balloon launched from Fort Churchill shortly after the associated flare appeared on the sun. The average flux of medium nuclei (6 ≤ Z ≤ 9) during the early part of the solar event was 12.0±1.8 particles m−2 ster−1 sec−1 in the energy interval from 120 to 204-Mev/nucleon, or about 40 times the normal galactic cosmic-ray medium nuclei flux at that particular time in the solar cycle. The helium-to-medium nuclei ratio in the same energy interval was 79±16. These values are consistent with those anticipated on the basis of the relative abundance of hydrogen, helium, and medium nuclei in other events. Some large nuclei (Z ≥ 10) were detected, but light nuclei were so rare that only an upper limit to their abundance could be set (L/M ≤ .07). The unbiased acceleration of multiply charged nuclei in every major solar event now seems more certain, and, therefore, it seems to be worth considering this feature in any theory of solar particle acceleration.

Three Nike-Apache sounding rockets were launched from Fort Churchill, Manitoba, Canada to study the composition and the characteristics of the September 2, 1966 solar flare event. These launches occurred approximately 9, 16, and 36 hours following the event. Nuclear emulsions were used to study all the components, namely, the singly, doubly, and multiply charged nuclei emitted in the solar event and propagated within the solar system. The helium nuclei were separated from protons by using grain density and range measurements and from multiply charged nuclei by δ-ray density and range measurements. The multiply charged nuclei were identified as to charge from δ-ray density and range measurements. The energies of these nuclei were determined mainly from their residual range in the emulsion. The energy spectra measured in the event are much steeper than those reported for events in which the composition studies were made previously. The measured ratio of helium to medium (C, N, O, F) nuclei for energies above 12.7 MeV/nucleon in the first flight is 54 ± 8 to 1. This value is in agreement with the previous measurements, which strongly supports the ideas held that the acceleration mechanism does not appreciably alter the spectroscopically observed abundance of the solar photosphere.

A search for point sources of γ rays has been made using oriented nuclear emulsions flown on high altitude balloons. The energy range examined was from 10 to 2×103 Mev, and the detection efficiency in the 10- to 50-Mev region averaged about 10%, which is almost a factor of 10 greater than that of previous experiments with counter systems. No measurable flux of γ radiation was observed for the regions of the sky examined, which included Cygnus A, the Spiral Arm In, Taurus A, Gemini, the Sun, and the Galactic Anticenter. A summary of the results of this and other experiments and theoretical considerations strongly suggest the need for further research in γ-ray astronomy.

Source spectra and composition of cosmic rays implied by analysis of interstellar and interplanetary travel

Solar cosmic rays, discussing flare association, solar particle acceleration, recurrence and low energy solar particle events

Low energy cosmic ray composition and energy spectra measured by sounding rockets carrying nuclear emulsions

Recent measurements of low-energy galactic cosmic rays obtained on sounding rockets and satellites exhibit a composition different from that obtained for intermediate and high-energy radiation obtained at balloon altitudes. In particular the ratio of light to medium nuclei is observed to be 0.2–0.3 in the 50–100 MeV/nucleon interval as compared with values near 0.5 in the 200–500 MeV/nucleon region. Lower values of the ratios C/O, N/O, F/O, and odd-Z/even-Z are also found. In the light of these new measurements and of new measurements on the fragmentation cross sections for cosmic-ray nuclei in interstellar space, an attempt has been made to calculate the composition expected if similar source spectra are assumed. It is found that neither passage through a fixed amount of material nor an equilibrium condition (exponential path-length distribution) is adequate to explain the observed features. The effects of including other mechanisms such as rigidity-dependent escape from the galaxy and Fermi acceleration in interstellar space are evaluated.

Flux and energy spectrum of solar hydrogen, helium and heavy nuclei is measured during three rocket firings, using nuclear emulsions techniques

A digitized spark chamber has been developed for gamma-ray and charged particle experiments on balloons and satellites. The spark chamber is assembled from modular units, which are stacked to the desired number of levels. Each module consists of parallel spark planes formed from two orthogonal wire grids. Ferrite memory cores, strung on the wires of both grids, record the X-Y coordinate of the spark. Factors affecting the operating characteristics of these modules are discussed. A gamma-ray detector of 32 module levels has been assembled and balloon-flown in a search for cosmic gamma-ray point sources. The construction of this detector is described and its performance briefly discussed.

A wealth of data on many aspects of solar cosmic rays has been collected over the last decade. One of the most striking features of these events has been the tremendous variation in many of their characteristics and the related difficulty of precise interpretation. However, one feature of the solar cosmic rays which does seem to be constant from event to event and within an event is the relative abundances of the multicharged nuclei with the same charge: mass ratio. This paper will concentrate on observations of the composition of solar particles and their interpretation. A brief discussion on the propagation of the energetic solar particles including a few comments on the source region will also be given.

Electromagnetic radiation from the Crab nebula were observed, showing that the Crab is unique among strong X-ray sources in that major component in the low energy range (1 to 10 KeV) shows little or no temporal variation. Observations of the Crab above 35 MeV were made with the high energy gamma ray telescope flown on SAS-2. The detector and technique are described in detail.

The present paper reports on the low-energy gamma-ray bursts observed by the plastic scintillator anticoincidence dome of the Small Astronomy Satellite-2 (SAS-2) gamma-ray telescope. SAS-2 detected two events observed by other satellites and discovered one which was subsequently confirmed by other satellite observations. Two events seen by other satellites were not detected by SAS-2, probably due to earth occultation. The event detection threshold for SAS-2 was almost two orders of magnitude lower than that of the Vela satellites.

An oriented nuclear-emulsion detector capable of time resolution was exposed in Gemini Flight XI to investigate the primary cosmic-ray nuclei above the earth's atmosphere. This was the first satellite exposure of an emulsion apparatus designed to collect 103 high-quality tracks of heavy nuclei under a negligible thickness of matter (0.07 g/cm2). Time resolution was obtained by moving a lower stack, consisting of emulsions of various sensitivities, with respect to a shallower, sensitive upper stack at the rate of 25 microns/minute. It was thus possible to separate the "useful" tracks–formed during the oriented portion of the flight–from those formed at other times. Preliminary data are presented on the relative abundances of individual chemical elements in the high-energy cosmic radiation above the earth's atmosphere. These measurements are compared to published results obtained on balloon flights at similar latitudes. When sufficient data become available in a later phase of this experiment, particular attention will be directed towards the Be and B abundances, the N and F content relative to C and O, and the relative number of iron-group nuclei compared to the lighter ones.

The proton spectra of a solar cosmic radiation event were measured at Font Churchill, Manitoba, Canada. The event occurred at 2100 U. T. Sept. 3, 1960; it was caused by a solar flare at 0040 U. T. Nike-Cajun sounding rockets were fired at 1408 and 1730 U. T. and reached an altitude of 130 km. Emulsion and Geiger counter data were presented, for proton energies of 2 to 250 Mev. Riometer data were given for the total beam. (T.F.H.)

A study of the energy spectrum of the cosmicray hydrogen and helium nuclei as a function of the period in the solar cycle provides one of the better means of gaining insight into the mechanism that modulates the galactic cosmic radiation. An examination of these nuclei in the low-energy region, where the modulation effects are greatest, has the particular merit of permitting the separation of velocity and rigidity effects, since for a given rigidity the helium nuclei have a markedly different velocity from that of the protons. Considerable information related to this problem had been obtained before 1960 for the most recent period of increasing solar activity [Webber, 1962]. In order to continue the study of the problem of the variation of the galactic cosmic-ray proton and helium nuclei spectrums and extend it to lower energies, a series of balloon flights, beginning in 1961, has been made from Fort Churchill, Canada. It is the aim of this letter to present the results of the nuclear emulsion studies of the hydrogen and helium nuclei obtained in 1962 and 1963 and compare them to those obtained in 1961 [Fichtel et al., 1964].

The spectra of nuclei with 1 < Z < 18 in a solar cosmic radiation event were measured. The event occurred at 2100 U. T. Sept. 3, 1960; it was caused by a solar flare at 0040 U. T. Emulsion data from a Nike-Cajun sounding rocket, which was fired at 1408 U. T. and reached an altitude of 130 km, were presented. A range and delta -ray analysis of these data yields the charge and rigidity of the nuclei. (T.F.H.)

The Gamma Ray Observatory (GRO) is currently planned for a launch from the space shuttle in 1990. After the long hiatus in high-energy gamma-ray astronomy since the end of the COS-B mission in 1982, the Soviet missions Granat and Gamma-1 and the NASA mission GRO will resume observations in the energy range from below 100 keV and extending to above 10 GeV. GRO will carry four instruments designed to cover this range of over five decades in photon energy. It is planned to perform a complete sky survey above 1 MeV in the first year of the GRO mission. Data from this survey will be used to study galactic and extragalactic sources of gamma radiation as well as the galactic and extragalactic diffuse emissions. Additionally, measurements of gamma ray bursts will be performed. The angular and spectral resolution of the GRO instruments is significantly improved with respect to previous experiments. The sensitivity for point sources will be better by an order of magnitude, and the location of strong, high energy sources will be determined to about 0.1°–0.2°. After a brief description of the complement of GRO instruments, a detailed discussion of the high-energy telescope EGRET, its design and scientific objectives, is presented in this review.

The current status of gamma-ray-telescope technology for ground, airborne, and space observations is surveyed and illustrated with drawings, diagrams, and graphs and tables of typical data. For the low- and medium-energy ranges, consideration is given to detectors and detector cooling systems, background-rejection methods, radiation damage, large-area detectors, gamma-ray imaging, data analysis, and the Compton-interaction region. Also discussed are the gamma-ray interaction process at high energies; multilevel automated spark-chamber gamma-ray telescopes; the Soviet Gamma-1 telescope; the EGRET instrument for the NASA Gamma-Ray Observatory; and Cerenkov, air-shower, and particle-detector instruments for the TeV and PeV ranges. Significant improvements in resolution and sensitivity are predicted for the near future.

Solar cosmic ray particle composition, charge and energy spectra of nuclear components and isotopic composition of H and He

The significance of high energy (≳30 MeV) γ-ray astronomy and its relationship to cosmic rays and many of the high energy processes of the universe has been realized for over a decade. Through the last six to eight years, searches for point sources, mostly from balloon experiments, but also Explorers 11 and OSO-3, have been unsuccessful in clearly establishing the existence of any point source. Recently on OSO-3, Clark et al. (1968) have obtained positive evidence for a celestial γ-ray flux which is anisotropic with a higher intensity in the direction of the galactic center region. In this talk, I wish to summarize our balloon results relating to both of these questions and indicate what we hope to accomplish with our new large γ-ray detector over the coming months.

Measurements of the flux of helium nuclei in the 24 January, 1971, event and of helium and (C, N, O) nuclei in the 1 September, 1971, event are combined with previous measurements to obtain the relative abundances of helium, (C, N, O), and Fe-group nuclei in these events. These data are then summarized together with previously reported results to show that, even when the same detector system using a dE/dx plus range technique is used, differences in the He/(C, N, O) value in the same energy/nucleon interval are observed in solar cosmic ray events. Further, when the He/(C, N, O) value is lower the He/(Fe-group nuclei) value is also systematically lower in these large events. When solar particle acceleration theory is analyzed, it is seen that the results suggest that, for large events, Coulomb energy loss probably does not play a major role in determining solar particle composition at higher energies (> 10 MeV). The variations in multicharged nuclei composition are more likely due to partial ionization during the acceleration phase.

Large area (0.5 m/spl times/0.5 m) drift chambers developed for the imaging system for the Advanced Gamma Ray Astronomy Telescope Experiment (AGATE), sensitive in the energy range 20 MeV to 100 GeV, have been tested at the Saskatchewan Accelerator Laboratory (SAL) in March 1995. Testing was performed with the instrument configured in 3 different angles relative to the incident photon beam using 2 different distributed pair production material thicknesses over three different incident photon tagging energy ranges. An instrument trigger with less than 1.5 ns FWHM jitter was implemented. Spatial resolution using Argon (85%)-Isobutane (15%) for the drift gas mixture is presented herein.

We report upper limits to the high-energy gamma-ray emission from the millisecond pulsars (MSPs) in a number of globular clusters. The observations were done as part of an all-sky survey by the energetic Gamma Ray Experiment Telescope (EGRET) on the Compton Gamma Ray Observatory (CGRO) during Phase I of the CGRO mission (1991 June to 1992 November). Several theoretical models suggest that MSPs may be sources of high-energy gamma radiation emitted either as primary radiation from the pulsar magnetosphere or as secondary radiation generated by conversion into photons of a substantial part of the relativistic e(+/-) pair wind expected to flow from the pulsar. To date, no high-energy emission has been detected from an individual MSP. However, a large number of MSPs are expected in globular cluster cores where the formation rate of accreting binary systems is high. Model predictions of the total number of pulsars range in the hundreds for some clusters. These expectations have been reinforced by recent discoveries of a substantial number of radio MSPs in several clusters; for example, 11 have been found in 47 Tucanae (Manchester et al.). The EGRET observations have been used to obtain upper limits for the efficiency eta of conversion of MSP spin-down power into hard gamma rays. The upper limits are also compared with the gamma-ray fluxes predicted from theoretical models of pulsar wind emission (Tavani). The EGRET limits put significant constraints on either the emission models or the number of pulsars in the globular clusters.