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Acceleration of energetic ions in solar flares produces narrow and broad nuclear γ-ray de-excitation lines, by nuclear interactions with the ambient solar atmosphere. Observations of several solar flare γ-ray spectra have been made since 1972 by satellite-borne γ-ray spectrometers on OSO 7, HEAO 3, SMM, HINOTORI, GRANAT, YOHKOH and CGRO. The line intensities depend directly on the ambient and accelerated particle abundances. In addition, production of neutrons in several reactions leads to emission of the neutron–proton capture γ-ray line, from the photosphere, at an energy of 2.223 MeV. The rate of decay in intensity of this line depends on the photospheric 3He/H abundance ratio. Current analysis gives a photospheric 3He/H abundance ratio of (2.3±1.2)×105. In addition, a study of narrow γ-ray line fluxes from several flares suggests that the ratio of (Mg + Si + Fe) to (C + N + O) abundances changes with time during the flare. Since this ratio measures the abundance ratio of low first ionization potential (FIP) elements to high FIP elements, the γ-ray production site may start in the lower chromosphere and move, as the flare progresses, into the corona where the low/high FIP abundance ratio increases. A study of broad γ-ray line fluxes also permits a determination of the abundance of flare-accelerated heavy ions.

The Liquid Xenon Gamma-Ray Imaging Telescope (LXeGRIT) is the first balloon-borne instrument developed to validate the concept of a monolithic detector with 3D imaging capability as a Compton telescope for MeV astrophysics. The geometrical area is about 350 cm 2 , an order of magnitude smaller than that of COMPTEL and the thickness of sensitive LXe is 7 cm, of equivalent stopping power as COMPTEL D2 detector. The spectroscopic and imaging response of LXeGRIT has been fully characterized in calibration experiments on the ground and during balloon flight experiments. During its most successful flight campaign of 27 h from Ft Sumner, in Fall 2000, the LXeTPC was operated without any external shield. The γ-ray background, measured at float altitude in the 0.5–10 MeV energy band, is well explained by the known atmospheric γ-ray flux. Results on the LXeGRIT in-flight performance, effective area, minimum flux sensitivity and background level are presented in this paper.

A liquid xenon time projection chamber (LXeTPC) has been developed to image cosmic γ-rays in the energy band 0.2–20 MeV. Its performance as Gamma Ray Imaging Telescope (LXeGRIT instrument) has been tested during a high altitude balloon flight (Spring ’99, New Mexico). The detector, with 400 cm2 area and 7 cm drift gap, is filled with 7 l high purity LXe. Both ionization and scintillation light signals are detected to measure the energy deposits and the three spatial coordinates of individual γ-ray interactions within the sensitive volume. During the pre-flight calibration experiments the LXeGRIT instrument was extensively tested with γ-ray sources in the laboratory: a 10% FWHM energy resolution at 1 MeV was determined, scaling with 1/ E . The detector shows a linear response in the energy range 511 keV–4.4 MeV.

Particle acceleration is intrinsic to the primary energy release in the impulsive phase of solar flares, and we cannot understand flares without understanding acceleration. New observations in soft and hard X-rays, γ-rays and coherent radio emissions are presented, suggesting flare fragmentation in time and space. X-ray and radio measurements exhibit at least five different time scales in flares. In addition, some new observations of delayed acceleration signatures are also presented. The theory of acceleration by parallel electric fields is used to model the spectral shape and evolution of hard X-rays. The possibility of the appearance of double layers is further investigated.

LXeGRIT is the first prototype of a novel concept of Compton telescope, based on the complete 3-dimensional reconstruction of the sequence of interactions of individual gamma rays in one position sensitive detector. This balloon-borne telescope consists of an unshielded time projection chamber with an active volume of 400 cm$^2 \times 7$ cm filled with high purity liquid xenon. Four VUV PMTs detect the fast xenon scintillation light signal, providing the event trigger. 124 wires and 4 anodes detect the ionization signals, providing the event spatial coordinates and total energy. In the period 1999 -- 2001, LXeGRIT has been extensively tested both in the laboratory and at balloon altitude, and its response in the MeV region has been thoroughly characterized. Here we summarize some of the results on pre-flight calibration, event reconstruction techniques, and performance during a 27 hour balloon flight on October 4 -- 5. We further present briefly the on-going efforts directed to improve the performance of this prototype towards the requirements for a base module of a next-generation Compton telescope., 13 pages, 11 figures, SPIE 2002 Proceedings, Conf. Vol. 4851 - 140; corrected references

LXeGRIT is a balloon-borne Compton telescope employing a large volume liquid xenon time projection chamber (LXeTPC) as the central gamma-ray detector. It is designed to image gamma rays in the energy range of 200 keV to 20 MeV, with an angular resolution of about 3 degrees (1 sigma) at 2 MeV, within a field-of-view (FOV) of about 1 sr. The detector's energy and three-dimensional spatial resolution as measured during pre-flight calibration experiments, are $\Delta E_\mathrm{lxe}/E=8.8% : \sqrt{1\MeV /E}$ and < 1mm RMS, respectively. The detection efficiency for Compton events varies between 1.5% and 4% depending on energy and event selection. We describe the instrument as flown on May 7, 1999 and review its overall performance at balloon altitude. The launch occurred at 13:26:54 UT from Ft. Sumner, New Mexico and the flight was terminated about 9 hours later. The Crab was in the instrument FOV for a few hours. Analysis of these data is in progress., Comment: 16 pages, 15 figures; To appear in: X-Ray and Gamma-Ray Instrumentation for Astronomy XI, 2000; Proc. SPIE, vol. 4140; K.A. Flanagan & O.H. Siegmund, eds

As part of our ongoing research program to develop a liquid xenon gamma-ray imaging telescope (LXe-GRIT) for medium energy astrophysics, we have built a liquid xenon time projection chamber (LXeTPC) with a total volume of 10 liters and a sensitive are of 20 cm by 20 cm. The detector has been successfully tested with gamma-ray sources in the laboratory and is currently being prepared as balloon-borne payload for imaging MeV gamma-ray emission from the Crab Nebula, Cygnus X-1 and the Orion molecular cloud region. The LXe-TPC, sensitive to gamma-rays from 300 keV to 30 MeV, measures the energy and the 3-D location of each gamma-ray interaction with a resolution of 6% FWHM and 1 mm RMS at 1 MeV, within a 1 sr FOV. Its detection efficiency for Compton events is about 4% in the 1 - 3 MeV, an energy band of great astrophysical interest for both continuum and line emission. Its 3 sigma continuum sensitivity of 1.8 multiplied by 10-7 ph cm-2s-1keV-1 for a nominal 10 hr observation time, will allow us to study a variety of sources with an imaging accuracy as good as 1 degree. We plan to pursue a vigorous program of balloon flights with this telescope to achieve the maximum science return while continuing a strong R&D laboratory program on LXe technology. The ultimate goal is an optimized design of a satellite implementation of a liquid xenon gamma-ray imaging instrument that will lead to drastic improvements in sensitivity and angular resolution in the 0.3 - 30 MeV band and beyond.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We describe a unique γ-ray imaging telescope operating in the energy range 0.3-10 MeV. The basic element of the telescope is a liquid xenon time projection chamber (LXe-TPC) as γ-ray spectrometer and 3-D imager. Location and energy analysis of multiple Compton scattering events in the chamber will permit a reconstruction of the incoming γ-ray direction, thus allowing a significant reduction of background. A 10-liter LXe-TPC prototype, with 400 cm2 active area and 5 cm drift gap has been built and is currently being tested with γ-ray sources in the laboratory. Initial results are presented. The combination of the LXe-TPC with a coded aperture (LXe-CAT), results in a telescope with superior detection efficiency, angular and energy resolution, and excellent background rejection capability. Simulation results on the instrument’s ability to image the Crab Nebula region in a balloon observation are presented, as well as to image the 1.809 MeV 26A1 distribution from the Vela region, in a satellite observation.

We describe a conceptual plan for the development of a comprehensive long duration solar space observatory, the Advanced Solar Observatory (ASO). The ASO is intended to provide solar astronomers with the observational power (spectral, spatial, and temporal resolution, sensitivity, and breadth of wavelength coverage) necessary to address fundamental problems relating to the solar convection zone and activity cycle, the thermal and nonthermal processes which control the transport of energy, mass and magnetic flux in the solar atmosphere, the generation of the solar wind, and the dynamics of the inner heliosphere. The Advanced Solar Observatory concept encompasses three proposed Space Station based instrument ensembles: (i) the High Resolution Telescope Cluster which includes far ultraviolet, extreme ultraviolet and x-ray telescopes, (ii) the Pinhole/Occulter Facility which includes Fourier transform and coded aperture hard x-ray and gamma ray telescopes and occulted ultraviolet and visible light coronagraphs, and (iii) the High Energy Facility which contains neutron, gamma ray and low frequency radio spectrometers. Two other facilities, the Orbiting Solar Laboratory (OSL), which will contain high resolution visible and ultraviolet telescopes on a free-flying platform, and a package of Global Dynamics Instruments will, with the Space Station ensembles, form a comprehensive capability for solar physics. We describe the scientific program of the ASO, current instrument concepts for the Space Station based ASO instrument ensembles, and pians for their accommodation on the Space Station. subject terms: solar atmosphere, soft x-ray, XUV, extreme ultraviolet (EUV), far ultraviolet (FUV), hard x-rays, gamma rays, neutrons

A conceptual plan for the development of a comprehensive long duration solar space observatory, The Advanced Solar Observatory (ASO) is described. The ASO is intended to provide solar astronomers with the observational power necessary to address fundamental problems relating to the solar convection zone and activity cycle; the thermal and nonthermal processes that control the transport of energy, mass, and magnetic flux in the solar atmosphere; the generation of the solar wind; and the dynamics of the inner heliosphere. The ASO concept encompasses three proposed Space Station-based instrument ensembles: (1) the High Resolution Telescope Cluster, which includes far ultraviolet, extreme ultraviolet, and X-ray telescopes; (2) the Pinhole/Occulter Facility, which includes Fourier transform and coded aperture hard X-ray and gamma ray telescopes and occulted ultraviolet and visible light coronagraphs; and (3) the High Energy Facility, which contains neutron, gamma ray, and low frequency radio spectrometers. Two other facilities, the Orbiting Solar Laboratory, and a package of Global Dynamics Instrumentation, will, with the Space Station ensembles, form a comprehensive capability for solar physics. The scientific program of the ASO, current instrument concepts for the Space Station based ASO instrument ensembles, and plans for their accommodation on the Space Station are described.

The Gamma-Ray Spectrometer on the SMM satellite has recorded high-energy gamma-ray and neutron emission from the flare on June 3, 1982. During the 65-s impulsive phase the gamma rays above 10 MeV contain emissions from both primary electron bremsstrahlung and nuclear pion decay. Hence the impulsive phase acceleration process must produce both primary electrons with energies above 60 MeV and ions above 500 MeV. This flare also has a extended emission phase lasting more than 1000 s which is most easily observed at gamma-ray energies above 10 MeV. After removing the counting rates from the more slowly moving neutrons produced at earlier times, the resulting gamma ray spectrum can be entirely explained by nuclear pion production. Over 70 percent of the pions were produced in the extended emission phase. In contrast, more than 70 percent of the high-energy primary electron bremsstrahlung and the nuclear line emission produced by ions below 30 MeV occurred in the 65-s impulsive phase. This represents the first clear observation of a new acceleration process which produces an electron-deficient, very hard ion spectrum extending beyond 1000 MeV.

A search is made for anisotropic X-ray bremsstrahlung photon production from relativistic electrons by studying the heliocentric angular dependence of 53 flares detected at energies above 300 keV. No evidence is found for a higher rate of detectable flares near the limb at the 80 percent confidence level. This result implies that the X-ray directivity as defined by the ratio of photon intensity at 75 deg and 0 deg of heliocentric angle is less than 1.5 above 300 keV and strongly rejects any flare model predicting X-ray production from a radial 'beam' of energetic electrons.

A directional detector for gamma-ray astronomy has been developed to image sources in the energy range 0.1 to 5 MeV. An array of 35 gain stabilized bismuth germanate detectors, together with a coded aperture mask based on a uniformly redundant array allows imaging in 4 deg square sky bins over a 16 x 24 deg field-of-view. The position of a strong point source, such as the Crab Nebula, can be determined to within not more than about 1 deg. A complementary 'anti-mask' greatly reduces systematic effects arising from nonuniform background rates amongst the detectors. The telescope has an effective area of 190 sq cm and an energy resolution of 19.5 percent FWHM at 662 keV. Results of laboratory tests of the imaging system, including the ability to image multiple sources, uniformity of response over the field-of-view, and the effect of the 'anti-mask', are in good agreement with computer simulations. Features of the flight detector system are described and results of laboratory tests and computer simulations are reviewed. A balloon flight of the telescope is planned for the fall of 1982.

A new solar gamma-ray telescope is described which is intended to take advantage of current long-duration ballon facilities such as the RACOON system. The primary scientific objective is to detect and measure gamma-ray lines from solar flares, along with the associated low-energy continuum. The proposed instrument is centered on a multiheaded Ge system and is designed to operate over the energy range 50 keV to 200 200 MeV. In the nuclear transition energy region, the average energy resolution of the primary detectors is over 20 times better than that achieved with the gamma-ray spectrometer on the Solar Maximum Mission satellite.

A 3" × 3" NaI(Tl) gamma ray (0.3 to 10 MeV) spectrometer with a CsI(Na) charged particle and anti-Compton shield has been developed for the ORBITING SOLAR OBSERVATORY (OSO-7) which was launched September 30, 1971. The instrument, designed for a rotating wheel compartment, utilizes a 377 channel quadratic PHA with accumulation times of 3, 1, or 0.5 minutes. Quick look and calibration data obtained via a direct data link to a minicomputer at the UNH campus allows near real time monitoring and control of the experiment. Various commands changing the operating mode can be executed. The functions which can be commanded include: rotation of the quadrants in which data is collected by 90°; gain adjustment of the central detector over a 6:1 range; manual or automatic sequencing of calibrations; variation of accumulation times by telemetering selected channels; and selection of reference directions. A small X-ray detector covering the range 7.5 to 120 keV is also included.

The energy and angular response of the University of New Hampshire solar gamma ray monitor on the OSO-7 spacecraft has been measured. The fully loaded and rotating spacecraft was exposed to strong gamma sources while data was collected through the spacecraft telemetry system by a minicomputer under real time control. The response function was measured for gamma ray energies of 0.393, 0.662, 1.12 and 2.75 MeV for typical angular increments of 15°. The actual means of carrying out the measurements are described in detail, since these techniques may be valuable for the calibration of future experiments (e.g., on HEAO-A) where the properties of the spacecraft itself must be taken into account. The results and interpretation of the energy dependent absolute response function are given.

Positron annihilation radiation has been observed from the 21 June 1980 and 3 June 1982 flares by the gamma‐ray spectrometer on the Solar Maximum Mission satellite. The observed 0.511 MeV line fluences from the flares were 14.6±3.3 γ/cm2 and 103±8 γ/cm2, respectively. Measurement of the line width allows us to set an upper limit to the temperature in the annihilation region of 3×106 °K. The time dependence of the 0.511 MeV line during the 21 June 1980 flare is consistent with Ramaty et al.’s calculations for positrons created in the decay of radioactive nuclei. The time dependence of 0.511 MeV line for the 3 June 1982 flare is more complex and requires more detailed study.

We discuss broad‐band continuum spectroscopy of 17 gamma‐ray bursts above 0.3 MeV. The spectra were fitted by 3 trial functions, none of which provided an adequate fit to all the spectra. Most were too hard for a thermal bremsstarhlung function. Harder functional forms, such as thermal synchrotron or power‐law, provide better fits for most of the spectra. The strong emission observed above 1 MeV raises some interesting theoretical questions.

The Gamma Ray Spectrometer on the SMM satellite has observed solar cosmic energetic photon transients since 17 February 1980. Using the data available through 1981, new results have been obtained on ion acceleration phenomena in solar flares. It now is evident that both ion and electron acceleration can take place impulsively, simultaneously or within seconds of one another. That the impulsive acceleration process can produce ions with energies as high as GeV/nucleon is directly shown by observations of neutrons at the Earth with energies of several hundred MeV. These two facts and the relative timing of hard X-ray emissions provide new constraints on solar flare particle acceleration theory. New flare spectra have also been observed showing new nuclear γ-ray lines not previously observed from 24 Mg, 20 Ne and 56 Fe as well as from other elements. These spectral observations provide new information on the relative abundances of the accelerated and target nuclei. Following a review of the solar data and implications for flare theories we will also give a brief review of the results obtained on nonsolar γ-ray bursts. Most such bursts have photon spectra extending to MeV energies but with little, if any, evidence for spectral features.

Results are presented for the flare of July 1, 1980, which started at approximately 1627 UT and in which simultaneous measurements were made of X-ray, gamma-ray, and optical continuum emission for the entire duration of the flare. The X-ray and gamma-ray observations were made by the Gamma-Ray Spectrometer on the Solar Maximum Mission satellite. The optical measurements were taken at the Sacramento Peak Observatory and the Big Bear Solar Observatory (Zirin and Neidig, 1981). It is found that the major white-light emission that occurs in the late phase of the flare could not have been due to heating by electron or ion precipitation. This conclusion derives from the fact that the X-ray and gamma-ray flux peaks approximately 1 minute before the maximum of the optical continuum mission emission. It is also found that approximately 73 percent of the optical continuum emission, representing a spatially and temporally distinct bright point, follows this maximum with little or no X-ray or gamma-ray emission in the same period.

The region of the Galactic anticenter, including the Crab Nebula, was observed during a balloon flight of the University of New Hampshire Directional Gamma-Ray Telescope employing the coded-aperture imaging technique to image celestial gamma-radiation between 160 keV and 9.3 MeV. The background systematics are treated with a simple and relatively straightforward correction procedure. The results demonstrate that the coded-aperture procedure is a viable approach for imaging not only point sources of radiation, but also extended sources of emission. The results for the Crab's photon spectrum are consistent with a power-law spectrum. Upper limits on the flux levels of line emission at 405 keV and 1050 keV and on the flux from the X-ray binary source A0535 + 26 and diffuse Galactic emission from the anticenter region are derived.

A balloon-borne coded-aperture telescope, measuring gamma-ray photons in the 160 keV to 9.3 MeV range, was used to observe the Cygnus region of the sky on October 1 and 2, 1984. In the 2-9.3-MeV band, evidence is found for a hard spectral component with a mean flux level at the top of the atmosphere of 7.4 + or - 2.5 x 10 to the -7th photons/sq cm per s per keV, inconsistent with the predictions of the inverse Compton models normally used to describe the X-ray emission. Both Cyg X-1 and Cyg X-3 could be observed simultaneously with the telescope. The results are used to establish 1-sigma upper flux limits on the spectral emission from Cyg X-3.