Search

We present results on the search for two-neutrino double-electron capture (2νECEC) of 124Xe and neutrinoless double-β decay (0νββ) of 136Xe in XENON1T. We consider captures from the K shell up to the N shell in the 2νECEC signal model and measure a total half-life of T2νECEC1/2=(1.1±0.2stat±0.1sys)×1022yr with a 0.87kgyr isotope exposure. The statistical significance of the signal is 7.0σ. We use XENON1T data with 36.16kgyr of 136Xe exposure to search for 0νββ. We find no evidence of a signal and set a lower limit on the half-life of T0νββ1/2>1.2×1024yrat90%CL. This is the best result from a dark matter detector without an enriched target to date. We also report projections on the sensitivity of XENONnT to 0νββ. Assuming a 275kgyr 136Xe exposure, the expected sensitivity is T0νββ1/2>2.1×1025yrat90%CL, corresponding to an effective Majorana mass range of ⟨mββ⟩

International audience; In the era of multi-messenger astronomy it is of paramount importance to have in space a gamma-ray monitor capable of detecting energetic transients in the energy range from 0.1 MeV to a few hundred MeV, with good imaging capabilities. The All-Sky-ASTROGAM mission proposal aims to place into an L2 orbit a gamma-ray instrument ($\sim$ 100 kg) dedicated to fast detection, localization, and gamma-ray spectroscopy of flaring and merging activity of compact objects in the Universe, with unprecedented sensitivity and polarimetric capability in the MeV range. The instrument is based on the ASTROGAM concept, which combines three detection systems of space-proven technology: a silicon tracker in which the cosmic gamma rays undergo Compton scattering or a pair conversion, a scintillation calorimeter to absorb and measure the energy of the secondary particles, and an anticoincidence system to veto the prompt reaction background induced by charged particles. The gamma-ray imager and the platform will be connected through a boom and will have almost no occultation, making possible a continuous monitoring of every single gamma-ray source in the sky during the entire mission lifetime.

The XENON1T liquid xenon time projection chamber is the most sensitive detector built to date for the measurement of direct interactions of weakly interacting massive particles with normal matter. The data acquisition system (DAQ) is constructed from commercial, open source, and custom components to digitize signals from the detector and store them for later analysis. The system achieves an extremely low signal threshold below a tenth of a photoelectron using a parallelized readout with the global trigger deferred to a later, software stage. The event identification is based on MongoDB database queries and has over 97% efficiency at recognizing interactions at the analysis energy threshold. A readout bandwidth over 300 MB/s is reached in calibration modes and is further expandable via parallelization. This DAQ system was successfully used during three years of operation of XENON1T., 14 pages, 6 figures, submitted to JINST; Version 2 with minor updates to text

In Fig. 5 of our original article, we compared measurements and predictions of the charge yield Qy. In that figure, the LUX points were misrepresented, and therefore we present here in Fig. 1 the corrected points from Ref. [1].

n our paper, we presented searches for solar axions and galactic axionlike particles (ALPs) in the data collected by the XENON100 experiment (with an exposure of 224.6 days). We recently found a bug in the code to calculate the exclusion limit for galactic ALPs. This resulted in an underestimation of the ALP expected rate, which in turn led to an overly conservative limit, compared to what it should really be. We corrected the code, and the result of the XENON100 90% C.L. exclusion limit on galactic ALPs (shown in Fig. 1) was reevaluated. The corrected limit is stronger than the one previously published by approximately a factor of 5 across all masses and sets the best published limit on the axion-electron coupling, $g_{Ae}$, in the (1–40) keV/$c^2$ mass range.

Silicon Photomultipliers as photosensors for liquid xenon detectors are an attractive alternative to photomultiplier tubes: liquid xenon detectors employed in low background experiments or compact Compton cameras can take advantage of the small sensor mass and dimensions, the larger light collection coming from a more packed tiling of the readout plane, and possibly a smaller cost per area. We investigate the operation in liquid xenon of samples developed by Ketek GmbH. To provide sensitivity to the liquid xenon scintillation light wavelength, 178 nm, the inactive entrance layer of the device has been thinned. The operation of such devices at temperatures as low as −100°C has been verified, characterizing dark count rate and after-pulsing as well. In order to not deteriorate the liquid xenon purity, we selected a substrate and a bonding adhesive measuring the outgassing properties of the parts. Finally, we realized a test station to assess the photon detection efficiency at a wavelength of 178 nm. In a cryogenic vacuum-insulated vessel an 241Am α-source and up to four samples are immersed in liquid xenon, allowing an event-by-event comparison of the response of three Silicon Photomultipliers and a 1” Photomultiplier Tube used as reference.

We report the detection of the Sc-44 nuclear decay lines at 67.9 keV and 78.4 keV associated with the nuclear decay of Ti-44 in Cassiopeia A. The line emission was observed by the PDS instrument on board BeppoSAX, which recently observed the supernova remnant for over 500 ks. The detection of the line emission with a flux of (2.1 +/-0.7) 10^{-5} ph/cm^2/s in each line (90% confidence) is at the 5 sigma significance level, if we can assume that the 12-300 keV continuum is adequately represented by a single power law. However, as the nature of the continuum is not clear we investigate various other possibilities. A more conservative estimate of the line flux is made by assuming that a power law continuum is at least a good approximation to the continuum emission for a narrower 30-100 keV energy range. With this limitation the measured line flux is (1.9 +/- 0.9) 10^{-5} ph/cm^2/s, with the detection still at the 3.4 sigma significance level. We suggest that together with the CGRO-COMPTEL measurement of the Ca-44 line at 1157 keV of (3.5 +/- 0.9) 10^{-5} ph/cm^2/s a flux for all three lines of (2.5 +/- 1.0) 10^{-5} ph/cm^2/s for Cas A can be adopted. This implies an initial Ti-44 mass of (0.8 - 2.5) 10^{-4} Msun.

LXeGRIT is a balloon-borne Compton telescope for MeV gamma-ray astrophysics, based on a liquid xenon time projection chamber with charge and light readout. The energy and direction of an incident gamma-ray is reconstructed from the three-dimensional locations and energy deposits of individual interactions taking place in the homogeneous detector volume. While the charge signals provide energy information and X-Y positions, the fast xenon scintillation light signal is used to trigger the detector. The drift time measurement, referred to the time of the trigger signal, gives the Z-position with the known drift velocity. The light is detected by four UV-sensitive photomultiplier tubes (PMTs). The logical OR of the PMT signals triggers the data acquisition system with an efficiency which depends on the event energy and location, as well as on the discriminator thresholds used on the individual PMTs. Results from experiments with a tagged 22Na source give the spatial distribution of the light trigger efficiency for 511 keV gamma-rays. When averaged over the whole sensitive volume amd all PMTs, the trigger efficiency is 47% or 40% for two discriminator windows used during the LXeGRIT balloon flight of 1999. These values are strongly affected by the different sensitivity of each PMT. The corresponding average efficiency at 511 keV for the best of the four PMTs is in fact 63%, and approaches 100% for interactions taking place in a small volume right above the PMT.

LXeGRIT is a balloon-borne Compton telescope for MeV gamma-ray astrophysics, based on a liquid xenon time projection chamber (LXe-TPC) with charge and light readout. The first balloon flights in 1997 revealed limitations of the trigger electronics and the data acquisition (DAQ) system, leading to their upgrade. New electronics was developed to handle the xenon scintillation light trigger. The original processor module was replaced by a commercial VME processor. The telemetry rate was doubled to 2 x 500 kbps and onboard data storage on hard disks was implemented. Relying on a robust real-time operating system, the new DAQ software adopts an object oriented design to implement the diverse tasks of trigger handling, data selection, transmission, and storage, as well as DAQ control and monitor functions. The new systems performed well during two flights in Spring 1999 and Fall 2000. In the 2000 flight, the DAQ system was able to handle 300-350 triggers/s out of a total of about 650 Hz, including charged particles.

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.

The instrumental line background of the Compton telescope COMPTEL onboard the Compton Gamma-Ray Observatory is due to the activation and/or decay of many isotopes. The major components of this background can be attributed to eight individual isotopes, namely 2D, 22Na, 24Na, 28Al, 40K, 52Mn, 57Ni, and 208Tl. The identification of instrumental lines with specific isotopes is based on the line energies as well as on the variation of the event rate with time, cosmic-ray intensity, and deposited radiation dose during passages through the South-Atlantic Anomaly. The characteristic variation of the event rate due to a specific isotope depends on its life-time, orbital parameters such as the altitude of the satellite above Earth, and the solar cycle. A detailed understanding of the background contributions from instrumental lines is crucial at MeV energies for measuring the cosmic diffuse gamma-ray background and for observing gamma-ray line emission in the interstellar medium or from supernovae and their remnants. Procedures to determine the event rate from each background isotope are described, and their average activity in spacecraft materials over the first seven years of the mission is estimated., Comment: accepted for publication in A&A, 22 pages, 21 figures

The imaging Compton telescope COMPTEL aboard NASA's Compton Gamma-Ray Observatory has opened the MeV gamma-ray band as a new window to astronomy. COMPTEL provided the first complete all-sky survey in the energy range 0.75 to 30 MeV. The catalogue, presented here, is largely restricted to published results. It contains firm as well as marginal detections of continuum and line emitting sources and presents upper limits for various types of objects. The numbers of the most significant detections are 32 for steady sources and 31 for gamma-ray bursters. Among the continuum sources, detected so far, are spin-down pulsars, stellar black-hole candidates, supernova remnants, interstellar clouds, nuclei of active galaxies, gamma-ray bursters, and the Sun during solar flares. Line detections have been made in the light of the 1.809 MeV 26Al line, the 1.157 MeV 44Ti line, the 847 and 1238 keV 56Co lines, and the neutron capture line at 2.223 MeV. For the identification of galactic sources, a modelling of the diffuse galactic emission is essential. Such a modelling at this time does not yet exist at the required degree of accuracy. Therefore, a second COMPTEL source catalogue will be produced after a detailed and accurate modelling of the diffuse interstellar emission has become possible., Comment: 50 pages including 4 figures; accepted for publication in A&A Supplements

COMPTEL observations of the Orion/Monoceros region have shown distinct evidence for excessive 3-7 MeV emission that was attributed to nuclear de-excitation lines from accelerated 12C and 16O nuclei. Unfortunately, we must conclude now that this appears to be a spurious result. This conclusion follows from a better understanding of the instrumental background, from a better exposure of the region, and from an improved analysis method. We show here how the impact of each of these gradually reduces the signal to a less than 3 σ result. The prime underlying cause seems to be 24Na activation in and around the upper COMPTEL detectors. Combining all available data, we now set a 2 σ flux upper limit on the 3-7 MeV emission of Orion of 3 × 10-5 γ cm-2 s-1, to be compared with the previously derived flux of ~10-4 γ cm-2 s-1.

The COMPTEL imaging telescope has measured γ-ray lines originating from decay of the radioactive isotopes 26Al and 44Ti: Imaging of the full sky in the 1.809 MeV line has presented a glow of nucleosynthesis from the plane of the Galaxy, yet with distinct irregularities on top of the expected emission from average nucleosynthesis. The measured sky profile can be understood as a composite of plausible sources. Alternatively, however, it may be interpreted as a mapping of regions of prominent nucleosynthetic activity from very massive stars. Current results favour massive stars as candidate sources, as supported from indications of spiral structure and from association of prominent emission with known massive-star rich regions. — The COMPTEL measurement of 44Ti gamma-rays from the Cas A supernova remnant presents the first detection of this isotope from supernova nucleosynthesis. Recent observations from CGRO Cycle 5 independently confirm the detection, consistent with previous results. With these data, further assessment of systematic effects is expected to yield a better absolute flux value from Cas A.

The Galactic supernova remnant Cas A is believed to be the result of a recent (∼ 300 yrs) and nearby (D ∼ 3 kpc) explosion of a massive star. The discovery of gamma-ray line emission due to radioactive 44 Ti with COMPTEL supports the massive star origin of Cas A. However, the ejection of 44 Ti should be accompanied by large amounts of 56 Ni and 57 Ni, which would have made this a very bright supernova. No 17th century sightings of this event exist, with the notable exception of the ∼ 6th mag star 3 Cas reported by Sir John Flamsteed. This mystery may be solved by assuming that the supernova was enshrouded by a dusty envelope, perhaps provided by a pre-supernova wind. X-ray observations with ROSAT provide evidence for this scenario through the simultaneous analysis of absorption along the line of sight and the extended X-ray scattering halo around Cas A. The supernova shock destroyed the dust, reducing the line-of-sight opacity to a value consistent with a 3 kpc path through an average ISM. ROSAT detects the evaporated dust as excess absorption.

26Al radioactivity is believed to originate predominantly from massive stars, ejected into interstellar medium in wind phases and/or supernova events. With its million-year decay time, penetrating γ-rays from 26Al decay measure the massive-star history averaged over a time scale of ≃million years, thus extending times cales accessible otherwise. The COMPTEL 1.809 MeV all-sky data from 5 years of observations show irregularities and features at intermediate latitudes, which may have a more local origin (≃ 1 kpc). We find that the large scale emission can be characterized by a Galactic scale height of ≃ 130 pc, and a Galactocentric scale radius of ≃ 5 kpc, with features from spiral structure. Catalogues from massive-star related objects do not significantly improve the description of COMPTEL data above this. Emission associated with nearby structures such as the Gould Belt, Loop I, or stellar aggregates, is indicated, yet cannot be clearly detected. Combined with our imaging results, this suggests that 26A1 yields from massive star ensembles depend on specifics of those stars and their history. Further 26A1 γ-ray studies are underway to help mapping of the massive star history in the solar vicinity.

We estimate the 1.8 MeV luminosity of the Sco-Cen association due to radioactive decay of 26A1 to (4 – 15) 10−5ph cm−2 s−1. We propose a low surface brightness, limb brightened bubble for the 1.8 MeV intensity distribution. The detectibility of this distribution with existing γ-ray telescopes is discussed.

Laboratory experiments searching for galactic dark matter particles scattering off nuclei have so far not been able to establish a discovery. We use data from the XENON100 experiment to search for dark matter interacting with electrons. With no evidence for a signal above the low background of our experiment, we exclude a variety of representative dark matter models that would induce electronic recoils. For axial-vector couplings to electrons, we exclude cross-sections above 6x10^(-35) cm^2 for particle masses of m_chi = 2 GeV/c^2. Independent of the dark matter halo, we exclude leptophilic models as explanation for the long-standing DAMA/LIBRA signal, such as couplings to electrons through axial-vector interactions at a 4.4 sigma confidence level, mirror dark matter at 3.6 sigma, and luminous dark matter at 4.6 sigma., 4 pages, 4 figures, with supporting online material

The COMPTEL gamma-ray telescope has been operating in low Earth orbit for six years, since the launch of the Compton Gamma-Ray Observatory in April 1991. Comparisons of data for different orbits and epochs show evidence of activation on time scales from minutes (/sup 27/Mg, /spl tau//sub 1/2/=9.5 min) to years (/sup 22/Na, /spl tau//sub 1/2/=2.58 yr). The activation is correlated with both the orbital altitude and solar cosmic-ray modulation. Because it requires coincident measurements in two different detectors, COMPTEL is most susceptible to instrumental background events in which two or more photons are produced simultaneously.

Imaging arrays of thick CdTe detectors, with millimeter spatial resolution and better than 10 keV energy resolution at 1 MeV, are very promising for the calorimeter section of an Advanced Compton Telescope. Measurements with small CdTe detectors with Schottky contacts, and advances in fabrication technology, have led us to the design of an 8×8 pixel array, with 2×2 mm2 pixel size, as a first prototype module for large arrays. The detectors are arranged with the electric field direction perpendicular to the incoming gamma-ray direction, allowing several centimeters deep arrays, with the minimum amount of passive material. With the development of the 8×8, 10 mm deep array, we aim to demonstrate the feasibility of the concept for gamma-ray imaging.

The observation of the interstellar 1.809 MeV decay-line of radioactive 26 Al by the imaging gamma-ray telescope COMPTEL have let to the conclusion, that massive stars and their subsequent core-collapse su­ pernovae are the dominant sources of the interstellar 26 Al abundance. Massive stars are known to affect the surrounding interstellar medium by their energetic stellar winds and by the emission of ionising radia­ tion. We present a population synthesis model allowing the correlated investigation of the gamma-ray emission characteristics with integrated matter, kinetic energy and extreme ultra-violet radiation emission of as­ sociations of massive stars. We study the time evolution of the various observables. In addition, we discuss systematic as well as statistical un­ certainties affecting the model. Beside uncertainties in the input stellar physics such as stellar rotation, mass loss rates or internal mixing mod­ ifications due to a unknown binary component may lead to significant uncertainties.

The star forming process in the Milky Way is non-uniform in time and space. The scale of star forming regions ranges from groups within a few pc to large segments of spiral arms with linear dimension of order kpc. When many stars form in a relatively small volume over a short duration, a localized starburst ensues. The energetic impact of such a burst of star formation can severely affect the dynamic structure of the gaseous disk. Stellar winds and supernova explosions drive an expanding superbubble, whose size eventually exceeds the scale height of the disk and thus drives a disk-wind blowing metal enriched gas into the halo. We discuss the basic scenario of superbubble evolution, emphasizing the associated gamma-ray line signatures. In particular, we discuss nuclear line emission from 26Al and 60Fe in the Cygnus region., Comment: 5 pages, 3 figures, to appear in Proc. of Gamma-Ray Astrophysics 2001, to be published in AIP Conf. Proc. Series

LXeGRIT is a balloon-borne Compton telescope based on a liquid xenon time projection chamber (LXeTPC) for imaging cosmic \g-rays in the energy band of 0.2-20 MeV. The detector, with 400 cm$^2$ area and 7 cm drift gap, is filled with high purity LXe. Both ionization and scintillation light signals are detected to measure the energy deposits and the three spatial coordinates of individual \g -ray interactions within the sensitive volume. The TPC has been characterized with repeated measurements of its spectral and Compton imaging response to \g -rays from radioactive sources such as \na, \cs, \yt and Am-Be. The detector shows a linear response to \g -rays in the energy range 511 keV -4.4 MeV, with an energy resolution (FWHM) of $ \Delta E/E=8.8% \: \sqrt{1\MeV /E}$. Compton imaging of \yt \g -ray events with two detected interactions is consistent with an angular resolution of $\sim$ 3 degrees (RMS) at 1.8 MeV., Comment: To appear in: Hard X-Ray, Gamma-Ray and Neutron Detector Physics XI, 2000; Proc. SPIE, vol. 4140; K.A. Flanagan & O.H. Siegmund, eds

The feasibility of a large-volume liquid xenon Compton telescope based on full event imaging in a time projection chamber has been demonstrated with the development of the balloon-borne instrument LXeGRIT. With its 400 cm2 sensitive area and 7 cm drift gap, the liquid xenon detector images γ-rays in the energy range from 200 keV to 25 MeV. The precise 3-dimensional localization of γ-ray interactions within the sensitive volume provides excellent background reduction capabilities. Together with the large efficiency of a homogeneous detector volume, LXeGRIT addresses the primary instrumental limitations encountered in this energy band. Following engineering tests at balloon altitude in 1997, LXeGRIT has been upgraded with a new trigger and data acquisition system, integrated with the existing readout electronics. Enhanced data transfer capability and onboard data storage were also implemented. LXeGRIT was successfully flown from Ft. Sumner, NM on May 7, 1999. The instrument worked as expected at balloon altitude. The large amount of data gathered during the 9.5 h flight are currently being analyzed. We describe the instrument and present some results from calibration data and preliminary results from the flight.

The instrumental-line background of the Compton telescope COMPTEL onboard the Compton Gamma-Ray Observatory is due to the activation and/or decay of a number of different isotopes. The major components of the COMPTEL instrumental-line background can be attributed to eight individual isotopes, namely 2D, 22Na, 24Na, 28Al, 40K, 52Mn, 57Ni, and 208Tl. In addition, evidence for the presence of 27Mg has been obtained in the search for gamma-ray lines from supernovae. The identification of the instrumental lines with specific isotopes is based on the line energies as well as on the variation of the activity with time, cosmic-ray intensity and deposited radiation dose during passages through the South-Atlantic Anomaly. The characteristic variation of the activity due to an individual isotope depends on its life-time, orbital parameters, and the solar cycle.

The COMPTEL map of the 1.809 MeV line, which is attributed to the radioactive decay of 26Al, shows significant excess emission in the Cygnus region. We investigate counterparts of this emission, based on COMPTEL data accumulated over 8 years. Previous studies suggested that this flux is due to the integrated, but unresolved, nucleosynthesis from young, massive objects like Wolf Rayet (WR) stars and core collapse supernovae. Beside a static scenario of known point sources-WR stars and SNRs-we consider OB associations in a dynamic scenario to model the observed flux in the Cygnus region. We carry out population synthesis studies to determine the time-dependent production of 26Al and also kinetic as well as radiative energy. Furthermore we consider the impact of enhanced 26Al yields probably occurring in a fraction of massive close binary systems. We compare the nucleosynthesis results of our OB association model with recent measurements in the Cygnus region.

44Ti is one of the few radioactive nuclei produced abundantly in supernovae (SNe). Because its life time is comparable to the typical time between supernovae in the Galaxy, γ-ray lines from 44Ti provide a powerful tool for the search for optically obscured supernovae that occurred during the last few 100 years. We present our analysis of COMPTEL data (Cycle 1–6), comparing γ-ray line images and flux distributions with Monte Carlo simulations of SNe in the Galaxy. Assuming standard SNe distribution models and theoretical estimates of the 44Ti yields, we constrain the Galactic supernova rate. We compare these constraints with those obtained from the historical record covering the last millennium. Relating the current supernova rate to the observed 44Ti solar abundance through chemical evolution models, agreement can be obtained only if there is a special type of supernova (e.g., helium detonations) that produces large amounts of 44Ti, but with a very low event rate.

We present an updated version of COMPTEL’s 1.809 MeV sky survey. Based on eight years of observations we compare results from different imaging techniques using background from adjacent energy bands. We confirm the previously reported characteristics of the galactic 1.809 MeV emission, specifically an extended galactic ridge emission, mainly concentrated towards the inner galaxy, a peculiar emission feature in the Cygnus region, and a low-intensity ridge extending towards Carina and Vela. Because this gamma ray line is due to the decay of radioactive 26Al, predominantly synthesized in massive stars, one anticipates flux enhancements aligned with regions of recent star formation. This is born out by the observations. In particular the Cygnus feature, first presented in 1996 based on three years of COMPTEL data, is confirmed. Based on the stellar population we distinguish three prominent areas in this region, for which we separately derive fluxes, and discuss interpretations.

XENA is a new Compton telescope concept, designed to image about 50% of the gamma-ray sky with a sensitivity that would significantly surpass CGRO/COMPTEL's multi-year sensitivity with a 2 weeks balloon flight from the Southern Hemisphere. The detector, based on liquid xenon time-projection chambers, is optimized for approximately 0.3 - 10 MeV and combines high efficiency within a 3 sr field-of-view with approximately 1 degree(s) angular resolution and excellent background reduction capability. XENA's primary scientific goal is the discovery and mapping of 60Fe radioactivity from the Galaxy, which is pivotal for understanding nucleosynthesis. XENA will detect 60Fe even if current predictions are 7X overestimated. At 1.8 MeV, XENA's sensitivity (6 10-6 cm-2 s-1) will significantly refine the COMPTEL 26Al mapping along the Southern Milky Way. Also, XENA would be the first instrument capable to decide whether the 3 - 7 MeV excess seen in Orion is indeed due to nuclear lines from 12C and 16O, and it could discover the predicted lower-energy lines. The scanned sky area includes many continuum (gamma) -ray sources as well, such as pulsars and numerous (gamma) -ray AGNs. Secondary scientific objectives include also supernova remnants, gamma-ray bursts, and solar flares.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We estimate the 1.8 MeV luminosity of the Sco-Cen association due to radioactive decay of 26Al to (4-15) 10e-5 ph cm**-2 s**-1. We propose a low surface brightness, limb brightened bubble for the 1.8 MeV intensity distribution. The detectibility of this distribution with existing gamma-ray telescopes is discussed.

26Al emission at 1.809 MeV has been measured by several different instruments. The constraints set by each are very different, due to differing sky exposures and background estimation methods. We report on a first combined analysis of data from the COMPTEL, OSSE, and SMM instruments. The COMPTEL 1.8 MeV image as deconvolved with the Maximum-Entropy algorithm was tested for its flux normalization and its fine structure in the inner Galaxy, using data from SMM and OSSE. We find consistency of the OSSE data with COMPTEL’s image structure, in particular the choice of the ‘stopping point’ of the iterative deconvolution method. The SMM data suggest substantially more total flux than that given by the COMPTEL map. We discuss possible explanations, in particular large-scale emission outside the plane of the Galaxy, and outline future work on combined data from the three instruments.

Nucleosynthesis models predict the production of 60 Fe by the same massive stars which are responsible for 26 Al synthesis. With a radioactive decay time similar to 26 Al , the gamma-ray line emission at 1.173 and 1.332 MeV is predicted to be ∼16% of the 1.809 MeV 26 Al line intensity, from the same source regions. We investigate with COMPTEL all-sky data from CGRO Phases 1–5 whether this source of 60 Fe can be detected, using the known spectral signature plus the spatial distribution as imaged with COMPTEL 26 Al measurements. Uncertainties in spatial signature of the instrumental and continuum background limit the sensitivity, such that only an upper limit of ∼44% (2σ) is quoted at this time.

The all-sky data from CGRO Phases 1–5 provide the most detailed constraints to date on the 26Al distribution in the Galaxy. With improved modelling of the instrumental background, different source distribution models can be distinguished. We derive large-scale parameters from first-order models, i.e., a characteristic scale height of 130 pc, and a Galactocentric scale radius of 5 kpc. Spiral structure of the 26Al source distribution improves the fits, as molecular gas provides a better description than geometrical models in the inner Galaxy. We note that infrared emission from dust also matches the 26Al emission quite well. Hints for contributions from more local and localized 26Al components such as Loop I and the Gould Belt are compatible with a massive star origin of 26Al.

The propagation and absorption of waves in the vicinity of the lower hybrid resonance are investigated under linear conditions in a cesium plasma generated by a Q-machine. The results of the measurements are compared with linear theories based on the fluid model. It is shown that in the case of high ion mass plasma only a theory including a finite k||-spectrum yields an appropriate description of wave absorption near the lower hybrid resonance.