Medium Energy Gamma-ray Research Articles

Rotation-powered pulsars in the soft- and medium energy gamma-ray bands

Rotation-powered pulsars in the soft- and medium energy gamma-ray bands

All-sky Medium Energy Gamma-ray Observatory eXplorer mission concept

The All-sky Medium Energy Gamma-ray Observatory eXplorer (AMEGO-X) is designed to identify and characterize gamma rays from extreme explosions and accelerators. The main science themes include: supermassive black holes and their connections to neutrinos and cosmic rays; binary neutron star mergers and the relativistic jets they produce; cosmic ray particle acceleration sources including Galactic supernovae; and continuous monitoring of other astrophysical events and sources over the full sky in this important energy range. AMEGO-X will probe the medium energy gamma-ray band using a single instrument with sensitivity up to an order of magnitude greater than previous telescopes in the energy range 100 keV to 1 GeV that can be only realized in space. During its three-year baseline mission, AMEGO-X will observe nearly the entire sky every two orbits, building up a sensitive all-sky map of gamma-ray sources and emission. AMEGO-X was submitted in the recent 2021 NASA MIDEX Announcement of Opportunity.

Constraining the axion mass through gamma-ray observations of pulsars

We analyze 9 years of PASS 8 $\textit{Fermi}$-LAT data in the 60$-$500 MeV range and determine flux upper limits (UL) for 17 gamma-ray dark pulsars as a probe of axions produced by nucleon-nucleon Bremsstrahlung in the pulsar core. Using a previously published axion decay gamma-ray photon flux model for pulsars which relies on a high core temperature of 20 MeV, we improve the determination of the UL axion mass ($m_a$), at 95 percent confidence level, to 9.6 $\times$ 10$^{-3}$ eV, which is a factor of 8 improvement on previous results. We show that the axion emissivity (energy loss rate per volume) at realistic lower pulsar core temperatures of 4 MeV or less is reduced to such an extent that axion emissivity and the gamma-ray signal becomes negligible. We consider an alternative emission model based on energy loss rate per mass to allow $m_a$ to be constrained with $Fermi$-LAT observations. This model yields a plausible UL $m_a$ of 10$^{-6}$ eV for pulsar core temperature $<$ 0.1 MeV but knowledge of the extent of axion to photon conversion in the pulsar $B$ field would be required to make a precise UL axion mass determination. The peak of axion flux is likely to produce gamma-rays in the $\leq$ 1 MeV energy range and so future observations with medium energy gamma-ray missions, such as AMEGO and e-ASTROGAM, will be vital to further constrain UL $m_a$.

The e-ASTROGAM space mission: a major step forward for supernova physics

Abstracte-ASTROGAM is a gamma-ray observatory operating in a broad energy range, 0.15 MeV – 3 GeV, recently proposed as the M5 Medium-size mission of the European Space Agency. It has the potential to revolutionize the astronomy of medium-energy gamma-rays by increasing the number of known sources in this domain by more than an order of magnitude and providing gamma-ray polarization information for many of these sources. In these proceedings, we discuss the expected capacity of the mission to study the physics of supernovae, both thermonuclear and core-collapse, as well as the origin of cosmic rays in SN shocks.

Position sensitivity in large spectroscopic LaBr3:Ce crystals for Doppler broadening correction

The position sensitivity of a large LaBr3:Ce crystal was investigated with the aim of correcting for the Doppler broadening in nuclear physics experiments. The crystal was cylindrical, 3in×3in (7.62cm x 7.62cm) and with diffusive surfaces as typically used in nuclear physics basic research to measure medium or high energy gamma rays (0.5MeV<Eγ<20MeV). The crystal was coupled to Position Sensitive Photomultipliers (PSPMT). The signals from the 256 segments of the four PSPMTs were acquired grouping them into 16 elements. An event by event analysis was performed and a positon resolution of the order of 2cm was found. It was verified that this allows an important reduction of the Doppler broadening induced by relativistic beams in Nuclear Physics experiments.

A nine-pixel matched collimator for low- and medium-energy SPECT imaging

For low-and medium-energy SPECT imaging without collimator exchange, a wide-aperture parallel-hole collimator—called a ‘nine-pixel matched collimator (9-PMC)’—was developed. The hole size of the 9-PMC is matched to nine detector pixels, giving nine (3 × 3) pixels per collimator hole. The 9-PMC was designed to reduce the penetration of medium-energy gamma rays. The performance of the 9-PMC for 99mTc and 111In gamma-ray energy was experimentally evaluated and compared with that of a low-energy collimator. A CdTe detector with 1.4 mm pitch was used in conjunction with the 9-PMC. A four-pixel matched collimator (‘4-PMC’ hereafter) with hole size matched to four detector pixels was also used as a high-sensitivity low-energy collimator. Hole pitch, hole length and septum thickness of the 4-PMC/9-PMC were 2.8/4.2 mm, 26.0/33.4 mm and 0.4/1.0 mm, respectively. The system sensitivities of the 4-PMC and 9-PMC for 99mTc gamma rays were 221 cps MBq−1 and 211 cps MBq−1, respectively. For the 4-PMC, system sensitivities for 171 and 245 keV 111In gamma rays were 232 cps MBq−1 and 503 cps MBq−1, respectively, while for the 9-PMC, the system sensitivities for 171 and 245 keV 111In gamma rays were 145 cps MBq−1 and 106 cps MBq−1, respectively. Spatial resolution of the collimator was analyzed by using modulation transfer functions (MTFs) obtained from the line-source profiles. The results of this analysis demonstrate that the 9-PMC for 99mTc had frequency ranges for similar (or higher) and lower MTF values compared with those for the 4-PMC, while the 9-PMC for 111In had higher MTF values than those for the 4-PMC at a wide frequency range. Hot-sphere phantom images showed that the image contrast of the 9-PMC for 99mTc gamma rays is comparable to that of the 4-PMC, while that of the 9-PMC for 111In gamma rays is higher than that of the 4-PMC. The developed SPECT with the 9-PMC and CdTe detector provided higher image quality for 111In gamma rays, or comparable image quality for 99mTc gamma rays, compared with that of a low-energy collimator.

Balloon flight test of a Compton telescope based on scintillators with silicon photomultiplier readouts

We present the results of the first high-altitude balloon flight test of a concept for an advanced Compton telescope making use of modern scintillator materials with silicon photomultiplier (SiPM) readouts. There is a need in the fields of high-energy astronomy and solar physics for new medium-energy gamma-ray (~0.4–10MeV) detectors capable of making sensitive observations of both line and continuum sources over a wide dynamic range. A fast scintillator-based Compton telescope with SiPM readouts is a promising solution to this instrumentation challenge, since the fast response of the scintillators permits both the rejection of background via time-of-flight (ToF) discrimination and the ability to operate at high count rates. The Solar Compton Telescope (SolCompT) prototype presented here was designed to demonstrate stable performance of this technology under balloon-flight conditions. The SolCompT instrument was a simple two-element Compton telescope, consisting of an approximately one-inch cylindrical stilbene crystal for a scattering detector and a one-inch cubic LaBr3:Ce crystal for a calorimeter detector. Both scintillator detectors were read out by 2×2 arrays of Hamamatsu S11828-3344 MPPC devices. Custom front-end electronics provided optimum signal rise time and linearity, and custom power supplies automatically adjusted the SiPM bias voltage to compensate for temperature-induced gain variations. A tagged calibration source, consisting of ~240 nCi of 60Co embedded in plastic scintillator, was placed in the field of view and provided a known source of gamma rays to measure in flight. The SolCompT balloon payload was launched on 24 August 2014 from Fort Sumner, NM, and spent ~3.75h at a float altitude of ~123,000ft. The instrument performed well throughout the flight. After correcting for small (~10%) residual gain variations, we measured an in-flight ToF resolution of ~760ps (FWHM). Advanced scintillators with SiPM readouts continue to show great promise for future gamma-ray instruments.

Position sensitivity in 3"×3" Spectroscopic LaBr3:Ce Crystals

The position sensitivity of a thick, cylindrical and continuous 3" × 3" (7.62 cm × 7.62 cm) LaBr3:Ce crystal with diffusive surfaces was investigated. Nuclear physics basic research uses thick LaBr3:Ce crystals (> 3cm) to measure medium or high energy gamma rays (0.5 MeV < Eγ< 20 MeV). In the first measurement the PMT photocathode entrance window was covered by black absorber except for a small window 1 cm × 1cm wide. A complete scan of the detector over a 0.5 cm step grid was performed. The data show that even in a 3" thick LaBr3:Ce crystal with diffusive surfaces the position of the full energy peak centroid depends on the source position. The position of the full energy peak centroids are sufficient to identify the collimated gamma source position. The crystal was then coupled to four Position Sensitive Photomultipliers (PSPMT). We acquired the signals from the 256 segments of the four PSPMTs grouping them into 16 elements. An event by event analysis shows a positon resolution of the order of 2 cm.

Gamma-ray rejection, or detection, with gadolinium as a converter

Gadolinium is a competent neutron conversion material for neutron detection due to its extremely high neutron capture cross section. It differs from the other neutron reactive materials by emitting large amounts of low-energy electrons for the consequent signal generation in a detector. Such low-energy electrons, though abundant, are prone to be contaminated by internal and/or external gamma rays, such as the activated 43.0 keV K-X rays, given the high atomic number of gadolinium. While the 43.0 keV K-X ray ought to be rejected as it originates in part from the external gamma rays when neutron detection is concerned, the ability to separate this energy line from other signals points to a practical mode of gamma-ray detection by a thin-film semiconductor with gadolinium as a converter. In this paper, a gamma-ray discrimination scheme for neutron detection is studied, which also provides insight into gamma-ray detection with a small semiconductor device with gadolinium as a converter, in line with the same principle of isolating the K-X rays activated by high- or medium-energy gamma rays.

Nonthermal Radiation from Pulsar Wind Nebulae

We study the nonthermal emission of pulsar wind nebulae (PWNe) from radio to TeV gamma-ray energies with a simplified time-dependent injection model. In this model, a relativistic wind of particles driven by a central pulsar with a spin-down power L(t) is blown into the ambient medium and a shock wave is formed, which accelerates the particles to very high energies through the Fermi acceleration mechanism in the PWN. The relativistic particles in the PWN therefore consist of two components, one coming directly from the pulsar magnetosphere and the other from shock acceleration in the PWN. The model PWN spectra follow a broken power law with different indices and a break energy E-b. The accelerated particles produce nonthermal photons through synchrotron radiation and inverse Compton scattering off soft photon fields. We apply this model to the Crab Nebula, the PWN in MSH 15 - 52, and HESS J1825 - 137, which have all been found to emit very high energy (VHE) photons. Our results indicate that ( 1) the observed data, ranging from radio to VHE gamma-rays, for the Crab Nebula can be reproduced by this model, where the emission from radio to medium-energy gamma-rays is from the synchrotron emission due to the injected electrons, whereas the high-energy photons primarily come from inverse Compton scattering of the high-energy electrons on synchrotron photons; and ( 2) the TeV emission from the PWN in MSH 15 - 52 and HESS J1825 - 137 mainly comes from the inverse Compton scattering of the high-energy electrons on infrared photons.

The MEGA Project for Medium Energy Gamma-ray Astronomy

The Medium Energy Gamma-ray Astronomy (MEGA) telescope concept will soon be proposed as a MIDEX mission. This mission would enable a sensitive all-sky survey of the medium-energy gamma-ray sky (0.4–50 MeV) and bridge the huge sensitivity gap between the COMPTEL and OSSE experiments on the Compton Gamma Ray Observatory and the visionary Advanced Compton Telescope (ACT) mission. The scientific goals include compiling a much larger catalog of sources in this energy range, performing far deeper searches for supernovae, better measuring the galactic continuum and line emissions, and identifying the components of the cosmic diffuse gamma-ray emission. MEGA records and images gamma rays by completely tracking Compton and pair creation events in a stack of double-sided Si strip detectors surrounded by a pixellated CsI calorimeter. A prototype instrument has been developed and calibrated in the laboratory and at a gamma-ray beam facility. We present calibration results from the prototype and describe the proposed satellite mission.

Some Aspects of Galactic Cosmic Ray Acceleration

AbstractI give a synopsis of two aspects of the Galactic Cosmic Ray (GCR) acceleration problem: the importance of the medium energy gamma-ray window, and several specific astrophysical sources which merit further investigation. NOTE: figures may be found in the on-line version only: astro-ph/0309758.

Computed radiography as a gamma ray detector—dose response and applications

Computed radiography (CR) can be used for imaging the spatial distribution of photon emissions from radionuclides. Its wide dynamic range and good response to medium energy gamma rays reduces the need for long exposure times. Measurements of small doses can be performed without having to pre-sensitize the computed radiography plates via an x-ray exposure, as required with screen-film systems. Cassette-based Agfa MD30 and Kodak GP25 CR plates were used in applications involving the detection of gamma ray emissions from technetium-99m and iodine-131. Cassette entrance doses as small as 1 µGy (140 keV gamma rays) produce noisy images, but the images are suitable for applications such as the detection of breaks in radiation protection barriers. A consequence of the gamma ray sensitivity of CR plates is the possibility that some nuclear medicine patients may fog their x-rays if the x-ray is taken soon after their radiopharmaceutical injection. The investigation showed that such fogging is likely to be diffuse.

Development of silicon strip detectors for a medium energy gamma-ray telescope

We report on the design, production, and testing of advanced double-sided silicon strip detectors under development at the Max-Planck-Institute as part of the Medium Energy Gamma-ray Astronomy (MEGA) project. The detectors are designed to form a stack, the “tracker,” with the goal of recording the paths of energetic electrons produced by Compton-scatter and pair-production interactions. Each layer of the tracker is composed of a 3×3 array of 500 μm thick silicon wafers, each 6 cm×6 cm and fitted with 128 orthogonal p and n strips on opposite sides (470 μm pitch). The strips are biased using the punch-through principle and AC-coupled via metal strips separated from the strip implant by an insulating oxide/nitride layer. The strips from adjacent wafers in the 3×3 array are wire-bonded in series and read out by 128-channel TA1.1 ASICs, creating a total 19 cm×19 cm position-sensitive area. At 20°C a typical energy resolution of 15–20 keV FWHM, a position resolution of 290 μm, and a time resolution of ∼1 μs is observed.

Development of the TIGRE Compton telescope for intermediate-energy gamma-ray astronomy

Gamma-ray observations in the low and medium energy range (0.1-100 MeV) with sufficiently sensitive telescopes will provide unique insights into many outstanding high-energy astrophysics questions. The University of California, Riverside (UCR) Tracking and Imaging Gamma-Ray Telescope (TIGRE) Compton gamma-ray telescope uses multilayers of silicon strip detectors to, for the first time, track the Compton electron and CsI(Tl)-photodiode detectors to measure the scattered photon energy. By combining the Compton telescope's inherent imaging capability with improved background discrimination, a larger field-of-view and improved spectral and spatial resolutions, a significant improvement in sensitivity over Compton Gamma-Ray Observatory (CGRO) and INTEGRAL can be achieved. The well-type calorimeter design also enhances the instrument as a gamma-ray polarimeter. The development and flight of a robust Compton telescope represents a unique opportunity to continue the momentum of recent discoveries in low and medium energy gamma-ray astrophysics with CGRO and an absolutely essential step to an extended satellite mission by 2010.

Gamma-ray self attenuation calculations in neutron activation ananlysis: A problem overlooked

Neutron activation analysis has for many years been considered to be a method that relatively is free from matrix interference. While this may be true for the majority of samples such as liquid, biological, air filters and geological aluminum-silicate specimens, there are many other types of samples that contains high Z materials in relatively high abundance that do attenuate low and medium energy gamma-rays. Samples such as heavy metal contaminated soils, ore concentrates and meteorites are prime examples. One can minimize the gamma-ray attenuation phenomena by using small masses of sample. This is not possible when using field environmental samples that are more than often not homogenous, and more sample mass is needed. Using a program called SELFABS, we have developed a very careful evaluation of the self-attenuation factors for a several of low and high Z reference materials. Special attention was given to the effect of the sample mass. Our conclusions indicate that while a great effort is given to various QA/QC procedures little literature information is ever published on the need or use for the calculation of these self-attenuation factors.

OSSE Observations of Blazars

Abstract : Results are reported on observations obtained with the Oriented Scintillation Spectrometer Experiment (OSSE) instrument on the Compton Gamma Ray Observatory (CGRO) of 17 active galactic nuclei known to exhibit blazar properties at other wavelengths. These observations span the period from 1991 June through 1994 May. Of the 33 high-confidence EGRET detections of blazars during CGRO observing phases 1 and 2 (1991 May 16 - 1993 Sept. 7), OSSE has observed 8 and detected 5, namely 3C 273, 3C 279, PKS 0528+134, CTA 102, and 3C 454.3. Additionally, OSSE has detected the BL LAC sources H 1517+65.6 and PKS 2155-304, which were not detected with EGRET. Variablility in the energy band 50 -150 keV is observed for all of the detected AGN. The OSSE blazar sources are all well described by simple power law models with photon number indices,,, varying from 1.0 to 2.1 among sources. When combined with available, although not necessarily contrmporaneous, COMPTEL and EGRET observations, 4 out of 5 detected blazars show clear evidence for spectral breaks between the hard X-ray and medium energy gamma-ray bands. The exception is the combined OSSE/EGRET data for 3C 279 during 1991 October, where a simple power-law with, approximately equal 1.9 works equally well. Gamma-ray evidence for beaming in CTA 102, PKS 0528+134, and 3C 454.3 is presented.

Nucleon polarization and cross section in photodisintegration of aligned and polarized deuterons at low and medium energies.

Effects of polarization and alignment of the deuteron on the polarization and cross section of the outgoing nucleons in deuteron photodisintegration have been investigated for low- and medium-energy gamma rays. Meson exchange currents and relativistic effects are included and the numerical calculations for the Paris potential are carried out. A comparison with the published results of Arenh\"ovel and Schmitt is made.

Digital signal processing for BGO detectors

Abstract The electronics for a medium-energy gamma ray spectrometer based on a large BGO crystal are described. The BGO signals are digitized and processed off-line to reduce spectral distortions due to pulse pile-up. An improvement of a factor of two to three is achieved in the minimum resolving time for pile-up identification as compared to standard analog techniques.

A ring Compton scatter camera for imaging medium energy gamma rays

The characteristics of a new ring Compton scatter camera are described. This camera is designed to image medium energy (0.5 to 3 MeV) gamma-ray fields. It consists of two positron sensitive detector arrays, i.e. a 4*4 planar array of high purity germanium (HPGe) crystals and a ring array of up to 64 NaI(Tl) crystals. Past evaluations of Compton cameras have employed a planar second detector, which is subjected to a large flux of gamma rays that either pass directly through or undergo small angle scatter in the first detector. A ring array significantly reduces direct and small angle scattered events in the second detector. An analytical model for ring camera systems is developed to predict angular resolution and efficiency, and is benchmarked against measurements made with a prototype system consisting of the 4*4 HPGe array and an eight-element ring. Predictions are made for a system with 64 crystals in the second detector ring.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>