Search

Nuclear astrophysics presents an extraordinary scientific potential for the study of the most powerful sources and the most violent events in the Universe. But in order to take full advantage of this potential, telescopes should be at least an order of magnitude more sensitive than present technologies. Today, Laue lenses have demonstrated their capability of focusing gamma-rays in the 100 keV–1 MeV domain, enabling the possibility of building a new generation of instruments for which sensitive area is decoupled from collecting area. Thus we have now the opportunity of dramatically increase the signal/background ratio and hence improve significantly the sensitivity. With a lens, the best detector is no longer the largest possible within a mission envelope. The point spread function of a Laue lens measures a few centimeters in diameter, but the field of view is limited by the detector size. Requirements for a focal plane instrument are presented in the context of the Gamma-Ray Imager mission (proposed to European Space Agency, ESA in the framework of the first Cosmic Vision AO): a 15–20 cm a side finely pixellated detector capable of Compton events reconstruction seems to be optimal, giving polarization and background rejection capabilities and 30 arcsec of angular resolution within a field of view of 5 arc min.

The Nuclear Compton Telescope (NCT) is a balloon-borne telescope based on the 3D-positioning germanium detectors. It is designed to study astrophysical sources of gamma-ray emission in the energy range of 0.2 MeV to 10 MeV. The data readout system of NCT is designed to amplify, digitize and collect signals from a germanium detector according to a certain trigger scheme. It also has an interface to the NCT flight computer to receive commands and transfer data. This paper contains the design and the scientific test result of the readout system.

The Nuclear Compton Telescope (NCT) is a balloon-borne telescope designed to study astrophysical sources of nuclear line emission and polarization at soft gamma-ray (0.2-10 MeV) energies. NCT uses high-purity germanium strip detectors for 3D tracking of photon interactions. Compton imaging enables effective background rejection, resulting in a compact but highly efficient instrument. The NCT prototype completed a successful flight from Fort Sumner, New Mexico in 2005. We have since integrated additional detectors, updated the readout electronics, and improved other flight systems. Two flights of the full instrument are upcoming: a conventional flight in New Mexico and a long duration flight from Australia. We give an overview of the instrument and its status prior to the planned balloon flights.

GRIPS is an envisioned space telescope for Gamma-Ray burst Investigation via Polarimetry and Spectroscopy. It will be capable of imaging gamma rays in the energy range from roughly 200 keV up to at least 50 MeV via Compton scattering and pair creation. GRIPS will also provide unprecedented sensitivities for nuclear-line science. With a field-of-view of roughly 50° HWHM, energy resolutions of ∼5% FWHM at 511 keV and ∼2% at 1809 keV, angular resolutions of 3.3° at 511 keV and 1.4° at 1809 keV, and effective areas after background cuts of ∼450 cm 2 at 511 keV and ∼200 cm 2 at 1809 keV, GRIPS will achieve all-sky narrow-line point-source sensitivities of 9 × 10 −6 ph/cm 2 /s for 511 keV and 4 × 10 −7 ph/cm 2 /s at 1809 keV after five years mission time.

Considerable interest has been manifested for the use of high-brilliance X-ray synchrotron sources and X-ray Free-Electron Lasers for the investigation of samples.

Focusing of γ-rays in the nuclear-line energy regime is a feasible and promising approach for high-sensitivity γ-ray (line) studies of individual sources. This approach combines two unique advantages over “traditional” instrumentation for γ-ray astronomy: arcminute resolutions and a decoupling of collection and detection areas, with the corresponding potential for increased sensitivity. Laue Lenses use diffraction on lattice planes in the volume of a crystal to concentrate photons. Suitable crystals for MeV lenses exist, and lenses with >100 keV bandpasses and ∼arcmin fields-of-view are feasible today. Fresnel phase shift lenses present another, longer-term possibility for γ-ray focusing. The current status of instrumentation development, lens missions under consideration, and some of the nuclear astrophysics observations such instruments could enable are described.

We report on a prototype detector system designed to perform imaging and spectroscopy on 20–250 MeV neutrons. The detection techniques employed can be applied to measurements in a variety of disciplines including solar and atmospheric physics, radiation therapy and nuclear materials monitoring. The detector measures the energy and direction of neutrons by detecting double neutron–proton scatters and recording images of the ionization tracks of the recoil protons in a densely packed bundle of scintillating plastic fibers stacked in orthogonal layers. The scintillation tracks are detected and imaged by photomultipliers and image intensifier/CCD camera optics. By tracking the recoil protons from individual neutrons, the kinematics of the scatter are determined. This directional information results in a high signal-to-noise measurement. The self-triggering and track imaging features of a prototype for tracking in two dimensions are demonstrated in calibrations with 14–65 MeV neutrons, 20–67.5 MeV protons, and with cosmic-ray muons. Preliminary results of phantom imaging measurements using a proton beam are also presented. We discuss several applications for this detector technique and outline future development work.

The Gamma-Ray Imager (GRI) is a novel mission concept that will provide an unprecedented sensitivity leap in the soft gamma-ray domain by using for the first time a focusing lens built of Laue diffracting crystals. The lens will cover an energy band from 200 - 1300 keV with an effective area reaching 600 cm2. It will be complemented by a single reflection multilayer coated mirror, extending the GRI energy band into the hard X-ray regime, down to ~10 keV. The concentrated photons will be collected by a position sensitive pixelised CZT stack detector. We estimate continuum sensitivities of better than 10^-7 ph/cm2/s/keV for a 100 ks exposure; the narrow line sensitivity will be better than 3 x 10^-6 ph/cm2/s for the same integration time. As focusing instrument, GRI will have an angular resolution of better than 30 arcsec within a field of view of roughly 5 arcmin - an unprecedented achievement in the gamma-ray domain. Owing to the large focal length of 100 m of the lens and the mirror, the optics and detector will be placed on two separate spacecrafts flying in formation in a high elliptical orbit. R&D work to enable the lens focusing technology and to develop the required focal plane detector is currently underway, financed by ASI, CNES, ESA, and the Spanish Ministery of Education and Science. The GRI mission is proposed as class M mission for ESA's Cosmic Vision 2015-2025 program. GRI will allow studies of particle acceleration processes and explosion physics in unprecedented detail, providing essential clues on the innermost nature of the most violent and most energetic processes in the Universe., 11 pages, to be pubslihed in the SPIE proceedings

The nuclear Compton telescope (NCT) is a balloon- borne soft gamma-ray (0.2 MeV-10 MeV) telescope designed to study astrophysical sources of nuclear line emission and polarization. A prototype instrument was successfully launched from Fort Sumner, New Mexico on June 1, 2005. The NCT prototype consisted of two 3D position sensitive high-purity germanium strip detectors (GeDs) fabricated with amorphous Ge contacts. We are currently working toward two balloon flights: another conventional balloon flight from Fort Sumner, New Mexico in September 2008, and a long-duration balloon flight (LDBF) from Alice Springs, Australia in December 2009. The NCT instrument is being upgraded to include all twelve planned GeDs. The electronics for all twelve detectors have been redesigned for smaller size, lower power consumption, and lower noise, and are now being fabricated and tested. Here we present our current progress in preparing for the flights.

The Nuclear Compton Telescope (NCT) is a balloon-borne soft gamma-ray (0.2MeV-10MeV) telescope designed to study astrophysical sources of nuclear line emission and polarization. A prototype instrument was successfully launched from Ft. Sumner, NM on June 1, 2005. The NCT prototype consists of two 3D position sensitive High-Purity-Germanium (HPGe) strip detectors fabricated with amorphous Ge contacts. The novel ultra-compact design and new technologies allow NCT to achieve high efficiencies with excellent spectral resolution and background reduction. Energy and positioning calibration data was acquired pre-flight in Fort Sumner, NM after the full instrument integration. Here we discuss our calibration techniques and results, and detector efficiencies. Comparisons with simulations are presented as well.

International audience; With focusing of gamma rays in the nuclear-line energy regime establishing itself as a feasible and very promising approach for high-sensitivity gamma-ray studies of individual sources, optimizing the focal plane instrumentation for gamma-lens telescopes is a prime objective. The detector of choice for a focusing nuclear-line spectroscopy mission would be the one with the best energy resolution available over the energy range of interest: Germanium. Using a Compton detector focal plane has three advantages over monolithic detectors: additional knowledge about (Compton) events enhances background rejection capabilities, the inherently finely pixellated detector naturally allows the selection of events according to the focal spot size and position and could enable source imaging, and Compton detectors are inherently sensitive to gamma-ray polarization. Suitable Ge-strip detectors that could be assembled into a sensitive high-resolution focal plane for a gamma-ray lens are available today. They have been extensively tested in the laboratory and flown on the Nuclear Compton Telescope balloon from Ft. Sumner in 2005. In the course of the ACT vision mission study, an extensive simulation and analysis package for Compton telescopes has been assembled. We leverage off this work to explore achievable sensitivities for different Ge Compton focal plane configurations - and compare them to sensitivities achievable with less complex detectors - as a step towards determining an optimum configuration.

MAX is a proposed Laue lens gamma-ray telescope taking advantage of Bragg diffraction in crystals to concentrate incident photons onto a distant detector. The Laue lens and the detector are carried by two separate satellites flying in formation. Significant effort is being devoted to studying different types of crystals that may be suitable for focusing gamma rays in two 100 keV wide energy bands centered on two lines which constitute the prime astrophysical interest of the MAX mission: the 511 keV positron annihilation line, and the broadened 847 keV line from the decay of 56Co copiously produced in Type Ia supernovae. However, to optimize the performance of MAX, it is also necessary to optimize the detector used to collect the source photons concentrated by the lens. We address this need by applying proven Monte Carlo and event reconstruction packages to predict the performance of MAX for three different Ge detector concepts: a standard coaxial detector, a stack of segmented detectors, and a Compton camera consisting of a stack of strip detectors. Each of these exhibits distinct advantages and disadvantages regarding fundamental instrumental characteristics such as detection efficiency or background rejection, which ultimately determine achievable sensitivities. We conclude that the Compton camera is the most promising detector for MAX in particular, and for Laue lens gamma-ray telecopes in general., Comment: accepted for publication in special issue of Experimental Astronomy for Proceedings of Gamma-Wave 2005 held at Bonifacio, France; A&A style file

The Nuclear Compton Telescope (NCT) is a balloon-borne soft gamma ray (0.2-10 MeV) telescope designed to study astrophysical sources of nuclear line emission and gamma ray polarization. A prototype instrument was successfully launched from Ft. Sumner, NM on June 1, 2005. The NCT prototype consists of two 3D position sensitive High-Purity-Germanium (HPGe) strip detectors fabricated with amorphous Ge contacts. The novel ultra-compact design and new technologies allow NCT to achieve high efficiencies with excellent spectral resolution and background reduction. We have completed our preliminary calibrations of both the energy and the 3D position of interactions for the prototype instrument. Determination of both the energy and the position is crucial for Compton imaging, and minimizing the errors in each improves the angular resolution. Because of the compact design of the detectors and the high spectral resolution of germanium, we expect the position uncertainties to dominate over energy uncertainties when determining the angular resolution. Detailed depth calibrations and a preliminary determination of angular resolution as a function of energy are described. We determine how measurement uncertainties and physical limitations (energy uncertainty, position uncertainty, Doppler broadening, and systematics) constrain the ultimate angular resolution.

We report the scientific motivation for and performance measurements of a prototype detector system for SONTRAC, a solar neutron tracking experiment designed to study high- energy solar flare processes. The full SONTRAC instrument will measure the energy and direction of 20 to 200 MeV neutrons by imaging the ionization tracks of the recoil protons in a densely packed bundle of scintillating plastic fibers. The prototype detector consists of a 12.7 mm square bundle of 250 micrometer scintillating plastic fibers, 10 cm long. A photomultiplier detects scintillation light from one end of the fiber bundle and provides a detection trigger to an image intensifier/CCD camera system at the opposite end. The image of the scintillation light is recorded. By tracking the recoil protons from individual neutrons the kinematics of the scattering are determined, providing a high signal to noise measurement. The predicted energy resolution is 10% at 20 MeV, improving with energy. This energy resolution translates into an uncertainty in the production time of the neutron at the Sun of 30 s for a 20 MeV neutron, also improving with energy. A SONTRAC instrument will also be capable of detecting and measuring high-energy gamma rays greater than 20 MeV as a 'solid-state spark chamber.' The self-triggering and track imaging features of the prototype are demonstrated with cosmic ray muons and 14 MeV neutrons. Design considerations for a space flight instrument are presented.