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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.

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.

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.

The Advanced Compton Telescope (ACT) has been selected by NASA for a one-year “vision mission” study. The study’s main goal is to determine feasible instrument configurations to achieve ACT’s sensitivity requirements, and to give recommendations for technology development. Space-based instruments operating in the energy range of nuclear lines are subject to complex backgrounds generated by cosmic rays, earth albedo radiations, trapped particles, and diffuse gamma rays; typically measurements are significantly background-dominated. Therefore accurate, detailed simulations of the background induced in different ACT configurations, and exploration of event selection and reconstruction techniques for reducing these backgrounds, are crucial to determining the capabilities of a given instrument configuration. The ACT simulation team has assembled a complete suite of tools that allows the generation of particle backgrounds for a given orbit, their propagation through any instrument and spacecraft geometry – including delayed photon emission from instrument activation – as well as the selection and reconstruction of Compton events in the given detectors. We describe here the scope of the ACT simulation effort and the suite of tools used.

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.