White paper for US National Academy of Sciences Decadal Survey "Astro2010"
Gamma-rays at MeV energies provide a unique window on the high energy Universe, especially so for nuclear astrophysics. The potential for significant contributions, e.g., to the understanding of SNe Ia as well as the large potential for new discoveries has long been recognized, but technical progress in this challenging energy band has been slow. The groundbreaking discoveries of CGRO’s COMPTEL, however, have inspired and driven the development of powerful new instrumentation over the past decade. Novel detector technologies developed in the US and Japan enable compact Compton telescopes, greatly improving the efficiency and field-of-view achievable with this technique successfully employed by COMPTEL. In parallel, Laue diffraction lenses developed in Europe have demonstrated the ability to focus MeV !-rays over broad energy bands in a telescope configuration. The combination of these two developments, Laue lenses and compact Compton telescopes, has paved the way for the first-ever focusing mission for !-ray astronomy. In such a configuration, the Compton telescope serves dual roles simultaneously, both as an optimal focal-plane sensor for deep focused observations (with the Compton imaging effectively collimating the focal plane), and as a sensitive wide-field Compton imager in its own right for all-sky surveys and monitoring. This combination addresses the issue of mass and cost inherent in previous Compton-only designs, yet can provide very good sensitivity for deep observations. Our international collaboration is studying this mission with the goal of proposing this as the next !-ray astrophysics mission. Gamma-ray astronomy presents an extraordinary scientific potential for the study of the most powerful sources and the most violent events in the Universe. In order to take full advantage of this potential, the next generation of instrumentation for this domain will have to achieve an improvement in sensitivity over present technologies of at least an order of magnitude. The DUAL mission concept takes up this challenge in two complementary ways: a very long observation of the entire sky, combined with a large collection area for simultaneous observations of Type Ia SNe. While the Wide-Field Compton Telescope (WCT) accumulates data from the full !-ray sky (0.1-10 MeV) over the entire mission lifetime, the Laue-Lens Telescope (LLT) focuses on 56Co emission from SNe Ia (0.8-0.9 MeV), collecting !-rays from its large area crystal lens onto the WCT. Two separated spacecraft flying in formation will maintain the DUAL payloads at the lens' focal distance. Focusing instruments have two tremendous advantages: first, the volume of the focal plane detector can be made much smaller than for non-focusing instruments, and second, the residual background, often time-variable, can be measured simultaneously with the source, and can be reliably subtracted. The concept of a Laue !-ray lens holds the promise of extending focusing capabilities into the MeV range. In order to achieve the ultimate sensitivity for the !-ray lens mission, the focal plane detector must be designed to match the characteristics of the lens’ focal spot. A Compton telescope is a good solution, because the focal plane is intrinsically finely pixellated, optimized for MeV !-ray detection, and because the direction of incident !-rays can be determined by the Compton reconstruction to enable discrimination !-rays coming from the lens (“electronic collimation”)
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