1242 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 56, NO. 3, JUNE 2009 Gamma-Ray Lenses for Astrophysics—and the Gamma-Ray Imager Mission GRI Cornelia B. Wunderer, Peter v. Ballmoos, Nicolas Barriere, Angela Bazzano, Steven E. Boggs, Finn Christensen, Filippo Frontera, Margarida Hernanz, Jürgen Knödlseder, and Andreas Zoglauer Abstract—Observations of the gamma-ray sky reveal the most powerful sources and the most violent events in the Universe. While at lower wavebands the observed emission is generally dominated by thermal processes, the gamma-ray sky provides us with a view on the non-thermal Universe. Here particles are accelerated to ex- treme relativistic energies by mechanisms which are still poorly understood, and nuclear reactions are synthesizing the basic con- stituents of our world. Cosmic accelerators and cosmic explosions are major science themes that are addressed in the gamma-ray regime. While Fermi will take the next step in surveying the high-en- ergy ( GeV) sky, and NuSTAR will pioneer focusing observations at hard X-ray energies (to 80 keV), there is currently no suc- cessor mission planned to ESA’s INTEGRAL observatory which currently provides important new insights into the MeV sky, albeit at much more modest sensitivities. There will be clearly a growing need to perform deeper, more focused investigations of gamma-ray sources in the 100-keV to MeV regime. Recent technological advances in the domain of gamma-ray fo- cusing using Laue diffraction and multilayer-coated mirror tech- niques have paved the way towards a gamma-ray mission, pro- viding major improvements compared to past missions regarding sensitivity and angular resolution. Such a future Gamma-Ray Im- ager will allow the study of particle acceleration processes and ex- plosion physics in unprecedented detail, providing essential clues on the innermost nature of the most violent and most energetic pro- cesses in the Universe. Index Terms—Astronomical satellites, Compton focal plane, gamma-ray astronomy detectors, imaging, Laue lens. I. INTRODUCTION G AMMA-RAY observations provide us with direct views of some of the most violent events in our universe, and afford unique views of the accelerators at work in the cosmos. Manuscript received June 30, 2008; revised October 03, 2008. Current ver- sion published June 10, 2009. This work was supported in part by NASA Grant NNG05WC28G and in part by MEC Grant ESP2007-61593. C. B. Wunderer, S. E. Boggs, and A. Zoglauer are with the Space Sciences Laboratory, University of California at Berkeley, CA 94708 USA (e-mail: wun- derer@ssl.berkeley.edu; boggs@ssl.berkeley.edu; zog@ssl.berkeley.edu). P. v. Ballmoos and J. Knödlseder are with the Centre d’Etude Spatiale des Rayonnements, 31028 Toulouse, France (e-mail: pvb@cesr.fr; kn- odlseder@cesr.fr). N. Barriere and A. Bazzano are with the IASF Roma, Via Fosso dell Cavaliere 100, 00133 Roma, Italy (e-mail: nicolas.barriere@iasf-roma.inaf.it; angela.baz- zano@iasf-romainaf.it). F. Christensen is with the National Space Institute, Danish Technical Univer- sity, Juliane Maries Vej 30, 2100 Copenhagen, Denmark (e-mail: finn@dsri.dk). F. Frontera is with the University of Ferrara, Physics Department, Via Saragat 1, 44100 Ferrara, Italy (e-mail: frontera@fe.infn.it). M. Hernanz is with the Institut de Ciencies de l’Espai (CSIC-IEEC), Campus UAB, 08193 Bellaterra (Barcelona), Spain (e-mail: hernanz@ieec.fcr.es). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TNS.2009.2013855 Deep, sensitive observations are necessary to help us answer fundamental questions such as “How do stars explode and how do their remnants evolve?”, “How are the elements formed and how are they fed back into the matter cycle?”, or “How and where are particles accelerated?”. The earth’s atmosphere is highly opaque to X-rays and all but the highest-energy gamma rays, necessitating high-altitude bal- loon or (better yet) satellite-based instruments. Past and present observatories, such as the COMPTEL [1] and EGRET [2] in- struments on CGRO and SPI [3] and IBIS [4] on INTEGRAL, have provided us with important and often surprising insights. The upcoming NuSTAR [5] and recently launched Fermi [6], [7] missions will follow up with deeper observations at hard X-ray and GeV gamma-ray energies. The energy ranges covered by these two instruments, how- ever, will leave a gap spanning several decades in energy in the 80 keV to 20 MeV region. This gap is where isotope-specific nuclear deexcitation lines provide a unique tool for diagnostics of nucleosynthetic processes in cosmic explosions such as novae or supernovae, where annihilation radiation from positrons is visible in the light of the 511 keV line and positronium con- tinuum, and where spectra from state transitions in galactic com- pact objects and e.g. a census of AGN cutoff energies could pro- vide us with new insights into the inner workings of accreting BH systems and hopefully help resolve the origin of the soft gamma-ray cosmic diffuse background. Many of the most pressing questions to be addressed by ob- servations in this soft gamma-ray regime require very sensitive observations of individual sources, rather than surveys of large sky fields. Traditionally, gamma-ray observatories have very large fields-of-view, utilizing either mechanical obscuration (coded aperture and/or collimator) or tracing of photon interactions (Compton or pair telescopes) to obtain directional information about the measured photons. The instrument’s collection area is equal to or a fraction of the detector area. With sensitivities dominated by detector backgrounds, and these backgrounds scaling with detector volume and/or area, order-of-magnitude sensitivity increases are very hard to obtain in these systems. Concentrating the photons from a large collection area onto a small detection plane, the approach taken at longer wavelengths, has only recently been proven a feasible approach for energies above 100 keV—using Laue lenses. They enable space-borne photon concentrators operating effectively up to at least 1 MeV. Laue lenses are also under consideration in the context of medical imaging [8], [9], and could in principle be used for longer-range detection of nuclear materials in a region or ob- ject of very small angular extent. 0018-9499/$25.00 © 2009 IEEE