The measurement of the direction of WIMP-induced nuclear recoils is a compelling but technologically challenging strategy to provide an unambiguous signature of the detection of Galactic dark matter. Most directional detectors aim to reconstruct the dark-matter-induced nuclear recoil tracks, either in gas or solid targets. The main challenge with directional detection is the need for high spatial resolution over large volumes, which puts strong requirements on the readout technologies. In this paper we review the various detector readout technologies used by directional detectors. In particular, we summarize the challenges, advantages and drawbacks of each approach, and discuss future prospects for these technologies.Comment: 58 pages, 26 figures, accepted by Physics Report

No abstract

Since the discovery of nuclear gamma-rays, its imaging has been limited to pseudo imaging, such as Compton Camera (CC) and coded mask. Pseudo imaging does not keep physical information (intensity, or brightness in Optics) along a ray, and thus is capable of no more than qualitative imaging of bright objects. To attain quantitative imaging, cameras that realize geometrical optics is essential, which would be, for nuclear MeV gammas, only possible via complete reconstruction of the Compton process. Recently we have revealed that “Electron Tracking Compton Camera” (ETCC) provides a well-defined Point Spread Function (PSF). The information of an incoming gamma is kept along a ray with the PSF and that is equivalent to geometrical optics. Here we present an imaging-spectroscopic measurement with the ETCC. Our results highlight the intrinsic difficulty with CCs in performing accurate imaging, and show that the ETCC surmounts this problem. The imaging capability also helps the ETCC suppress the noise level dramatically by ~3 orders of magnitude without a shielding structure. Furthermore, full reconstruction of Compton process with the ETCC provides spectra free of Compton edges. These results mark the first proper imaging of nuclear gammas based on the genuine geometrical optics.

To explore the sub-MeV/MeV gamma-ray window for astronomy, we have developed the Electron-Tracking Compton Camera (ETCC), and carried out the first performance test in laboratory conditions using several gamma-ray sources in the sub-MeV energy band. Using a simple track analysis for a quick first test of the performance, the gamma-ray imaging capability was demonstrated with clear images and 5.3 degrees of angular resolution measure (ARM) measured at 662 keV. As the greatest impact of this work, a gamma-ray detection efficiency on the order of 10 −4 was achieved at the sub-MeV gamma-ray band, which is one order of magnitude higher than our previous experiment. This angular resolution and detection efficiency enables us to detect the Crab Nebula at the 5σ level with several hours observation at balloon altitude in middle latitude. Furthermore, good consistency of efficiencies between this performance test and simulation including only physical processes is very important; it means we achieve nearly 100% detection of Compton recoil electrons and means that our predictions of performance enhancement resulting from future upgrades are more realistic. We are planning to confirm the imaging capability of the ETCC by observation of celestial objects in the SMILE-II (Sub-MeV gamma ray Imaging Loadedon-balloon Experiment II). The SMILE-II and following SMILE-III project will be an important key of sub-MeV/MeV gamma-ray astronomy.

For MeV gamma-ray astronomy, we have developed an electron-tracking Compton camera (ETCC) as a MeV gammaray telescope capable of rejecting the radiation background and attaining the high sensitivity of near 1 mCrab in space. Our ETCC comprises a gaseous time-projection chamber (TPC) with a micro pattern gas detector for tracking recoil electrons and a position-sensitive scintillation camera for detecting scattered gamma rays. After the success of a first balloon experiment in 2006 with a small ETCC (using a 10×10×15 cm 3 TPC) for measuring diffuse cosmic and atmospheric sub-MeV gamma rays (Sub-MeV gamma-ray Imaging Loaded-on-balloon Experiment I; SMILE-I), a (30 cm) 3 medium-sized ETCC was developed to measure MeV gamma-ray spectra from celestial sources, such as the Crab Nebula, with single-day balloon flights (SMILE-II). To achieve this goal, a 100-times-larger detection area compared with that of SMILE-I is required without changing the weight or power consumption of the detector system. In addition, the event rate is also expected to dramatically increase during observation. Here, we describe both the concept and the performance of the new data-acquisition system with this (30 cm) 3 ETCC to manage 100 times more data while satisfying the severe restrictions regarding the weight and power consumption imposed by a balloon-borne observation. In particular, to improve the detection efficiency of the fine tracks in the TPC from ∼10% to ∼100%, we introduce a new data-handling algorithm in the TPC. Therefore, for efficient management of such large amounts of data, we developed a data-acquisition system with parallel data flow.