Photon imaging for MeV gammas has serious difficulties due to huge backgrounds and unclearness in images, which are originated from incompleteness in determining the physical parameters of Compton scattering in detection, e.g., lack of the directional information of the recoil electrons. The recent major mission/instrument in the MeV band, Compton Gamma Ray Observatory/COMPTEL, which was Compton Camera (CC), detected mere ∼ 30 persistent sources. It is in stark contrast with ∼2000 sources in the GeV band. Here we report the performance of an Electron-Tracking Compton Camera (ETCC), and prove that it has a good potential to break through this stagnation in MeV gamma-ray astronomy. The ETCC provides all the parameters of Compton-scattering by measuring 3D recoil electron tracks; then the Scatter Plane Deviation (SPD) lost in CCs is recovered. The energy loss rate (dE/dx), which CCs cannot measure, is also obtained, and is found to be indeed helpful to reduce the background under conditions similar to space. Accordingly the significance in gamma detection is improved severalfold. On the other hand, SPD is essential to determine the point-spread function (PSF) quantitatively. The SPD resolution is improved close to the theoretical limit for multiple scattering of recoil electrons. With such a well-determined PSF, we demonstrate for the first time that it is possible to provide reliable sensitivity in Compton imaging without utilizing an optimization algorithm. As such, this study highlights the fundamental weak-points of CCs. In contrast we demonstrate the possibility of ETCC reaching the sensitivity below 1×10 −12 erg cm −2 s −1 at 1 MeV.

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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.

We have investigated correlations of coincident ΛN pairs from the stopped K − reaction on 4 He, and clearly observed Λp and Λn branches of the two-nucleon absorption process in the ΛN invariant mass spectra. In addition, non-mesonic reaction channels, which indicate possible exotic signals for the formation of strange multibaryon states, have been identified.

Muon spin methods are very sensitive to nanoscale defects such as trace elements and vacancies in metals. This sensitivity is required when investigating Al-Mg-Si alloys, a complicated system in which diffusion-controlled phase transformations are responsible for the most important hardening mechanisms. We present muon spin relaxation experiments conducted on Al-Mg-Si alloys at measurement temperatures in the range 20-300 K. Varying the alloy composition and heat treatment, we find differences in muon depolarization in several temperature regimes. This reflects differences in concentration of several types of muon-trapping defects. We identify free solute atom and vacancy regimes, and confirm that the concentration of these defects decrease when an alloy is annealed at low temperature. We further attribute one regime to Mg-Si-vacancy clustering, a mechanism required for precipitation hardening during aging. After storage at room temperature, muon trapping in this regime is more pronounced for a Mg-rich alloy than a Mg-Si-balanced alloy.