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.
Highlights
Nuclear gamma-rays were discovered in 1890s, and since many scientists have made a great effort to invent a fine imaging method of nuclear gammas with little success
In order to attain proper imaging of nuclear gammas based on genuine geometrical optics, we have developed Electron-Tracking Compton Camera (ETCC) (Tanimori, T. et al.[13], hereafter referred to as TT), which outputs the two angles of an incident gamma, ζand η, by measuring the direction of a recoil electron, and provides the brightness distribution of gammas with a resolution of the PSF
We have to use the annulus as a probability distribution function to conserve the number of events, which means that any event distribution on the imaging plane measured by CCs is highly smeared by the wide angle of θ
Summary
Nuclear gamma-rays were discovered in 1890s, and since many scientists have made a great effort to invent a fine imaging method of nuclear gammas with little success. This complex interaction of a gamma with a matter makes proper imaging of MeV gammas very challenging. In the early phase of the project, optimization algorithms, such as Maximum Entropy method (MEM), were adopted in order to compensate the lack of information of the azimuthal angle (ηin Fig. 1a and c) of the incident gammas They successfully detected three sources in a pre-launch laboratory experiment, where the background was ~3 times as intense as the sources, by CC analysis (blue).
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