Title: First On-Site True Gamma-Ray Imaging-Spectroscopy of Contamination near Fukushima Plant,
Abstract: We have developed an Electron Tracking Compton Camera (ETCC), which provides
a well-defined Point Spread Function (PSF) by reconstructing a direction of
each gamma as a point and realizes simultaneous measurement of brightness and
spectrum of MeV gamma-rays for the first time. Here, we present the results of
our on-site pilot gamma-imaging-spectroscopy with ETCC at three contaminated
locations in the vicinity of the Fukushima Daiichi Nuclear Power Plants in
Japan in 2014. The obtained distribution of brightness (or emissivity) with
remote-sensing observations is unambiguously converted into the dose
distribution. We confirm that the dose distribution is consistent with the one
taken by conventional mapping measurements with a dosimeter physically placed
at each grid point. Furthermore, its imaging spectroscopy, boosted by
Compton-edge-free spectra, reveals complex radioactive features in a
quantitative manner around each individual target point in the
background-dominated environment. Notably, we successfully identify a "micro
hot spot" of residual caesium contamination even in an already decontaminated
area. These results show that the ETCC performs exactly as the geometrical
optics predicts, demonstrates its versatility in the field radiation
measurement, and reveals potentials for application in many fields, including
the nuclear industry, medical field, and astronomy.