First On-Site True Gamma-Ray Imaging-Spectroscopy of Contamination near Fukushima Plant

TL;DR

This study demonstrates the first on-site gamma-ray imaging spectroscopy near Fukushima using an Electron Tracking Compton Camera, accurately mapping contamination and identifying residual hotspots in a complex environment.

Contribution

The paper introduces a novel on-site gamma-ray imaging method with ETCC that reconstructs gamma-ray directions and spectra, enabling detailed contamination mapping and hotspot detection.

Findings

Accurate dose distribution mapping consistent with traditional measurements.

Identification of a residual caesium hotspot in a decontaminated area.

Demonstration of ETCC's effectiveness in complex, background-rich environments.

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.