Continuous mapping of radionuclides on the seafloor using a towed gamma ray spectrometer

An instrument to continuously map the distribution of radionuclides on the seafloor has been developed and applied to survey radioactive discharge from the Fukushima Dai-ichi Nuclear Power Plant following the M9.0 earthquake and tsunami that struck the east coast of Japan on March 11 2011. The towed gamma ray spectrometer has been deployed off Fukushima and has to date, been used to monitor radionuclide distribution over a total distance of more than 120 km. Here, we introduce the results of measurements made along three transects within an 80km radius of the plant, focusing on a 22km long transect off Iwaki. While the levels of the natural gamma ray emitter ⁴⁰K remained constant, the levels of ¹³⁷Cs and ¹³⁴Cs were found to vary significantly with location. The in situ measurements show good agreement with laboratory analyzed samples obtained during the survey, indicating that the technique described provides an effective solution for rapid, low cost monitoring of radioactive material on the seafloor.