Observation of gamma-rays from fallout collected at Ibaraki, Japan, during the Fukushima nuclear accident

Abstract

Gamma-ray pulse height distributions were measured for a fallout sample collected at Ibaraki, Japan, during the Fukushima accident in March 2011. The fallout was collected in a pan of water and then evaporated to dryness on a stainless-steel holder. The sample was measured by a germanium detector three times over a year. In the pulse height distribution of the initial measurement, approximately 140 peaks were observed in the 50–2048keV energy region. Most of these peaks were either total absorption peaks or sum peaks of Te, I or Cs isotopes. Unlike fallout samples at the past nuclear accidents, nuclides such as Ce and Ru were not detected whereas 110mAg was prominently observed. The radioactivity concentration of 137Cs was determined to be at least 1.4×104Bqm−2, approximately 14% of which was attributed to rainout.