Experiences on high energy radiation dosimetry

The problem on the radiation monitoring of an accelerater with energy of several Me.V. or more is high energy radiation dosimetry, especially, fast neutron dosimetry. In this paper, several experiences are presented on the neutron dose measurement in the Institute for Nuclear Study of University of Tokyo using a cyclotron. The shielding effects of paraffin, water, earth, concrete, iron and lead to high energy neutrons have been measured with the neutron beam generated from a copper plate irradiated by 50 MeV proton beam from a F.M. cyclotron. The attenuation length of each material presents different value corresponding to the thickness of the material. The neutron beam propagating effect on the ground by skyshine has been measured with F.F. and F.M. cyclotrons. The dose rate of neutrons has been found to decrease in proportion to the -2.6 power of the distance from the cyclotron. The dose rates of gamma ray and neutron beam generated from a target irradiated with the cyclotron have been measured. The dose rate of neutrons relies upon the energy as well as the kinds of injected particles. The measurement results with area monitors showed that there exist only neutron radiation around the site boundary. (Tai, I.)