Development of Neutron Detectors and Measurement of Cosmic-Ray Neutrons

Abstract

We have developed several neutron spectrometers and dosemeters, such as Phoswich-type spectrometer, Bonner sphere spectrometer and two-type dosemeters of high-efficiency type and lightweight type.

The Phoswich-type spectrometer consists of EJ309 organic liquid scintillator fully covered with EJ299-13 plastic scintillator. Both scintillators emit light outputs of different decay time, 3.5 ns of EJ309 and 285 ns of EJ299-13. Signals of neutrons and photon events are derived from only the inner liquid scintillator and those from charged particles are the sum of both scintillators. Signals from non-charged and charged particles therefore have small and large tail components, respectively, which enables the identification of incident particles. This detector was used to measure cosmic-ray neutrons and photons on board the aircraft over Japan in 2008.

We also developed Bonner sphere spectrometer using only four polyethylene moderators, 1,5 cm, 3 cm, 6 cm, and 9 cm thickness, covered with a spherical ³He counter for environmental neutron measurement on earth. This compact-type detector was modified for use in space by JAXA (Japan Aerospace Exploration Agency) in Japan Experiment Module (JEM) Kibo of ISS (International Space Station) and is continuously operating to send the cosmic-ray neutron data from ISS above about 400 km from the earth.

High-efficiency dosemeter was used for balloon experiment in 2004. The altitude variation of cosmic neutron dose rates was measured up to 25 km altitude over Japan. Recently, a lightweight dosemeter of only 2 kg has been developed and its energy response was investigated to neutrons from thermal to 300 MeV energies. This lightweight dosemeter can be widely used in nuclear facilities and accelerator facilities even in space.