Cosmic-ray Neutron Spectrometry and Dosimetry

Abstract

When primary cosmic rays consisting of galactic cosmic rays and solar particles come into the earth’s atmosphere, secondary neutrons generated through nuclear reactions with atmospheric atoms reach the ground. Over the past 10 years, there has been increasing concern about the exposure of air crews to atmospheric cosmic radiation. At aviation altitudes, the neutron component of the secondary cosmic radiation contributes about half of the dose equivalent. Recently, an accumulation of the semiconductor device greatly increases and the soft-errors of SRAM and DRAM on the ground level caused by high-energy cosmic-ray neutrons become a serious problem in the world. Under these circumstances, cosmic-ray neutron spectrometry and dosimetry are presented here in the terrestrial and space environments where neutrons and protons coexist.The neutron detection methods for use in this mixed field are described; 1) multi-moderator spectrometer (Bonner Ball), 2) organic liquid scintillation spectrometer, 3) dose-equivalent counter (rem counter) and 4) Phoswich-type detector. Using these detectors, neutron energy spectra and dose-equivalent rates have been measured on the ground at sea level and at mountain level, aboard an airplane and in space. The space experiments were done using a balloon, space shuttle and space station. The neutron spectrum on the ground has three major peaks, thermal energy peak, evaporation peak around 1 MeV and cascade peak around 100 MeV. While on the other hand, the neutron spectrum apart from the ground has no thermal neutron peak that comes from the albedo neutron effect backscattered from the terrestrial surface. The time-sequential experimental results in Japan, Europe and U.S.A. are described with the experimental procedures by paying attention to variation with latitude, altitude and solar activity.