Dosimetry and Spectrometry of Cosmic-ray Neutrons in Aircraft (DOSCONA Experiment)

Exact information on the cosmic-ray environment in spacecraft and aircraft are important for radiological protection of astronauts and aircraft crew. They are exposed to various types of primary and secondary cosmic-ray particles having the broad energy ranges. These particles could react in complicated manners inside a crew body and such reactions at deep organs would bring some critical adverse effects on the crew’s health. To discuss this risk, it is necessary to know the accurate energy spectra and its spatial distribution of each species of cosmic-ray particles, particularly penetrating components such as high-energy neutrons which occupy a considerable fraction of total dose. It is very difficult, however, to precisely measure the neutrons with energies greater than 10-20 MeV because they are mixed with and hardly separated from co-existing particles such as protons and muons in the thin atmospheric environment. Under these circumferences, we repeated in-flight measurements of cosmic-ray neutrons onboard a business jet aircraft near Japan from 2006 to 2008, during the solar minimum. The onboard experiments were supported as the 8th Ground-based Research Program for Space Utilization promoted by Japan Space Forum; this program was operated with a financial support of Japan Aerospace Exploration Agency (JAXA). The experimental flights were operated by Diamond Air Service Inc. For cosmic-ray neutron measurements, we employed several neutron monitors including an original, newly developed neutron spectrometer. The spectrometer probe is a phoswich-type scintillation detector and can measure a high-energy neutron spectrum from 10 MeV to about 200 MeV, by separating well from cosmic-ray protons and others. It was an important object of this study to verify the effectiveness of this spectrometer at high altitude. As results of the onboard measurements using selected rem counters, it was verified that cosmic-ray neutron doses can be successfully determined by numerical model simulations using the PHITS-based analytical model (PARMA). Also, it was confirmed that the new spectrometer works well for the determination of cosmic-ray neutron spectrum in the energy range from 10 to 200 MeV. The obtained spectrum of the high-energy neutrons was well connected with that of the low-to-middle energy neutrons (<15 MeV) evaluated with the Bonner ball neutron spectrometer. This is the first successful onboard experiment in which energy spectrum of neutrons with a wide energy range from thermal to about 200 MeV was presented with high energy resolution. The obtained peak of the high-energy neutrons, however, appeared around 70 MeV which is considerably lower than those (about 100 MeV) reported in previous Bonner ball measurements, though this result agrees well with the neutron spectra measured with organic liquid scintillator on the ground and with double-scatter neutron telescope at the top of atmosphere. Though this discrepancy was discussed in comparison with model simulations in the present study, further discussion is strongly required by performing onboard long-haul flight measurements at high geomagnetic latitude. After such sufficient verification, tests on the International Space Station are desirable with developing a more sophisticated instruments regarding safety, energy efficiency and transportability.