Design of Active-Type Personal Dosemeter for High-Energy Neutrons
*In order to design an active-type personal dosemeter having sufficient sensitivity to high-energy neutrons up to 100 MeV, the characteristic response of several types of silicon (Si) semiconductor detectors has been investigated experimentally and theoretically. At first, the response of each detector has been checked to protons accelerated by a Van de Graff accelerator installed at Kobe University. A fully-depleted Si detector was selected as a promising detector element owing to a simple structure and comprehensive characteristics. In the next step, neutron irradiation experiments were carried out in a monoenergetic reference field in JAEA. On the other hand, theoretical distributions of the deposited energy were estimated in a simplified geometry consisting of Si and PE layers. An agreement in the relative shape was confirmed between the theoretical and experimental results. Applying this calculation procedure to other neutron energies and various thickness of sensitive layer, it was found that the energy dependence could be improved by the two-window technique by setting different discrimination levels on the pulse height. Another important characteristic of the angular dependence was also discussed for practical application of a personal dosemeter for high energy neutrons.
Application of Monte Carlo Simulation to Design of Sampler and Detector in Radiation Monitoring System
In this study, the applicability of Monte Carlo code particle and heavy ion transport code system (PHITS) [Sato et al. (2013, “Particle and Heavy Ion Transport Code System PHITS, Version 2.52,” J. Nucl. Sci. Technol., 50(9), pp. 913–923)] to the equipment design of sampler and detector in the radiation monitoring system was evaluated by comparing calculation results with experimental results obtained by actual measurements of radioactive materials. In modeling a simulation configuration, reproducing the energy distribution of beta-ray emitted from specific nuclide by means of Fermi Function was performed as well as geometric arrangement of the detector in the sampler volume. The reproducing and geometric arrangement proved that the calculation results are in excellent matching with actual experimental results. Moreover, reproducing the Gaussian energy distribution to the radiation energy deposition was performed according to experimental results obtained by the multi-channel analyzer. Through the modeling and the Monte Carlo simulation, key parameters for equipment design were identified and evaluated. Based on the results, it was confirmed that the Monte Carlo simulation is capable of supporting the evaluation of the equipment design.
Natural and Induced Environment in Low Earth Orbit
Abstract Estimating the long-term exposure of astronauts on the developingInternational Space Station (ISS) requires an accurate knowledge ofthe internal exposure environment for human risk assessment and otheronboard processes. The natural environment is moderated by the solarwind, which varies over the solar cycle. The neutron environment withinthe Shuttle in low Earth orbit has two sources. A time dependent modelfor the ambient environment is used to evaluate the natural and inducedenvironment. The induced neutron environment is evaluated usingmeasurements on STS-31 and STS-36 near the 1990 solar maximum. Introduction The commitment of astronauts to long-term exposure to the space environment on the InternationalSpace Station (ISS) requires resolution of issues concerning ionizing radiation. For the high inclinationof the ISS (51.6°), computational models indicate that about half of the ionizing radiation exposure nearsolar minimum results from Galactic Cosmic Rays (GCR, 233 µSv/d), and the bulk of the remainder isfrom trapped particles (166 µSv/d, Wu et al. 1996). There are, of course, contributions from the neutronalbedo of 25 to 54 µSv/d (varies with solar cycle) excluding effects of intervening material (Wilson et al.1989). Within the craft, the environment is a complex mixture of surviving primary particles and secon-dary radiations produced in the spacecraft structure. Various arrangements of detectors have been used tostudy the composition of the internal radiation fields within the spacecraft which need to be understood interms of computational models to allow a better understanding of the local environment of the astronauts’critical tissues. As a result, a number of studies of the low Earth orbit (LEO) environment have beenmade to better understand the nature of the radiations within a spacecraft (Dudkin et al. 1992; Keith et al.1992; Badhwar et al. 1995a; and Dudkin et al. 1995) and to understand these results in terms of computa-tional models (Badhwar et al. 1995b; Shinn et al. 1995; and Shinn et al. 1998).Measurements of neutrons on Cosmos-2044, flown at an 82° inclination between 216–296 km, re-sulted in 35 µSv/d using nuclear emulsion (Dudkin et al. 1992) and compares favorably with the neutronalbedo model of 25 µSv/d estimated for near polar orbits at the cycle 20 solar minimum (Wilson et al.1989). Similar measurements within the Spacehab on STS-57 in a 28.5° inclination orbit at 462 km yield174 µSv/d compared to 12.5 µSv/d from the albedo neutrons near solar maximum. Unlike the Cosmos-2044 spacecraft, the Shuttle is itself a strong source of neutrons, especially within the massive Spacehabmodule in the Shuttle bay. Indeed, time-resolved neutron measurements on the Mir and Salyut stations(Lobakov et al. 1992) reveal strong neutron levels, mainly within the South Atlantic Anomaly (SAA)passage through the trapped proton belt against a lower background of neutrons in the remainder of timeoutside the SAA.Neutron measurements using Bonner spheres and gold foils were made (Keith et al. 1992) near solarmaximum in the low inclination (28.5°) with high altitude (617 km) flight STS-31 in April 1990 and inthe high inclination (62°) with low altitude (246 km) flight STS-36 in February 1990. The neutron doseequivalent on STS-36 was found to be 45 µSv/d compared to 25 µSv/d from the albedo model, and onSTS-31 the measurements were 345 µSv/d compared to 12.5 µSv/d from the albedo model, again show-ing the Shuttle to be a strong source of neutrons. Small spacecraft have relatively few locally producedneutrons as seen on Cosmos-2044 and also on the Orbiting Geophysical Observatory (OGO-6) satellitewhere only 3 to 4 percent corrections of the albedo neutron measurements resulted from neutrons pro-duced locally in the spacecraft materials (Jenkins et al. 1971; see also Dudkin et al. 1992).