A total dose measurement technique using RADFETs in spacecraft environment

This thesis mainly describes a total dose measurement technique using radiation-sensitive field effect transistors (RADFETs) in a spacecraft environment along with flight data analysis results obtained using the technique. A RADFET is a specially designed P-channel metal oxide semiconductor (PMOS) transistor that is optimized for increased radiation sensitivity and which has a thick gate oxide. Chapter 1 presents the background of this research. The necessity for total dose environment measurement in the spacecraft is explained. One reason is that radiation-hardened electronic parts have become difficult to obtain in light of the increasing use of commercial off-the-shelf products (COTS), which are vulnerable to radiation environments. Another reason is that the design for total dose has been overestimated. For these reasons, total dose measurement techniques to be used inside spacecraft have become important. Calibration methods for RADFETs are discussed in chapter 2. First, dosimeters, which measure the total dose environment, were investigated. The superiority of RADFETs used in this study was discussed. Gamma-ray irradiation test results were compared to those of electron and proton irradiation tests. Results show that a gamma-ray irradiation technique is used for a standard calibration method for RADFETs used in space. The fading phenomenon (or annealing effect), by which the RADFET voltage decreases with time, was evaluated. The annealing effect can be corrected using a deconvolution technique. Regarding irradiation dispersion, temperature effects on irradiation were analyzed; the measurement error attributable to these factors was discussed. Chapter 3 presents results of flight data analysis of the Dosimeter (DOS) onboard the Mission Demonstration test Satellite -1 (MDS-1). The MDS-1, launched on 2 February 2002, has experimental devices to verify the function of commercial parts and components, and to collect space environment data. The satellite has a highly elliptical orbit (GTO; Geostationary Transfer Orbit). The MDS-1 flight data were calibrated using the correction technique described in chapter 2. The results were compared with calculation data using the SHIELDOSE-2 based on the space radiation model (AE8 and AP8: standard trapped radiation belt models), and electron and proton data obtained from the on-board radiation spectrometer (SDOM). The model calculation is considered to overestimate the total dose in the shield of less than 2.7 mm thickness. This result might reflect the expanded use of COTS. On the other hand, flight data for the shield thicker than 8 mm shows a greater total dose than the model data. However, the value is lower by a factor of 10 than that for the thinner shield. Therefore, overestimation is not considered to be serious. A method to use these results in satellite design for total dose is discussed in chapter 4. Design margins are also reported for some shield thicknesses. Finally, the results of this research are summarized in chapter 5.理工学研究科博士論文(理学) ; 学位取得日: 平成20年3月25