The Development of a Personal Dosemeter for Use by Aircraft Crew

This paper describes preliminary work to develop a cosmic-radiation dosemeter for use by military aircraft crew. The dosemeter is based on a combination of CR-39 etched-track detectors and TLD-700 thermoluminescent detectors. It is intended that the CR-39 be used to assess the neutron dose, while the TLD700 is used to assess the photon and charged particle dose. The sensitivity of CR-39 to the neutron component of cosmic radiation was estimated by irradiating samples of the plastic at the CERN-CEC High Energy Reference Field Facility. This facility produced a radiation field with a neutron spectrum resembling that of the neutron component of cosmic radiation at typical airflight altitudes. The response of the CR-39 was linear over the range of doses studied (0.2-6.0 mSv) and there was no significant fading in the six-month period after irradiation. The TLD-700 component of the dosemeter was calibrated using Cs gamma rays. The response of the TLD-700 was linear over the range of doses studied (0-1.1 mSv) with no significant fade in the six-month period after irradiation. It was concluded that a combination of CR-39 and TLD-700 detectors would provide an effective cosmic-radiation dosemeter for use by military aircraft crew. INTRODUCTION Aircraft crew are exposed to above average levels of background radiation because the dose rate due to cosmic radiation increases significantly with altitude (1). It is estimated that civilian aircrew receive an annual occupational effective dose of 2-4 mSv (depending on flight routes and number of flights), compared to an average occupational dose in the United Kingdom of 0.8 mSv (2). The revised Basic Safety Standards Directive (BSS96/29) (3), which has to be incorporated into the national legislation of European Union member states before 13 May 2000, requires that aircrew liable to receive more than 1 mSv per year should have their radiation exposure assessed. In the case of civilian aircrew, this may be achieved by combining staff roster information with computer estimates of route doses. However, the situation is more complex for military aircrew as they do not have regular flight patterns. In this case, it is desirable to have a reliable personal dosemeter. The radiation field at airflight altitudes (typically 10-12 km) is complex consisting of neutrons, protons, electrons, pions, muons and photons (4). However, it is possible to measure the absorbed dose in this type of radiation field using a combination of etched-track and thermoluminescence (TL) detectors (4, 5); the etchedtrack detectors record the dose due to the neutron component of the cosmic radiation, while the TL detectors record the dose due to the photons and charged particles. This paper describes the development of a dosemeter consisting of CR-39 and TLD-700 detectors. The CR-39 component of the dosemeter was calibrated using the CERN-CEC High-Energy Reference Field Facility (6). This facility produces a radiation field whose neutron component resembles that of cosmic radiation at commercial airflight altitudes. The TLD-700 detectors were calibrated using Cs gamma rays as this is considered to give a reasonable estimate of the effective dose for the non-neutron component of cosmic radiation (7). The dose response and fading properties of CR-39 and TLD-700 were also studied using the CERN-CEC facility. MATERIALS AND METHODS DESIGN OF THE DOSEMETER The dosemeter consists of a polypropylene holder containing two CR-39 elements and a TLD-700 chip. The CR-39 was manufactured by Track Analysis Systems Limited (TASL) of Bristol, England. Each sheet of CR-39 was cut into 108 numbered elements of size 25 mm × 20 mm and thickness 1.5 mm. A quality check on background track density and sensitivity was carried out for each sheet. Only those sheets with a mean background track density less than 50 tracks cm and a sensitivity greater than 200 tracks cm mSv were used in the study. The TLD-700 chips were obtained from NE Technology of Reading, England. Prior to use, the chips were pre-annealed for 1 h at 400 C followed by 24 h at 80 C. TLD-700 was chosen because it has a relatively low sensitivity to neutrons. The neutron-to-gamma efficiency of a TL material is defined as the ratio of the TL per unit absorbed dose for neutrons to that for gamma-rays (8). It has been reported (8, 9) that TLD-700 has a neutron-to-gamma efficiency of approximately 0.1 for neutron energies between 0.1 and 10 MeV, rising to 0.2