Thermoluminescent Detectors for Neutron Dosimetry at High Altitudes

At high altitudes in the Earth’s atmosphere cosmic-ray induced neutrons become the dominant contributors to the biologically relevant dose equivalent. Depending on the geomagnetic latitude and the solar activity cycle, neutron dose onboard aircraft amounts to about 50 to 70 % of the overall dose equivalent. Neutron detection in complexly mixed radiation environments by means of active, i.e. power-consuming, detectors is commonly aggravated by the fact that a variety of charged particles cannot be discriminated and hence biases the measurement. Passive thermoluminescence dosemeters are proposed as a convenient alternative. The simultaneous application of specific detector types is known in neutron dosimetry as the Pair Method. Neutronsensitive TLD-600 (LiF:Mg,Ti) is combined with TLD-700 (LiF:Mg,Ti) which shows almost negligible neutron response. As the detector efficiencies to charged particles, photons and neutrons > 200 keV are practically identical for both types, simple subtraction of the recorded glow curves reveals the dose from thermal and intermediate neutrons. The Pair Method can be extended for utilization at high altitudes in the atmosphere, such as onboard aircraft or atop high-altitude mountains, if the calibration of the detectors is performed in a reference field whose fractional thermal and intermediate component of dose equivalent is similar to that at the measurement location. For our purposes, we used the CERN-EU High-Energy Reference Field (CERF) which simulates in reasonable proximity the cosmic-ray induced neutron spectrum. Measurements with Bonner sphere spectrometers at the CERF facility and onboard aircraft and subsequent comparison with FLUKA Monte Carlo calculations verified the similarity of both spectra. The Extended Pair Method was successfully applied during extensive in-flight measurements as well as atop high-altitude mountains. INTRODUCTION The Earth is continuously bombarded by galactic and solar cosmic radiation characterized by complex particle and energy spectra. In cascade-like interaction processes with the geomagnetic field and the atmosphere, a secondary radiation field is created whose composition differs essentially from the primary cosmic-ray spectrum. The dose equivalent contributions from the many constituents vary with altitude and latitude. At high altitudes greater than about 3 km above sea level neutrons, protons and electromagnetic showers become dominant. Because of their high biological effectivness, neutron radiation is of particular interest to radiation protection dosimetry. However, neutrons are effective not only in causing biological hazards, but may as well affect aircraft electronics containing several gigabytes of semiconductor memory by producing so-called single event effects (SEE), e.g. bit-flips.The shape of the cosmic ray-induced neutron spectrum is relatively independent from altitude and characterized by two peaks around 1 MeV and 100 MeV. Due to the complexity of the mixed radiation environment encountered in the high atmosphere, the detection of neutrons by conventional active, i.e. power-consuming devices is usually aggravated by the insufficient discrimination of charged particle-induced effects in the detector signal which biases the measurement. Thermoluminescence (TL) dosemeters are proposed as a convenient alternative. As passive detectors they do not require air-worthiness certification, as they need no power supply, contain no flammable gases and emit no electromagnetic radiation which could potentially interfere with aircraft electronics. Their small size on the order of some mm and low mass makes them ideal tools for routine applications. INSTRUMENTATION AND METHODOLOGY The simultaneous application of specific TL detector types is well-known in neutron dosimetry as the Pair Method. TLD-600 (LiF:Mg,Ti) and TLD-700 (LiF:Mg,Ti) detectors show almost identical responses to photons and charged particles, but very different neutron efficiencies < 200 keV (1). This property is related to the Li(nth,α)H reaction dominating the TLD-600 neutron response. The reaction cross section is 943.2 barn for 0.0253 eV neutron energy, compared with a total neutron cross section value of 14.7 barn for Li and the same energy (Figure 1a). The TL signal induced from thermal and intermediate neutrons is ascertained by subtraction of the TLD-700 from the TLD-600 TL glow curve (Figure 1b). Hence, by arranging TLD-600 and TLD-700 dosemeters in pair, the net neutron dose < 200 keV may be determined in mixed radiation fields without the influence of charged particles and photons. Energy (eV) 10-2 10-1 100 101 102 103 104 105 106 107 C ro ss se ct io n (b ar n) 10-1 100 101 102 103 104 Li total neutron cross section Li total neutron cross section (a) Temperature (°C) 100 200 300 400 TL -e m iss io n (c ts )