Achievements in workplace neutron dosimetry in the last decade: lessons learned from the EVIDOS project

The availability of active neutron personal dosemeters has made real time monitoring of neutron doses possible. This has obvious benefits, but is only of any real assistance if the dose assessments made are of sufficient accuracy and reliability. Preliminary assessments of the performance of active neutron dosemeters can be made in calibration facilities, but these can never replicate the conditions under which the dosemeter is used in the workplace. Consequently, it is necessary to assess their performance in the workplace, which requires the field in the workplace to be fully characterised in terms of the energy and direction dependence of the fluence. This paper presents an overview of developments in workplace neutron dosimetry but concentrates on the outcomes of the EVIDOS project, which has made significant advances in the characterisation of workplace fields and the analysis of dosemeter responses in those field

Application of workplace correction factors to dosemeter results for the assessment of personal doses at nuclear facilities

Ratios of Hp(10) and H*(10) were determined with reference instruments in a number of workplace fields within the nuclear industry and used to derive workplace-specific correction factors. When commercial survey meter results together with these factors were applied to the results of the locally used personal dosemeters their results improved and became within 0.7 and 1.7 of the reference values or better depending on the response of the survey meter. A similar result was obtained when a correction was determined with a prototype reference instrument for Hp(10) after adjustment of its response. Commercially available survey instruments both for photon and neutron H*(10) measurements agreed with the reference instruments in most cases to within 0.5-1.5. Those conclusions are derived from results reported within the EC supported EVIDOS contrac

REFLECT – Research flight of EURADOS and CRREAT: Intercomparison of various radiation dosimeters onboard aircraft

Aircraft crew are one of the groups of radiation workers which receive the highest annual exposure to ionizing radiation. Validation of computer codes used routinely for calculation of the exposure due to cosmic radiation and the observation of nonpredictable changes in the level of the exposure due to solar energetic particles, requires continuous measurements onboard aircraft. Appropriate calibration of suitable instruments is crucial, however, for the very complex atmospheric radiation field there is no single reference field covering all particles and energies involved. Further intercomparisons of measurements of different instruments under real flight conditions are therefore indispensable. In November 2017, the REFLECT (REsearch FLight of EURADOS and CRREAT) was carried out. With a payload comprising more than 20 different instruments, REFLECT represents the largest campaign of this type ever performed. The instruments flown included those already proven for routine dosimetry onboard aircraft such as the Liulin Si-diode spectrometer and tissue equivalent proportional counters, as well as newly developed detectors and instruments with the potential to be used for onboard aircraft measurements in the future. This flight enabled acquisition of dosimetric data under well-defined conditions onboard aircraft and comparison of new instruments with those routinely used. As expected, dosimeters routinely used for onboard aircraft dosimetry and for verification of calculated doses such as a tissue equivalent proportional counter or a silicon detector device like Liulin agreed reasonable with each other as well as with model calculations. Conventional neutron rem counters underestimated neutron ambient dose equivalent, while extended-range neutron rem counters provided results comparable to routinely used instruments. Although the responses of some instruments, not primarily intended for the use in a very complex mixed radiation field such as onboard aircraft, were as somehow expected to be different, the verification of their suitability was one of the objectives of the REFLECT. This campaign comprised a single short flight. For further testing of instruments, additional flights as well as comparison at appropriate reference fields are envisaged. The REFLECT provided valuable experience and feedback for validation of calculated aviation doses

The microdosimetric variance-covariance method used for beam quality characterization in radiation protection and radiation therapy

Radiation quality is described by the RBE (relative biological effectiveness) that varies with the ionizing ability of the radiation. Microdosimetric quantities describe distributions of energy imparted to small volumes and can be related to RBE. This has made microdosimetry a powerful tool for radiation quality determinations in both radiation protection and radiation therapy. The variance-covariance method determines the dose-average of the distributions and has traditionally been used with two detectors to correct for beam intensity variations. Methods to separate dose components in mixed radiation fields and to correct for beam variations using only one detector have been developed in this thesis. Quality factor relations have been optimized for different neutron energies, and a new algorithm that takes single energy deposition events from densely ionizing radiation into account has been formulated. The variance-covariance technique and the new methodology have been shown to work well in the cosmic radiation field onboard aircraft, in the mixed photon and neutron fields in the nuclear industry and in pulsed fields around accelerators. The method has also been used for radiation quality characterization in therapy beams. The biological damage is related to track-structure and ionization clusters and requires descriptions of the energy depositions in nanometre sized volumes. It was shown that both measurements and Monte Carlo simulation (condensed history and track-structure) are needed for a reliable nanodosimetric beam characterization. The combined experimental and simulated results indicate that the dose-mean of the energy imparted to an object in the nanometre region is related to the clinical RBE in neutron, proton and photon beams. The results suggest that the variance-covariance technique and the dose-average of the microdosimetric quantities could be well suited for describing radiation quality also in therapy beams

Range-shifter effects on the stray field in proton therapy measured with the variance–covariance method

Measurements in the stray radiation field from a proton therapy pencil beam at energies 70 and 146 MeV were performed using microdosimetric tissue-equivalent proportional counters (TEPCs). The detector volumes were filled with a propane-based tissue-equivalent gas at low pressure simulating a mean chord length of 2 mu m in tissue. Investigations were performed with and without a beam range shifter, and with different air gaps between the range shifter and a solid water phantom. The absorbed dose, the dose-mean lineal energy, and the dose equivalent were determined for different detector positions using the variance-covariance method. The influence from beam energy, detector- and range-shifter positions on absorbed dose, LET, and dose equivalent were investigated. Monte Carlo simulations of the fluence, detector response, and absorbed dose contribution from different particles were performed with MCNP 6.2. The simulated dose response for protons, neutrons, and photons were compared with, and showed good agreement with, previously published experimental data. The simulations also showed that the TEPC absorbed dose agrees well with the ambient absorbed dose for neutron energies above 20 MeV. The results illustrate that changes in both dose and LET variations in the stray radiation field can be identified from TEPC measurements using the variance-covariance method. The results are in line with the changes seen in the simulated relative dose contributions from different particles associated with different proton energies and range-shifter settings. It is shown that the proton contribution scattered directly from the range shifter dominates in some situations, and although the LET of the radiation is decreased, the ambient dose equivalent is increased up to a factor of 3

Determinations of H*(10) and its dose components onboard aircraft

Aircrew is in general receiving a higher average annual dose than other occupationally exposed personnel, and about half of the effective dose is deposited by high-LET neutron secondaries. A recent investigation of the cancer incidence following the atomic bombs at Hiroshima and Nagasaki has put forward the possibility that the relative biological efficiency for neutrons could be underestimated. If so, the effective dose to aircrew from this component would increase and the estimation of this component will become even more important. Different ambient dose equivalent measurement techniques and calculation methods have recently been compared on a dedicated flight. The experimental results are compared with calculations made with the codes EPCARD 3.2 and an updated version of FLUKA and different galactic proton spectra. The aircraft circulated within the target areas at two constant altitudes with a flight route variation of only about 1° in longitude and latitude to reduce the influence from variations in atmospheric and geomagnetic shielding. The instrumentation consisted of tissue-equivalent proportional counters (TEPC) and a silicon diode spectrometer. Measurements were performed for 2 h to reduce the statistical uncertainties in the results. The TEPCs were evaluated either according to single-event analysis techniques or the variance-covariance method. Besides the total ambient dose equivalent, the instruments can be evaluated to reveal the low- and high-LET components. The EPCARD and FLUKA simulations can determine the contribution from each type of particle directly. The ratio between the calculated and the measured average value of the ambient dose equivalent rate was 1.00 ± 0.08 with all instruments included for EPCARD and 0.97 ± 0.07 when FLUKA was used. The measured high-LET component and the calculated neutron component are not quite identical, but should be similar. The agreement was always within 20%. The high-LET component contributed with about 57% at N57 E8 and 48% at N42 E12. ©The Author 2007. Published by Oxford University Press. All rights reserved

A comparison of ambient dose equivalent meters and dose calculations at constant flight conditions

Ambient dose-equivalent results from an in-flight comparison between different tissue-equivalent proportional counters and silicon diode spectrometers from seven European institutes are presented and compared with calculations using the EPCARD computer program. The measurements were performed on 40 000 and 32 000 ft in narrow target areas at latitudes N57° and N42° . Keeping the altitude and geographic position almost constant provided unique conditions for comparisons. The different measuring systems as well as the calculations are in remarkably good agreement, with an average standard deviation in the ambient dose equivalent between 6% and 21%. The ratio between calculated and measured ambient dose-equivalent rates varies between 0.91 and 1.09, with an average of $1.00 \pm 0.08\ (1s)$. Nevertheless some systematic differences in the experimentally determined ambient dose equivalent and its low-LET and high-LET components are noticed and discussed. It is concluded that the standard deviation between different instruments can through optimization and harmonization of the calibration procedures be reduced by up to a factor of two