Abstract The ICRU is considering amending the definition of the operational quantity for personnel monitoring. The present work investigates the impact of the proposed change on the PHE neutron personal dosemeter, which utilizes electrochemically etched PADC and is currently optimized in terms of Hp(10). The energy-dependent dose response characteristics of the dosemeter, and its performance in realistic workplace neutron fields, are calculated and compared for both the current and proposed dose quantities, for both frontal and rotationally isotropic fields. Adoption of the proposed quantity would make the dosemeter more sensitive to normally incident neutrons, but it would require some modification to ensure that it is able to meet the recommendations of the current ISO standard at all energies and its directional dependence of response would be poorer. The implications of this are discussed. INTRODUCTION Individuals occupationally exposed to sources of ionizing radiation within controlled environments are legally required to wear personal dosemeters, which are typically designed to respond in terms of operational dose quantities defined by the International Commission on Radiation Units and Measurements (ICRU). Since Report 39(1), ICRU have recommended the use of ‘personal dose equivalent’, Hp(d,θ), where d denotes the specified depth in soft tissue and θ is the angle of incidence of the radiation; for penetrating radiation, a depth of d = 10 mm is recommended. As a result of limitations in the use of Hp(10,θ), a new definition of the operational quantity is currently being proposed for personal monitoring(2, 3). The updated quantity will have the suggested symbol Hp(Ω), where Ω is the angle of incidence in the horizontal plane. However, since the new quantity will be based on effective dose calculated in the reference voxel phantoms(4), it will no longer be calculated at a specified depth, and as well as having a modified energy dependence, it will have significantly different dependence on the angle of incidence. Clearly, changing or abandoning Hp(10,θ), in favour of Hp(Ω), will affect all whole body dosemeters. The current work considers how the proposed change will affect the characteristics of the Public Health England (PHE) neutron personal dosemeter, both in terms of its energy-dependence of response and its performance in realistic workplace fields. In particular, will the dosemeter’s response be made better or worse by the proposal, and if the latter, would its performance still be acceptable against established criteria? THE PHE NEUTRON DOSEMETER PHE has been operating an approved neutron personal dosimetry service based on poly-allyl diglycol carbonate (PADC or CR-39™) since 1986, covering 1000s of workers and serving major nuclear sites in the UK and abroad(5). The dosimetry system uses a long-established method of electrochemical etching to reveal etchable tracks in the PADC structure that are produced by neutron damage, with the thermal response produced by 14N(n,p)14C reactions in the dosemeter’s nylon holder; the relationship between dose and areal track density is well-characterized. The dosemeter provides good Hp(10,θ) response to neutrons spanning a range from thermal to MeV energies (Figure 1a and b), and is characterized using exposures at PHE, the UK’s National Physical Laboratory (NPL), iThemba LABS (South Africa) and the high-energy CERF facility at CERN. The dosemeter is calibrated to the mean of its 0° and rotational (ROT) responses in a number of workplace energy distributions, relative to a traceable 241Am–Be exposure(6). Good performance of the PHE dosimetry system has been demonstrated in, for example, EURADOS-organized intercomparison exercises(7). Figure 1. View largeDownload slide (a) Fluence response of the PHE neutron dosemeter to exposures from the front (left axis), and Hp(10,0°)/Φ and Hp(0°)/Φ (≡E(AP)) conversion coefficients (right axis). (b) Fluence response of the PHE neutron dosemeter to rotational exposures (left axis), and Hp(10,ROT)/Φ and Hp(ROT)/Φ (≡E(ROT)) conversion coefficients (right axis). Figure 1. View largeDownload slide (a) Fluence response of the PHE neutron dosemeter to exposures from the front (left axis), and Hp(10,0°)/Φ and Hp(0°)/Φ (≡E(AP)) conversion coefficients (right axis). (b) Fluence response of the PHE neutron dosemeter to rotational exposures (left axis), and Hp(10,ROT)/Φ and Hp(ROT)/Φ (≡E(ROT)) conversion coefficients (right axis). DOSE QUANTITIES A main drawback of Hp(10,θ) identified by ICRU is that for some energies and geometries it significantly under or over estimates the protection quantity effective dose, E, defined by the International Commission on Radiological Protection (ICRP)(8). These discrepancies can readily be shown (Figure 1a and b) from a comparison between the values of the conversion coefficients from fluence, Φ, to E(8) and to Hp(10,θ)(9). Furthermore, the Hp(10,θ)/Φ data were derived using the kerma approximation; this is technically incorrect, but proper calculation using full secondary charged particle transport within Monte Carlo models in a vacuum would not provide conservative assessments of E at high energies. Together, these limitations undermine Hp(10,θ) as a convenient calibration surrogate for E for operational purposes, designed to ensure that individuals’ doses in penetrating radiation fields can be correctly and conservatively monitored. The proposed quantity Hp(Ω) will instead be defined explicitly in terms of effective dose. Specifically, at a given energy the value of Hp(Ω)/Φ will be equated with the maximum value of E(Ω)/Φ, where Ω represents the geometry of the exposure and the maximum is taken over the values from the left and right directions at that angle(3). The current article focusses on anterior–posterior (AP, i.e. 0°) and rotational (ROT) exposures, because these are the most relevant geometries for personal dosimetry for which conversion data are tabulated in ICRP 116(8): values for plane-parallel exposures at angles other than the cardinal directions are not yet tabulated. Thus left-right asymmetries are not relevant for the present work, and numerically Hp(0°)/Φ ≡ E(AP)/Φ and Hp(ROT)/Φ ≡ E(ROT)/Φ. Comparisons of Hp(0°)/Φ and Hp(10,0°)/Φ, and Hp(ROT)/Φ and Hp(10,ROT)/Φ, are shown in Figure 1a and b, respectively, alongside the corresponding fluence responses of the PHE neutron dosemeter in those fields. Clearly, the degrees to which the performance of the dosemeter matches the dose quantities are both energy and geometry dependent, and its dose responses will depend strongly on the choice of quantity against which it is calibrated. DOSMETER RESPONSE IN CALIBRATION AND WORKPLACE FIELDS The responses of the PHE dosemeter to both the current and proposed dose quantities have been determined by applying the appropriate dose per fluence conversion coefficients to its fluence responses, (Figure 1a and b) for 0° (AP) and ROT fields, respectively. These results are shown in Figure 2, with the data given in terms of the expected number of counted tracks per unit dose equivalent, for typical dosemeter sensitivity. The values of Hp(Ω)/Φ at low energies are smaller than those of Hp(10,θ)/Φ (Figure 1a and b), so the dosemeter appears more sensitive in that range for the new quantity. However, and as anticipated from their respective definitions(1, 3), the variation in dosemeter response with exposure geometry is much greater for Hp(Ω) than it is for Hp(10,θ), and the Hp(ROT) sensitivity is significantly lower than the Hp(0°) result at all energies considered. Figure 2. View largeDownload slide Dose equivalent responses of the PHE neutron dosemeter, for both 0° and ROT exposures and for both the current and proposed dose quantities. Figure 2. View largeDownload slide Dose equivalent responses of the PHE neutron dosemeter, for both 0° and ROT exposures and for both the current and proposed dose quantities. In addition to its responses to monoenergetic sources, the performance of the dosemeter has also been estimated in realistic workplace fields. This was achieved by convolving the fluence response characteristics of the dosemeter (Figure 1a and b) with the nineteen neutron fluence-energy distributions that were determined during the EVIDOS project(10), using a bespoke algorithm that folds the relevant data pointwise across a suitably fine energy grid, dividing by the mean Hp(Ω)/Φ or Hp(10,θ)/Φ for that field. The responses of the dosemeter to an 241Am–Be neutron distribution(10) were also calculated using the same folding method. All responses were then normalized to the corresponding calibration source response, conventionally chosen for the PHE dosemeter as the response to 241Am–Be(11) at 0°. The mean of all 40 relative response results for a given dose quantity was subsequently determined to provide the calibration response; the mean response relative to 241Am–Be at 0° was 0.79 for Hp(d,θ), and 1.05 for Hp(Ω). Finally, the individual relative responses for the workplace fields were normalized to the appropriate Hp(d,θ) or Hp(Ω) calibration factor. These final results are shown in Figure 3a for Hp(10,θ) and in Figure 3b for Hp(Ω), with the x-value for each data point being the mean Hp(10,θ)/Φ or Hp(Ω)/Φ of the corresponding workplace or 241Am–Be field. Figure 3. View largeDownload slide (a) Normalized Hp(10,θ) responses to the neutron distribution fields. (b) Normalized Hp(Ω) responses to the neutron distribution fields. Figure 3. View largeDownload slide (a) Normalized Hp(10,θ) responses to the neutron distribution fields. (b) Normalized Hp(Ω) responses to the neutron distribution fields. In both cases, a wide range (± ~50%) of results is observed, though the standard deviation of the distribution for Hp(10,θ) is a little smaller than that for Hp(Ω), being 25 and 34%, respectively. Moreover, and similar to Figure 2, it is apparent that the Hp(0°) responses are considerably higher than the Hp(ROT) data, whilst this trend appears less defined for Hp(10,θ). For Hp(10,θ), the mean conversion coefficients for the ROT fields are typically much lower in value than for the 0° exposures, but the Hp(Ω) data do not exhibit this shift to such an extent. This observation demonstrates that changing the dose quantity effectively changes the perceived characteristics of the fields, as well as those of the dosemeters used in them. The above mean calibration factors have also been applied to the monoenergetic response characteristics (Figure 2) to derive the recalibrated relative responses of the dosemeter across the energy range of interest for both the current and proposed dose quantities. The results for the 0° exposures are shown in Figure 4. The performance of personal dosemeters can be judged against criteria specified in ISO 21 909-1:2015(12), which recommends that the relative response, r, of a dosemeter exposed at normal incidence should lie within the limits 0.6 < r < 1.7 for thermal neutrons and across the energy range from 144 keV to 14.8 MeV. Those limits are superimposed in Figure 4, indicating the energies at which they are met/not met by the PHE dosemeter. Figure 4. View largeDownload slide Calibrated Hp(10,0°) and Hp(0°) relative responses of the PHE neutron dosemeter. Solid datapoints indicate the energies at which the ISO standard applies. Figure 4. View largeDownload slide Calibrated Hp(10,0°) and Hp(0°) relative responses of the PHE neutron dosemeter. Solid datapoints indicate the energies at which the ISO standard applies. SUMMARY/CONCLUSIONS The proposed quantity Hp(Ω) provides a better estimate of risk than the current quantity Hp(10,θ), with the ‘conservatism’ inherent in the latter reduced. However, adoption of Hp(Ω) would be both advantageous and disadvantageous for the PHE dosimetry service. On the one hand, the dosemeter will be more sensitive at normal incidence, and for ROT at lower energies, leading in turn to an enhanced signal to background noise ratio. Conversely, the angle dependence of response will be worse, because Hp(10,θ) is lower for larger angles of incidence than Hp(Ω), and the fluence response of the dosemeter falls with increasing angle of incidence. This angle dependence will lead to larger uncertainties overall due to the PHE calibration method, which is in terms of averaged responses to AP and ROT workplace field exposures, because the latter will give much lower results. Of course, this conclusion should be viewed in the context that the discrepancy currently exists in the estimate of risk, rather than in the estimate of dose equivalent. However, it might imply that two dosemeters may need to be worn by individuals, one on their front and one on their back, and the results combined to give a proper assessment of risk. In general, the normalized Hp(Ω) response of the PHE dosemeter (Figure 4) will satisfy the energy-dependent recommendations of the ISO standard(12). Crucially, it will not under-estimate risk by more than 40% at any of the specified energies; however, its over-response will exceed the 70% limit for thermal neutrons and around 1 MeV. These problems might be overcome by modifications of the shape, size and material composition of the dosemeter’s holder, with more filtration providing an easy solution to the thermal neutron problem. There will be a significant problem at higher energies: the effective dose (and hence, Hp(Ω) by association) conversion coefficients extend far beyond 20 MeV (Figure 1a and b), where it becomes increasingly hard for track detectors to respond well. The dosemeter’s response will be much lower than for the fields discussed in this report, but there are also no performance requirements covering those energies. Many of the effects described in this article for the PHE dosemeter would be common to other track-based dosimetry systems, though they may be quite different from those experienced by other types, such as albedo dosemeters. These remarks might lead to the suggestion that the performance criteria recommended for personal dosemeters may, in time, need updating to better reflect the demands that would result from adopting Hp(Ω) over Hp(10,θ): a higher tolerance for thermal neutron responses, for instance, might be considered. ACKNOWLEDGMENTS The authors thank former colleague David Bartlett (formerly NRPB & HPA, and currently ICRU RC26) for private discussions on the proposed change, and are also grateful to colleagues within PHE Radiation Metrology Group and NPL for providing the irradiations used to characterize the dosemeter. REFERENCES 1 International Commission on Radiation Units and Measurements (ICRU) . Determination of dose equivalents resulting from external radiation sources. ICRU Report 39 ( 1985 ). 2 Endo , A. The operational quantities and new approach by ICRU. In: Proceedings of the Third International Symposium on the System of Radiological Protection. 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Passive neutron dosimetry systems—Part 1: Performance and test requirements for personal dosimetry ( 2015 ). © Crown copyright 2018. This article contains public sector information licensed under the Open Government Licence v3.0 (http://www.nationalarchives.gov.uk/doc/open-government-licence/version/3/).
Radiation Protection Dosimetry – Oxford University Press
Published: Aug 1, 2018