Harmonization of IEC type testing requirements and test methods for active area dosimeters in environmental monitoring

Highlights

• Active area dosimeters are frequently used for environmental monitoring.

• Analysis of existing IEC standards related to area monitoring was performed.

• Harmonized influence quantity ranges and response limits of variation were proposed.

• Area dosimeter performance for environmental monitoring was examined.

Abstract

Advancements in area monitoring caused by development of environmental monitoring networks which utilise active electronic dosimeters have led to widespread use of active area dosimeters. Currently, there are various sets of requirements for area dosimeters available in IEC standards. A clear distinction is not always made between requirements for area dosimeters used for workplace monitoring and that used for environmental monitoring. Performance testing methods related to the radiation characteristics were examined in a number of standards. Out of this, criteria in terms of relative response limits of variation have been derived for photon energy, angle of incidence and dose rate non-linearity for both dosimeter use cases. Testing of several active area dosimeters which are used for environmental monitoring has been performed in a wide range of photon energy, angle of incidence and dose rate. The collected requirements for environmental and workplace area dosimetry imply that some harmonization of IEC standards for area dosimeters is needed.

Introduction

The basics of ionising radiation protection practice include individual monitoring and ambient (area) monitoring, which is carried out with the help of active and passive radiation protection instrumentation (RPI). To ensure that the acquired dosimetry data is reliable, individual dosimeters are calibrated, producing metrologically traceable data, while dosimeter types are approved for use by being type-tested. Type testing of dosimeters is done by following defined type test procedures and criteria of a selected International Electrotechnical Commission (IEC) standard covering specific area monitoring situation (Ambrosi et al., 2004). The range of applications for area dosimeters can vary, and same device types are often used for different workplace or environmental monitoring situations. End users of such area dosimeters may use workplace area dosimeters in environmental monitoring, even though devices are type tested according to IEC standards which define criteria for workplaces. Therefore, devices which are currently used for environmental monitoring may not exhibit satisfactory performance under different environmental conditions, encountering influence quantity values which are unaccounted for, even if they have satisfactory performance and are type-tested according to some of the IEC area monitoring standards.

The rapid development and expansion of official radiological early warning networks as well as non-governmental environmental radiation monitoring networks following recent nuclear accidents have led to the introduction of a great number of devices produced by various manufacturers. Many devices employed in non-governmental networks are low-cost and easily accessible to the wider population due to their simplistic design. In a previous research a number of such devices termed as Monitoring Instruments used in Non-governmental Networks (MINNs) were subjected to performance tests in order to examine their behaviour during acquisition of dosimetry data under various environmental monitoring conditions (Morosh et al., 2021). The performance of these devices was evaluated according to the acceptance criteria defined in the RPI IEC standard IEC 60846–1:2009 related to area monitoring (International Electrotechnical Commission, 2009). Produced dosimetry data in terms of ambient dose equivalent rate (H*(10)) may be unrealiable, due to the use of insufficiently characterised devices as well as inadequate performance testing criteria related to dose rate measurements under environmental conditions.

Currently, many international standards for radiation protection instrumentation pertaining to the same operational dosimetry quantity (H*(10)) exist. A number of these standards were analysed in this work with the aim to compare existing type testing procedures defined for area workplace monitoring and area environmental monitoring (both common and specific) and derive acceptability criteria. Based on these criteria a type testing procedure was developed. To check the validity and applicability of performance testing method for area environmental dosimeters, the measurement programme was conducted on several device types which are used in non-governmental environmental monitoring networks along with a representative environmental spectrodosimeter.

The IEC standards that are related to RPI are maintained by the subcommittee SC45B, covering all fields of measurements in the workplace or in the environment, under regular or accidental conditions, during external or internal exposure to radiation of the population or professionally exposed personnel (Voytchev and Radev, 2018). The recent edition of the IEC 62387 (International Electrotechnical Commission, 2020) standard sets the performance requirements for passive dosimetry systems and hybrid dosimeters (such as direct ion storage dosimeters) which are to be used for individual monitoring or area monitoring. In this standard the criteria related to area monitoring are further subdivided to workplace and environmental monitoring. On the other hand, the instruments used in early warning networks and especially the non-governmental networks are mostly active electronic dose rate meters, mainly due to their direct real-time dose (rate) readout and incorporated visual or audible alarm level functionality, implying that a set of criteria should be applicable for environmental applications of active electronic ambient dosimeters. IEC 60846-1 (International Electrotechnical Commission, 2009) defines the performance testing procedures and acceptability criteria for portable devices for general area monitoring in regular workplace conditions. Depending on the instrument application, the overall range encompasses a dose rate range from 100 nSv·h⁻¹ up to 10 Sv·h⁻¹. The second standard in this series, IEC 60846-2 (International Electrotechnical Commission, 2015), extends the area workplace monitoring scope covering the criteria for high range portable instruments for accident and post-accident situations, with the dose rate range being from 1 mSv·h⁻¹ to 10 Sv·h⁻¹. Similarly, IEC 60532 (International Electrotechnical Commission, 2010) applies specifically to installed dose rate meters and warning assemblies at workplaces which are in the establishment of nuclear power plants (NPP) or other nuclear facilities (NF) related to nuclear fuel and waste processing and storage, either under normal operating conditions or preceding possible incident such as minor radioactive release or minor fuel degradation. In the more recent standard IEC 61017 (International Electrotechnical Commission, 2016), the possibility of environmental radiation dose evaluation is recognised, where the criteria for active portable, mobile, or installed dosimeters in the dose rate range from 30 nSv·h⁻¹ to 30 μSv·h⁻¹ are defined.

Various effects can modify the dose rate value indicated by the dosimeter. These effects are quantified with different influence quantities which can be classified into radiation, environmental, electrical, or mechanical characteristics of the dosimeter. The influence quantities can be of type S (sum; introducing a deviation ($D$) effect on the indicated value) or of type F (factor; introducing a change in relative response ($r$)). The value measured by the dosimeter $M$ can be represented with a general measurement model function (International Electrotechnical Commission, 2009):

$$M=N_0\frac{G-{\sum }_{p=1}^l{D}_p}{{\prod }_{q=1}^m{r}_q}$$

where $G$ represents the value indicated by the instrument, $N_0$ the calibration coefficient, $D_p$ deviations due to influence quantities of type S (such as electromagnetic disturbance) and $r_q$ relative response values for the influence quantities of type F (such as photon energy). Relative response of the dosimeter represents the normalised absolute response value determined under certain conditions to the absolute response value determined under reference (often calibration) conditions. Absolute response is determined as the quotient of the indicated value and the reference (conventional quantity) value measured by the metrologically traceable secondary or working standard. The minimum rated range of an influence quantity is defined as the range of values for which the dosimeter is obliged to fulfil the limits of variation (of relative or absolute response) in order to comply with a certain IEC standard (International Electrotechnical Commission, 2009). Among others, influence quantities regarding radiation characteristics (dosimeter non-linearity, energy response and angular response) and environmental characteristics (ambient temperature, relative humidity, atmospheric pressure) were analysed in the beforementioned IEC standards considering area workplace and environmental monitoring. Mechanical and electromagnetic influence quantities were not included in this research. Compiled radiation characteristic influence quantity reference values, minimum rated rages, and response limits of variation defined in the standards for area monitoring are presented in Table 1, Table 2.

Effects of the photon energy encountered in real poly-energetic fields in individual and area monitoring on the dosimeter response have been studied taking into account the criteria defined by certain standards in radiation protection in both continuous (Boziari and Hourdakis, 2007; Cardoso et al., 2016; Kržanović et al., 2017) and pulsed radiation fields (Ankerhold et al., 2009; Zutz et al., 2012). Depending on the dosimeter application and according to the IEC standard scopes, a wide range of possible photon energies is defined ranging from low-photon energies of 10 keV encountered in the medical applications of ionising radiation, up to high-energy photons of 10 MeV characteristic for nuclear applications. The common minimum rated range for most of the analysed IEC standards is 80 keV up to 1.5 MeV, a range that covers general area monitoring and the photons most encountered in radiation protection in various fields of applications. Depending on the radiation application specificity, the minimum rated range can be extended in compliance with the scope of the chosen standard. The energy dependence performance tests are evaluated according to the criteria given by the limits of variation of either relative response $r\left(E\right)$, or absolute response $R\left(E\right)$. Requirements given in terms of relative response disregard individual dosimeter calibration and are characteristic for type testing procedures, whereas absolute response is used for field measurements purposes where capability of manufacturer calibration is included in performance testing. The reference radiation quality used for this test is usually the ISO 4037-1 (International Organisation for Standardisation, 2019) Cs-137 standard radiation quality (abbreviated as S–Cs). IEC 60846-1 (International Electrotechnical Commission, 2009) defines two minimum rated ranges depending on the nature of workplace. The “industrial” range covers the photon energies from 80 keV up to 1.5 MeV, while the “medical” range excludes usage of high-energy photons, applying exclusively to the medical diagnostics photons in the energy range from 20 keV to 150 keV. For such a narrow energy range, absolute response is normalised to ISO 4037-1 (International Organisation for Standardisation, 2019) radiation quality N-100 (with 83.3 keV mean energy). In this standard the limit for relative response variation due to photon energy is set to 0.71 to 1.67 (International Electrotechnical Commission, 2009). Since IEC 60846-2 extends the scope of the previous standard for nuclear facility monitoring applications, the additional minimum rated range covers energies from 1.5 MeV up to 7 MeV, with the relative response limits of variation 0.62 to 2.50. This criterion is defined specifically for external telescopic and remote probes (International Electrotechnical Commission, 2015). On the other hand, in case of IEC 60532 (International Electrotechnical Commission, 2010) and IEC 61017 (International Electrotechnical Commission, 2016) the criteria are expressed in terms of absolute response limits in the same minimum rated range of photon energies of 80 keV to 1.5 MeV. In these standards, performance tests propose testing in certain radiation fields (such as S–Am, N-100, S–Co) other than the whole energy range, comparing the absolute response values for these qualities with the one obtained for S–Cs. The minimum rated range covers main natural and artificial sources of radiation encountered in environmental monitoring. Limits of variation for the possible extensions of the minimum rated range are not clearly defined in these standards and are to be set by the device manufacturer.

Incident photon angle as an influence quantity can significantly affect the dosimeter response due to the different trajectory of incident radiation, interacting with structural components of the dosimeter and its electronics, ultimately not depositing a significant part of its initial energy in the active volume of the dosimeter. The directional dependence of the dosimeter response is an effect more pronounced at low-photon energy, as the high-energy photons may have lower interaction probabilities with the dosimeter material other than active detection volume constituents (Ćeklić et al., 2014; Kržanović et al., 2017). The general area workplace monitoring standard (International Electrotechnical Commission, 2009) considers a minimum rated range of angles of incidence (0°; ±45°) with a possible extension up to ±90° for wide range monitors (abbreviated as “w” in the standard). The relative angular (and energy) response for the minimum rated ranges of photon energy and angle of incidence is in the range of 0.71–1.67. In the special case of external probes, three angular ranges with different criteria are recognised (International Electrotechnical Commission, 2015). For (0°; ±60°) and (180°; 180° ± 60°) the same relative response criterion applies as for regular area monitors. In the angular range (±60°; ±120°), a more relaxed relative response limit of variation is proposed, being 0.62 to 2.50. Specifically for the „critical“ (depending on the device geometry) angular range (90°; 90° ± 10°) the suggested lower limit of relative response is as low as 0.50. This angular response criteria reasoning could be of use in defining criteria for angular dependence of environmental area monitoring dosimeters, where ideally omnidirectional devices would be used. In the new version of passive dosimetry systems type-test standard (International Electrotechnical Commission, 2020) the need for distinction between workplace and environmental monitoring is well defined by having different minimum rated ranges of incident angles as well as more relaxed criterion for larger values of angles of incidence (±60°; ±120°). In these standards, relative energy and angular response $r(E,\Omega )$ is determined by normalising the absolute response values for a certain energy and angle of incidence $R(E,\Omega )$, to the value determined for Cs-137 and 0° $R_0(E_0,0°)$. In the active dosimeter environmental standard (International Electrotechnical Commission, 2016), the need for 4π geometry monitoring is recognised (emphasizing practical and technical limitations), with the minimum rated range being (0°; ±120°). In this standard the compliance criteria are expressed in terms of absolute angular response $R\left(\text{Ω}\right)$. The normalisation of calculated responses is performed for each radiation quality separately, where the absolute response deviation of ±20% is defined for angular dependence testing in the reference field of S–Cs. If testing in other radiation qualities is required, the limits of variation are not clearly defined in the standard, and are subject of agreement between the device manufacturer and end-user.

Deviation of the ambient dosimeter indication from the conventional quantity value can differ depending on the dose rate variation over the instrument measurement range (Morosh et al., 2021). Depending on the measurement range and the monitoring application, the dose rates used for non-linearity testing should include several measurement points per order of magnitude of the measurement range (e.g., 30% and 70%). The reference dose rate of 10 μSv·h⁻¹ should be included into the dose rate dependence tests. The relative response limits of variation, normalised to the response at reference dose rate, are defined as 0.85 to 1.22 (International Electrotechnical Commission, 2009). If the specificity of application considers extreme monitoring conditions, such as accidental situations, higher dose rates such as 10 Sv·h⁻¹ should be mandatory as well (International Electrotechnical Commission, 2015). Additionally, a dose rate overload should be recognisable. On the other hand, for environmental monitoring purposes it would be beneficial for the monitoring instrument to measure low background level dose rates (below 100 nSv·h⁻¹) with sufficient accuracy. Further dose rate dependence testing including determination of background radiation level components including instrument inherent background, response to terrestrial and secondary cosmic radiation would contribute to the detailed characterisation of environmental monitoring devices (Neumaier and Dombrowski, 2014; Morosh et al., 2021).

Air density parameters are important during routine dosimeter calibrations since they can affect the determination of conventional true value measured with secondary standard ionisation chambers, and are corrected for during reference air kerma rate measurements. Environmental effects are less pronounced on the area dosimeter performance under regular ambient conditions, roughly close to the reference values of 20 °C, 101.3 kPa and 65% for ambient temperature, atmospheric pressure and relative humidity, respectively. Since area monitors usually have confined internal atmosphere in the active volume, atmospheric pressure variations do not influence the dosimeter response, therefore the minimum rated range for pressure is defined only in IEC 60846 series (International Electrotechnical Commission, 2009; International Electrotechnical Commission, 2015). Depending on the dosimeter application and the ambient conditions that are expected to be encountered in a certain workplace or in the environment, different temperature and relative humidity ranges, with different limits of variation are defined in the IEC standards. In the IEC 60846 series (International Electrotechnical Commission, 2009), temperature ranges (−10 °C; +40 °C) and (+5 °C; +40 °C) are defined for outdoor and indoor area workplace monitoring with the relative response limits 0.87 to 1.18, while for extreme monitoring conditions the range (−25 °C; +70 °C) is proposed with broader limits of variation 0.77 to 1.43 (International Electrotechnical Commission, 2015). The environmental area monitoring standard considers two temperature ranges, one for regular, temperate climates (−10 °C; +40 °C), and the other for more harsh climates (−25 °C; +55 °C) where limits of absolute response deviation of ±20% and ±50% are defined for the lesser and broader range, respectively (International Electrotechnical Commission, 2016). The limits of variation for relative humidity do not significantly differ between IEC standards although the minimum rated ranges depend on the dosimeter application, being up to 85% for regular workplace monitoring (or up to 93%, 95% in case of nuclear facility monitoring) or 40%–90% for environmental monitoring.

Several area dosimeter models produced by various manufacturers available on the market have been examined with the aim to determine if the available dosimeter specifications conform to general area monitoring needs of the end-users in various area workplace monitoring situations as well as the possibility of use in environmental area monitoring.

Depending on the specificity of the workplace, dosimeters based on one or multiple radiation detectors with different measurement ranges, radiation and environmental characteristics are used. These active electronic dosimeters utilise radiation detectors based on different mechanisms of radiation detection such as gas-filled detectors (G-M tubes, proportional counters, pressurised ionisation chambers), scintillation detectors (CsI(Tl), plastic scintillators) and semiconductor detectors (silicon diode). The wide spectra of area workplace monitoring situations covers applications in e.g. industry, nuclear safety (power plants, waste disposal sites, etc.), or medicine which includes emergency situations monitoring, post-accidental situations monitoring, healthcare (various diagnostic radiology and radiotherapy modalities and applications), education and civil defence.

Most of the dosimeter manufacturer specifications are derived from type testing requirements according to the IEC 60846 (International Electrotechnical Commission, 2009) standard series, which considers area workplace monitoring. Dosimeters available on the market from renowned manufacturers are thoroughly tested for certain cases of radiation applications. All of the area workplace dosimeters should comply with the relative response limits of variation in the minimum rated range which pertain to general area workplace monitoring. For specific applications of ionising radiation e.g., for many medical applications of ionising radiation monitoring extensive performance testing in low-energy photon radiation fields should be considered, or in the vicinity of nuclear facilities a high dose rate measurement range and high-photon energy occurrences should be taken into account. Also, it should be noted that properly type-tested and characterised MINNs could be used as workplace dosimeters if they comply with the set requirements of the appropriate IEC standard. The rated ranges of photon energies usually span from 60 keV up to 1.25 MeV for most area workplace dosimeters, with extensions down to approximately 20 keV or up to 10 MeV, for monitoring of low-energy photons (in healthcare), or NF high-energy photons, respectively. The specified angle of incidence rated ranges differ depending on the regular workplace conditions of use, as well as if the dosimeter is a portable/mobile survey meter or an installed monitoring assembly, usually corresponding to the IEC 60846-1 (International Electrotechnical Commission, 2009) minimum rated range up to ±45°, where explicitly stated by the manufacturer. Depending on the device geometry and workplace requirements the specified angle ranges up to ±120° are also observed.

Dose rate measurement ranges greatly differ depending on the application and can cover dose rate ranges such as 100 nSv·h⁻¹ up to 10 Sv·h⁻¹. Often more than one detector is employed in the dosimeter, allowing for automatic dose rate measurement range switching, covering a large number of orders of magnitude. Additionally, a few area workplace dosimeters are designed having in mind the use in pulsed radiation fields characteristic for medical diagnostic modalities. The temperature and relative humidity rated ranges specified by the manufacturers usually include a wide range of these influence quantities, considering various environmental conditions in different workplace monitoring situations, covering temperature ranges from −20 °C up to +50 °C and relative humidity values up to 98%.

For an area dosimeter to be used in environmental monitoring, it would be necessary that the dosimeter has such a measurement range that it is sensitive to minor changes in the ambient dose equivalent rate in the order of magnitude corresponding to background radiation level (˂100 nSv·h⁻¹). Certain analysed dosimeters' specifications display a measurement range with the lower limit of 10 nSv·h⁻¹ which is a desirable dose rate measurement property for environmental measurements, often achievable by employing CsI(Tl) scintillators or energy compensated G-M tubes. Some of the devices use two detectors with different sensitivities, such as CsI(Tl) scintillator paired with Si semiconductor diode, or two G-M tubes, while other detectors have a possibility to connect external probes in order to cover a wider dose rate range than the properties of the main measurement unit allow. Photon energies defined in the rated ranges provided by the manufacturers consider regular area monitoring conditions covering the range from 60 keV up to 1.5 MeV. The ideal environmental dosimeter should be able to detect incident radiation from any direction in a natural multidirectional radiation field, achieving 4π monitoring. Due to the practical device design limitations, an environmental area dosimeter should be able to detect incident photons in a wide planar range (2π monitoring). Unfortunately, in the analysed devices’ manufacturer specifications the rated range of angles of incidence is either up to ±45° (which is the minimum rated range for area workplace dosimeters in IEC 60846 series (International Electrotechnical Commission, 2009)), or the minimum rated range of angles is not clearly defined, either way resulting in an angular range which is unsuitable for environmental monitoring.

Another important feature of environmental dosimeters is the reliable operation under various environmental conditions, i.e. in a wide range of ambient temperature and relative humidity values (atmospheric pressure variation would be considered as well if the dosimeter does not have enclosed internal atmosphere). Environmental conditions rated ranges defined for temperature and relative humidity are similar to the ones defined for area workplace dosimeters, while some of the environmental dosimeter manufacturer specifications include a very large temperature working range from −40 °C up to +60 °C, covering extreme environmental conditions encountered in some parts of the world or if exposed to direct sunlight.

Besides the renowned manufacturer's device specifications, the increase in the public awareness following the recent nuclear accidents and the release of radiation in the environment, induced rapid development of non-governmental networks. These networks contain low-cost devices primarily based on G-M tubes, which are mostly not energy compensated, providing unreliable dosimetry data. Monitoring instruments used in non-governmental networks (MINNs) are usually not thoroughly characterised for their use as general area monitors, and even though they are widely used as environmental monitors in these networks (Morosh et al., 2021), they are not complying with the requirements of the standards pertaining to environmental monitoring, such as IEC 61017 (International Electrotechnical Commission, 2016) (whose requirements on the other hand are not strict enough and less binding than e.g.in IEC 60846 (International Electrotechnical Commission, 2009) series or IEC 62387 (International Electrotechnical Commission, 2020)). MINN manufacturer specifications are mostly incomplete not providing all relevant information in terms of radiation and/or environmental characteristics.