ORIGINAL ARTICLE Year : 2018  Volume : 41  Issue : 2  Page : 7783 Assessment of the performance characteristics of an optically stimulated luminescence dosimetry system in Gabon Philippe Ondo Meye^{1}, Cyril Schandorf^{2}, Roger Ondo Ndong^{3}, Hans Essone Obame^{3}, ^{1} General Directorate of Radiation Protection and Nuclear Safety, Ministry of Water and Energy, Libreville, Gabon ^{2} Department of Medical Physics, School of Nuclear and Allied Sciences, Accra, Ghana ^{3} Département Sciences Physiques, Ecole Normale Supérieure, Libreville, Gabon Correspondence Address: To ensure the reliability of the performance of the optically stimulated luminescence (OSL) dosimetry system used for individual monitoring of occupationally exposed workers in Gabon, some key performance indicators were evaluated. The performance indicators assessed included: measurement uncertainty, linearity, coefficient of variation, and photon energy and angular dependence. The results for uncertainty assessment were in very good agreement with the International Commission on Radiological Protection (ICRP) and International Atomic Energy Agency (IAEA) requirements for overall accuracy. The maximum relative overall uncertainty in absolute value in the region of the recording level was 45%. In the region beyond the recording level, the maximum relative overall uncertainty was 42% (in absolute value). The results for linearity assessment met the requirement for linearity of International Electrotechnical Commission (IEC) 62387 standard. The results for coefficient of variation assessment were within the relevant requirement of IEC 62387 for doses beyond the recording level. The results obtained for photon energy and angle of incidence were in good agreement with the IAEA and IEC 62387 corresponding requirements. The OSL dosimetry system can be relied on to provide acceptable monitoring data for the individual monitoring service in Gabon.
Introduction Between 2013 and 2016, the Individual Monitoring Service (IMS) of the General Directorate of Radiation Protection and Nuclear Safety (DGRSN), Ministry of Water and Energy in Gabon, participated in four regional intercomparison exercises on measurements of the personal dose equivalent Hp(10) in photon fields. Two of them were organized in 2013 and 2016 by the International Atomic Energy Agency (IAEA) through the regional technical cooperation projects RAF/9/043,[1] Strengthening the Transfer of Experience Related to Occupational Radiation Protection of the Nuclear Industry and other Applications Involving Ionizing Radiation, and RAF/9/053, Strengthening Technical Capabilities for Patients and Occupational Radiation Protection in Member States. The first exercise in 2013 involved 24 countries from Africa and three countries from outside this continent. Seventeen African countries participated in the second exercise. One intercomparison exercise was organized in 2013 by Landauer Europe [2] to assess the performance of some of the African laboratories that use Landauer OSL dosimetry systems. Five laboratories from five African countries participated in this exercise. The last intercomparison exercise was organized in 2016 by IMSs of the DGRSN and that of the Radiation Protection Institute of the Ghana Atomic Energy Commission. Only Gabon and Ghana participated in this exercise. The purpose of this work was to highlight the results obtained by Gabon during these intercomparison exercises and to assess some performance indicators using data from these intercomparisons and relevant international standards for personal monitoring. The work was limited to data from IAEA and Landauer Europe intercomparisons. The intercomparison organized by national IMSs in Gabon and Ghana is discussed in reference.[3] Materials and Methods Materials The dosimetry system considered here is an InLight dosimetry system (Landauer, Inc., USA). It is designed to assess personnel exposure to beta particles and photons using OSL.[4],[5],[6] The system consists of individual passive dosemeters, a reader, a highintensity light annealer, and a software installed on a personal computer. Reader The reader under consideration is the portable InLight Microstar (32.7 cm width × 23.2 cm depth × 10.9 cm height,[4],[6] serial number 11040681, date of installation, April 2012). The OSL readout of each detector is performed using an array of 38 green LEDs (532 nm wavelength) operated in continuous waveOSL mode during about 1 s. All the 38 LEDs are used for the stimulation of low doses (strong beam), whereas only 6 LEDs are used for high doses (weak beam). The dose range is evaluated before the measurement using one LED and the cutoff point is an adjustable parameter. Dosemeters The dosemeter considered comprises a case and a slide. The latter contains four detector elements of Al2O3: C cut into round pieces of ~5 mm in diameter sandwiched between two layers of polyester for a total thickness of 0.3 mm. The detectors are located in read positions 1 (E1), 2 (E2), 3 (E3), and 4 (E4). When the slide is inside the case, each detector is positioned behind different filters providing different radiation attenuation conditions. The signal from each OSL detector is used in conjunction with a dose algorithm to evaluate different dosimetric quantities (Hp(10), Hp(0.07), and Hp(3)).[6] This dose algorithm inherently uses individual calibration factors (ICFs). For wider and homogeneous radiation fields, the chest OSLD badge (element E3) may also act as a surrogate for eye lens doses. Methods Details on Landauer and the 2013 and 2016 International Atomic Energy Agency intercomparison exercises During the 2013 and 2016, IAEA regional intercomparison on measurements of the personal dose equivalent Hp(10) in photon fields, the dosemeters were irradiated at the surface of slab phantoms using values of dose rates calculated from the measured air kerma determined with the Secondary Standard Dosimetry Laboratory (SSDL) reference instrument. For the 2013 exercise,[1] irradiations were performed by the SSDL of the Nuclear Research Centre of Algiers as follows: Irradiations using the 137 Cs source to dose values varying from 0.5 to 10 mSv in normal incidence (linearity verification –dosemeters irradiated in group of 3)Irradiations at the fixed dose of 2 mSv using three Xray qualities (N60, N80, and N150) (energy response – dosemeters irradiated in group of 3)Irradiations with SCs quality at the fixed dose of 2 mSv and angular incidences of 0°, 45°, and 60° (angular response – dosemeters irradiated in group of 2) andIrradiations at different doses (0.7–150 mSv), energies (N150, SCs, and SCo), and in mixed qualities (N150 + SCs) (Blind test – dosemeters irradiated in group of 2). For the 2016 exercise, irradiations were carried out as follows: Irradiations using the 137 Cs source to dose values lying between 0.2 and 10 mSv in normal incidence (linearity verification – dosemeters irradiated in group of 4)Irradiations at the fixed dose of 2 mSv using three Xray qualities (N60, N80, and N120) (energy response – dosemeters irradiated in group of 4)Irradiations with N60 and SCs qualities at the fixed dose of 2 mSv and angular incidences of 0°, 45°, and 60° (angular response – dosemeters irradiated in group of 2); andIrradiations with SCo quality and mixed qualities (N60 + SCs) at different doses (20 and 5 mSv, respectively) (Blind test – dosemeters irradiated in group of 4). Landauer Europe intercomparison was limited to the assessment of dosimetry systems involved using a linearity response test.[2] Irradiations were performed by Landauer Europe using a 137 Cs source in normal incidence. The doses delivered were 0.19 mSv, 6.27 mSv, and 19.90 mSv (dosemeters irradiated in group of 3). Uncertainty assessment The analytical method described in International Electrotechnical Commission (IEC) technical report TR 62461 was followed.[7] Three steps were required: The construction of a model function, the collection of data and existing knowledge, and the calculation of uncertainty associated with the result of the measurement. The model function used in this study was derived from that given in the standard IEC 62387:[9] Hp (10) = N0 KnKE,φ Kenv f(H¯m)(1) Where Hp(10) is the measuring quantity personal dose equivalent, N0 is the reference calibration factor, Kn is the correction factor for nonlinearity, KE,φ is the correction factor for photon energy and angle of incidence, Kenv is the correction factor for environmental conditions, H¯m is the mean indicated value, mean reading of the dosemeter in units of Hp(10), f(H¯m) is a function derived from the calibration curve. This model function is similar to that presented in reference.[3] The notable difference is that the correction factor for photon energy and angle of incidence in Equation (1) is a function of energy. Therefore, an “if” algorithm with four branches was used.[8] The main data and required information needed in the present work for the uncertainty analysis were provided by the standard IEC 62387[9] (Kenv), the IEC technical report TR 62461 (N0), the three intercomparison exercises considered here (Kn and KE,φ), and IAEA safety guide No. RSG1.3.[10] In particular, IEC TR 62461 suggests limits of ±5% with a triangular distribution for N0, and IAEA RSG1.3 assumes a rectangular distribution for type B uncertainties. These assumptions were used in this study. As H¯m is a mean value of several measurements, its standard deviation is the standard deviation of a single measurement divided by the square root of the number of measurements. This standard deviation of the mean is then taken as the standard uncertainty associated with H¯m. The standard uncertainty σf associated with f(H¯m) is then derived using the uncertainty propagation formula [INLINE:1] Where σH¯m is the standard uncertainty associated with H¯m. f(H¯m) depends on the calibration curve. For the purpose of this work, f(H¯m) was chosen so that [INLINE:2] f(H¯m) is the same for Landauer and 2013 IAEA intercomparison exercises as they occur in the same period. To calculate the uncertainty associated with Hp(10), first sensitivity coefficients of input quantities were computed using the formula [INLINE:3] Where Xi= {N0, Kn, KE,φ, Kenv} denote the true values (capital letters) and xi= {n0, kn, kE,φ, kenv} their respective expectation values (small letters). cxi is the sensitivity coefficient associated with xi. Thereafter, the uncertainty contribution to the output quantity Hp(10) was calculated as follows: [INLINE:4] Where sxi is the standard uncertainty associated with xi. The total standard uncertainty u associated with hp(10) was obtained by taking the geometrical sum of these contributions: [INLINE:5] Finally, the complete result of the measurement is given by [INLINE:6] Where kcov is a coverage factor. A comparison of the results with International Commission on Radiological Protection (ICRP) and IAEA [10],[11],[12] requirements for overall accuracy was performed. These are: relative uncertainty limits of –33% and +50% (a factor of 1.5 in either direction) in the region near the relevant dose limitrelative uncertainty limits of ±100% (factors of 2 and 0) in the region of the recording level. These requirements are met by the trumpet curve given by [INLINE:7] Where Ht is the conventional true value (or delivered dose provided by an SSDL) and H0 (=0.05 mSv for the dosimetry system considered) is the recording level. Data used were collected from intercomparison exercises results for linearity response (quality: SCs; dose rage: 0.19–19.9 mSv), energy and angular response (qualities: N60, N80, N120, N150 and SCs; angles: 0, 30, 45, and 60°; dose: 2 mSv), and blind test (qualities: N150, N150+ SCs, SCs, SCo, N60+ SCs; angles: 0° and 30°; doses: 0.7, 2.5, 5, and 20 mSv). Linearity and coefficient of variation Linearity The procedure from IEC 62387 was followed.[9] The dosemeters are irradiated at known dose equivalents and the variation of the response due to the change of the dose equivalent shall not exceed the values given in the standard. This requirement is met only if the relationship below is valid, [INLINE:8] Where Ht, i is the conventional true value of the personal dose equivalent delivered to dosemeters of group iHt, ref is the reference conventional true value of dose equivalentH¯m, i is the mean of the indicated values of dosemeters of group iH¯m, ref is the mean of indicated values of dosemeters of the reference groupUcom is the expanded uncertainty of H¯m, i/H¯m, refUt, com is the combined relative expanded uncertainty of Ht, ref/Ht, i The corresponding limits in terms of mean response were derived (0.88 and 1.15). Ucom was computed following the procedure given in Annex A. Coefficient of variation According to the IEC 62387 standard, this test shall be performed together with the linearity test. The statistical fluctuations of the indicated value shall fulfill the following requirement: [INLINE:9] Photon energy and angle of incidence IEC 62387 requirement was followed:[9] the variation of relative response due to a change of radiation energy and angle of incidence within the rated ranges shall not exceed the value given for Hp(10) in the standard. The radiation qualities N60, N80, N100, N150, SCs (137 Cs), and SCo (60 Co) are part of the qualities used during the 2013 and 2016 IAEA intercomparisons. The above requirement is met if, for every radiation quality, the inequality is valid, [INLINE:10] Where rmin (0.69 and 0.71 for 48 keV and energies ≥65 keV, respectively) and rmax (1.82 and 1.67 for 48 keV and energies ≥65 keV, respectively) are, respectively, the minimum and maximum relative response given in the standard. The other quantities are as defined above. From equation (11), limits in terms of mean response were derived. The IAEA [10] requirements for radiation energy and angle of incidence were also tested in this work. Requirements on energy and directional dependence are combined and given by equation [INLINE:11] Where R¯E,φ is the mean response at photon energy E and angle of incidence φ between 0 and 60° by step of 20°. For energy E, all ISO series can be used. [Table 1] summarizes the recommended ranges of all the parameters that have been investigated in the study.{Table 1} Results and Discussion Uncertainty assessment [Table 2] shows uncertainty budget for the reference measured dose of 2013 intercomparison exercise. From all the uncertainty budgets of each measured dose, it is observed that the maximum relative overall uncertainty in absolute value in the region of the recording level is 45%. In the region beyond the recording level, the maximum relative overall uncertainty is 42% (in absolute value). These results are in very good agreement with ICRP and IAEA requirements for overall accuracy. These results are also close to that obtained in reference [3] where only one energy (662 keV) at normal incidence was used. [Figure 1] and [Figure 2] show the results in terms of response for the 2013 IAEA exercise. In particular, it can be seen that more accurate values of dose were obtained when using correction factors. This trend was also observed for the 2016 IAEA and Landauer exercises (not shown here). In addition, for each intercomparison exercise considered here, 100 % of the data points were within the trumpet curve.{Table 2}{Figure 1}{Figure 2} Linearity and coefficient of variation [Figure 3] presents the results obtained for linearity. It is observed that they are in very good agreement with IEC 62387 requirements for linearity. Data points and their associated statistical uncertainty near the recording level could be significantly improved by increasing the number of dosemeters in the irradiation process when delivering a given dose. Indeed, statistical or type A uncertainty can, in principle, be reduced by increasing the number of measurements. Furthermore, Annex A of IEC 62387 was used to compute statistical uncertainty. In this approach, the statistical uncertainty is obtained by multiplying the standard deviation of a specific group of measurements by the ratio of a coverage factor by the square root of the number of measurements. The coverage factor is obtained from the Student's tdistribution. For a number of 2, 3, and 4 measurements, this ratio is 8.98, 2.48 and 1.59, respectively, while for a number of 6 and 10 measurements, the ratio is 1.05 and 0.715, respectively. Furthermore, the results obtained in this study are comparable to that obtained in reference.[3] [Figure 4] presents the results for the coefficient of variation according to the IEC 62387 standard. All the data points are within the required limits apart from two data points near the recording level. Again, it is believed that the results could be improved if more dosemeters were used during the irradiation process for each delivered dose.{Figure 4} Photon energy and angle of incidence [Table 3] and [Table 4] present the results obtained according to the IEC 62387 standard. All the data points are within the required limits. However, the statistical overall uncertainty limits associated with mean responses at (662 keV, 45°) and (48keV, 45°) for the 2013 and 2016 intercomparisons, respectively, are clearly outside the required limits. Mean responses at 662 keV, 60° and 118 keV, 30°, for the 2013 IAEA exercise, and 662 keV, 45° for the 2016 IAEA exercise have their lower statistical overall uncertainty limit outside the required limits by a small margin. Increasing the number of dosemeters will help improve these results. [Table 5] presents the results obtained according to the IAEA (safety guide No. RSG1.3) requirements for radiation energy and angle of incidence. The results show that the requirement is met since all the data points lie within the required limits.{Table 3}{Table 4}{Table 5} Conclusions The OSL dosimetry system used for the national IMS in Gabon performed satisfactorily with respect to the performance indicators assessed, namely, measurement uncertainty, linearity, coefficient of variation, photon energy and angular dependence. The results obtained can be improved if appropriate correction factors are applied. The results for uncertainty assessment were in very good agreement with ICRP and IAEA requirements for overall accuracy. The results for linearity were in good agreement with the requirement for linearity specified in the IEC 62387 standard. The results for coefficient of variation met the relevant requirement specified in the IEC 62387 standard, except for doses near the recording level. The results obtained for photon energy and angle of incidence were in good agreement with the IAEA and IEC 62387 corresponding requirements. It is believed that all the results obtained could be improved if the number n of measurements (n ≥ 6) was increased. Finally, the OSL dosimetry system can be relied on to provide acceptable monitoring data for the IMS in Gabon. To ensure the longterm performance and reliability of the OSL dosimetry system, QC checks recommended by the manufacturer must be followed in between regional and international intercomparisons. Financial support and sponsorship Nil. Conflicts of interest There are no conflicts of interest. References


