SPANISH INTERCOMPARISON OF APPROVED PERSONAL DOSIMETRY SERVICES USING PHOTON RADIATION BEAMS

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1 SPANISH INTERCOMPARISON OF APPROVED PERSONAL DOSIMETRY SERVICES USING PHOTON RADIATION BEAMS I. Villanueva (1), I. Amor (1), A. Brosed (), R. Villanueva () M. Ginjaume (3), X. Ortega (3), M. Roig (3) (1) Consejo de Seguridad Nuclear, E-8006 Madrid, Spain. () Unidad de Metrología de las Radiaciones, CIEMAT, E-80 Madrid, Spain. (3) Institut de Tècniques Energètiques. Universitat Politécnica de Catalunya, E-0808 Barcelona, Abstract In order to supervise quality control and reliability of approved dosimetry services, the Spanish Nuclear Safety Council (CSN) organizes periodic national comparisons. The main results of the 001 comparison organised by the CSN among approved Personal Dosimetry Services are presented. All participants used TLD systems and measured H p (10) and H p (0.07). Irradiations were performed according to the ISO Standard using the recommended ISO water phantom and the ISO narrow spectra x-ray qualities from 33 to 00 kev and a gamma 137 Cs beam. Reference doses ranged between 0.3 msv and 7 msv. Results were evaluated using the so-called trumpet curve together with ANSI It is shown that 90 of the dose estimates fulfilled the established requirements. However, some difficulties were still found for the correct assessment of H p (0.07) for low-energy photons. 1. Introduction Exposed workers in Spain are controlled by Personal Dosimetry Services (PDSs), which have been approved by the Spanish Nuclear Safety Council (CSN). In 001 about 85,000 people were controlled, which represented a collective dose of 4,375 man msv. At the moment, there are 4 approved PDSs for external dosimetry and these services are subject to periodic inspections as well as interlaboratory comparisons as part of the control procedure. CSN comparisons are programmed, approximately every 5 years, with the collaboration of the National Calibration Laboratory (CIEMAT) and the accredited laboratory at the Technical University of Catalonia (INTE-UPC) [1]. The last intercomparison was performed during 001 with the aim of verifying the reliability of the approved services as well as comparing quality as regards the previous intercomparison performance in Furthermore, as the new ISO standard for personal dosemeter calibration has just been adopted [, 3], several PDSs were chosen to estimate the influence of the calibration procedure in their dose assessment. The previous calibration procedure was based on ICRU-47 [4], namely the use of a 30-cm x 30-cm x 15-cm PMMA slab and back-scatter correction factors for differences in back-scattering between ICRU tissue and PMMA.. Material and methods Twenty two PDSs participated in the intercomparison. All of them used TLD systems, but employed different badges as summarized in Table I and Figure 1. Among the participants, five services were in charge of occupational dosimetry of exposed workers in nuclear power plants and one service was dependent on the Health Ministry and thus, in charge of the dosimetry of most of the exposed workers from public hospitals and health centres. 1

2 Table I. List of badge types used by the participating personal dosimetry services (PDS) and number of PDSs that used each of them. Type of badge and TL material Nº PDS Vinten 860/E/10 with a 4 TLD700 card 3 Vinten 860/N/5 with a 4 TLD100 card 1 Harshaw 8814 with a TLD100 card 7 Harshaw 8814 with 3 6 LiF and 1 7 LiF Alnor OY with 6 LiF and 7 LiF 1 Alnor OY with LiF 1 Alnor with 3 TLD700 1 Panasonic UD 80AR LiB + CasO 5 Teledyne C-300 with 4 SO4 1 FIG.1. Photograph of the badges that were used in the comparison Radiation qualities used, in 001, were the ISO narrow x-ray series N-40, N-80 and N-50 and a 137 Cs beam with mean energy of 33, 65, 07 and 66 kev, respectively [5]. The irradiations were performed on a 30-cm x 30-cm x 15-cm water-filled PMMA phantom, as stated in ISO , with normal incidence of the beams on the phantom surface at a distance of.5 m for x-rays and 4.0 m for 137 Cs. Seven different calibration points were selected according to the dose range indicated in Table II. H p (10) was determined at random the fixed range and the corresponding H p (0.07) was

3 calculated, except for 137 Cs. The personal dose equivalents, H p (10) and H p (0.07), were derived from the air kerma reference measurements using the personal dose equivalent conversion coefficients tabulated in ISO Table II. Radiation qualities chosen for the intercomparison with corresponding personal dose equivalent range. Calibration Point Radiation Quality H p (10) (msv) 1 N (± 30 ) N (± 30 ) 3 N-80.0 (± 30 ) 4 N (± 30 ) Cs Cs 1.5 (± 30 ) Cs 5.0 (± 30 ) Reference irradiations were performed at the CIEMAT National Calibration Laboratory for 137 Cs and at the INTE-UPC secondary standard laboratory for x-ray qualities. The intercomparison was conducted in two phases. The PDSs were asked to provide 10 dosemeters for each calibration point and 15 control dosemeters for background and transit control. 3. Results and discussion Results were evaluated on the basis of the accepted uncertainties for routine monitoring stated in ICRP-60 [6], using the so-called trumpet curve [7,8] for a monthly recording level of 0.1 msv, which is now applied in Spain. Results were also analysed following the ANSI recommended performance, establishing a tolerance level of 0.40, both for H p (10) and H p (0.07) [9]. Table III summarises the number of TL dose estimates for each radiation quality the established by the trumpet curves, considering all the PDSs together and Figure illustrates the performance of the participating services according to these. In Figure 3, a summary of the evolution of the results in the 16 services that participated in the last two intercomparisons is shown. In this case, the tolerance recommended by ANSI [9] was used for the evaluation of the performance, since it was the adopted criterion in Results show that most measurements (>90 ) fulfil the established requirements, but that some difficulties are still found for the correct assessment of H p (0.07) for low-energy photons. Although some correlation services that were using the same type of batch, TLD material and software, was observed, it could not be concluded that a specific type of system was always better than another. Some participants obtained excellent results, the reference personal dose equivalent uncertainty. 3

4 Table III. and percentage of TL dose estimates the trumpet curve, classified by radiation mean energy and operational quantity. Quantity x-ray mean energy 33 kev 65 kev 07 kev 137 Cs H p (10) H p (0.07) Within Within except for Hp(0.07) at N40 Within in some qualities Don't comply with the established 8 Stable - good results Improvements - good results Stable- some difficulties at low energies Worsening - needs improvement FIG.: Distribution of SDP as regards compliance FIG. 3: Summary of the evolution in performance with ICRP performance. of the participants in the 1995 and 001 comparison. Analysis of results indicates that 75 of the participants still present a stable performance compared with The best services have maintained their standards and two other services have improved their procedures in this period. On the other hand, eight services, five belonging to nuclear power plants, still show difficulties in the low-energy range, according to ANSI which are more restrictive than the ICRP proposal for a specific radiation quality. This is a problem often found, but can be solved by modifying the dose estimate algorithm. Such a consideration should be taken into account, in particular, by services which control workers that are concerned with low-energy-range radiation. No significant differences were found in the dose estimate, irrespective of the two tested calibration procedures. Detailed results of this comparison are given in Ginjaume et al. [3]. 4. Conclusions The 001 intercomparison has shown good reliability performance for most of the approved dosimetry systems. However, it has also identified two services which should revise and improve their measurement procedure. To this effect, it is believed that a 4-5 year period for organizing intercomparison exercises on a national basis is a good compromise between quality assurance and disturbance of the normal work of a PDS on account of such exercises. 4

5 These type of audits together with the participation of some of the approved Personal Dosimetry Services in international intercomparisons have proven to be very interesting tools to ensure worldwide reliability in occupational dosimetry. Moreover, the recent approval of an international standard for calibration in this field will also guarantee greater harmonisation in personal dosimetry. References 1. VILLANUEVA, I., HERNÁNDEZ, A., AMOR, I., MUÑOZ, M.J., Spanish intercomparisons for individual monitoring. Radiat. Prot. Dosim. 96 (001) ISO X and gamma reference radiation for calibrating dosemeters and doserate meters and for determining their response as a function of photon energy. Part 3 : Calibration of area and personal dosemeters and the measurement of their response as a function of energy and angle of incidence. International Organization for Standardization, Geneva, Switzerland (1999). 3. GINJAUME, M., BROSED, A., ORTEGA X., ROIG M., VILLANUEVA, I., MUÑOZ, M.J., AMOR, I., Photon personal dosemeter calibration based on ISO (Proc. European Congress IRPA 00), Florence (00). 4. ICRU 47. International Commission on Radiation Units and Measurements. Measurements of Dose Equivalents from External Photon and Electron Radiations, ICRU Report 47. Bethesda, Maryland (199). 5. ISO X and gamma reference radiation for calibrating dosemeters and doserate meters and for determining their response as a function of photon energy. Part 1 : Radiation characteristics and production methods. International Organization for Standardization, Geneva, Switzerland (1996). 6. ICRP 60. International Commission on Radiological Protection, 1990 Recommendations of the ICRP, Publication 60, Annals of the ICRP 1, 1-3, Pergamon Press, New York (1991). 7. BÖHM, J., AMBROSI, P., Mandatory type tests of solid state dosimetry systems as an appropriate aid to quality assurance in individual monitoring. Radiat. Prot. Dosim. 34 (1990) IAEA, International Atomic Agency. Assessment of occupational exposure due to external sources of radiation. Safety Guide Nº RS-G1.3, IAEA, Viena (1999). 9. ANSI nº American National Standard for dosimetry - personnel dosimetry performance - for testing. New York (001). 5