Implementation of personnel dosimetry systems using thermoluminescence dosimetry

Transcription

1 Implementation of personnel dosimetry systems using thermoluminescence dosimetry J. C. Rojas 1, S. O. Benavides 1,2, and H. Olaya 3 1 Radiation Safety Committee, Fundación Clínica Valle del Lili, A.A. 2338, Cali, Colombia comiterad@clinicalili.org 2 Department of Physics, Universidad del Valle, A.A. 2536, Cali, Colombia oriana@calima.univalle.edu.co 3 Laboratory of Radiation Metrology, INGEOMINAS, Av. El Dorado Cra. 5, Bogotá, Colombia Abstract. In the present work, we describe the implementation methodology of a personal dosimetry service applied to the personnel occupationally exposed to ionizing radiation at the Fundación Clínica Valle del Lili (FCVL), in Cali. For the measurements of dosimetry, type 8814 holders and thermoluminescent crystals, TLD- 1, were used. These were measured in a Harshaw 35 TM reader. The methodology involved the preparation of thermoluminescent crystals after being received from the manufacturer, involving thermal profile and irradiations on the crystals for calibration. For the determination of the equivalent doses, we illustrate the method of the statistical treatment of measurements to determine the values of Element Correction Coefficient (ECC) and Reader Calibration Factor (RFC) of crystals, the elaboration of personal control cards of received dosages, and the monthly report of radiation dosage. The doses reported by all the personnel occupationally exposed at FCVL in nuclear medicine, catheterism, diagnostic images, radiotherapy, endoscopy and surgery rooms were measured monthly. The measurements of the monthly exposition were taken simultaneously using identical dosimeters from a dosimetry service contracted through the Secondary Laboratory of Dosimeter Calibration at the Institute of Investigation and Information of Geological, Environment, Mining, and Nuclear Sciences, INGEOMINAS, a laboratory certified by the International Atomic Energy Agency (IAEA). Our procedures were revised by them and our measurements compared with their reports. 1. Introduction The personnel dosimetry quantifies the equivalent dose received by the personnel occupationally exposed (POE). The limits with respect to the radiation exposure of POE are regulated by the last criteria recommended in the publication number 6 of the International Commission on Radiological Protection (ICRP) [1]. The thermoluminescence phenomenon (TL), classified as a phosphorescence phenomenon, consists on the emission of light of a semiconductor or insulating material, previously exposed to ionizing or non ionizing radiation, and later stimulated with heating energy [2]. The thermoluminescence process begins with the previous exposition of the material at a temperature T, followed by a thermal stimulation up to a temperature T > T, which is generally linear; with the objective of measure the light intensity emitted as function of heating temperature of the thermoluminescence material, providing a TL-curve commonly called "glow curve" [3]. The glow curve presents one or several peaks (TL-bands) as consequence of the recombination processes of charge carriers thermally activated. The possible applications of thermoluminescence dosimetry (TLD) are very diverse; it includes the radiation measurements in exposed workers, the monitoring of environmental radiation and medical applications [4,5]. In this work we show the methodology used with the purpose of to implement a system of whole body personnel dosimetry and to compare our dose report with the report given by the Secondary Laboratory of Dosimeter Calibration (LSCD) at the Institute of Investigation and Information of Geological, Environment, Mining, and Nuclear Sciences, INGEOMINAS, which is a laboratory certified by the International Atomic Energy Agency (IAEA).

2 2. Methodology Two calibration protocols were compared for the TLD-1 crystals. The protocol 1 consisted on the calculation of the ECC (Element Correction Coefficient) and RCF (Reader Calibration Factor) factors of crystals for analysis of the thermoluminescence response (electric charge reading, in nano-coulomb, by a reader BICRON Harshaw 35) using a well-known dose. The second protocol calculates the RFC factor of the batch using the linearity of the TL-response of crystals with respect to the given radiation dose. The crystals were irradiated in a linear accelerator, SIEMENS MEVATRON MX2 of 6 MeV, at 1cGy. The assembly of the irradiation process is shown in Figure 1. DFS Central axis 15 mm 5 cm TLD Crystals Phantom of solid water Stretcher FIG. 1. Scheme of the irradiation process of TLD crystals in the lineal accelerator Mevatron MX2 using a phantom of solid water at a distance source-surface of 1 cm, with the crystals located to 1.5 cm (distance of maximum dose). Field 1 cm x 1 cm, dose: 1 cgy PROTOCOL 1: The crystals received the known dosage D and we obtained, in the reader, the measurements of the TLresponse or charge value: R m D R IA i i ( ) = j = 1 ; where (IA) j = Individual Average of crystal j (1) m

3 Then, we obtain the calibration factor (RCF) for the batch as: RCF = N j= 1 ( IA) N j (2) and the calibration factor of each crystal (ECC) is: RCF ( ECC ) j = (3) IA ( ) j PROTOCOL 2: We carried out irradiations to several dose on groups of crystals, so that: Reading TL: R [nc] = RCF [nc/cgy] Dose [cgy] (4) 3. Results The calibration carried out by each protocol provided the following factors: PROTOCOLO 1: Measurements 1st and 3rd period: Batch 1: RCF 1 = [msv/nc]; ECC (average) = 1.5 Measurements 2nd and 4th period: Batch 2: RCF 1 = [msv/nc]; ECC (average) = 1.3 PROTOCOL 2: RCF 2 =.237 [msv/nc] For this study we gave our dosimeters to the users simultaneously with the dosimeters of INGEOMINAS. With the calculated factors we elaborated monthly reports of personal dosimetry by each protocol. We compared 4 periods of measurements with the reports of equivalent personal dosimetry, H p (1), carried out by the LSCD. The performance of protocols were analyzed in terms of "trumpet curves" in accordance with the norm ISO 14146d [6], based on the recommendations of the International Commission on Radiological Protection ICRP-6 [7] and ICRP-35 [8], which are used for the intercomparison of personnel dosimetry services. In agreement with this analysis, a performance of 1% is acquired when all the obtained measurements are inside the following range: 2 H H ev H ( ) (5) 1.5 H + H P H P 2 H + H P where H P is the reference dose of the intercomparison (in this work is the LSCD report), H ev is the dose measured by the service that will be evaluated (in this work are protocol 1 and protocol 2), and H =.2 msv corresponds to the typical value of detection threshold of a dosimetry service. The

4 performance of each protocol is evaluated by means of the percentage relation between the number of irradiation points outside of the trumpet curve limits and the total number of points. Keeping in mind that below the detection limit (.2 msv) the evaluation criterion is outside of range, we only have used points above.2 msv for the evaluation. Figure 2 shows the behavior of measurements in comparison with the trumpet curve criterion. According with this criterion while protocol 1 reached a percentage performance of 1%, the protocol 2 obtained only a 69%. Figure 2 shows these results graphically. a. b. FIG. 2 a. Comparison of the dose evaluated by the protocols 1 and 2 (H ev ) using trumpet curve criterion for 4 report periods. b. Percentage performance for each protocol. 4. Conclusions From the results of this work and the LSCD reports show that any crystal (user) was subjected to an overexposure during the respective period of measurement. Inside the validity range of the trumpet curve criterion and inside the range of monthly reported measurements, a percentage performance of 1% was obtained for the protocol 1, while with the protocol 2 was only 69%. This implies a better behavior of the protocol 1 with respect to the protocol 2. Dosimetric intercomparison process allows to detect fails in the evaluation systems and to standardize the dosimetric magnitudes with the purpose of offering reliability to the service and finally to the users.

5 Acknowledgements This work was realized in the frame of the project "Physical aspects of the insurance of quality of radiations in radiotherapy" cod: COL/6/1 that execute the Universidad del Valle and the Fundación Valle del Lili, financed by the International Atomic Energy Agency (IAEA). References 1. International Commission on Radiological Protection (ICRP) Publication No. 6 (199), Recommendations of the International Commission on Radiological Protection. Nueva York: Pergamon Press, (1991). 2. McKeever, S.W.S. Moscovitch, Marko., Townsend, Peter D. Thermoluminescence Dosimetry Materials: Properties and Uses. (1995) Nuclear Technology Publishing McKeever, S.W.S. Thermoluminescence of Solids. (1985) Cambridge Solid State Science Series. 3. Oberhofer, M. and Sharmann, A., Techniques and Management of Personnel Thermoluminescence Dosimetry Services. ECS, EEC, EAEC, Brussels and Luxemboug. (1993). 4. Harshaw Bicron Radiation Measurement Products. Model 35. Manual TLD Reader. User s manual. Publication No. 35--U July 3, (1993). pp Villanueva I, Hernández A, Amor I, Muñoz MJ. Radiat Prot Dosimetry, 96(1-3):21, (21) 6. Criteria and Performance Limits for Periodic Testing of External Individual Dosimetry for X and Gamma Radiation. International Standarization Organization ISO 14146d. (1996). 7. General 199 Recommendations of the Internacional Comisión on Radiological Protection. ICRP reporte 6. Pergammon Press. (199). 8. General Principles of Monitoring for Radiation Protection Workers. International Commision on Radiological Protection. ICRP report 35 Pergammon Press. (1975).