TLD ALBEDO DOSIMETER PERFORMANCE ON A Hp(10) NEUTRON DOSE IAEA INTERCOMPARISON

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1 2007 International Nuclear Atlantic Conference - INAC 2007 Santos, SP, Brazil, September 30 to October 5, 2007 ASSOCIAÇÃO BRASILEIRA DE ENERGIA NUCLEAR - ABEN ISBN: TLD ALBEDO DOSIMETER PERFORMANCE ON A Hp(10) NEUTRON DOSE IAEA INTERCOMPARISON M. M. Martins 1,2, C. L. P. Maurício 1 and A. X. da Silva 2 1 Instituto de Radioproteção e Dosimetria (IRD / CNEN) Av. Salvador Allende, s/nº - Caixa Postal Rio de Janeiro, RJ marcelo@ird.gov.br; claudia@ird.gov.br 2 Programa de Engenharia Nuclear/COPPE/UFRJ Caixa Postal CEP: Av. Horácio Macedo, 2030, Bloco G, Sala 206 Cidade Universitária CEP: Rio de Janeiro, RJ ademir@con.ufrj.br ABSTRACT A new TLD albedo individual monitor system took part in the International Intercomparison of Personal Dosimeters for Mixed Neutron-Photon Radiation for the Quantity Personal Dose Equivalent, sponsored by IAEA from 2003 to This exercise was performed in two steps: a type test intercomparison and a simulated work place intercomparison. In Part I, the neutron individual monitors were irradiated in selected calibration fields in order to improve the dosimetric procedures of the participants. In Part II, the dosimeters were irradiated in radiation fields similar to those in workplace fields for final check of the performance. The neutron irradiations were performed at the facilities of the PTB, German, and the IRSN, France, with energies ranging from thermal to 5 MeV, on an ISO water slab phantom. The readings reported by the participants were compared with the H p (10) reference neutron values ( msv) derived with primary standards, with an uncertainty of 4.2% (k=2), at the two irradiating laboratories. Using data of Part I to improve algorithm and calibration, the new TLD albedo system has obtained 100% of results within the acceptance limits in the Part II performance test. The albedo individual monitor is also quickly described and the system performance is analyzed. 1. INTRODUCTION The neutron individual monitoring is performed with a wide variety of active and passive detectors. Since 1983, Instituto de Radioproteção e Dosimetria (IRD) runs a thermoluminescent detector (TLD) albedo-based neutron monitoring service using a onecomponent monitor [1]. Recently, a two-component albedo individual monitor has been developed [2]. In addition to the albedo component, it has an incident neutron component that enables correction in its calibration factor. In both components, TLDs are used for neutron and gamma detection. Among other requirements, its development aims a detection limit lower than the Brazilian monthly recording level (H 0 ) of 0.20 msv [3], even for bare 241 Am-Be source, which is not true for the actual routine albedo system. The new TLD based albedo system has been calibrated [2] according to international standard ISO [4]. Nowadays, it is being characterized according to ISO standard [5]. Among the sets of reference neutron radiations described in the international standard ISO [6], those from accelerator (monoenergetic) are difficult to be available. First, the new TLD based albedo system could be calibrated only in neutrons from radionuclide sources

2 because the other neutron radiation qualities are not commonly available. In 2003, a unique opportunity to access these neutron radiation fields, and test its global performance, has appeared at The International Intercomparison of Personal Dosimeters for Mixed Neutron- Photon Radiation for the Quantity Personal Dose Equivalent, sponsored by International Atomic Energy Agency (IAEA) [7]. 2. H p (10) NEUTRON DOSE IAEA INTERCOMPARISON The IAEA International Intercomparison of Personal Dosimeters for Mixed Neutron-Photon Radiation has been proposed not only as a pure performance test of the existing neutron dosimetric service. The three main objectives of this intercomparison were: 1. To assess the capabilities of the dosimetry services in measuring the quantity Hp(10) in mixed neutron-gamma fields; 2. To assist Member States in achieving sufficiently accurate dosimetry and, if necessary; 3. To provide guidelines for improvements. This exercise was performed in two steps: a type test intercomparison (Part I) and a simulated work place intercomparison (Part II). In Part I, the dosimeters were irradiated in selected calibration fields in order to improve the dosimetric procedures of the participants (Table 1). It comprises particular standard irradiation conditions, including monoenergetic beams, in order to investigate the energy and angular dependence for neutrons and to check home calibrations. In Part II, the dosimeters were irradiated in radiation fields similar to those in workplace fields for final check of its performance (Table 2). The irradiations were performed at two European facilities: Physikalisch-Technische Bundensansalt (PTB), in Germany, and Institut de Radioprotection et de Sûrete Nucléaire (IRSN), in France. The irradiation laboratories gave the reference values for neutron radiation H p (10), using wellestablished procedures. At both facilities, the conventionally true value of neutron H p (10) was measured with an uncertainty of 4.2% (k=2). Irradiations were performed on an ISO water slab phantom, except for radiation qualities 15 and 16 [7] on Part I, when individual monitors were attached on an ISO slab phantom. Some monitors were chosen for background control (non-irradiated monitors): In Part I, one monitor to follow the group sent to PTB and another one to IRSN. In Part II, around 3 control monitors are sent for each irradiating laboratory. In order to reduce the total irradiation time, up to four individual monitors were irradiated together, fixed to the surface of the same phantom [4]. 3. RESULTS After being irradiated at PTB and IRSN, IAEA returned the whole individual monitors group to each participant. The TLDs from irradiated and non-irradiated monitors were evaluated following normal IRD routine procedures, in a manual Harshaw 3500 TLD reader together with control TLDs kept in laboratory. At both intercomparison parts, the background control monitors received a small neutron dose from cosmic radiation due to the air transport between each laboratory participant and IAEA. Using results of Part I to improve algorithm and calibration, the new TLD albedo system has passed with 100% of approval in the performance test of Part II [7, 8].

3 3.1. Part I: Type Test Intercomparison Table 1 summarizes the neutron results obtained in Part I. The reference values of neutron H p (10) are given as and the reading values expressed as Mn. The response Rn is defined by equation 1. Mn Rn = (1) Table 1. Albedo response on type test Nº Neutron Radiation Quality Mn Rn* 1 Thermal kev kev kev MeV MeV a Cf, 0º Cf, 45º Cf, 60º AmBe AmBe (+Pb) Cf + 60 Co kev + 60 Co * - Italic values refer to those outside trumpet curves limits (Figure 1) Part II: Simulated Workplace Field Intercomparison The results obtained in Part I were extremely useful to calibrate the albedo monitor system and to improve its algorithm. Albedo monitors (as other neutron individual monitors) have strong energy dependence in the large occupational energy range, which covers from thermal to around 20 MeV [4, 5]. Thus, they always need some information about the neutron field in order to apply the correct calibration factor. When the monitors returned after Part II irradiations, IAEA stated only the application area [8]. Usually, IRD neutron individual monitoring service considers being necessary more data about where and how the albedo monitors were irradiated, like: reactor, radionuclide and others. Even so, the IRD twocomponent albedo system had a good performance, except for radiation quality nº 7, where the monitor returned to IRD without two TLDs, making its evaluation impossible. The ratio from incident to albedo component values was applied in the algorithm to correct the neutron

4 reading. Neutron results of the intercomparison Part II are shown in Table 2. The Canel quality is a reactor neutron spectrum from IRSN. Table 2. Albedo response on simulated workplace field Nº Neutron Radiation Quality Mn 1 Canel / 0º Canel + W250 b / 0º Scattered neutrons behind a cone shadow/ isotropic Cf(moderated)/ 0º + scattered neutrons behind a cone shadow/ isotropic a Rn Cf(moderated)/ 0º Cf(moderated)/ ± 75º Cf(moderated) + Cs / 60º Cf(moderated) + Cs / ± 75º a. Reading not available (TLDs lost). b. W wide X-rays spectra Trumpet Curves Trumpet curves have been proposed to define the limiting interval around the conventional true value or reference value (H ref ) of the individual monitoring quantity in which its measured value should lie [9]. They are a useful tool to visualize the overall response of an individual monitoring system s intercomparison, the dosimetric service performance and others. Due to overall uncertainties and the state-of-the-art associated with neutron individual monitoring, trumpet curves for neutron are wider than for photon [9]. Exclusively in neutron individual monitoring, the upper part of the trumpet is different from the lower part. The upper limit for neutron personal dose equivalent limit is shown in equation 2. The lower limit for neutron personal dose equivalent limit is shown in equation 3, where H 0 is the recording level. = 2 (2) ul ll 1 2. H 0 =. 1 2 H 0 + H ref (3) Together with, Mn and R from the two intercomparison parts, both trumpet limits (upper and lower) are presented in Figure 1.

5 Response Upper Unitary Lower Part I Part II Hp(10) Figure 1. Neutron responses from H p (10) IAEA Intercomparison s Part I and II. 4. CONCLUSIONS In several intercomparisons for neutron or mixed (neutron + gamma) fields throughout the last two decades, albedo monitors have shown excellent performance [10]. The IRD twocomponent TLD albedo system showed a good performance (100% results inside trumpet curve limits) in Part II of the IAEA International Intercomparison of Personal Dosimeters for Mixed Neutron-Photon Radiation for the Quantity Personal Dose Equivalent, where simulated workplace neutron fields are used. Only 20% (6/30) of the participating laboratories showed this same performance [8]. Participation in Part I of its IAEA intercomparison proved to be fundamental for the final characterization of the tested albedo system [2]. A 252 Cf(D 2 O-moderated) calibration factor [6] was applied to 70, 144, 565 kev spectra in Part I, with had result in the 3 points out of trumpet curves limit. Nowadays, in addition due to Part I, IRD has calibration factors for 70, 144, 565 kev, 1.2 and 5 MeV. Without this intercomparison, IRD will never access these calibration fields so quickly. Unfortunately in Part II, two TLDs were lost in the monitor used in radiation quality nº 7, it also would allow evaluating the Canel quality (reactor) in a mixed neutron-photon radiation quality. At present, the authors are employing their best efforts to fulfill all specific ISO [5] performance requirements for thermoluminescence albedo and to allow for this twocomponent TLD albedo individual monitor system to be ready for routine use in IRD.

6 ACKNOWLEDGMENTS The authors are grateful to International Atomic Energy Agency by the invitation and by opportunity in taking part in the intercomparison. REFERENCES 1. S.A. Gonçalves, C.L.P. Maurício, J. Moura Júnior, M.M. Martins, N.F. Meira, R. Diz, R.P.G. Seda, Monitoração Individual de Nêutrons: 18 Anos de Experiência, Proceedings of the 2002 INAC (International Nuclear Atlantic Conference), Rio de Janeiro, August, Paper CD-rom E01_566.pdf (2002). 2. M.M. Martins, C.L.P. Maurício, K.C.S. Patrão, A.X. Silva, Calibration of a TLD Albedo Individual Neutron Monitor, to be presented at this Conference. 3. Comissão Nacional de Energia Nuclear, Restrição de Dose, Níveis de Referência Ocupacionais e Classificação de Áreas, Posição Regulatória 3.01/004 rev. 01, CNEN, Rio de Janeiro (2005). 4. International Organization for Standardization, Reference Neutron Radiations Part 3: Calibration of Area and Personal Dosimeters and the Determination of Their Response as a Function of Neutron Energy and Angle of Incidence, International Standard ISO , ISO, Geneva (1998). 5. International Organization for Standardization, Passive Personal Neutron Dosemeters Performance and Test Requirements, International Standard ISO 21909, ISO, Geneva (2005). 6. International Organization for Standardization, Reference Neutron Radiations Part 1: Characteristics and Methods of Production, International Standard ISO , ISO, Geneva (2001). 7. H. Schuhmacher, R.C. Suáres, J.L. Pochat, Intercomparison on Measurements of the Quantity Personal Dose Equivalent Hp(d) in Mixed (Neutron-Gamma) Fields Analysis of Results from Part 1, IAEA, Vienna (Austria), March (2004). 8. A. Zimbal, H. Schuhmacher, Intercomparison on Measurements of the Quantity Personal Dose Equivalent Hp(10) in Mixed (Neutron-Gamma) Fields Analysis of Results from Phase 2, IAEA, Vienna (Austria), April (2005). 9. J. Böhm, V.N. Lebedev, J.C. McDonald, Performance Test of Dosimetry Services and Its Regulatory Aspects, Radiat. Prot. Dosim., 54, pp (1994). 10. R.E. Swaja, Performance Characteristics of Neutron Personnel Dosemeters Used in the Oak Ridge Intercomparison Studies, Rad. Prot. Dos., 23, pp (1988).