CALIBRATION OF A TLD ALBEDO INDIVIDUAL NEUTRON MONITOR

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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: CALIBRATION OF A TLD ALBEDO INDIVIDUAL NEUTRON MONITOR M.M. Martins 1,2, C.L.P. Maurício 1, K. C. S. Patrão 1 and A.X. da Silva 2 1 Instituto de Radioproteção e Dosimetria (IRD/CNEN) Caixa Postal CEP: Av. Salvador Allende, s/n CEP: Rio de Janeiro, RJ marcelo@ird.gov.br; claudia@ird.gov.br; karla@ird.gov.br 2 Programa de Engenharia Nuclear/COPPE/UFRJ Caixa Postal CEP: Av. Horácio Macedo, 2030, Bloco G, Sala 206 CEP: Rio de Janeiro RJ ademir@con.ufrj.br ABSTRACT Neutron individual monitoring is considerably less established than photon individual monitoring. The main difficulty is the impossibility to find a detector that covers the large occupational neutron energy range. All commercial individual monitors have strong energy dependence. Another difficulty is to calibrate an individual neutron monitor in the ISO established energy operational range: from thermal to 20 MeV. Since 1983, IRD uses a TLD one-component albedo neutron monitor, which has to be calibrated specifically for each neutron field. To improve its energy response, a two-component albedo monitor was developed, which measure the thermal neutron component besides the albedo one. Using the ratio between both components, it is possible to make some energy response correction. However, the construction of this energy correction curve depends on the accessing of several different traceable calibration fields that are not easily available. The twocomponent monitor has already been calibrated in reference neutron fields: thermal, 5 accelerator-produced monoenergetic beams (70, 144, 565, 1200 and 5000 kev) and 5 radionuclide sources ( 252 Cf, 252 Cf(D 2 O), 241 Am- Be, 241 Am-B and 238 Pu-Be) at several distances. Four different energy correction curves have been adjusted for use with different neutron sources. Nowadays, the neutron system is in test for routine application. Its main application is occupational neutron monitoring of workers who handle neutron sources at different distances and moderation, such as well logging and calibration facilities. 1. INTRODUCTION Instituto de Radioproteção e Dosimetria (IRD) has large experience with neutron individual monitoring. Since 1983, IRD runs a neutron individual monitoring system with an onecomponent albedo monitor, which employs a Harshaw 6 LiF:Mg,Ti and 7 LiF:Mg,Ti thermoluminescent detectors pair (TLD-600 and TLD-700) [1]. This system has shown a good performance, mainly in slow neutron fields. Like other pure TLD albedo neutron monitors, its response varies strongly with neutron energy, dropping down very quickly after some hundreds of kev. Its lower detection limit (LDL) is higher for fast neutrons. To improve the neutron response of IRD albedo system, a two-component TLD albedo individual monitor has been developed with the following characteristics [2]: Capability to measure incident and albedo components with the two pairs of 6 LiF:Mg,Ti and 7 LiF:Mg,Ti TLDs; Detection limit lower than the recording level (H 0 ) of 0.20 msv for bare 241 Am-Be;

2 Interchangeability of the monitor parts, making possible the reuse of a specific part, should other part be damaged. The monitor design (cross section in Figure 1) includes [3]: o Black holder: case to shield light and to hold TLDs, divided in 2 parts (frontal and albedo); o Moderator: additional neutron moderator material (polyethylene based), used to improve monitor sensitivity; o Shield: boron based layer that captures the thermal incident neutrons, differentiating the incident component from the albedo one; o Belt cavity: space where a belt could be attached to fix the monitor on the abdomen. In both frontal side black holder and moderator, there are two cavities to insert, respectively, albedo detectors and incident neutrons detectors (a pair of TLD600 and TLD700). Figure 1. Two-component TLD albedo individual monitor cross section. In addition to the albedo component, an incident neutron component enables an energy correction in the calibration factor (CF), using the ratio of incident to albedo neutron response (IA). This correction is only applicable to moderated neutron fields from the same primary neutron source [4]. Before using this albedo monitor in the routine IRD neutron individual monitoring service, it should be calibrated and tested accord ISO Standard s requirements [5]. The measured quantity is the personal dose equivalent - H p (10). The reference neutron radiations described in ISO 8529 [6, 7, 8] are given in Table 1. Table 1. Reference neutron radiations according ISO 8529 Standard [6, 7, 8] Neutrons from Spectra Type Radionuclide Sources Wide 252 Cf, 252 Cf(D 2 O moderated), 241 Am-B, 241 Am-Be Accelerator-produced Monoenergetic thermal, 2, 24, 144, 250, 565, 1200, 2500, 2800, 5000, 14800, kev Reactor-produced Wide or quasi-monoenergetic thermal, 2, 24, 144 kev Reactor and accelerator-produced neutrons are expensive and difficult to be available for radiation protection purpose. In the beginning, the new TLD based albedo system was

3 calibrated only in neutrons from radionuclide sources available in IRD. In 2003, there was a unique opportunity: The International Intercomparison of Personal Dosimeters for Mixed Neutron-Photon Radiation for the Quantity Personal Dose Equivalent, sponsored by International Atomic Energy Agency (IAEA) [9]. The 70, 144, 565, 1200 and 5000 kev radiation qualities were used. 2. CALIBRATION SETUP For calibration, all irradiations were performed on an ISO water slab phantom [7, 8]. The IRD calibrations were done in a low scattering room of the Neutron Laboratory (LN/IRD), which has the following dimensions: 7.8 x 15.3 m 2 and 4.5 m height. The irradiations were carried out on the 2 nd floor at 2.2 m height. The radionuclide neutron sources used and their characteristics are shown in Table 2. Table 2. LN/IRD radionuclide neutron sources used for albedo calibration Neutron source Half-life (year) Activity a Sources shape Hp(10)-average energy b (MeV) 252 Cf(D 2 O moderated + Cd) Cylinder Cf Cylinder Am-B(α,n) Cylinder Am-Be(α,n) ; 185; 592 Cylinders Pu-Be(α,n) ; 1850 Disks 4.2 a. For 252 Cf sources, the specific quantities are related to the mass (µg) of californium. For other sources, they are related to the α emission (GBq). (In both cases when they were made) b. Stated value. It depends on the neutron spectrum from the manufacturer. In the low scattering room, up to 5 phantoms can be irradiated simultaneously. A neutron irradiation setup sketch for 5 phantoms is shown in Figure 2(a). The calibration distance (d) is the distance between the source center and the frontal face of the phantom. A Monte Carlo MCNP computer code is been used to simulate which is the minimum distance from a neutron source that may cause interference among individual monitors. Thus, in our work, usually 3 phantoms had been used for CF determination with cylindrical sources. For 238 Pu- Be disk shape sources, only 2 phantoms were used due to their main axial axis. For the albedo individual neutron monitor calibration, it was carried out irradiations with distances from 50 cm to 4 m. The given dose was around 3 msv. In each phantom, 4 individual monitors were attached to its frontal face. The monitors were positioned on a 15 cm diameter circle centered on the phantom frontal face (Figure 2(b)) [8].

4 Figure 2. Neutron irradiation setup at LN/IRD low scattering room: (a) Sketch of the phantoms. (b) Sketch of the individual monitors on each phantom. The neutron irradiations of the IAEA International Intercomparison of Personal Dosimeters for Mixed Neutron-Photon Radiation for the Quantity Personal Dose Equivalent were performed at Physikalisch-Technische Bundesanstalt (PTB), German and at Institut de Radioprotection et de Sureté Nucléaire (IRSN), France, also on ISO water slab phantoms [9]. The following irradiations conditions were used for IRD albedo calibration: Thermal neutron field at 0 o ; Accelerator-produced monoenergetic neutron fields at 0 o (ISO energies of 70 kev, 144 kev, 565 kev, 1.2 MeV, 5 MeV). 3. CALIBRATION RESULTS The 241 Am-B source available at LN/IRD is too weak. For instance, in order to irradiate at 75 cm with 1 msv, it is necessary to irradiate the individual monitors during around 1160 h. Thus, it has only been evaluated the albedo response at 50 cm from this source. For this neutron field ( 241 Am-B), the LDL is around 0.12 msv. The calibration factors for different distances from the LN/IRD s radionuclide neutron sources are shown in Table 3. Calibration factor versus IA is plotted for each neutron source in Figure 3. For 75 cm distance, the LDL are about: msv, 0.06 msv, 0.09 msv and 0.08 msv, respectively for 252 Cf(D 2 O), 252 Cf, 241 Am-Be and 238 Pu-Be. The calibration factors, IA and LDL for different IAEA Intercomparison neutron fields (thermal and the 5 monoenergetic radiation qualities) are shown in Table 4. In this Table, it is also included the same data for LN/IRD thermal radiation quality. The greater epithermal component of LN/IRD thermal radiation quality and its shorter irradiation distance comparing to IRSN thermal spectra explains the difference in their CF values (much higher for LN/IRD).

5 Table 3. Calibration factors for LN/IRD radionuclide neutron sources CF (msv/mgy 60 Co) d (cm) Source 252 Cf(D 2 O moderated+cd) Cf Am-B(α,n) Am-Be(α,n) Pu-Be(α,n) CF (msv/mgy 60 Co) i/a ratio x Cf(D2O moderated) 252Cf 241AmBe 238PuBe Monoenergetic Figure 3. Calibration factor (CF) versus incident to albedo ratio (i/a) for neutron fields. Table 4. Calibration factors, IA and LDL for thermal and accelerator-produced monoenergetic neutron sources Radiation Irradiation d CF IA x LDL Facility Quality Laboratory (cm) (msv/mgy 60 Co) 100 (msv) Thermal IRD FTP Thermal IRSN SIGMA kev PTB Accelerator kev PTB Accelerator kev PTB Accelerator MeV PTB Accelerator MeV PTB Accelerator

6 4. CONCLUSIONS Nowadays, the IRD two-component monitor has already calibration factors for: thermal neutrons, 5 monoenergetic fields (70, 144, 565, 1200 and 5000 kev) and 5 radionuclide sources ( 252 Cf, 252 Cf(D 2 O), 241 Am-Be, 241 Am-B and 238 Pu-Be). It is also necessary to calibrate it in 2, 24, 250, 2500, 2800, 14800, kev monoenergetic fields, in order to understand the albedo response in the whole ISO established energy operational range: from thermal to 20 MeV. Although it was not possible to achieve the LDL of 0.20 msv for 5 MeV radiation quality, this is not very important because it is unlikely that any worker being directly exposed to such accelerator neutron beam. ACKNOWLEDGMENTS The authors are grateful to International Atomic Energy Agency by the invitation and by opportunity in taking part in the International Intercomparison of Personal Dosimeters for Mixed Neutron-Photon Radiation for the Quantity Personal Dose Equivalent. 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 (Brazil), August, Paper CD-rom E01_566.pdf (2002). 2. M.M. Martins, C.L.P. Maurício, Factors that Affected the Development of a New Albedo Individual Monitor at IRD, Proceedings of the V Regional Congress on Radiation Protection and Safety, Recife (Brazil), April, CD-rom, trab-c 065fv.pdf (2001). 3. M. M. Martins, C. L. P. Maurício, E. S. Fonseca, Monitor Individual de Nêutrons Passivo Tipo Albedo de Duas Componentes para Corpo Inteiro, Depósito de Patente de Modelo de Utilidade MU , INPI (2005). 4. M. Oberhofer, A., Scharmann, A. (eds), Techniques and Management of Personnel Thermoluminescence Dosimetry Services, chapter 6, Brussels and Luxembourg, Kluwer Academic Publishers (1993). 5. International Organization for Standardization, Passive Personal Neutron Dosemeters Performance and Test Requirements, Standard ISO 21909, ISO, Geneva (2005). 6. International Organization for Standardization, Reference Neutron Radiations Part 1: Characteristic and Methods of Production, Standard ISO , ISO, Geneva (2001). 7. International Organization for Standardization, Reference Neutron Radiations Part 2: Calibration Fundamentals of Radiation Protection Devices Related to the Basic Quantities Characterizing the Radiation Field, Standard ISO , ISO, Geneva (2000). 8. 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, Standard ISO , ISO, Geneva (1998). 9. M.M. Martins, C.L.P. Maurício, A.X. Silva, TLD Albedo Dosimeter Performance on a Hp(10) Neutron Dose IAEA Intercomparison, to be presented at this Conference.