STUDIES ON DOSIMETRIC TESTS APPLYING SOURCE IRRADIATING FORCE OF CS-137 FOR USING IN CHAMBERS FOR CALIBRATION AND TLD TYPE DOSIMETERS.

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1 2011 International Nuclear Atlantic Conference - INAC 2011 Belo Horizonte,MG, Brazil, October 24-28, 2011 ASSOCIAÇÃO BRASILEIRA DE ENERGIA NUCLEAR - ABEN ISBN: STUDIES ON DOSIMETRIC TESTS APPLYING SOURCE IRRADIATING FORCE OF CS-137 FOR USING IN CHAMBERS FOR CALIBRATION AND TLD TYPE DOSIMETERS. Laila Lorena X. Ribeiro 1, Rugles César Barbosa 2, Rosângela S. Côrrea 3 1 Depto. Mat., Fís., Quí. e Eng. de Alimentos, Química, Pontifícia Univ. Católica de Goiás _ PUC-GO Av. Universitária, 1069 Setor Universitário caixa postal 86 CEP Goiânia, Go laila_bala@hotmail.com.br 2 Centro Regional de Ciências Nucleares do Centro Oeste (CRCN-CO/CNEN) Laboratório de Radioproteção BR 060, Km 174,5 - Abadia de Goiás - Go CEP rbarbosa@cnen.gov.br 3 Centro Regional de Ciências Nucleares do Centro Oeste (CRCN-CO/CNEN) Laboratório de Imagens e Dosimetria BR 060, Km 174,5 - Abadia de Goiás - Go CEP rcorrea@cnen.gov.br ABSTRACT The West Central region of Brazil does not have a basic infrastructure for research, development, training programs, and personnel dosimetry education. All of them applied to environmental, industrial and medical uses. Service deployment for irradiance of TLD, via 137 Cs irradiator J. L. SHEPERD model 28-8A (444 activity GBq) in CRCN-CO, it is necessary to introduce procedures for calibration of the radiator and other procedures related to dosimetry and calibration. Such procedures should be repeated periodically, as necessary to introduce techniques that make the service of the CRCN-CO a template, and that meet all standards requirements for radioprotection and operation of dosimetry and calibration. The objective of this work was to evaluate the radiation field of Cs-137, and the automatic system which systematizes the calibration procedures attached to a system control target for the radiator/calibration of monitors, and portable dosimeters. 1. INTRODUCTION The calibration laboratory of the CRCN-CO is made up of an armoring with reinforced special concrete facility, with a set of equipments that need to be adjusted and calibrated, according to special techniques and standards based on the type of radiator (opening

2 geometry and type of radionuclide), and target (geometry and composition of support as well as geometry and composition of the target itself). The work presented here must be consistent with the purposes and goals established in the institutional project of the CRCN-CO/CNEN, and its scope of this project to research and development of dosimetry and calibration addressing matters, since the requirements of radioprotection, control of film, orientation and alignment of the radiator system/target, making of softwares and procedures for assessment of irradiation field of Cs-137 and dosimetry control. This work can be summarized in three stages, where the first is about radioprotection regarding Cs-137 source. The second approach is the alignment of the radiator/target, where it is used table flush with a leveler for horizontal and vertical bubble beyond using a horizontal vertical laser system of SKILL model 500 with 650 nm (with horizontal and vertical beam). It employed also supports staples for ionization chambers or support for dosimeters (thermoluminescent dosimeters - TLD) case box. The adjustment in the alignment between the source positioned at the apex of the cone (Figure 1), and the supporters (of latches or kits), was used to flattener and laser system (with associated accuracy values and respectively). The alignment, with the adjustments of the Bureau and of the supports, was verified and adjusted to the dose relationship Kerma in air and distance, with a portable SHP 270 probe with monitor and E-600 and compared with the calibration data held previously (see citation). After getting the relationship between dose and distance, it was checked for uniformity of the field. The goal of this measure is information of variation, in a particular position in a plane perpendicular to the axis of the cone of opening dose for the scaling of the suitcase and verification of perturbations effects produced by the presence of metallic or cardboard support. And with the information scale the project of the cassette port TLDs. The third step was reading achievement of dosimeters TLDs that are constituted pos 50 tablets of fluoride and lithium (LiF) TLD type 100 subjected to thermal treatment (400 C before being irradiated and 100 C before reading and after being irradiated) and the irradiation of a source of Cs-137. Reading was conducted using the equipment Thermo model 5500 model of automatic reading and heat treatment carried out with the oven. The purpose is verification and collation of lots of inserts of homogeneous TLDs (same common physical characteristics) and linear response between the reading of load (nano Coulomb) and dose data provided by the radiator's manual [4] and by dose of kerma in air in various configurations of distance and shielding the previously performed calibration. 2. METHODOLOGY To run the irradiation in a controlled manner is necessary knowledge, and has certification [5], the values of font exposure rate in its various configurations of distance and shielding. The distance and shielding from the source to the target can be varied easily, but is necessary for the adjustment with the geometrical axis of the cone of opening of the source. The distance from the source (fixed) to target (mobile) can be varied according to the placement of the object/target system, but you will need to adjust the center of the object (puntual) with the central axis of the Conic opening of 36 degrees in a solid angle of π/20 as Figure 1.

3 2r Figure 1- Conical cavity and shutter system A D Figure 2 - Conical cavity system and shutter inside the shielding Assuming that the caesium-137 source be puntform, where the shutter diameter 2r = 0, 5 cm. We can assume a beam of uniform irradiation to a fixed distance D and in a plane perpendicular to the axis of the cone. Experimental assessment of uniformity for field was leveled and fixed rectangular platform 4 x 35 where was fixed a tape measure that is taken as an indicator of distance from the origin at source to the target. This platform has been adjusted a mobile support with metal staple. Were used the following processes and equipment: -Source of Cs-137: J.L. Shepherd, model 28-8A, 444 GBq. -Radioproteção control: the font is automatically triggered via remote via an electromechanical device that controls the exposure time. Have a system of security against unauthorized access in the room. -Control of film, orientation and alignment adjustment via beam: lasers system so that the center of the detector sensitive volume coincides with the geometrical centre of irradiation system collimator.

4 -Dose Control: dose measurements for distance and shielding with probe SHP 270 and monitor Eberline E The behavior of the radiation beam field was compared with previous measurements using probe PTW LS-01, traced to international standards, and eletrômetro 617A KEITHLEY. Dosimeters Control Thermoluminescent Dosimeters TOP-LEVEL DOMAIN. -Consist of lithium fluoride crystals (LiF) that present the phenomenon of Thermoluminescence [3] when heated after they have been irradiated. The light emitted by TLDs is proportional to the ionizing radiation received. -Type Crystals Harshaw TLD-100 were irradiated and measured in TLD reader to verify the potential of calibration RESULTS Control of security of radiation. Originally the radiator did not have an automatic system of triggering the shutter to opening of caesium -137 sources. So was designed a system for the firing of the shutter opening lever via a electromechanical device called "plunger" that elevates the lever of equipment via locking barriers system shutter Radiator and preview cameras and monitoring of installations. Simultaneously settled a cable to the interlock for disabling or closing (font loading) shutter to unauthorized entry into the enclosure of irradiation. Detail of installation of automation system of radiator with Cesium source -137

5 Figure 3 - illustration of automation scheme Starter shutter opening and closing of Cs-137 source. Photo 1 - Radiator without security system. Photo 2 - Radiator with security system Measure control, orientation and alignment of the radiator system/target. Measurement system using lasers for measurement model and brand SKILL model 500 and media systems with special characteristics of the material (metal or cardboard) and employee in the same geometry.

6 Differences (mgy/h) Measurement of uniformity with lasers in relation to the center of the radiator Adjustment via laser system, alignment of the coordinates of the radiator with the coordinates of the target (the probe). Photo 3 Horizontal alignment adjustment With respect to the field of radiation beam and dosimetry control and calibration Measurements made for various doses in varying distances and shielding are in concordance with the earlier measures undertaken with the probe PTW LS-01 and electrometer [1], within the limitations of the equipment. Comparisons between reading Chamber PTW-LS01 versus perpendicular Probe SH270 50% 40% 30% 20% 10% 0% -10% -20% 1 1,25 1,5 2 2,5 Distance (m) 100-X 10-X 4-X 2-X Unshielded Graph 1 - Read Differences dose rates between probe PTW LS-SH 270/01 and Eberline E600 depending on distance and shielding.

7 Field uniformity. The maximum fluctuation in doses suffered radial measures mgy/h and mgy/h using support for the probe/target of metallic material (Z high) and cardboard (Z), respectively (A) (B) Graph 2 - Curves of Isodoses: probe into metallic support (A) and made of cardboard (B) To make is set cases. The case has dimension of per cm2 (see Photo 4) and an area containing number of 100 small cavities of 0.25 cm diameter for accommodation of 100 tablets (rods and chip cubes). From data of fluctuations in doses 2 graphics, 4 and 6 chose an exhibition area of TLDs of 0.6 cm2, for Wells, based on the assumption that reduction of fluctuation in the value of fluctuation that took place in the various exposure settings were tested. Photo 4 - Photo of kit cases used as support where they were put up TLD s of LiF.

8 Irradiated TLDs. Preparation of rods and chip cubes 100 TLDs Initially makes a visual inspection by microscope, and separates the rods and chip cubes that present some irregularity. Later performs an evaluation of tablets weighing individually each chewable tablet. From this selection performs grouping by lots of chips with similar characteristics and each of these lots may be split into groups. A given lot serves for a dosimetry with specific energy (Rays X, Co-60, Cs-137 and etc.) for a specific activity (occupational medicine X - ray Tomography, and etc.) and to a group of people (split TLDs in groups). Photo 5 - Visual inspection of pellets of TLDs for selection and removing a lot of TLDs There is a linear response between reading TLD (load) and the doses irradiated and establish a calibration factor of reading (loading)/dose. D (mgy) = 0.35 Q (nc) : R 2 = 0.91 (1)

9 Dose (mgy) (Charge,Dose) Curve adjustment 0 47,23 91,45 181,41 363,25 Charge (nc) Graph 4-Charge (ncoulomb) vs. Dose Irradiated TLDs. 3. CONCLUSIONS Tests performed with the equipment and Eberline-600 type with camera perpendicular SHP 270 compared to calibrations using Ionization Chamber and the PTW-LS01 617A KEITHLEY, electrometer # [1] nc-scale demonstrates conformity and concordance of values and quantities within the limits of quantities and errors of readings recorded by equipment mentioned above. It was held to build cases for irradiation of TLD s and developed systems for automation of roll shutter opening and closing of the radiator of caesium-137 were adequate and worked well, inside of the estimates, subject to the conditions of Calibration Laboratories (Irradiation source) and Dosimetry Laboratory (dosimetry read measures). ACKNOWLEDGMENTS Authors wish to thanks to financial support from Council National Council Scientific and technological development CNPq. Special thanks to technical support in review this paper, for Dr. A. P. Bakker. REFERENCES 1. KEITHLEY INSTRUMENTS, INC. Instruction Manual Digital Electrometer Model 616, (1977). 2. ATTIX, F. H. Introduction to Radiological Physics and Radiation Dosimetry.JohnWiley&Sons, (1986). 3. P.D. Townsend, M. Moscovitch, S.W. McKeever, Thermoluminescence Dosimetry Materials: Properties and uses. Nuclear Technology Publishing, (1995).

10 4. J.L. Shepherd & Associates, Operating Manual for Model 28 8A 444 GBq Single Source Beam Calibration System With PT 1 Timer and Model IT 1 Calibration Table, Cs- 137: model 28-8A, 444GBq, (2004). 5. Erik Catugal, Source Calibration Certification Model Model 28 8A 444 GBq Single Source Beam Calibration System With PT 1 Timer and Model IT 1 Calibration Table, Cs- 137: J.L. Shepherd & Associates, (2004). 6 Attenuator Certification Model 28 8A 444 GBq Single Source Beam Calibration System With PT 1 Timer and Model IT 1 Calibration Table, Cs-137: J.L. Shepherd & Associates.