Calibration of Radioprotection Instruments and Calibrated Irradiation: Characterization of Gamma Beam of 137 Cs and 60 Co

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1 Calibration of Radioprotection Instruments and Calibrated Irradiation: Characterization of Gamma Beam of 137 Cs and 60 Co Pirchio Rosana a*, Lindner Carlos a, Molina Laura a and Vallejos Matías a. a Comisión Nacional de Energía Atómica, CAE, Pro. Luis González y Aragón N 15, B180AYA, Buenos Aires, Argentina. Abstract. Radioprotection Laboratory belongs to Dosimetry Reference Regional Centre of Atomic Energy National Commission, C.A.E. This laboratory offers the service of calibrations for radioprotection instruments as Geiger Muller detector, ionisation chamber, probe, proportional counters, electronic personal dosimeters and others used in nuclear medicine, radiotherapy centres, nuclear power plants, industry and in other applications of ionising radiation. Also it offers the service of calibrated irradiations. An gamma Irradiator and a Stabilipam 300 X ray are the equipments of the Radioprotection Laboratory used for calibrations. Hopewell Designs Irradiator was installed to improve the quality of services in 005. The irradiator has a 137 Cs source of 10 Curies and a 60 Co source of 1 Curie, approximately. Theoretical and experimental studies were done to analyse symmetry, flatness, penumbra and secondary radiation of photon beams. For symmetry, flatness and penumbra X-OMAT-V films and termoluminiscent dosimeters (TLDs) were used. Films were placed to 0.7 and 1 meter from collimator exit receiving 10, 40, 80, 100 and 10mGy. TLDs were placed to 1 and 1.80 meters from collimator on a surface higher than 137 Cs beam cross section. Also studies were done to a distance of 1m and 1.80 from exit collimator using LiF powder in capsules. Irradiations were done without attenuators and with a collimator aperture of 8º. Results were compared to those obtained with Monte Carlo simulation (MCNP5c code). For secondary radiation calculation three methods were employed: Shadow-shield, Multiple distance and Monte Carlo simulation. Finally, from theoretical and experimental studies could conclude that the secondary radiation resulted to be lower than 3.5%, total radiation, symmetry and flatness were higher than 90% and penumbra was lower than 13 mm. Those results agree to recommendations ISO 4037 Standard. KEYWORDS: Radioprotection; irradiator; dosimetry; flatness, Monte Carlo; dispersion. 1. Introduction Instruments and personal dosimeters are able to detect and measure directly ionizing or indirectly ionising radiation and to provide information to control radiological hazards. Radiation survey meters should be calibrated to measure different dosimetric magnitudes such as air Kerma, ambient equivalent dose, personal equivalent dose and exposition in a confident way. Radiation protection instruments, such as dosemeters and rate dosemeters are used in nuclear medicine, brachytherapy, nuclear installations and industries. Radiation dose measurements must have international traceability and for that reason the International Atomic Energy Agency (IAEA) and the World Health Organization (WHO) created a Network of Secondary Standards Dosimetry Laboratories (SSDLs) in SSDLs are the link in the traceability chain for radiation metrology users. They provide calibrations at specific radiation qualities appropriate for the use of radiation measuring instruments. The calibration of instruments by the SSDLs mean that these instruments are traceable to the Primary Standards Dosimetry Laboratories (PSDLs) and the Bureau International des Poids et Mesures (BIPM). [1] In Argentina irradiations and calibrations in cobalt and cesium beam are done at Radioprotection Laboratory of Dosimetry Reference Regional Center of Atomic Energy National Commission some years ago. This is a Secondary Standard Dosimetry Laboratory. * Presenting author, pirchio@cae.cnea.gov.ar 1

2 Calibration means the determination of the instrument response in a specified radiation field delivering a known dose (or other magnitude) at the point reference of the instrument. The procedure used to calibrate radiation protection instruments in a radiation field is by substitution [-3]. This method consists in two steps; in the first the reference dosimeter is placed at the calibration point to determine the reference output rate of the beam. In the second step it is replaced by the dosimeter to be calibrated, and reading are taken. The goal to acquire the new gamma irradiator Hopewell Designs, Inc. Model G with 137 cesium and 60 cobalt source and a new calibration bench was mainly to offer a better and more services to the users, and to have a more reliable measurement system. Instead of using sources with different activities, the air Kerma rate may also be varied by means of lead attenuators for collimated beams of cesium and cobalt. The attenuators shall be placed in close vicinity to the diaphragm. A sequence of lead attenuator with different thickness leads to a reduction in air Kerma, or other magnitude by successive orders of magnitude for cesium and cobalt. In this work were studied irradiator performance, beam characteristics and other aspects. The facilities available for the laboratory to calibrate instrument are an irradiator, X ray equipment, reference instruments and others. Ionization cylindrical chamber 600cm 3 Nuclear Enterprises and an electrometer Farmer are secondary standards traceable to a primary laboratory. The irradiator has a 137 Cs source of 10 Curies and a 60 Co source of 1 Curie, approximately. Theoretical and experimental studies were done to analyse symmetry, flatness, penumbra and secondary radiation of photon beams as ISO 4037 suggests it [3-4].. Materials and methods.1 Irradiator description Hopewell Designs, Inc. Model G Gamma Beam Irradiator produces gamma beam that is used to calibrate instruments for radiation detection and to irradiate personal dosimeters [5]. Irradiator is cylindrical and it contains two gamma sources shielded by lead. External diameter is 17.78cm and height is 71.1cm, the thickness lead assure that the value of exposition rate on the surface be lower than 5mR/h. Sources movement is possible by an automatic system based on vacuum. Base Irradiator is made of iron and height is 68.5cm. Beam centerline is 110cm on the floor. Collimator it has tungsten disks specially design to absorb scatter radiation according to ISO , 1999 [4]. Aperture collimator is equal to 8º. In Figure 1 is showed a schematic diagram of the irradiator. Figure 1: Schematic diagram of the irradiator

3 Cylindrical sources have an active nucleus of 0.53cm and 1.84cm, diameter and length. They have a double encapsulation and sealed hermetically. External encapsulation is made of stainless type 304 with 1.8cm and 1.9cm of external diameter and height, respectively. In Figure is showed the irradiator belongs to the Radioprotection Laboratory. Figure : Irradiator of the Radioprotection Laboratory of Atomic Energy Commission. Penumbra, flatness and symmetry..1 TLDs measurements External Dosimetry Section of Constituyentes Atomic Center provided termoluminiscent dosimeters (TLDs). They were used to study penumbra and homogeneity of the photon beam. TLDs LiF 700 Bicron/Harshaw with dimension 3x3x0.89mm 3 were selected with a sensibility factor lower than 10%. They were inside a capsule. A total of 10 dosimeters were fixed on backside of acrylic slab of 5mm thickness and surface 50x50cm. They were distributed on Z and Y axis, perpendicular to beam axis, each 5cm on the entire surface. This slab was used to provide electronic equilibrium; its centre was positioned to 1.80 meters from exit collimator. Irradiation beam was 137 Cs and the time was the necessary to give an air Kerma of mgy, approximately. A total of 5 dosimeters were used to repeat the irradiation but to 1 meters from exit collimator. External Dosimetry Section read the TLDs in a Harshaw 3500 Reader following procedures written at Laboratory. Dose received by dosimeters were normalized to the maximum value and a dispersion value (max-min) and the uncertainty were calculated. Other studies were done to a distance of 1m from exit collimator using an ICRU slab phantom. LiF- 100 powder in capsules was fixed each 5 cm to the thinner and greater surface of the phantom and on the Z and Y axis. They were read in Harshaw Automatic reading. It was done for cesium and cobalt and the personal equivalent dose received was msv... Films measurements Dosimetric films X-Omat-V were positioned on a perpendicular direction of beam axis to 0.7 and 1m from exit collimator to film. For 0.7 meters only a film was used and for 1m 5 films were used to cover the radiation field, they were fixed to a slab acrylic of surface 50x50cm and thickness of 5mm. 3

4 They film to 0.70m received 10, 40, 80, 100 and 10 mgy to built calibration curve Optical density Vs Air Kerma. Films were developed and read at Spanish Hospital..3 Measurements out axis radiation beam According to the suggested in ISO measurements out axis radiation beam were done. Cylindrical reference chamber of 600cm 3 NE TYPE 575C Model 467 and FARMER DOSEMETER Model 70/1 Series Nº 1163 electrometer were used. Accumulated charges were collected to 0.70 and 1m distances from collimator exit-reference point of the chamber and to 0, 15, 30, 45, 60 and 75cm along an axis perpendicular to radiation beam. Those distances are the maximum possible due to the proximity of a wall. A percentage was calculated respect to the maximum value. Before using calibration equipment radioactive check and leakage control were done to assure the good performance..4 Scattering radiation measurement Scattering radiation was evaluated by 3 methods: Shadow-shield, Multiple distance and Monte Carlo simulation Shadow-shield Method. In this method air Kerma rate ( K & ) due to scattering radiation is assumed constant on all distances exit collimator-point reference. In this method a cone of a high atomic number (Z) is located between the source and the chamber to avoid that primary photon reach the chamber. The chamber is completely shielded of primary radiation and only reaches the chamber scattering radiation. Dispersion value is obtained by the quotient between charge without and with shielding. Shielding dimension should be he minimum required to reduce the primary radiation intensity in detector position less than % of value without shielding. Height cone is enough to provide attenuation and it is not so close of chamber to avoid scattering from the cone. A cylindrical ionisation chamber of 600cm 3, NE 551, Series Nº 490 with build up, an electrometer PTW-UNIDOS Model Series Nº 0119 were used. Dimension of cylinder made in lead were 8cm and 0cm, of diameter and length, respectively. Charge accumulated was collected at 0.7, 1, 1., 1.4, 1.5,,.5, 3, 3.5, 5 and 5m from exit collimator and reference point of chamber. For each position were taken 3 readings of 60 seconds each one, and an average value was calculated. Those reading corresponded to total radiation, primary plus secondary. Air Kerma rate was measurements in Then accumulated charge was collected for the same distances but interposing a cylinder, to approximately middle distance exit collimator-reference point of the chamber. For each position were taken 3 readings of 300 seconds each one, and an average value was calculated. In this way those values correspond to secondary radiation. Current values were corrected by a factor of pressure and temperature and corrected by drift. The difference between total and secondary radiation gave the primary radiation. The quotients between primary and secondary radiation and total and secondary radiation were calculated for different distances..4..analytical Method of multiple distances Readings to different distances show the inverse quadratic law and it assumes a constant dispersion. It is essential in this method that distances be precise and correct to derivate the correction c that gives the true distance exit collimator (center source)-chamber (d ). Where d = d + c d is the true distance center chamber center source. 4

5 d is the apparent distance center chamber center source. c reading error. The contribution of scattering radiation to K& total, K& S, is included in K& measured ( K & (d ) ). The value of K& due only primary photons K& P (d ) K & P ( d ) = K& ( d ) K& (1) total S & P ( d ) d = ( K& total ( d ) K S ) ( d + c) () Constant = K & Clearing gives Cte & total ( d ) = K& S + or y a + K ( d + c) b = (3) ( d + c) Air Kerma rate was calculated starting from the values got in the before point for 0.7, 1, 1., 1.4, 1.5,,.5, 3, 3.5, 5 and 5m from exit collimator and reference point of chamber. The curve obtained for Current vs distance for each beam was adjusted by a function which equation gave the value constant of the secondary radiation. Mathematic software and Curve Table software were used to obtain a, b, c values..4.3 Theoretical calculation with MCNP5c. Field size of radiation field, flatness and penumbra were calculated by Monte Carlo simulation. Photon flux was calculated on different point of radium radio cm in air using MCNP5c code [6]. Tally f5, flux at a point detector (particles/cm), was used for different distances from collimator exit, Z- axis, and on the transversal plane of the axis beam. The center of the point detectors were at 0, 5, 10, 13, 15, 17, 0, 3, 5 y 30 cm from the center of the beam for combinations using the same positions for Y axis. All was surrounded by an air sphere of radium 6 meters and outside was vacuum. The same was repeated for 0.70, 1 and 1.80 m from collimator exit for cesium and cobalt. For 1.80 m was considered more points on transversal plane because the size field was greater (0, 5, 10,13, 15,17, 0, 3,5, 30, 33, 34, 36, 37, 38, 39, 40, 4, 44, 45 and 50cm). Irradiator s geometry was obtained from manual given by manufacturer. Densities considered were g.cm -3, 19.3 g.cm -3 and x 10-3 g.cm -3 for lead, tungsten and air, respectively. Air composition by mass was 75.50, 3.176, 1.88, and % nitrogen, oxygen, argon, and carbon respectively. No variance reduction techniques were used. Enough photon histories were run to pass statistics test proposed for the code. The appropriate photon (MCNPLIB04) and electron (EL3) cross-section libraries were chosen from the DLC-0 library ENDF/B-VI Release 8 [7]. The ENDF/B-VI.8 photo atomic and atomic relaxation data are in turn based on the EPDL97 library, RSICC Data Library DLC Results 3.1 Penumbra, flatness and symmetry TLDs measurements Personal equivalent dose, Hp(10) was evaluated in units of milisievert (msv) and normalized to the value of maximum dose. Dispersion was calculated and resulted 19.4%. Uncertainty was 10% (k=). In Table 1 and is showed the percentage of photon flux (kerm or dose) for cesium and for 180cm, 100cm from collimator exit, respectively, using TLDs rods. 5

6 Table 1: Percentage of photon flux (kerm or dose) for cesium and 180cm from collimator exit using TLDs rods. Y/Z Table : Percentage of photon flux (kerm or dose) for cesium and 100cm from collimator exit using TLDs rods. Y/Z According to the table 1 all dosimeters were inside radiation field and penumbra could not be measured. Beam homogeneity on a surface of 50x50cm to 1.80 meters from exit collimator 80.6%, size field is 5cm of radium and less of 15cm for 1meter. Penumbra is less than 3 cm. Difference between the value of the air kerm given and the read it is less than 10%. Respect to measurements made with powder LiF, the most of them showed standard deviation greater than 5% so they could not be taken into account Films measurements In Figure 3 is showed several imaging s, the first is for cesium to 1 meter from collimator exit and it is more diffuser than others. Second and third figure are for cobalt to 0.7m and 1 meter from exit collimator. Radiation field had a radium of 19cm for cobalt, cesium and penumbra resulted of 0.7cm. For 1 meter the results were similar for cesium and cobalt, 16cm of radium and penumbra of 0.4cm. Symmetry and flatness were greater than 90%. 6

7 Figure 3: Developed films for cesium to a distance of 1meter, cobalt to 0.70m and 1 meter from collimator exit, respectively. 3. Measurements out axis radiation beam Air Kerma was calculated for 0.7 and 1m from exit collimator to reference point of chamber and values were normalized to the maximum. It was realized for cesium and cobalt. In Table 3 is showed the percentage of dose for cesium and cobalt for 0.7m and 1m from collimator exit. Its showed that the size field to 1m has a radium of 15cm and to 0.7m is less than 10cm. Table 3: Percentage of dose for cesium and cobalt for 0.7m and 1m from collimator exit. 60 Co (1m) 137 Cs (1m) 60 Co (0.7m) 137 Cs (0.7m) Distances (cm) % % % % Scattering radiation measurement Shadow-shield method Results of that part showed that the scattering radiation was lower than 4%, below that 5% suggested for ISO Analytical Method of multiple distances In Table is showed of a, b, c and regression coefficient values for equation (3) for cesium and cobalt sources. Table 4: a, b, c and regression coefficient values for equation (3) for cesium and cobalt sources. a b c r 1-Cesio Cesio Cesio Cobalto Cobalto

8 Remembering that y is the total current, a is the scattering current (secondary), c is the error from distance and b is a constant. Secondary intensity varies from 0.13% to 4.7% of total intensity for 60 Co and for 0.7 and 5 meters, respectively. For the case of 137 Cs the values varies from 0.1% to 4.9% for 0.7 and 5 meters. Differences in Table 4 are due that different distances between 0.7 and 5 meters were considered, in some cases they were from 1 to 4 meters Monte Carlo Simulation Results of that part showed that the scattering radiation was lower than 4.%, below that 5% suggested for ISO Results for 137 cesium, 60 cobalt showed a radium for radiation field of 0cm and 17cm for 100cm from collimator exit. For 180cm the results were 33cm and 30cm respectively. For 0.7m the results were 15cm and 13cm respectively. Penumbra was of.5cm approximately and beam homogeneity was 85%. Size field results agree to the ones from films. For penumbra the differences are greater. 4. Conclusion An exhausted study was done to the irradiator and this showed the good performance of itself. Differences from theoretical and experimental studies of radiation field size and penumbra, specifically with TLDs, are going analysed. Values of secondary radiation are acceptable as well as homogeneity beam. In a future work the secondary radiation is analysed which part is due to the floor, walls and ceiling using Monte Carlo simulation. Respect to the future of the Radioprotection Laboratory, this is going to have an intern audit to intent to pass next year and audit of Accreditation Argentinean Organism. For that reason instructions and procedures are being written at this time. Acknowledgements The authors wish to thank Mrs. Rosana Sansogne from Spanish Hospital and Mr. Horacio Sanchez for their collaboration. REFERENCES [1] INTERNATIONAL ATOMIC ENERGY AGENCY, Calibrations of Radiation Protection Monitoring Instruments, Safety Reports Series No. 16, Vienna (000). [] NACIONAL COUNCIL ON RADIATION PROTECTION AND MEASUREMENTS, Calibrations of Survey Instruments used in Radiation Protection for the Asessment of Ionizing Radiation Fields and Radioactive Surface Contamination, NCRP Report No. 11, Bethesda, (1991). [3] INTERNATIONAL ORGANIZATION FOR STANDARIZATION, X and gamma reference radiation for calibrating dosimeters and dose rate meters and for determining their response as a function of photon energy. Part : Dosimetry for radiation protection over the energy ranges 8keV to 1,3MeV and 4MeV to 9MeV. ISO 4037-, Geneva, (1997). [4] INTERNATIONAL ORGANIZATION FOR STANDARIZATION, X and gamma reference radiation for calibrating dosimeters and dose rate meters and for determining their response as a function of photon energy. Part 1: Radiation characteristics and production methods. ISO Geneva, (1997). 8

9 [5] FERNANDES, E., et al., The radiation field characteristics of a 137 Cs source used for calibration of radiation protection instruments, Applied Radiation and Isotopes 61 (004) [6] MCNP A General Monte Carlo N-Particle Transport Code, Versión 5, X-5 Monte Carlo Team, Los Alamos National Laboratory, Los Alamos, NM (003). [7] RADIATION SAFETY INFORMATION COMPUTATIONAL, RSICC Data Library DLC- 0, Oak Ridge National Laboratory (005), 0.html. 9