Intercomparison of Ionization Chambers in Standard X-Ray Beams, Radioprotection Level

Transcription

1 Primer ongreso mericano del P 2006 /First merican P ongress 2006 XXV eunión nual de la SS y XV ongreso nual de la SN/XXV SS nnual eeting XV nnual SN ongress capulco éxico, del 3 al 8 de Septiembre 2006/ capulco exico, September 3-8., 2006 ntercomparison of onization hambers in Standard X-ay eams, adioprotection evel na.. essa and inda V. E. aldas nstituto de Pesquisas Energéticas e Nucleares (PEN) omissão Nacional de Energia Nuclear (NEN) v. Professor ineu Prestes, São Paulo, SP acmbessa@ipen.br; lcaldas@ipen.br bstract n ionization chamber intercomparison in X-ray beams, radioprotection level, was realized, using the calibration coefficients of the instruments as a comparison factor. esponse energy dependence curves were obtained for the ionization chambers. The ionization chambers tested in these X-ray beams were: 22 adcal chambers, models 10x5-1800, 10x5-180, 10x5-6, 10x5-6, 10x5-60, 20x5-3, 20x5-180, 20x5-6 and 10x5-6-3; a Smart on chamber, model 2100; an novision chamber, model 451; two Victoreen chambers, model 450P, and a abyline 81 chamber. mong these ionization chambers, the adcal chamber, model 10x5-60, presented the lowest energy dependence relative to the radiation quality N-80: 2.3% in the studied energy range. Victoreen ionization chambers, specially designed for use in X-ray beam intercomparisons, models 415, 415 and 415, all coupled to a PTW electrometer, model nidos, were also tested. The Victoreen ionization chambers presented, respectively, maximum energy dependence of 6.2%, 4.9% e % in the energy range from 49 to 117 kev. 1. NTTN The confidence of the readings of radiation monitoring instruments relies on the calibration of each unit. The nternational tomic Energy gency (E) recommends [1] that each individual instrument used in radiation monitoring should be calibrated before its first use, and then it should be calibrated periodically, usually every 12 to 14 months. n razil, this kind of calibration may be performed at the alibration aboratory of the nstituto de Pesquisas Energéticas e Nucleares (PEN). ntercomparisons can be defined as comparisons of measurements among instruments of the same metrological classification in order to compare their performance. comparison of instruments of the same model and of the same metrological level in different radiation beam qualities is important to determine the best application of each instrument model and the correction factors that should be applied when the instrument is utilized in a different beam quality than that in which it was calibrated. emorias capulco 2006 en 1/7 Proceedings capulco 2006 on

2 na.. essa and inda V. E. aldas, ntercomparison of onization hambers in Standard X-ay eams Since a great number of radiation monitoring instruments are calibrated in the alibration aboratory every year, an intercomparison was performed in X-ray beams, radioprotection level, using the calibration coefficients of the instruments as a comparison factor. The energy dependence of the response of the ionization chambers was studied in this work. 2. TES N ETHS The X-ray beams used in this work were the narrow series S beams, qualities N-60, N-80, N- 100 e N-150, according to the standard S [2]. These beam qualities are recommended by the nternational tomic Energy gency [1] for calibration of radiation protection monitoring instruments. The beams were established at a Seifert/Pantak X-ray system, model SVT 160. The characteristics of these narrow beams are shown in Table. Table haracteristics of the established standard radiation protection beams at the alibration aboratory of PEN. dditional Filtration [mm] ean Energy [kev] ir erma ate * [my.h -1 ] adiation Quality Voltage [kv] 1 st HV [mm u] 2 nd HV [mm u] N l + u N l + 2 u N l + 5 u N l Sn * ir kerma rates were measured at a focal-detector distance of 2.5 m and with a filament current of 20 m. The reference instrument used for the measurements of air kerma rates was a spherical 1 liter PTW ionization chamber, type 32002, coupled to a PTW electrometer, model nidos. For the monitoring of the intensity variations of the X-ray beams during the calibrations, a PTW transmission ionization chamber, model TW34014, and a PTW electrometer, model nidos E, were utilized. Victoreen ionization chambers, specially designed for use in X-ray beam intercomparisons, models 415, 415 and 415, all coupled to a PTW electrometer, model nidos, were tested. The ionization chambers, models 415 e 415, are cylindrical chambers, recommended for measurements in high and medium energy X-ray beam qualities. The ionization chamber model 415 has a cylindrical electrode and a ylar window, and it is for use in low energy X-ray beams. The 22 adcal ionization chambers tested in these X-ray beams were of models 10x5-1800, 10x5-180, 10x5-6, 10x5-6, 10x5-60, 20x5-3, 20x5-180, 20x5-6 and 10x Two Victoreen ionization chambers, model 450P, an novision chamber, model 451, a Smart on chamber, model 2100, and a abyline 81 chamber were also tested. The energy dependence is expressed according to E [3], which recommends that energy dependence shall be expressed as a variation of the measurement in a reference condition, as a emorias capulco 2006 en 2/7 Proceedings capulco 2006 on

3 Primer ongreso mericano del P 2006 /First merican P ongress 2006 radiation quality around the half of the energy range. n this work, the radiation quality N-80 was used as a reference quality for the study of the energy dependence of response of the ionization chambers. For the determination of the calibration coefficients [4] of an instrument under calibration in terms of air kerma rate, Equation 1 was utilized. f the quantity measured by the instrument under calibration differs from the quantity measured by the reference instrument, additional corrections factors should be applied. where N N ktp 1 = (1) k tp2 N is the calibration coefficient of the instrument under calibration in terms of air kerma (at reference conditions); is the measured value of the reference instrument; N is the calibration coefficient of the reference instrument in terms of air kerma (at reference conditions); k is the correction factor for differences in air density, from measurements of pressure and tp1 temperature, for the measured value of the reference instrument; is the measured value of the instrument under calibration; k is the correction factor for differences in air density, from measurements of pressure and tp2 temperature, for the measured value of the instrument under calibration. 3. ESTS The calibration coefficients of the Victoreen ionization chambers models 415, 415 and 415, obtained in standard radioprotection beams (Table ), are presented in Table. The energy dependence of these chambers is showed in Figures 1 to 3. The ionization chambers models 415, 415 and 415 presented maximum energy dependence, relative to the beam quality N-80, of 6.2, 4.9% and %, respectively. This is a low energy dependence, considering that the standard radiation protection beams used in this work include a wide energy range, from 49 kev to 117keV. Table alibration coefficients of the Victoreen ionization chambers models 415, 415 and 415, obtained in standard radioprotection beams of Table. adiation Quality 1 st HV [mm u] [y.µ -1 ] N N N N emorias capulco 2006 en 3/7 Proceedings capulco 2006 on

4 na.. essa and inda V. E. aldas, ntercomparison of onization hambers in Standard X-ay eams Half Value ayer [mm u] Figure 1 Energy dependence of the Victoreen ionization chamber model Half Value ayer [mm u] Figure 2 Energy dependence of the Victoreen ionization chamber model Half Value ayer [mm u] Figure 3 Energy dependence of the Victoreen ionization chamber model 415. emorias capulco 2006 en 4/7 Proceedings capulco 2006 on

5 Primer ongreso mericano del P 2006 /First merican P ongress 2006 The dispersion of the calibration coefficients obtained from the calibration data of the ionization chambers shown in Table in standard radiation protection beams recommended by S [2] is presented in Figure 4. Table also shows the maximum energy dependence relative to the radiation quality N-80 for each chamber model. N-60 N N-100 N Figure 4 ispersion of the calibration coefficients of the ionization chambers shown in Table obtained in the standard radioprotection beams of Table. The dashed lines show the variation of ± 20% relative to the conventional true value. emorias capulco 2006 en 5/7 Proceedings capulco 2006 on

6 na.. essa and inda V. E. aldas, ntercomparison of onization hambers in Standard X-ay eams Table alibration coefficients [N ] and maximum energy dependence relative to the radiation quality N-80 for each chamber model, in S [2] standard radioprotection beams. hamber Volume [cm 3 ] onitor N-60 [N ] N-80 [N ] N-100 [N ] N-150 [N ] Energy ependence adcal 10x adcal aby ine Smart on novision Victoreen 450P Victoreen 450P adcal 10x adcal adcal 10x adcal adcal 10x adcal adcal 10x adcal adcal 10x adcal adcal 20x adcal adcal 20x adcal adcal 20x adcal adcal 20x adcal adcal 20x adcal adcal 10x5-6 6 adcal adcal 10x5-6 6 adcal adcal 10x5-6 6 adcal adcal 10x adcal adcal 10x adcal adcal 10x adcal adcal 20x5-3 3 adcal ajor pplication adiation Protection Scattered adiation onventional iagnostic adiology adcal 10x adcal Fluoroscopy adcal 20x5-6 6 adcal adcal 20x5-6 6 adcal adcal 20x5-6 6 adcal ammography emorias capulco 2006 en 6/7 Proceedings capulco 2006 on

7 Primer ongreso mericano del P 2006 /First merican P ongress NSNS The Victoreen ionization chambers models 415, 415 and 415 showed low energy dependence in the S standard narrow radioprotection beams used in this study. The ionization chamber model 415 presented the best performance: only % of energy dependence. The two Victoreen ionization chambers model 450P presented energy dependence greater that the other chambers designed for the same use, in radioprotection beams: 25.3% and 15.4%. n relation to the spread of the calibration coefficients presented in Figure 4, one Victoreen chamber model 450P () showed the worst performance, since the measurements obtained for this ionization chamber in three of four radioprotection beams of this work presented a variation greater than 20% in relation to the conventional true value. The adcal chamber model 20x5-6 () showed a great energy dependence: 34.6% (in the N-100 beam), but this occurs because this chamber is designed for use in mammography, which use beams of lower energy than the narrow beams for radioprotection of this work. NWEENTS The authors acknowledge the partial financial support of Fundação de mparo à Pesquisa do Estado de São Paulo (FPESP) and onselho Nacional de esenvolvimento ientífico e Tecnológico (NPq), razil. EFEENES 1. nternational tomic Energy gency. alibration of radiation protection monitoring instruments. Vienna, (Safety eports Series 16). 2. nternational rganization for Standardization. X and gamma reference radiation for calibrating dosemeters and doserate meters and for determining their response as a function of photon energy Part 1: adiation characteristics and production methods. eneva, (S ). 3. nternational Electrotechnical omission. edical electrical equipment. s with ionization chambers and/or semi-conductor detectors as used in X-ray diagnostic imaging. eneva, (E 61674). 4. eghzifene,.; Shortt,.. alibration factor or calibration coefficient? SS Newsletter, 46, p (2002). emorias capulco 2006 en 7/7 Proceedings capulco 2006 on