Influence of the PMMA and the ISO Slab Phantom for Calibrating Personal Dosemeters

Transcription

1 Influence of the PMMA and the ISO Slab Phantom for Calibrating Personal Dosemeters M. Ginjaume, X. Ortega, A. Barbosa Institut de Tècniques nergètiques Universitat Politècnica de Catalunya, Barcelona - Sain INTRODUCTION Wide agreement has been reached among the main Bodies and Organizations involved in standardization of radiation rotection to define as oerational quantity for individual monitoring of external exosure, the ersonal dose equivalent, H (d) (1,2,3). However, as H (d) is defined in the human body, it is not a measurable quantity. In ractice, it has to be estimated from the indication of an individual dosemeter, which is worn on the trun and calibrated on a suitable hantom. The calibration hantom should rovide a bacscatter contribution, similar to that of the art of the body where the dosemeter is worn. Since its definition in 1985 (4), several reorts have been ublished to clarify and define the exerimental set u to be used for the calibration of dosemeters in terms of the above-mentioned quantity. Although the ICRU shere was first considered to be a good hantom, its ractical use resented so many difficulties that it was rejected. On the other hand, other hantoms such as the PMMA slab hantom of different sizes or the IAA cubic water hantom were adoted. In 1992, ICRU, in its reort 47 (5), gave a list of five different hantoms that were being used by several laboratories and were considered to be accurate enough within an acceted overall uncertainty in most radiation rotection measurements of 30 %. ICRU 47 also included a new quantity to be used for calibration, which is defined as H (d) in a hantom which has the comosition of ICRU tissue and the same size and shae as the calibration hantom. In the following text, this hantom will be named as the ICRU hantom. Nevertheless, to achieve uniformity in calibration rocedures, ICRU 47 recommended the 30 cm x 30 cm x 15 cm PMMA slab hantom. However, recently, the International Organization for Standardization (ISO), in ISO (3), has roosed another hantom, which will be named, in this text, the ISO hantom, which consists of a 30 cm x 30 cm x 15 cm water hantom with PMMA walls (front wall 2.5 mm thic). Many studies, such as the wors from Grosswendt (6) and Gualdrini and Morelli (7), have analysed the characteristics of several ractical calibration hantoms by means of Monte Carlo codes for hoton transort. Calculations confirm that the new ISO hantom is a better substitute of the ICRU hantom than the PMMA hantom, mainly at low hoton energies. The values for the ersonal dose equivalent for the ISO hantom are found to be very close to those calculated for the theoretical ICRU hantom, thus indicating that, in ractice, bacscatter correction factors are not needed in this new hantom. In 1995, the secondary calibration Laboratory from the Institute of nergy Technology (INT) at the Technical University of Catalonia in Barcelona, Sain, agreed with the Sanish National Laboratory to adot the ICRU recommended PMMA hantom, and to imrove calibration rocedure accuracy by introducing a correction factor for bacscatter differences in PMMA and ICRU tissue (8). Such corrections were of the order of 8 % for the low-energy X-ray qualities. In 1999, before imlementing the new ISO standard the INT laboratory carried out the resent wor, whose aim was to estimate the influence of both hantoms when calibrating ersonal dosemeters with hotons in the range 33 ev, 1.5 MeV. MATRIAL AND MTHODS The study was erformed in the INT secondary Standard Dosimetry Laboratory using the ISO reference radiation qualities defined in ISO (9), corresonding to the X-ray narrow sectrum series and 137 Cs. The X-ray beams were roduced in a Rich Seifert ISOVOLT 320D X-ray system for dosimetry whose outut stability was controlled with a PTW monitor chamber. The study aimed at analysing the influence of the calibration rocedure in the measurement of H (10) and H (0.07), for the TL dosemeter, used by the INT aroved ersonal dosimetry service and for the PD2 electronic ersonal dosemeter manufactured by SIMNS. The TLD holder had a 2-mm-thic PVC bac surface, whereas the SIMNS device had a metal case. Calibration rocedure 1: This rocedure is based on the use of the PMMA hantom, which is recommended by ICRU 47 as a ractical calibration hantom for ersonal dosemeters. The hantom is situated at a source-oint of test distance of 2.5 m, which ensures a comlete irradiation of the hantom. Dosemeters are held to the surface using doublesided stic aer on the front surface. When calibrating, four TLD holders were laced on the front face, centred within an 11-cm diameter circle and irradiated at the same time. Because of its size, only one unit of the electronic device could be calibrated at once. 1

2 As described in reference (8), a correction factor,, for bacscatter differences in a PMMA and an ICRU slab hantom was introduced when calibrating TLDs. was calculated according to the following exression: ( ( ν where B m is the bacscatter factor, at a given radiation quality, on a hantom of material m, in a media ν and ( m ) ν is the erma in a media m on the surface of a semi-infinite hantom of material ν. In exression (1), m is the dosemeter material, therefore, for tissue equivalent detectors, as variations of in air and tissue are within 1%, the following simlification is adoted: In the energy range considered in this study, the erma ratio indicated in equation (2) can be substituted by air collision erma ratio, which in turn can be aroximated to dose equivalent at 0 mm deth on the PMMA and on the ICRU tissue slab hantom. Values of for monoenergetic hoton have been obtained from Grosswendt data (6) as the ratio of the dose equivalent in the two hantoms. Values of for the considered X- ray qualities have been calculated by integrating our exerimental energy sectral distribution of air erma: ) ICRU Bm m ICRU, (1) PMMA Bm m ) PMMA ( ( ) ICRU ICRU. (2) ICRU ) PMMA max 0 max 0 air, air, d d (3) Radiation quality (1) Mean energy (ev) Calibration area diameter (2) (cm) h (10) (Sv/Gy) h (0.07) (Sv/Gy) N N N N N N N N N S-Cs Table 1. Radiation qualities used in the comarison of the erformance of the PMMA and the ISO hantoms, conversion coefficients and bacscatter correction factors. (1) Radiation qualities roduced in accordance to ISO (2) Diameter of the area of the front face, where the dose is aroximately within 98 % of the dose in the centre of the hantom (3). 2

3 Table 1 lists the radiation qualities chosen for this comarison, the corresonding mean energy averaged over the fluence exerimental sectra, the conversion coefficients from air erma to the ersonal dose equivalent in a deth of 10 mm and 0.07mm for the ICRU hantom, h (10) and h (0.07) (3) and the calculated values. When rocedure 1 is used, the calibration factor for the TL dosemeters, N TLD, is defined as: N TLD h ( d ) M a, (4) where: h, (d) is the conversion coefficient from air erma, a, to the dose equivalent H (d), M is the corrected dosemeter reading in arbitrary units, a is the air erma free in air, in the oint of test, when there is no hantom. Previous measurements showed that the SIMNS dosemeter did not detect the bacscatter radiation in the considered energy range. Therefore, the laboratory established that for electronic devices, which usually had thic cases, no bacscatter correction would be alied, thus the calibration factor, N PD2, is calculated as: N PD 2 h ( d) M a. (5) Calibration rocedure 2: The exerimental set-u is the same as in rocedure 1, with the excetion of the ISO hantom used in the calibration. As it is acceted that this hantom does not need any secial correction for bacscatter differences, the calibration factor for any tye of ersonal dosemeter, when alying rocedure 2, is obtained by using the same exression as in (5): N ISO h ( d) M a. (6) RSULTS Tables 2 and 3 show the calibration factors obtained with rocedure 1 and 2, resectively for the TLD and the Siemens PD2. The ratio of the calibration factors corresonding to each calibration rocedure, for H (10) and H (0.07), is also tabulated. ach calibration factor has been calculated, using as dosemeter reading, M, the mean value of the four TLDs or the mean value of three consecutive readings in the electronic dosemeter. The uncertainty associated with the calibration rocedures has been estimated following the ISO guide (3). The uncertainty comonents (1) that have been considered in the study are - : 1.5%, - dosemeter reading: 0.85 % for TLDs, 0.2 % for H (10) and 0.8 % for H (0.07) in the Siemens detector, - exerimental set-u (ositioning, outut stability) : 1.5 %, - dose equivalent coefficients: 2 %, - air erma reference measurement: 1.6 %. The overall uncertainties (1) associated to the different calibration factors are of the order of 3.4 % for N TLD and 3.1% for N PD2 and for N ISO. The uncertainty budget of the ratio of calibration factors between the two calibration rocedures (columns 4 and 7 of tables 2 and 3) does not include the uncertainty for the dose equivalent coefficients nor for the air erma reference measurement, since these arameters affect in the same way both calibration rocedures. Therefore, the uncertainty of the ratio of the two calibration factors is of the order of 2.9 % for the TLD and of 2.4 % for the PD2 (1). 3

4 Radiation quality N TLD H (10) H (0.07) N ISO N TLD /N ISO N TLD N ISO N TLD /N ISO N N N N N N N N N S-Cs Table 2: Calibration factors for H (10) and H (0.07) for the TL dosemeter of the INT aroved ersonal dosimetry sevice, which uses LiF as sensitive material. Comarison between rocedure 1 with a PMMA hantom (N TLD ) and rocedure 2 with an ISO hantom (N ISO ). (a.u stands for arbitrary units and corresonds to the TL reader number of counts for a given thermal treatment and sensitivity). エラー! フ ックマークが定義されていません Radiation quality H (10) H (0.07) N PD2 N ISO N PD2 /N ISO N PD2 N ISO N PD2 /N ISO N N N N N N N N N S-Cs

5 Table 3: Calibration factors for H (10) and H (0.07) for an PD2 SIMNS ersonal electronic dosemeter. Comarison between rocedure 1 with a PMMA hantom (N PD2 ) and rocedure 2 with an ISO hantom (N ISO ). (N PD2 and N ISO are dimensionless because the dosemeter reading is in msv). 5

6 For the TLD, the mean value of the calibration factor ratio and the corresonding standard deviation, in the studied energy range, was 1.02±0.01 for H (10), and, 1.03±0.02 for H (0.07). These results indicate a very good agreement, within uncertainties, between the two calibration rocedures. Table 2 highlights that, for the TLD holder and material used in the comarison, the introduction of a bacscatter correction factor,, imroves the coherence between rocedures 1 and 2, mainly for H (10) and the low energy qualities from N-40 to N-100. However, at higher energies, and for H (0.07) calibration, no significant imrovement associated with this correction could be roved. In all the radiation qualities, the calibration factor obtained with the PMMA hantom has been greater than the calibration factor from the ISO hantom, which imlies that the correction factors roosed by our Laboratory overestimate the contribution of this radiation comonent. In articular, for 137 Cs, as it is suggested by ISO, no bacscatter correction should be used when calibrating with a PMMA hantom. For the SIMNS device, the mean value of the calibration factor ratio, in the studied energy range, was 0.996±0.003 for H (10), and, 1.00±0.01 for H (0.07). Thus demonstrating a erfect coincidence between the two hantoms. This result confirms that the selected electronic ersonal dosemeter is not sensitive to bacscatter and therefore the same calibration factor would be found when calibrating free-in-air. CONCLUSIONS The results of this wor indicate that although, theoretically, the ISO water hantom is a better substitute of the ICRU hantom, in ractice, the differences between PMMA an ISO hantoms are not significant. This conclusion ensures that, in case of changing from the calibration rocedure recommended by ICRU to the rocedure recommended by ISO, it will not imly any imortant changes in former dose estimate. This statement which has been roven to be true for secific radiation qualities is even more clear in realistic mixed radiation fields. The acceted hoton energy resonse for ersonal dosemeters is of the order of ± 30% 137 Cs resonse, thus maing negligible the influence of the calibration hantom for the dose assessment. To this effect, from a radiation rotection oint of view, any of the roosed calibration rocedures ensures a satisfactory estimate of the effective dose and therefore the comliance with the established limits. However, for tye testing and for the comarison of the erformance of several dosemeters, it is very desirable to achieve international uniformity in calibration rocedures. This rincile which is general in metrology has been emhasised by ISO and ICRU. Since ISO and IC recommend the water hantom with PMMA walls for ersonal dosemeter calibration, accredited laboratories should gradually adot the new criteria to favour international harmonization and uniform roficiency testing results. One of the advantages of the ISO roosal is that, irresective of the dosemeter material, the bacscatter contribution at the oint of measurement is very similar to the bacscatter of a theoretical ICRU hantom, thus eliminating the need of estimating the bacscatter correction. Nevertheless, the water hantom is more cumbersome and difficult to maniulate than the solid hantom. Precautions have to be taen to guarantee a flat front surface and to revent leas and ressure changes in the hantom. RFRNCS 1. International Commission on Radiation Units and Measurements Quantities and units in radiation rotection. ICRU Reort 51. Bethesda, Maryland (1993). 2. International Commission on Radiological Protection. Conversion coefficients for use in Radiological Protection against xternal Radiation. ICRP Publication 74. Annals of the ICRP vol 26 nº 3-4. Pergamon Press, Oxford (1996). 3. International Organization for Standardization. X and gamma reference radiations for calibrating dosemeters and doserate meters and for determining their resonse as a function of hoton energy Part 3: Calibration of area and ersonal dosimeters and the determination of their resonse as a function and angle of incidence. International Standard ISO (ISO, Geneva) (1999). 4. International Commission on Radiation Units and Measurements Determination of dose equivalents resulting from external radiation sources. ICRU Reort 39. Bethesda, Maryland (1985). 5. International Commission on Radiation Units and Measurements Measurement of Dose quivalents from external hoton and electron Radiations. ICRU Reort 47. Bethesda, Maryland (1992). 6. Grosswendt B. The angular deendence and irradiation geometry factor for the does equivalent for hotons in slab hantoms of tissue equivalent material and PMMA. Rad. Prot. Dosim. 35, (1991). 7. Gualdrini G.F., Morelli B. Air erma to ersonal dose equivalent conversion factors for the ICRU and ISO recommended slab hantoms for hotons from 20 ev to 1 MeV. NA technical reort ref RT/AMB/96/15, Bologna (1996). 8. Ginjaume M., Ortega X., De la Corte N., IRPA.Conversion coefficients relating air erma to H (10) and 6

7 H (0.07) for ISO X Ray narrow sectrum series using a PMMA slab hantom International Congress on Radiation Protection. Proceedings, volume 4, (1996). 9. International Organization for Standardization. X and gamma reference radiations for calibrating dosemeters and doserate meters and for determining their resonse as a function of hoton energy Part 1: Radiation characteristics and roduction methods. International Standard ISO (Geneva: ISO) (1996). 7