RELATIONSHIP BETWEEN PERSONAL DOSEMETERS READINGS AND HUMAN DOSES*

Transcription

1 RELATIONSHIP BETWEEN PERSONAL DOSEMETERS READINGS AND HUMAN DOSES* Eri Hiswara** ABSTRACT RELA TlONSHIP BETWEEN PERSONAL DOSEMETERS READINGS AND HUMAN DOSES. The relationship between personal dosemeter reading and the effective male dose (equivalent), the effect of skin-dosemeter distance and the effect of angular response of dosemeters, were studied using a computational model developed in JAERI and a realistic body phantom. The relationship was calculated for five locations of dosemeters worn at the skin surface of the trunk. The calculation showed that the dosemeters at the left breast pocket gave the best estimation of the effective male dose (equivalent), although from the measurement it was shown that the same location had the worst angular horizontal response. The study on the effect of skin-dosemeter distance indicated a contradictory result which requires further investigation, whereas the angular vertical response showed less than 10% response variation even for 900 rotation of the dosemeter. ABSTRAK HUBUNGAN ANTARA BACAAN DOSIMETER PERORANGAN DAN DOSIS MANUSIA. Hubungan antara bacaan dosimeter perorangan dan dosis (ekivzlen) efektif pria. efek jarak kulit-dosimeter dan cfek tanggapan sudut dosimetcr teiah dipelajari dengan menggunakan model komputasi yang dikembangkan JAERI dan fantom tubuh realistik. Hubungan dihitung untuk lima lokasi dosimeter yang dipasang pada permllkaan dada. Perhitllngan menllnjukkan bahwa dosimeter yang dipasang pada saku kiri memberikan pcrkiraan terbaik dalam dosis (ekivalen) efektif, meskipun pengukuran pada lokasi yang sarna memiliki tanggapan sudut horisontal yang terburuk. Studi ten tang efek jarak kulitdosimeter menunjukkan hasil yang berlawanan yang memerlukan penyelidikan lebih lanjut, semcntara tanggapan sudut vertikal menunjukkan variasi tanggapan yang kurang dari 10% bahkan untuk rotasi dosimeter sejauh 90. INTRODUCTION For the purpose of a quantitative estimation of risk, the International Commission on Radiological Protection (ICRP) has introduced the effective dose quantity [J]. This quantity is defined as the sum of weighted equivalent doses in various organs and/or tissues, and is given by E where HT is the equivalent dose in the tissue T and WT is the weighting factor for each respective tissue. Since the effective dose cannot be measured directly, the International Commission on Radiation Units and (1) * ** This work was carried out at the Japan Atomic Energy Research Institute (JAERI), Tokai Research Establishement, Japan. Center for Standardization and Radiation Safety Research, SATAN 9

2 Measurements (ICRU) has introduced the operational quantities for area monitoring - H*(lO), H'(0.07,a) or H'(3,a), and for individual monitoring - Hp(lO), Hp(0.07) or Hp(3) [2]. Notation a in the case of area monitoring is referred to as the direction of the incident radiation field. For the individual monitoring quantities, the ICRU has given extensive conversion coefficients relating the personal dose equivalent to air kerma, exposure and fluence [3]. These coefficients, however, are applied only for personal dosemeters facing a uniform radiation field, whereas the actual exposures are mostly not aligned and expanded as required. This results in a problem of how to relate the personal dosemeter reading to individual monitoring quantity. AUSTERLITZ et.al [4] have previously studied the relationship between the effective male dose equivalent and readings of personal dosemeters worn at nine locations on a computational model. In their work, conversion coefficients for a wide range of photon energies were calculated applying the modified GSF-Adam computer code [5]. In the case of diagnostic radiology fluoroscopy, FAULKNER and MARSHALL [6] have measured the ratio of effective dose to film badge dosemeter reading. The use of a lead apron in diagnostic radiology causes some difficulties in interpreting the effective dose from a single personal dosemeter, so that their study suggested that in the future the use of two personal dosemeters, instead of one, is preferred. The objective of this study is to investigate the relationship between personal dosemeter reading and the effective male dose (equivalent) for dosemeters worn at five locations on the skin surface of the trunk. The effect of skin-dosemeter distance and the angular response of dosemeters were also investigated. Comparisons were made between experiment and computer simulation. MATERIALS AND METHOD A realistic body phantom and a computational model were used in this work. A brief description of the realistic body phantom is presented, while a complete description of the computational model has been reported elsewhere [7]. The realistic body phantom The realistic body phantom used in this study is for an adult Japanese. The assembly consists of an artificial skeleton embedded in a flexible plastic-based tissue substitute. Simulated internal organs (heart, lung, stomach, intestine, testis and breast) are included. The overall height of the phantom is 168 cm with a mass of 63 kg. In this study, however, 10

3 the breast was not attached to the trunk, since the object was for a male. Handling and use of dosemeters Phosphate glass dosemeters of GO-403 type were used throughout this study. These dosemeters have shown an excellent longterm stability with a measurement accuracy in the order of 2% for ambient temperatures up to 40 C during irradiation [8]. The GO-403 type glass dosemeter consists of an RPL glass card with four fluorescence detection positions, a capsule of 25 mm x 60 mm x 9 mm to accommodate the glass card, and a capsule holder (see Figure I). The holder, however, was not used during the experiment. All dosemeters received a pre-experiment calibration with a 137Cs gamma source to determine the calibration factor for each dosemeter. The dosemeters were mounted on the surface of a slab phantom of 40 cm x 40 cm x 15 cm and exposed with a dose of 6 msv in term of H* (10). Before reading was taken, all dosemeters were pre-annealed for 30 minute at 100 C for buildup and kept in room temperature for 2 hours. The read out were carried out by an FGO 502 reader. Post-annealing for 12 hours at 400 C was also performed to eliminate the residual dose information before the dosemeters were exposed. The next irradiation was carried out after the dosemeters were cooled down in room temperature for at least 12 hours. In this study, the effect of dosemeter location, the effect of variation of skin-dosemeter distance and the angular dependence of dosemeter were investigated. Table I lists the coordinates for the positions of the dosemeters at five locations in the realistic body phantom. The origin of the coordinate system was taken the same in this body phantom as in the computational model, i.e., at the center of the base of the trunk. The x-axis was directed to the left from the center of the phantom, the z-axis was directed up to the head and the y-axis was to the back (see Figure 2). Table 1 also lists the coordinates for the positions of the dose meters for computational model. The differences of the y-axis for the computational model compared to the body phantom were due to the thickness of the dosemeter which were taken into account in the model. Irradiation conditions A 137Cs gamma source of 740 GBq (20 Ci, August 1979) was used throughout the experiment. The source-skin distance was set to be cm to get a dose equivalent of 6 msv in the skin. To study the effect of horizontal angular dependence of dosemeter, the phantom was rotated clockwise in increments of 45, whereas the effect of vertical 11

4 angular dependence was investigated by rotating the dosemeter against the phantom in increments of 30. An isotropic 137Cs parallel beam source was assumed for simulating phantom irradiation in the computational model. Calculations were performed for whole-body irradiation and anterior-posterior (AP) geometry Personal dosemeter reading and the effective male dose Personal dosemeter reading is the dose equivalent measured at a certain position of the dosemeter, or the dose equivalent for soft tissue in the case of calculation. The quantity used in the dose equivalent in sievert (Sv), and denoted by R. The mathematical model available has three types of model : male, female and unisex. In this study only the male model was used so that the quantity calculated is called effective male dose equivalent denoted by HEm [10], or effective male dose (Em) [I]. In this context, the tissue weighting factors used slightly differ from those recommended by the ICRP (see Table 2). The relationship between R and HEm or Em is given by: f ~ or = HEm R f = Em (2) where f is the rati,) of personal dosemeter reading to the effective male dose equivalent or to the effective male dose. Error propagation The errors associated with the measurement and calculation were derived from the coefficient of variation. The propagation of error was treated by the method described by KNOLL [9]. For the measurements using the phosphate glass dosemeter, the uncertainty in personal dosemeter reading was in the range of %, the highest coefficient of variation value was detected for dosemeter positioned at 9 mm skin-dosemeter distance. The uncertainty in the calculation generally was within ± 1.5%. RESUL TS AND DISCUSSION The personal dosemeter response to the effective dose equivalent (R/HEm) and to effective dose (RlEm) is given in Figure 3. It shows the skin-dosemeter distance dependence of the response at five locations when the model was irradiated at em of source-to-skin distance with AP geometry. From Figure 3 it can be seen that R/Em is always higher than RlHEm. As mentioned by ZANKL et.al [11], the differences were mostly due to 12

5 the modified weighting factor of the remainder tissue. The weighting factor for remainder tissue in the latest ICRP recommendation [1] was reduced by a factor of 6 compared to the previous figure [10] (see Table 2), as well as its dose contribution is higher to HE than to E since it is averaged over the maximum five organs doses in case of HE and over ten fixed organs in case of E. Figure 3 also shows that the value recorded at the left breast pocket gives the best estimate of both Hm and Em. This can be compared with the results obtained by AUSTERLITZ et.al [4] who found those on the pocket and wrist locations result in the best approximation of the HEm. In all cases, the ratio of RlEm differs by a factor of 1.03 compared with RlHEm, whereas the personal dosemeter reading was reduced by a factor of 0.07 from the skin to the SO mm skin-dosemeter distance. The reduction is thought to be due to the backscatter from the body which becomes less as the dosemeter moves away from the skin. Figure 4 shows the personal dosemeter reading as a function of skin-dosemeter distance. While the calculated result, as it might be expected, showed a decrease of the relative dose as the distance increases, the result of measurement showed a completely different pattern. The increase in the relative dose with the skin-dosemeter distance in case of measurement seems to be caused by the fact that the phantom did not give much scattering, and the law of inverse square concerning with the dose was dominantly applied. The examination of the validity of the law reveals that all the measured results differ only by % from the calculation applying the inverse square law. Figure S shows the angular horizontal response of personal dosemeter. It is worth to mention that the objective of this study of angular response was not to evaluate the response of Toshiba GD-403 phosphate glass dosemeters used, but to evaluate the response of personal dosemeters worn at the realistic body phantom. The back location in Figure S is almost in the same axis as the middle of the chest, except that it was located at the back of the body (x = 0, z = 37, y = -14). As it is clearly seen from Figure S, the overall pattern of angular horizontal response for the back location was the same as for left breast pocket, middle chest and lower chest locations. The response at 180 for left breast pocket was found to be higher than that for middle chest and lower chest, and this is thought to be due to smaller linear attenuation coefficient of the lungs compared with soft tissue and bone. The phantom used in the experiment was held by a four-leg stand. It is interesting to note that the responses at 4So and 90 are always higher than those at 3ISo and 270, respectively, and the response at 4So itself 13

6 is also higher than that at 90. This is suspected to be due to the front leg of the stand on the right side of the phantom when it was irradiated from the front, which contributed more scatters than that on the left side. The above result was also observed with the dosemeter worn at left breast pocket, where the response at 90 was much higher than that at 270. The left arm is also thought to give and additional scatter. For dosemeters worn at the back, similar results were observed. However, in this case, the responses at 315 and 270 were higher than those at 45 and 90 respectively, and the response at 270 was higher than that at 315. By assuming that the responses at 315 and 270, or responses at 45 and 90 in the case of back location, were in a scatter-free condition, it can be concluded that the responses of personal dosemeters are at best when worn at the middle chest and at worst when worn at left breast pocket. The angular vertical response of personal dosemeter given in Figure 6 simulates a condition where the bottom part of a dosemeter is moving away from the body. This condition occurs frequently when the dosemeters are worn at a loose pocket. At the worst condition of 90, the response was only decreased by less than 10% relative to 0. Therefore, the angular vertical movement of the dosemeter has little effect to the response. CONCLUSIONS The relationship between personal dosemeter reading, R, and the effective male dose (equivalent), Em (HEm), was studied by means of a mathematical model developed in JAERI. In addition, two characteristics of personal dosemeter worn at a realistic body phantom, i.e, the effect of skin-dosemeter distance and the effect angular response to the dosemeter, were also investigated. The calculation on the relationship suggests that the left breast pocket location give the best estimation of the effective male dose (equivalent). However, at the same time, actual measurement revealed that the same location had the worst angular horizontal response. Further study is then required to find the optimum condition where the estimation is closest and the angular movement of the dosemeter does not affect the response. The effect of skin-dosemeter distance was studied experimentally and computationally by moving the dosemeters away from the skin of the phantom and the model. Since the results indicated a clear contradiction, i.e., experiment showed an incerase of relative dose with distance, whereas calculation showed a decrease, further study is then needed to clarify the result. The angular horizontal response of dosemeters studied using a realistic 14

7 body phantom showed that the response was at the best when a dosemeter was worn at middle chest, and at the worst when worn at left breast pocket. The angular vertical response, however, showed no substantial variation even for 900 rotation of the dosemeter. The present study was only applying a 137Cs gamma source. It might be interesting to investigate how the results will vary with the energy of the source. Further study applying a wide range of gamma energies is strongly suggested. ACKNOWLEDGMENTS The author wishes his thanks to Mr. Y. Yamaguchi and Mr. F. Takahashi of JAERI for their valuable advice and help. The work was carried out under Science and Technology Agency (Japan) fellowship. REFERENCES 1. ICRP, "1990 Recommendations of the International Commission on Radiological Protection". ICRP Publication 60. Pergamon Press, Oxford (1991) ICRU, "Determination of Dose Equivalents from External Radiation Sources - Part 2". ICRU Report 43. ICRU, Bethesda (1988) ICRU, "Measurement of Dose Equivalents for External Photon and Electron Radiations". ICRU Report 47. ICRU, Bethesda (1992) AUSTERLITZ, C., KAHN, B., EICHHOLZ, G.G., ZANKL, M. and DREXLER, G., "Calculation of the Effective Male Dose Equivalent Relative to the Personal Dose at Nine Locations with a Free-Arm Model". Radiat.Prot.Dosim. 36 (1) (1991) KRAMER, R., ZANKL, M. WILLIAMS, G. and DREXLER, G., "The Calculation of Dose from External Photon Exposures Using Reference Human Phantoms and Monte Carlo Methods, Part I : The Male (Adam) and Female (Eva) Adult Mathematical Phantoms", GSF- Bericht S-885. Neuherberg, GSF (1984) 6. FAULKNER, K. and MARSHALL, N.W., "The Relationship of Effective Dose to Personnel and Monitor Reading for Simulated Fluoroscopic Irradiation Conditions", Health Phys. 64 (5) (1993) YAMAGUCHI, Y. DEEP Code to Calculate Dose Equivalents in Human Phantom for External Photon Exposure by Monte Carlo Method. JAERI-M JAERl, Japan (1991) 15

8 8. PIESCH, E., BURGKHARDT, B. and VILGIS, M., "Progress in Phosphate Glass Dosimetry: Experiences and Routine Monitoring with A Modern Dosimetry System", Radiat. Prot.Dosim. 47 (1/4) (1993) KNOLL, G., "Radiation Detection and Measurement", 2nd Ed. John Wiley: New York (1989) ICRP, "Recommendations of the International Commission on Radiological Protection", lcrp Publication 26. Pergamon Press, Oxford ( 1977) 11. ZANKL, M., PETOUSSI, N. and DREXLER, G., "Effective Dose and Effective Dose Equivalent : The Impact of the New ICRP Definition for External Photon Irradiation", Health Phys. 62 (5) (1992)

9 Table I. Locations of the personal dosemeter.), '7 zyz '7 () 9 0 y () 0 10 x 0 () Body () 0 Computational phantoma Coordinates modelb -8 a : y coordinate 0 represents surface, whereas or 9, 26 and 49 represent skin-dosemeter distances in mm. b : y coordinate 0.25 represents surface, whereas of 1.25, 3.25 and 5.25 represent skin-dosemeter distances of 10, 30 and m,respectively. 17

10 Table 2. Tissue weighting factors used for computational model 1 - factors ICRP This study (testes) 0.12 Weighting (CRP 1977 (red) In the model it is called GI LLI. 2 In the model it is called skeleton. 18

11 Binary code ~ Capsule holder f)-window Glass card '"[-window Capsule Figure I. Sturcture of GD-403 phosphate glass dosemeter \ 1] '~'" -. \ ~ X Figure 2. Coordinate system in the computational model 19

12 1.134 (a) R/HEm '.~', (a) R/Em (b) R/HEm (b) R/Em (c) R/HEm (c) RlEm " (d) R/HEm (d) R/Em (e) R/HEm 1.07~ i I I I 1.05 I._~, --~ _. I o ~ (e) R/Em Skin-dosemeter distance (mm) Figure 3. Personal dosemeter reading per effective male dose equivalent, RJHEm, and per effective male dose, R/Em calculated at (a) left breast pocket, (b) upper chest, (c) middle chest, (d) right waist and (e) lower chest. 20

13 1.1 ~------_._--~ Q), I (a) 1.05J I (b) (d) (c) (e) Meas. Calc (a) Calc : cu c0"0 Qj./.. E - (c) Calc. c Q) r--~ ,, o Skin-dosemeter distance (mm). Figure 4. Personal dosemeter reading relative to reading on skin surface measured and calculated at (a) len breast pocket, (b) upper chest, (c) middle chest, (d) right waist and (e) lower chest, 21

14 Figure 5. Angular horizontal response relative to 0 measured at (a) left breast pocket. (b) middle chest. (c) lower chest and (d) back. Figure 6. Angular vertical response relative to 0 measured at (a) left breast pocket, and (b) middle chest. 22