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A User-Friendly Thyroid Monitor

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A User-Friendly Thyroid Monitor P-3a-173 A User-Friendly Thyroid Monitor Herman Cember & Wei-Hsung Wang Purdue University, School of Health Sciences, 1338 Civil Engineering Building, West Lafayette, IN 47907-1338, USA ABSTRACT Good radiation safety practice includes routine thyroid monitoring for workers who are occupationally exposed to iodine-131. This work describes the design, construction, and calibration of a simple and reliable thyroid monitoring system. This system consists of a single channel analyzer and a sodium iodide detector mounted to assure a fixed and reproducible counting geometry. The system was calibrated using a mock thyroid into which a known iodine-131 activity was injected. The mock thyroid was made by inserting one thyroid-lobe shaped sponge into each of two glass vials. The two vials were capped and placed into a neck phantom. At a 5 cm “skin” to detector distance, the lower limit of detection was about 142 Bq (3.84 nano-curie). This detector system provides an easy, economic, and effective means to measure the internal radioiodine deposition in radiation workers and also meets both the regulatory standards and the good health physics practice. INTRODUCTION Iodine-131 (I-131) is widely used in the application of health physics, nuclear medicine, veterinary science, biology, and chemistry. It is also a significant fission product in a nuclear power plant. Because of its high volatility under room temperature, iodine can easily be inhaled by radiation researchers and workers due to an unknown or subtle release of radioiodine to the atmosphere. Healthy subjects accumulate about 30% of the inhaled iodine in the thyroid gland which is the critical organ for iodine (1). The thyroid gland is subject to radioiodine carcinogenesis (2). The United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) report suggested that a total risk of radiation induced thyroid cancer is 0.0004 per year per gray (3). Hence, the most efficient way to determine the internal deposition of iodine-131 is to perform in vivo measurement of the iodine-131 thyroid burden, and thus to prevent overexposure. According to the United States Nuclear Regulatory Commission's Title 10 Code of Federal Regulation (CFR) Part 20.1502, it is required to monitor the occupational intake of radioactive material for radiation workers if they are likely to receive a yearly intake in excess of ten percent of the applicable annual limit on intake (ALI), as listed in Appendix B, 10 CFR 20. The inhalation ALI for I-131 is 50 µCi and the corresponding derived air concentration (DAC) is 2 x 10-8 µCi/mL. DAC means the concentration of a given radionuclide in air which, if breathed by the reference person for a working year of 2,000 hours under conditions of light work, results in an intake of one ALI. Kopp et al. (4) suggested using a high-purity germanium (HPGe) detector system for routine thyroid measurements of radioiodine in radiation workers. The cost of an HPGe detector system is much higher than a sodium iodide (NaI) scintillator monitoring system. The HPGe detector system has poorer counting efficiency than a NaI scintillator monitoring system. Besides, the HPGe detector must be operated at low temperature of 77 K (5). Tran (6) proposed to use two collimated 2" x 2" NaI detectors and a Canberra1 System 100 Multi-Channel Analyzer to calibrate the I-131 uptake in the thyroid in a neck phantom provided by the Canadian Bureau of Radiation and Medical Devices. This detector system costs about $5,500 without the computer related expense. The report presented here describes a simpler user-friendly thyroid monitor. The monitor consists of an ORTEC2 single channel analyzer (SCA) with a 1.25" diameter x 5/16" thick NaI detector that is calibrated for the assessment of I-131 uptake in the thyroid gland of radiation workers. This NaI scintillation counting system costs less and can also be applied to various other practices. This monitoring device provides an effective and economic way to demonstrate compliance with the regulatory limits. It also accomplishes good health physics practice to use radiation sources safely. MATERIAL AND METHODS COUNTING SYSTEM Scintillation counters have widespread uses in measuring gamma rays and low-energy beta rays. The NaI scintillator has a high sensitivity for the detection of gamma rays. When the analyzer of the scintillation counting system is in the differential mode, it operates as a single channel analyzer and allows only pulses of a certain size to be counted. The essentials of a single channel analysis system consist of a detector, a high voltage power supply, a linear amplifier, a pulse height analyzer, and a readout device (timer and counter). The main use of the SCA is to optimize the signal to noise ratio when measuring a low radioactivity source in the presence of a significant background (7). 1 Canberra Industries, 800 Research Parkway, Meriden, CT 06450 2 EG&G ORTEC, 100 Midland Road, Oak Ridge, TN 37830 1 P-3a-173 The sodium iodide counting system uses an ORTEC SCA to electronically process the signals from the iodine-131 radioactivity in the thyroid. A Harshaw3 1.25" diameter x 5/16" thick NaI detector which can measure the radioactivity at levels well below legal limits was utilized. Iodine-131 emits several photons of different energies. In this case, we counted the signals from the 364 keV photons with a radiation abundance of 82% (8). The use of the SCA effectively reduced the background count rate to a very low level relative to the I- 131 activity in the thyroid at levels of interest to the health physicist. PREPARATION OF THE COUNTING SYSTEM 1. Determination of the Operating Voltage Most photomultiplier (PM) tubes commonly used with a NaI crystal operate well in the 800-volt to 1,100-volt range. The appropriate high voltage setting is obtained by plotting the integral count rate vs. high voltage with the analyzer in the integral mode. In the integral mode, all pulses greater than a given size are counted. As a good rule of thumb, the operating voltage should be selected relatively close to the threshold voltage (within the lower 25% of the plateau). Since the plateau curves for different gamma-emitting nuclides vary somewhat, the operating potential must be determined individually for each isotope used (9). In order to obtain a good voltage curve, at least 30 minutes should elapse between successive increases in voltage to allow the PM tube to stabilize. This stabilization period helps minimize the variations in output voltage and ambient temperature due to the voltage shifts in the PM tube. The operating voltage should be chosen to avoid the rapidly rising thermal noise level at the higher voltages and to help preserve the life of the PM tube. The operating voltage is determined on statistical grounds as that voltage where the ratio of the square of the gross sample count rate to the total background count rate is at maximum (10). Results from this procedure are shown in Table 1 and Figure 1. From the data on Table 1, 925-volt was selected as the operating voltage. Voltage (volts) Gross (cpm) BKG (cpm) Net (cpm) (Gross)2/BKG 700 2378 34 2344 166320 725 10318 35 10283 3041746 750 118016 58 117958 240134073 775 144438 65 144373 320959013 800 163892 88 163804 305233951 825 191434 101 191333 362841350 850 208694 105 208589 414792244 875 222854 113 222741 439503587 900 243678 132 243546 449840664 925 254594 138 254456 469696412 950 260333 145 260188 467401868 975 267997 154 267843 466379169 1000 277738 166 277572 464689136 1025 286520 178 286342 461200620 1050 291948 189 291759 450971612 1075 296926 203 296723 434310589 1100 301490 210 301280 432839143 * Gross: gross sample counting rate in counts per minute ** BKG: background counting rate in counts per minute *** Net: net counting rate in counts per minute = (Gross - BKG) Table 1. Determination of the Operating Voltage 3 Harshaw Chemical Company, Cleveland, Ohio 2 P-3a-173 350000 Net Counting 300000 250000 200000 150000 100000 50000 counting rate (cpm) 0 0 200 400 600 800 1000 1200 voltage (volts) Figure 1. Plot of Integral Count Rate vs. High Voltage 2. Energy Calibration An SCA must be calibrated before performing any sorting of the pulse sizes produced in the analyzer. The 364 keV gamma photon from I-131 was chosen for the calibration. A window can be established using the base and upper discriminators of the analyzer. This window must be wide enough to allow a narrow range in pulse sizes to get through, but not too wide or the calibration will be inaccurate. A window whose width is about 5 - 10% of the full energy peak is usually acceptable. This window should be placed on the division scale of the base discriminator so that a division will correspond to a gamma ray with a given energy. For this experiment, the base discriminator was set at 354 and with a 20-division window and so the center was at 364. With this analyzer setup, pulses corresponding to energies between 354 and 374 keV would pass through this window. After the 364 keV photopeak had been defined, a calibration plot of I-131 was also established to provide the complete information on the shape and position of the photopeak and help confirm the discriminator settings. The amplifier's coarse and fine gains were adjusted to maximize the counting rates in the 354 - 374 keV channel. The data and the calibration plot of I-131spectrum are shown in Table 2 and Figure 2. Base Gross BKG Net Base Gross BKG Net 14 18857 25 18832 414 2898 1 2897 34 11537 11 11526 434 1412 1 1411 54 8568 5 8563 454 775 4 771 74 13818 12 13806 474 503 1 502 94 8486 18 8468 494 308 2 306 114 6212 6 6206 514 186 3 183 134 7251 6 7245 534 142 2 140 154 8853 6 8847 554 138 1 137 174 8415 5 8410 574 286 0 286 194 6410 10 6400 594 511 2 509 214 4184 2 4182 614 660 2 658 234 2948 6 2942 634 581 0 581 254 3642 6 3636 654 404 0 404 274 4694 3 4691 674 263 3 260 294 4399 2 4397 694 213 2 211 314 6859 3 6856 714 163 0 163 334 16727 4 16723 734 112 0 112 354 24660 2 24658 754 79 1 78 374 17493 3 17490 774 47 0 47 394 7664 2 7662 794 36 1 35 * Base: base discriminator setting ** All counting rate measurements are in counts per minute *** Instrument settings: high voltage = 925 volts; coarse gain = 1K; fine gain = 1.24 Table 2.
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