TY - JOUR
AU - Luo, Hailong
AB - Abstract Tissue-equivalent proportional counter (TEPC) is a very promising dosemeter for radiation protection in mixed radiation fields. In this article, a portable TEPC system including a spherical homemade TEPC detector and the compact low-power integrated circuit prototype was constructed and tested in gamma and neutron radiation fields. The microdosimetric spectra results displayed that the portable TEPC system could deal with four orders of magnitude voltage pulse signals. And the measured dose equivalent values agreed with reference values, which means the portable TEPC system had fulfilled the test and satisfied the requirement for radiation monitor. INTRODUCTION In previous work(1), a cylindrical tissue-equivalent proportional counter (TEPC) detector was constructed and tested in laboratory. Considering the portability and compactness of dosemeters for monitoring radiation absorbed dose (definition in reference(2, 3)) or dose equivalent (definition in reference(4)) in outer space or around the locale of nuclear facilities, it is necessary to develop a portable TEPC system such as πDOS TEPC used by Wissmann et al.(5, 6), or commercially available portable TEPC system named as HAWK(7, 8). For the LET spectrum measurement in low earth orbit, the most preferred active dosemeter is Johnson Space Center (JSC) TEPC(9, 10). In this work, a portable TEPC system was developed to achieve the property of HAWK system to monitor ambient dose equivalent in mixed fields of nuclear power plant or around the other nuclear facilities. The portable TEPC system consisted of two parts: (1) a spherical TEPC detector, whose response isotropy was better than the right cylindrical detector constructed in previous work; and (2) the coupled integrated circuit (IC) prototype, which can process four orders of magnitude voltage pulse signals according to the measurement range of TEPC detector (~0.4–1000 keV/μm). DESIGN AND TEST OF TEPC DETECTOR Configuration of TEPC detector If there are no field tubes to correct electric field in a proportional counter, distortions always appear at the ends of anode wire because of the wire support and insulator. So, one kind of spherical structure of TEPC detector named as Benjamin type was designed to keep the electric field constant along the length of anode by balancing the cathode diameter and the support diameter at the ends of anode wire(11), whose response isotropy was also verified. A spherical cathode structure similar to the Benjamin detector has been designed. The materials and size of detector components are listed in Table 1. As illustrated in Figure 1, the spherical sensitive volume of TEPC detector is circled by conductive TE plastic as cathode, which could be connected to ground or negative high voltage. An aluminum shell surrounds the TE plastic to provide electrostatic shielding and serves as a vacuum container. There is a valve to control the gas inlet/outlet. TE gas based on propone (C3H8 54.89%, CO2 39.6%, N2 5.51%) flows into Al shell and then enters into the sensitive cavity through the gas holes on the cathode wall (not shown in Figure 1). The gas pressure should be determined according to simulated site size and the temperature. For example, the gas pressure is 1015.7 Pa at 293.15 K for 2 μm site. Table 1. The materials and size of detector components.   Material  Size  Cathode shell  TE plastic (A-150)  Inner diameter (11.13 cm), thickness (0.3 cm)  Anode wire  Gold-plating tungsten wire  Diameter (60 μm)  Wire support  TE plastic (A-150)  Diameter (1.27 cm)  Insulator  Polyethylene  Diameter (2.86 cm)  Vacuum container  Aluminum  Diameter (13 cm), height (20 cm), thickness (0.16 cm)    Material  Size  Cathode shell  TE plastic (A-150)  Inner diameter (11.13 cm), thickness (0.3 cm)  Anode wire  Gold-plating tungsten wire  Diameter (60 μm)  Wire support  TE plastic (A-150)  Diameter (1.27 cm)  Insulator  Polyethylene  Diameter (2.86 cm)  Vacuum container  Aluminum  Diameter (13 cm), height (20 cm), thickness (0.16 cm)  Table 1. The materials and size of detector components.   Material  Size  Cathode shell  TE plastic (A-150)  Inner diameter (11.13 cm), thickness (0.3 cm)  Anode wire  Gold-plating tungsten wire  Diameter (60 μm)  Wire support  TE plastic (A-150)  Diameter (1.27 cm)  Insulator  Polyethylene  Diameter (2.86 cm)  Vacuum container  Aluminum  Diameter (13 cm), height (20 cm), thickness (0.16 cm)    Material  Size  Cathode shell  TE plastic (A-150)  Inner diameter (11.13 cm), thickness (0.3 cm)  Anode wire  Gold-plating tungsten wire  Diameter (60 μm)  Wire support  TE plastic (A-150)  Diameter (1.27 cm)  Insulator  Polyethylene  Diameter (2.86 cm)  Vacuum container  Aluminum  Diameter (13 cm), height (20 cm), thickness (0.16 cm)  Figure 1. View largeDownload slide The sketch of TEPC detector. Figure 1. View largeDownload slide The sketch of TEPC detector. TE plastic wall mold and assembly In order to fabricate ~5 inch spherical chamber of TEPC detector, it is needed to mold two big semicircle shells with granular TE plastic (A-150). There are two basic moulding processes that can be used, namely compression and injection moulding. The compression moulding was utilized in this work because of its technical simplification comparatively. The basic parts of a mold (Figure 1) are: base, inside form, outer body and the plunger. At the top of the outer body is a neck which is used to hold the excess granular plastic that does not fit inside the mold before moulding. In the compression process, the required amount of granular plastic is placed directly into the mold cavity. One end of the mold is open, and after the plastic is poured, a metal plunger is placed into that opening to seal it. After the mold reaches the proper temperature, the plunger is pushed into the mold body which forces the molten plastic to consolidate and take the shape of the mold. This pressure is held on the molten plastic, and the mold is allowed to cool to room temperature. The technical parameters such as temperature and pressure are controlled as recommendation in the reference(12). The whole assembly is heated to the plastic melting temperature at 152°C or 4 h till TE plastic should be shaped. The TE plastic moulding is not usually finished perfectly so that the finished mold parts may shrink and crack. After many tests and improvements, the moulding parts (Figure 2) are accomplished. Figure 2. View largeDownload slide TE plastic shells. Figure 2. View largeDownload slide TE plastic shells. In order to get a sphere chamber, two contact surfaces of semicircle shells were pressed together, and the leak was softened and amended by searing-iron. The polyethylene screw and Al clamp were used to fasten and fix the sphere on the bottom of vacuum container. There was no glue in assembly except sealing Al container with vacuum cement. The processes of cleaning, assembly and conditioning detector, such as outgassing, were implemented as recommended in reference(12). Calibration and test of TEPC detector The experimental method used to test the spherical TEPC detector is identical with previous work for a homemade cylindrical TEPC detector in reference(1). The TEPC electronic system in laboratory usually consists of a preamplifier (ORTEC 142PC), parallel two main amplifiers (ORTEC 572A), analog-to-digital converters (ADC FAST 7074) and multi-channel system (FAST MPA-3), which is illustrated in Figure 3. The 252Cf neutron source and 137Cs gamma source were all used to calibrate the measured linear energy spectra, namely the proton-edge 135 keV/μm (2 μm site) for the low-gain calibration and the electron-edge 10 keV/μm (2 μm site) for the high-gain calibration. The maximum deposited energy of proton was calculated by SRIM procedure according to the composition and density of TE gas in detector chamber. The experiments layouts are seen in Figures 4 and 5. Figure 3. View largeDownload slide Block diagram of the electronic system. Figure 3. View largeDownload slide Block diagram of the electronic system. Figure 4. View largeDownload slide Experiment layout in neutron fields. Figure 4. View largeDownload slide Experiment layout in neutron fields. Figure 5. View largeDownload slide Experiment layout in gamma field. Figure 5. View largeDownload slide Experiment layout in gamma field. The microdosimetric spectra measured in 137Cs reference radiation field were plotted in logarithmic linear energy ( y) intervals as shown in Figure 6. The microdosimetric spectra measured in 252Cf and 241Am–Be reference neutron radiation fields were plotted in logarithmic linear energy ( y) intervals as seen in Figure 7. Also, there was a measured microdosimetric spectrum of 241Am–Be neutron source enclosed in unknown capsule illustrated in Figure 9. In the region lower than 0.3 keV/μm, white noise and radiation particles were recorded meanwhile and could not be separated actually, which contributed to the detection threshold. This region below 0.3 keV/μm was the discrepancy of spectra measured by homemade detector with literature spectra(13) in the Figures 8 and 9. From the comparison in Figures 8 and 9, it can be seen that the measured spectra and literature data agreed well primarily. Concerning the 241Am–Be measured spectra in Figures 7 and 9, the main difference lied in the region 1–10 keV/μm due to the 59.5 keV gamma fraction which would be restrained by lead shield capsule when a neutron radiation source used as reference neutron radiation recommended in ISO 8529-1(3). Figure 6. View largeDownload slide Microdosimetric spectrum (2 μm site) measured in gamma fields. Figure 6. View largeDownload slide Microdosimetric spectrum (2 μm site) measured in gamma fields. Figure 7. View largeDownload slide Microdosimetric spectra (2 μm site) measured in neutron fields. Figure 7. View largeDownload slide Microdosimetric spectra (2 μm site) measured in neutron fields. Figure 8. View largeDownload slide 252Cf source microdosimetric spectra (2 μm site) comparison. Figure 8. View largeDownload slide 252Cf source microdosimetric spectra (2 μm site) comparison. Figure 9. View largeDownload slide 241Am–Be source microdosimetric spectra (2 μm site) comparison (including 59.5 keV gamma fraction). Figure 9. View largeDownload slide 241Am–Be source microdosimetric spectra (2 μm site) comparison (including 59.5 keV gamma fraction). In 252Cf and 241Am–Be reference radiation fields, the neutron absorbed dose and dose equivalent values should be estimated after the subtraction of gamma radiation contribution. The gamma contribution fraction was discriminated by normalization of the pure gamma ray spectrum (137Cs) to that of the mixed field(1, 14). And the pure neutron fraction spectra are shown in Figure 10.The dose equivalent rates were determined by the measured spectra as shown in Table 2. The TEPC results of dose equivalent rate were higher than reference values of ambient dose equivalent (definition in reference(4)) in neutron or gamma reference fields. So, TEPC measurements results should be revised with a factor in reference fields(6) for application in radiation protection. Table 2. Comparison of HTEPC measured values with H*(10) reference values (μSv/h).   252Cf  241Am–Be  137Cs  HTEPC  67  177  25  H*(10)  60  160  23  Relative deviation  11.7%  10.6%  8.7%    252Cf  241Am–Be  137Cs  HTEPC  67  177  25  H*(10)  60  160  23  Relative deviation  11.7%  10.6%  8.7%  Table 2. Comparison of HTEPC measured values with H*(10) reference values (μSv/h).   252Cf  241Am–Be  137Cs  HTEPC  67  177  25  H*(10)  60  160  23  Relative deviation  11.7%  10.6%  8.7%    252Cf  241Am–Be  137Cs  HTEPC  67  177  25  H*(10)  60  160  23  Relative deviation  11.7%  10.6%  8.7%  Figure 10. View largeDownload slide neutron fraction spectra (2 μm site) in neutron fields. Figure 10. View largeDownload slide neutron fraction spectra (2 μm site) in neutron fields. DESIGN OF IC PROTOTYPE In this work, a compact low-power IC prototype for the portable TEPC system (Figure 11) was developed, which replaced the separated electronic system. Generally, as illustrated in Figure 12, the pulse signals from proportional counter needed to be shaped and amplified by amplifiers, then processed and kept in storage by IC device or transmitted to personal computer by RS-232 serial port, or transformed into absorbed dose (or dose equivalent) values displayed on LCD screen. The spectra data would also be stored in the flash storage chip and read in computer. Figure 11. View largeDownload slide A portable TEPC system photo. Figure 11. View largeDownload slide A portable TEPC system photo. Figure 12. View largeDownload slide Block diagram of IC prototype. Figure 12. View largeDownload slide Block diagram of IC prototype. The primary concern on IC design was to amplify and shape the voltage pulse signals from TEPC detector. According to the characteristics of pulse signals from TEPC, a part of signals was near to noise level and could not be separated clearly. So the design of preamplifier unit was a key to analog circuit design, which was concerned on low noise, high anti-interference performance, low power and so on. The charge sensitive preamplifier (A250 AMPTEK Inc.) was chosen for a wide range of detectors including silicon, solid state detectors, proportional counters and others. Meanwhile, to permit optimization for a wide range of application, the input field effective transistor was external to the package and user selectable such as 2sk152 or 2N4416. The pulse signals from the preamplifier unit were alternating current coupled to post amplifier with a pole-zero differentiator. Because of the broad coverage of TEPC signals, the pulse from preamplifier would be continually amplified by two different gains, namely low-gain amplifier for the range of 2–1000 keV/μm and high gain two amplifiers for the range of 0–20 keV/μm. Then the pulse peaks would be kept respectively and lasted for enough time by peak-hold units to be collected and converted into digital signals by ADC unit. Finally, the pulses calibrated with a factor would be transformed into absorbed dose (or dose equivalent) rate values by micro-programmed control unit (MCU). In order to achieve low power consumption, the power management should be considered throughout the whole system design, including the choice of low power consumption chips, high efficiency voltage conversion, intelligent battery solution, the selection of ultra-low power MCU and so on. TEST OF THE PORTABLE TEPC SYSTEM Following the preparation of IC prototype enclosed in a Al alloy box (inner 16 cm × 16 cm × 18 cm), the TEPC detector was fixed on the box top panel. The calibration of portable TEPC system is similar to the method described in segment 1.3. As illustrated in Figure 13, microdosimetric spectrum was measured by this portable TEPC system in 241Am–Be neutron reference radiation field. Experimental results verified that this IC device was adequate to deal with 3–4 orders magnitude voltage signals from TEPC detector. Comparison the 241Am–Be neutron spectrum curve in Figure 13 with that in Figure 7 displayed a discrepancy in the region below 1 keV/μm because of the higher noise level. So far, the reason for higher noise level of IC prototype mainly lied in the filter of the high voltage supply module still needed to upgrade. The contribution of this low linear energy region to dose equivalent value was little, so after calibration in reference radiation fields, it can be used as a radiation monitor in neutron and gamma mixed radiation fields. Figure 13. View largeDownload slide Spectrum (2 μm site) measured in 241Am–Be field. Figure 13. View largeDownload slide Spectrum (2 μm site) measured in 241Am–Be field. CONCLUSIONS A portable TEPC system consisting of a detector and the IC prototype has been developed and tested in neutron and gamma fields. According to the measurements microdosimetric spectra and dose equivalent values, the results showed that the portable TEPC system accomplished the initial object to be a practical dosemeter in mixed fields. The improvement of IC prototype is still ongoing to restrain the noise level and expand the detection lower limit. REFERENCES 1 Zhang, W., Wang, Z., Liu, Y., Li, C., Xiao, X., Luo, H., Chen, J. and Li, W. TEPC performance for a reference standard. Radiat. 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TI - CONSTRUCTION AND TEST OF A PORTABLE TISSUE-EQUIVALENT PROPORTIONAL COUNTER SYSTEM
JF - Radiation Protection Dosimetry
DO - 10.1093/rpd/ncx214
DA - 2018-04-01
UR - https://www.deepdyve.com/lp/oxford-university-press/construction-and-test-of-a-portable-tissue-equivalent-proportional-bvoXP20D1T
SP - 95
EP - 100
VL - 179
IS - 1
DP - DeepDyve
ER -