TY - JOUR
AU1 - Morishita, Yuki
AU2 - Yamamoto, Seiichi
AU3 - Momose, Takumaro
AU4 - Kaneko, Junichi H
AU5 - Nemoto, Norio
AB - Abstract Plutonium dioxide (PuO2) is used to fabricate a mixed oxide fuel for fast breeder reactors. When a glove box containing PuO2 fails, such as by rupture of a glove or a vinyl bag, airborne contamination of plutonium (Pu) can occur. If a worker inhales PuO2 particles, they will be continually irradiating their lung tissue with alpha particles, and this could cause lung cancer. The nasal smear and nose blow methods are useful for checking workers for PuO2 intake in the field. However, neither method can evaluate the quantitative activity of Pu. No alpha-particle detector that can be used for direct measurements in the nasal cavity has been developed. For direct and quantitative measurement, it is required that a shape of the detector should be a fine bar which inserts itself in the nose to measure the accurate activity of Pu. Therefore, we developed a nasal monitor capable of directly measuring the activity of Pu in the nasal cavity to estimate the internal exposure dose of a worker. Prismatic-shaped 2 × 2 acrylic light guides were used to compose a detector block, and a ZnS(Ag) scintillator was adhered to the surface of these light guides. Silicon photomultiplier (SiPM) arrays with 8 × 8 channels were used as a photodetector. Actual PuO2 particles were measured using the nasal monitor. The nasal monitor could be directly inserted in the nasal cavities, and the activity distribution of Pu was obtained by the nasal monitor. The average efficiencies in 4-pi were 11.4 and 11.6% for the left and right nasal cavities, respectively. The influence of gamma and beta rays from Cesium-137 (137Cs) Strontium-90 (90Sr) on the detection of the alpha particles of Pu was negligible. The difference in the measured Pu activity between the ZnS(Ag) scintillation counter and the nasal monitor was within 4.0%. Therefore, it was considered that the developed nasal monitor could be used in direct Pu determination to estimate the internal exposure dose of workers. INTRODUCTION Plutonium dioxide (PuO2) is used to fabricate a mixed oxide fuel for fast breeder reactors(1). PuO2 is handled in a glove box to prevent it from leaking into the working environment(2, 3). When a glove-box failure occurs, such as by rupture of a glove or vinyl bag, Pu could be released as an airborne contaminant(4, 5). If the worker is not wearing a protective mask, this worker will inhale PuO2 particles. When deposited in the lung, these particles will continually irradiate the lung tissue with alpha particles, and this could cause lung cancer(6). Therefore, when Pu is released as an airborne contaminant, workers should be checked immediately for possible inhalation of Pu. A nasal smear was a useful method to test for the intake of PuO2 in the field(7). In this method, a nasal swab was used to obtain PuO2 from the surface of the nasal cavity, and an alpha counter was used to measure the alpha particles emitted from PuO2 on the swab. If the measured alpha count is significantly high, the worker has possibly inhaled PuO2. The nasal smear enables rapid estimation of the intake of PuO2. Several studies have attempted to evaluate the internal exposure dose from nasal swab samples(8, 9). However, a disadvantage of the nasal smear method was that it could not quantify the activity of Pu because the pressure of the finger varied from one user to another. Hence, the removal factor of Pu represented an extreme value, and the internal exposure dose evaluated via the nasal smear method was overestimated. Nose blow was another onsite method to check for the inhalation of PuO2 by workers(10, 11). PuO2 was blown out of the nasal cavity and subsequently measured using an alpha counter. This method could remove ~19% on average of PuO2 from the nasal cavity(12), however, it will be easily changed from person to person. A silver doped zinc sulfide (ZnS(Ag)) scintillation counter (also called ZnS(Ag) survey meter) is capable of measuring the quantitative activity of Pu at a work site(13). However, this survey meter was designed to detect surface contamination on work clothes and equipment. Because its active area was ~70 cm2(14), the ZnS(Ag) scintillation counter was not appropriate for conducting measurements in the nasal cavity directly. No alpha-particle detector that can be used for direct measurements in the nasal cavity has been developed. Lung monitoring and bioassays could be used to evaluate the activity of Pu inside a worker’s body. In lung monitoring, a CsI(Tl)–NaI(Tl) phoswich detector was used to measure ~17-keV L X-rays emitted from 239Pu under low-background conditions(15). However, the minimal detectable activity (MDA) of 239Pu using a lung monitor was limited to a few thousand Bq(15). In the bioassay method, a urine sample taken from a worker was measured using alpha spectrometry. The evaluation required a long time because of the complexity of the process and the long measurement time. Therefore, in this work, we developed a nasal monitor capable of directly measuring the activity of Pu in the nasal cavity to estimate the internal exposure dose of a worker. MATERIALS AND METHODS Nasal monitor It is required that nasal monitor has a function that measures its precise activity and distribution of Pu. That a shape of the detector enables to insert itself in the nose is favorable. The nasal monitor was composed of ZnS(Ag) scintillator, prismatic-shaped light guide and silicon photomultiplier (SiPM) arrays. Figure 1 shows a schematic drawing of the nasal monitor, and Figure 2 shows a photograph of the nasal monitor and an example of its use. A size of the prismatic-shaped light guide was 1.5 mm × 1.5 mm × 20 mm and 2 × 2 prismatic-shaped acrylic light guides (Acrylite No. 000, Mitsubishi Rayon Co., Ltd., Japan) were used to compose a detector block. A 3.25 mg/cm2 ZnS(Ag) scintillator was adhered to the surface of these light guides. The prismatic-shaped light guides were separated by a reflector (aluminum foil) to divide the scintillation light into four different positions. An aluminized Mylar was used as a reflector to shield external light. The bottom surface of the prismatic-shaped light guide was optically coupled to a 3-mm-thick acrylic light guide. The 3-mm-thick acrylic light guide was optically coupled to the surface of SiPM arrays (TSV-MPPC array, S12642-0404PA-50, Hamamatsu Photonics K.K., Japan) using silicone rubber (Shin-Etsu silicone, Shin-Etsu Chemical Co., Ltd., Japan). Alpha particles were irradiated onto the ZnS(Ag) scintillator to activate the scintillation light, which was transmitted to the SiPM array through the light guide. Two block detectors were prepared ~6 mm apart to allow them to be inserted into both nasal openings. The size of one channel of the SiPM array (3.0 mm × 3.0 mm) was larger than one channel of the nasal monitor (1.5 mm × 1.5 mm). Therefore, to divide each of channels of 2 × 2 of the nasal monitor, position calculation by using output signals of multiple SiPMs was required. The total channels of the SiPMs were 8 × 8. Figure 3 shows a block diagram of a data processing. Signals from the 8 × 8 SiPMs were fed to a weight-summing circuit that calculates the position signals of X+, X−, Y+ and Y−. These signals were transferred to analog to 100 MHz digital (A–D) converters. A center-of-gravity position was digitally calculated in a field-programmable gate array (FPGA) by Anger principle(16). Finally, information of the center-of-gravity position and energy were transferred to a personal computer (PC) via a LAN cable. The 2D distribution and energy of alpha particles was displayed by the PC. The voltage for the SiPMs was adjusted by −68.3 V. The temperature was monitored by a thermometer to compensate a gain in the circuit. Figure 1. View largeDownload slide Schematic drawing of the nasal monitor: cross-section (a) and top-view (b). Figure 1. View largeDownload slide Schematic drawing of the nasal monitor: cross-section (a) and top-view (b). Figure 2. View largeDownload slide Photograph of the developed nasal monitor (a) and an example of use (b). Figure 2. View largeDownload slide Photograph of the developed nasal monitor (a) and an example of use (b). Figure 3. View largeDownload slide Block diagram of a data processing. It is similar to a Positron Emission Tomography system(16). Figure 3. View largeDownload slide Block diagram of a data processing. It is similar to a Positron Emission Tomography system(16). 2D distribution of alpha particle A 2D distribution of alpha particle is useful to identify a location of Pu attached on the surface of the nasal cavity. To confirm the 2D distribution of the alpha particles, the alpha particles were uniformly irradiated to the nasal monitor. An alpha source of 241Am (727 Bq) which was collimated to 1 mmφ with 5.5 MeV alpha particle was then moved around the block detector, and the 2D distribution was obtained at each source position. Efficiency The efficiency of the nasal monitor needs to be determined to evaluate the activity of Pu in the nasal cavity. We evaluated the efficiency using a Pu point source with 19 Bq with 5.5 MeV (238Pu) and 5.2 MeV (239Pu and 240Pu) alpha particles. The point source was attached on a flat surface of the nasal monitor to measure the efficiency. The measurement was conducted three times, and each measurement was performed for 10 min. The efficiency was calculated from the measured count rate as follows:   Efficiency=Countpersecond(cps)Thesurfaceemissionrateofalphaparticles×100(%) (1) Also, the MDA was calculated using the following formulas(17):   ND=4.65NB+2.71 (2)  MDA=NDEfficiency(2−pi)×T×2(Bq) (3)where NB represents the background count and ND represents the detection limit. T represents measurement time in seconds. Efficiency for 2-pi direction is equal to half of efficiency for 4-pi direction(17). Each background measurement was conducted for 10 min, and three measurements were performed. Influence of gamma and beta rays When the nasal monitor is used for measurement of Pu activity in an MOX fuel fabrication facility, the influence of environmental background gamma and beta rays on the nasal monitor needs to be considered. A 137Cs source (54 Bq) with 662 keV gamma rays and a 90Sr source (48 Bq) with the maximum beta ray energy of 0.54 and 2.28 MeV (90Y, progeny of 90Sr) were used to irradiate gamma rays and beta rays to the nasal monitor from a distance of 2 cm. The measurement was conducted for 10 min and three times, and the average count rate and its standard deviation were evaluated. Actual measurements of PuO2 particles Two samples of PuO2 particles attached to a tape were prepared for the measurements of the nasal monitor. Figure 4 shows the distribution of PuO2 particles on samples taken by a ZnS(Ag) autoradiographic camera that combines a ZnS(Ag) scintillator with Polaroid film(18). One PuO2 particle was confirmed in Figure 4a; Figure 4b shows numerous PuO2 particles dispersed on the sample. These samples were wrapped around the nasal monitor and were measured for 5 min. The activity was calculated from the measurement count of the nasal monitor using the evaluated efficiency and its activity was compared with the activity measured using the ZnS(Ag) survey meter. Figure 4. View largeDownload slide Pu samples used for measurements of nasal monitor. These photos were taken by a ZnS(Ag) autoradiographic camera: a single PuO2 particle (a) and numerous PuO2 particles (b). Figure 4. View largeDownload slide Pu samples used for measurements of nasal monitor. These photos were taken by a ZnS(Ag) autoradiographic camera: a single PuO2 particle (a) and numerous PuO2 particles (b). RESULTS 2D distribution of alpha particle Figure 5 shows the 2D distribution of alpha particles when the nasal monitor was irradiated from all circumferences. The positions of each block detectors corresponding to each nasal cavities could be identified and four different positions in the block detector were clearly separated. Figure 5. View largeDownload slide 2D distributions of alpha particles for the left and right nasal cavities measured by the nasal monitor. The alpha particles were uniformly irradiated to the nasal monitor from all circumferences. Figure 5. View largeDownload slide 2D distributions of alpha particles for the left and right nasal cavities measured by the nasal monitor. The alpha particles were uniformly irradiated to the nasal monitor from all circumferences. Figure 6 shows the 2D distribution of the alpha particles radiated from different angles. The alpha source was moved around the block detector, which was irradiated with alpha particles from different angles. The results in Figure 6 confirm that the detected alpha-particle position corresponds to the source position. Figure 6 also shows the ratio of counts in four ROIs, which are set around four different positions. Figure 6. View largeDownload slide 2D distribution of the alpha particles measured by the nasal monitor and the ratio of counts in four ROIs. The source was moved to upper left (a), upper right (b), lower left (c) and lower right (d). Figure 6. View largeDownload slide 2D distribution of the alpha particles measured by the nasal monitor and the ratio of counts in four ROIs. The source was moved to upper left (a), upper right (b), lower left (c) and lower right (d). Energy spectrum Figure 7 shows energy spectrum of 5.5 MeV alpha particles measured by the nasal monitor. A range of channels from 5 ch to 120 ch was set as an energy window. Figure 7. View largeDownload slide Energy spectrum of 5.5 MeV alpha particles measured by the nasal monitor. The ZnS(Ag) scintillator produces a light pulse with the number of photons proportional to the alpha particle energy (5.5 MeV). The SiPM produces a voltage output pulse (pulse length ~200 ns) with amptitude proportional to the number of photons emitted. A clear peak of 5.5 MeV was not confirmed because the thickness of the ZnS(Ag) scintillator was thin and a part of energy of 5.5 MeV alpha particles was absorbed(19). Figure 7. View largeDownload slide Energy spectrum of 5.5 MeV alpha particles measured by the nasal monitor. The ZnS(Ag) scintillator produces a light pulse with the number of photons proportional to the alpha particle energy (5.5 MeV). The SiPM produces a voltage output pulse (pulse length ~200 ns) with amptitude proportional to the number of photons emitted. A clear peak of 5.5 MeV was not confirmed because the thickness of the ZnS(Ag) scintillator was thin and a part of energy of 5.5 MeV alpha particles was absorbed(19). Efficiency Figure 8 shows average efficiencies of individual nasal cavities evaluated using a PuO2 source. The average efficiencies in 4-pi were 11.4% for the left nasal cavity and 11.6% for the right nasal cavity. These efficiencies are inferior to the 16.3% reported for a commercial ZnS(Ag) survey meter(20). The MDA was calculated as 0.23 Bq. Figure 8. View largeDownload slide The average efficiencies in 4-pi direction for individual nasal cavities. Figure 8. View largeDownload slide The average efficiencies in 4-pi direction for individual nasal cavities. The influence of gamma and beta rays The average count rate of the gamma rays from 137Cs was 0.1 ± 0.082 cpm and that of the beta rays from 90Sr was 0 cpm. Energy depositions from these beta rays were negligible small because the thickness of ZnS(Ag) scintillator was very thin of 3.25 mg/cm2 (=7.95 μm), and output signals of these beta rays were below the discrimination level of an output signal to eliminate the influence of an electric noise. Therefore, the influence of the gamma and beta rays on the detection of the alpha particles of Pu was negligible. Measurement of PuO2 particle Figure 9 shows the 2D distribution of alpha particles from PuO2 samples. When a single PuO2 particle was measured, the alpha particles were detected at only one angle (Figure 9a); by contrast, when many PuO2 particles were measured, these alpha particles were detected at four angles (Figure 9b). To evaluate the difference between the ZnS(Ag) scintillation counter and the nasal monitor, two PuO2 samples with different activities were measured by using both detectors (Figure 10). The differences were within 4.0%. Figure 9. View largeDownload slide 2D distribution of a single PuO2 particle (a) and many PuO2 particles (b) measured by the nasal monitor. Figure 9. View largeDownload slide 2D distribution of a single PuO2 particle (a) and many PuO2 particles (b) measured by the nasal monitor. Figure 10. View largeDownload slide Comparison of activities measured by the ZnS(Ag) scintillation counter and the nasal monitor. Figure 10. View largeDownload slide Comparison of activities measured by the ZnS(Ag) scintillation counter and the nasal monitor. DISCUSSION The nasal monitor could be directly inserted in the nasal cavities, and clearly identify four different angles of alpha particles by the four light guides divided by the reflector. This separation provides the activity distribution of Pu in the nasal cavity, and the activity distribution will be helpful in estimating the internal exposure dose. When Pu in the nasal cavity is washed out from the nasal cavity for decontamination, the nasal monitor can effectively check the effectiveness of decontamination. A prismatic-shaped acrylic light guide with a width of 1.5 mm was used to compose the block detector of the nasal monitor, and position resolution was limited to 2 × 2. Although the diameter of the PuO2 particles is on the order of a few micrometers(21), more thin-width light guides will, in principle, improve the position resolution of the alpha particles. An improvement of the position resolution is valuable for obtaining a detailed Pu distribution in the nasal cavity. The International Commission on Radiological Protection (ICRP) offered a conversion factor (CF) for internal exposure dose estimation. The CF of 239Pu for inhalation is 4.7 × 10−2 mSv/Bq for activity median aerodynamic diameter (AMAD) of 1 μm and 3.2 × 10−2 mSv/Bq for 5 μm(22). If the worker inhales an aerosol with AMAD on the order of a few micrometers, deposition to the extrathoracic (ET) region will be dominant(23). The nasal monitor can measure the Pu activity mainly within the ET1 region, which is the anterior nasal passage. The respiratory tract model of ICRP publication 66 provides deposition fractions for each region(23). The effective dose to the worker can be estimated using the deposition fraction for the ET1 region. Also, Fukutsu et al.(24) reported a conversion factor to directly convert the Pu activity in the ET1 region to the effective dose. Previously, the nasal smear and the nose blow methods have been used to measure Pu activity in the nasal cavity(22). The disadvantage of these methods is that the quantitative removal factor for Pu was difficult to estimate. Also, the removal factor was strongly affected by AMAD of the aerosol. Hence, both methods included uncertainty of the estimated worker’s internal exposure dose. The nasal monitor developed in this work provides a direct measurement of the quantitative Pu activity in the nasal cavity. Also, the nasal monitor will enable real-time measurement for the worker when an accident occurs because it does not require preparation before the measurement. Therefore, the nasal monitor is helpful for a rapid estimation of possibility of worker’s inhalation and the effective dose based on its activity. The use of cascade impactor to measure AMAD in the working room would help to estimate the worker’s exposure dose(25). A gain of SiPM is affected by temperature variation. SiPM itself is not directly inserted for the measurement of the nasal cavities. However, there is a possibility of decreasing the gain of SiPM by temperature increasing. Therefore temperature compensation circuit is required to maintain stable count rate. A disadvantage of the nasal monitor is that self-absorption is occurred by nasal discharge in the nasal cavity. Blowing the nose is recommended before the measurement with the nasal monitor. The activity of the nose blow sample and that measured using the nasal monitor are both required to estimate the total activity in the nasal cavity by summation. CONCLUSIONS We successfully developed the nasal monitor for direct and quantitative measurement of the Pu activity in the nasal cavity. The activity measured by the nasal monitor was approximately equal to that measured by a ZnS(Ag) survey meter. The nasal monitor can solve the problem of uncertainty encountered with the nasal smear and the nose blow methods. 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TI - DEVELOPMENT OF A NEW DETECTOR SYSTEM TO EVALUATE POSITION AND ACTIVITY OF PLUTONIUM PARTICLES IN NASAL CAVITIES
JF - Radiation Protection Dosimetry
DO - 10.1093/rpd/ncx127
DA - 2018-03-01
UR - https://www.deepdyve.com/lp/oxford-university-press/development-of-a-new-detector-system-to-evaluate-position-and-activity-RyUN63Humu
SP - 414
EP - 421
VL - 178
IS - 4
DP - DeepDyve
ER -