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β radiation detector using wavelength and delayed fluorescence discrimination
Nuclear Instruments and Methods in Physics Research A 480 (2002) 626–635
a/b radiation detector using wavelength and delayed fluorescence discrimination T. Maekawa*, A. Sumita Power and Industrial Systems Research and Development Center, Toshiba Corporation, 4-1, Ukishima-cho, Kawasaki-ku, Kawasaki 210-0862, Japan Received 12 June 2000; received in revised form 13 March 2001; accepted 7 April 2001
Abstract This paper describes a novel two-layer radiation detector for a=b simultaneous counting for dust radiation monitoring in nuclear power plants. For a=b discrimination, wavelength and delayed fluorescence discrimination techniques were newly developed. To establish the wavelength discrimination, we adopted a two-layer scintillator consisting of the plastic scintillator (NE-111A) and Y2O2S(Eu) whose emission spectra are quite different. To reject the mixed b signal in the a detection layer, we used the delayed fluorescence characteristics of Y2O2S(Eu) in the signal processing. We manufactured the detector and tested its feasibility and the detection performance for dust radiation monitoring. Finally, we concluded that the performance of this new a=b detector using the new discrimination methods is suitable for dust radiation monitoring. r 2002 Elsevier Science B.V. All rights reserved. PACS: Detector scintillator: 29.40.m; Radiation detector: 07.57.k; 29.40; 85.25.p Keywords: Radiation discrimination; Wavelength; Delayed fluorescence; YOS(Eu); a/b simultaneous counting; Dust radiation monitor
1. Introduction A dust radiation monitor is an important apparatus for safe operation of nuclear power plants. Conventional monitors use b scintillation detectors because the main target of monitoring is b particles derived from artificial radionuclides. One problem concerning dust monitoring is that the concentration deviation of natural Rn daugh*Corresponding author. Tel.: 81-44-288-8173; fax: 81-44-2701809. E-mail address: [email protected] (T. Maekawa).
ter nuclides directly influences b counting rates. To suppress this influence, a particles emitted from Rn daughters should be monitored and discriminated against. The composition ratio of Rn daughter nuclides adsorbed on a filter paper varies depending on the time of adsorption. Therefore, the a=b measurement should be performed simultaneously. For particle discrimination using scintillators, various methods have been developed [1]. The basic properties of scintillators were reviewed and optimum processing systems have been developed theoretically [2]. The well-known methods are the ‘‘zero crossing method’’ [3–5], the ‘‘double
0168-9002/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 1 1 0 6 - 8
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integration method’’ [6,7], use of space charge saturation effect in photomultiplier tube (PMT) [8] and the rise time discrimination method called the ‘‘Kumahara–Kinbara method’’ [9,10]. These methods have been applied to various systems such as phoswich detectors [11–19]. To realize suppression of Rn daughter influences during the dust monitoring, the phoswich detector combined with ZnS(Ag) and a plastic scintillator is normally used with the rise time discrimination method. The two scintillators emit light of a similar wavelength spectrum and are set in layers on a photocathode of PMT. In such a method, there are two basic problems to be solved. The first problem concerns the electrical dynamic range of the pulse height. The number of photons emitted from ZnS(Ag) induced by an a particle is considerably greater than that emitted from the plastic scintillator induced by a b particle. Therefore, the gain of measurement circuits cannot satisfy output levels of both a and b particles, and should be adjusted to either a or b particle. S. Usuda et al. have reported the use of various scintillators and measurement techniques for phoswich detectors [20–24]. Use of a neutral density (ND) filter to reduce the light output of ZnS(Ag) and flatten the light output levels of both scintillators was reported [25]. If the main target to be detected is the a particle, this method is certainly effective. However, it is not always adequate for b particle detection, because a filter of this type absorbs a portion of b particle energy and degrades the b particle detection efficiency. The second problem concerns the thickness optimization of the first layer scintillator. To keep b and g sensitivity sufficiently low, the thickness of the a scintillator (the first layer) should be as thin as possible. On the other hand, it should be a finite thickness to keep the a particle within the scintillator thickness. Thus, in the design of the first layer, there is often a trade-off between a detection efficiency and b mixture level. To solve these problems, we have developed the wavelength and delayed fluorescence discrimination techniques, and realized a practical a=b particle detector using them. The wavelength discrimination method solves the dynamic range
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problem of the light output, and leads to a simplified measurement system. The delayed fluorescence discrimination method solves the trade-off problem concerning the first layer. This paper describes the principles of the new discrimination methods and how they are adopted for the practical detector system, and presents the detection performance achieved by the manufactured a=b detector. 2. Experimental setup 2.1. Detector design for wavelength discrimination Fig. 1 shows the schematic diagram of the newly developed a=b detector system using the wavelength and the delayed fluorescence discrimination methods. The two-layer scintillator consists of two kinds of scintillators whose emission wavelength spectra are entirely different. The first layer mainly detects a particles and the second layer is for the b particle detection. Two PMTs, each equipped with a color filter, and each filter having its transmission characteristics matched to the respective layer spectrum, can detect the light from the respective layers. Using this structure, simultaneous a=b counting is achieved. The bialkali photocathode has its maximum sensitivity at 400 nm (blue) range. Considering that the b particle detection is prior to the a particle detection in dust monitoring, we selected the NE111A plastic scintillator as the second layer for the b particle, which shows a relatively narrow emission spectrum of blue. Regarding selection of the first scintillator for a particle, it is necessary to satisfy the following conditions: *
*
*
the overlap region of the emission spectra of two scintillators should be minimized to avoid optical cross-talk, the effective atomic mass number should be minimized to reduce the b or g sensitivity, the emission spectrum should be sharp to match the sensitivity of the photocathode.
According to the above conditions, we selected Y2O2S(Eu) (hereinafter YOS) which is a red fluor used in color CRTs.
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Fig. 1. Schematic diagram of a=b detector. The composite light emitted from YOS(Eu) and NE-111A is detected by two PMTs, each equipped with a color filter. A conventional counting system is adopted for the b-PMT. The output of the a-PMT is connected to a scaler through the newly developed delayed fluorescence discriminator.
The sensitive area of the detector is a rectangle (26 75 mm2) and equivalent to the conventional circular filter paper (5 cm+) used in dust radiation monitors. The incident window sheet consists of an Al vaporized thin polymer film to shield the light. YOS particles (6 mm+) are fixed with polymer binder material on a transparent PET sheet (9 mm). The thickness of NE-111A is 1 mm in consideration of the need to maintain the b sensitivity and suppress the g sensitivity as much as possible. On the other hand, the thickness of the YOS sheet is designed based on a consideration of Rn daughter nuclides. The energy range of the a particles emitted from Rn daughters is from 5.5 MeV to 8 MeV. The total required thickness for full energy deposition is about 10 mg/cm2. In the case of a practical design, some energy deposition of the a particles occurs in the air and the incident window. Considering these effects, the thickness of YOS is designed to be about 5 mg/cm2. Thus, a 5.5–8 MeV a particle can reach the YOS layer and deposit its remaining energy. The NE-111A scintillation photons transmitted through the blue filter (1+ 5 mm-thickness, BFL-390, Sigma Koki) were detected by a bPMT (1+; R1924P, Hamamatsu Photonics). The
b-PMT is attached to the surface of NE-111A. The light output of the NE-111A is trapped in the scintillator by total internal reflection and transferred to the position of the b-PMT. The YOS scintillation photons transmitted through the red filter (1+ 5 mm-thickness, SCF-560, Sigma Koki) were detected by the aPMT (1+, R1924P, Hamamatsu Photonics). The light output of YOS cannot be transferred by the total internal reflection, because the YOS sheet is translucent. So, the a-PMT is distanced from the sheet with an air gap as shown in Fig. 1 and catches the radiation light emitted to the air from the YOS sheet. Fig. 2 shows the typical emission spectra of NE-111A and YOS together with the transmission characteristics of the adopted color filters. As shown in Fig. 2, the characteristics of these color filters match the respective scintillator spectrum well. 2.2. Signal processing for delayed fluorescence discrimination As the a particle does not reach the second layer due to the optimum thickness design, the light emitted from the second layer contains only the b
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Fig. 2. Emission spectra of NE-111A and YOS(Eu) and transmission characteristics of the adopted blue and red color filters. The left Y-axis represents the transparency of optical filters and the right Y-axis represents the relative intensity of emission spectra of the scintillators.
particle signal. On the contrary, the light emitted from the first layer contains not only the a signal, but also some amount of the b signal. This is an unavoidable problem of the two-layer scintillation detector. For good separation of a and b; a new method to reject the b signal within the signal of the first layer is required. Therefore, we newly proposed the delayed fluorescence discrimination for the first layer, as mentioned below. YOS is not widely used as a scintillator, but it is well known as the most popular red fluor for color CRTs. Generally, CRT fluors show delayed fluorescence (phosphorescence) characteristics depending on the density of the intense electron beam [26]. Therefore, we assumed that delayed fluorescence phenomena of YOS would differ even in the case of a and bðgÞ irradiation. To utilize the difference in delayed fluorescence characteristics, the output of the a-PMT is connected to the counter through the newly developed delayed fluorescence discriminator (DFD) as shown in Fig. 1. The normal integral output of the preamp is differentiated once again to split it into multiple pulses, and connected to the voltage comparator. On the other hand, the bPMT output is connected to a conventional counting system. Fig. 3 shows the basic timing chart of the DFD. When a radiation incidence occurs, the signal detection gate Ts is triggered. After a delay time Td ; the counting gate Tc is activated. While the former gate Ts is activated, the next trigger signal is inhibited. The time width of delay time Td is normally zero, but non-zero values are used in the
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Fig. 3. Basic timing chart of measurement for the delayed fluorescence characteristics. The signal detection gate (Ts ) is activated by the radiation incidence. The multiple pulses are counted during the active state of the counting gate (Tc ).
subsequent experiments. Furthermore, Ts and Tc can be set to various values independently for experiments. The final values of these parameters for the practical discrimination are presented below. The concept of the DFD is simple. The output of the DFD connected to the a signal scaler as shown in Fig. 1 is activated and counted as a signal, only if the number of observed multiple pulses during Tc exceeds the threshold (Cth). Thus, the b particle and the noise of the PMT signals from the a-PMT are almost entirely rejected by the optimized Cth. 2.3. Experimental items 2.3.1. Feasibility test for discrimination method We performed the following two experiments to confirm the basic feasibility of the two discrimination methods. 2.3.1.1. Pulse height distributions of two PMTs. The aim of this experiment was to confirm the discrimination effect of the color filters. In this experiment, either the NE-111A or the YOS scintillator was mounted on the detector shown in Fig. 1. The point radiation source (90Sr for b; 241 Am for a) was put on the sensing area at the center position of the target PMT. The preamp output was connected to a multi-channel analyzer (MCA) through the shaping amplifier (NAIG
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E-511). The pulse height distributions of YOS and NE-111A by b irradiation were measured using the b-PMT equipped with the blue filter, respectively. Using the a-PMT equipped with the red filter, the measurements by a irradiation were performed in the same way. 2.3.1.2. Delayed fluorescence characteristics of YOS. Using the measurement system shown in Fig. 1, we have measured the time decay curve and the probability distribution of the number of multiple pulses during the fluorescence in order to arrive at a detailed understanding of the phenomenon and optimize time parameters (Tc ; Ts ; Td ) of the DFD. The same radiation point sources of a and b are used and put on the sensing area at the center position of the a-PMT in these experiments. The output signal of the a-PMT was measured by a and b irradiation. For the measurement of time decay curve, the time parameters of the DFD were set to {Ts ¼ 1 ms, Tc ¼ 100 ms, Td ¼ 0B900 ms with 10 steps}. The number of multiple pulses generated during each 100 ms step was measured by the multiple pulses memory and scaler as shown in Fig. 1. To obtain the probability distribution, the time parameters of the DFD are set to {Ts ¼ 4 ms, Tc ¼ 500 ms, Td =0}. The frequency of the number of multiple counts generated by the radiation incident event was measured by the multiple pulses memory and scaler. The obtained data were normalized by the number of events. 2.3.2. Detector performance test After the above two measurements, the detector performance test for the dust radiation monitor was carried out as follows. *
Position dependence of the counting efficiency: The counting rates were measured at seven positions equally spaced along the detector length by the a and b signal scalers. The point radioactive source (90Sr for b; 241Am for a) was put directly on the sensitive area. Normalizing measured counting rates by the rate at the center position of the entire sensitive area, we obtained the position dependence of the relative
*
*
a counting efficiency at the a-PMT and the relative b counting efficiency at the b-PMT. Intrinsic counting efficiency and mixture level for a and b particles: The test was performed using the plate radioactive sources. A natural uranium oxide U3O8 source (26 75 mm2, 12.9 Bq/ cm2) for b and an Am-241 source (26 37.5 mm2, 183 Bq/cm2) for a were used. The source was set about 5 mm above the sensitive area and the active area of the b source is covered with the Al sheet (27 mg/cm2) to shield a and low energy b particles emitted from U3O8. Each count rate was measured by the a and b signal scaler during irradiation. For a efficiency, measurements for sensitive areas R and L were performed twice, as shown in Fig. 7, because the active area of the a plate source cannot cover the entire sensitive area at one time. Then, we obtained the count rate of a particle by averaging the two values for area R and L. Finally, we calculated the intrinsic counting efficiencies (%/2p) using the count rates and the calibration activities of the radioactive sources. Effectiveness of DFD: The detector was set under the known dose rate conditions at g (60Co) irradiation facility. The DFD output was measured by the a signal scaler. The Compton scattering electrons generated by g irradiation can be regarded as equivalent to b irradiation for the scintillator. Using the g source, bðgÞ rejection function of the DFD can be confirmed under various dose rate conditions.
3. Results of experiments 3.1. Results of feasibility test for discrimination method 3.1.1. Pulse height distributions of two PMT outputs The measurement results are shown in Fig. 4. Fig. 4(a) shows the pulse height distributions of NE-111A and YOS using the b-PMT equipped with the blue filter by b irradiation and (b) shows the results for the a-PMT equipped with the red filter by a irradiation. X-axis represents pulse
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height and it is relatively proportional to the number of photons generated per radiation incident event. Regarding these spectra, we paid attention to the maximum distributed channel. The sensitivity characteristic of R1924P (bialkali photocathode of curve number 400 K) matches the NE-111A emission spectrum. The NE-111A signal is definitely bright in the case of the b-PMT with the blue filter shown in Fig. 4(a). Even in the case of the a-PMT, the maximum pulse height of YOS is greater than that of NE-111A by the red color filter and the pulse height of NE-111A is drastically reduced, compared to (a). Thus, we con-
Fig. 4. Pulse height distribution of NE-111A and YOS. The preamp output of the b-PMT with the blue filter by b irradiation and that of the a-PMT with the red filter by a irridiation were measured.
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firmed the feasibility of wavelength discrimination whereby each target light was filtered to be dominant and unnecessary light was absorbed by the filter adopted at the respective PMT.
3.1.2. Delayed fluorescence characteristics of YOS Fig. 5 shows the comparator output at the upper column and preamp output at the lower column as the typical signal trace of YOS. In the case of the b particles, the simple exponential decay curve is observed, related to the integral time constant of the preamp (B20 ms). In particular, in the case of an a particle that has an ionizing density higher than that of a b particle, multiple pulses are emitted at a burst as the comparator output. Fig. 6 shows the results of measurement for characteristics of delayed fluorescence of YOS. Fig. 6(a) shows the time decay characteristic. The X-axis represents the time in 100 ms units after the radiation incidence. This decay curve can be represented by the two exponential decay functions. We found that the dominant component for a particles shows slow decay characteristic with a time constant of B450 ms. For b- and g-ray, fast decay component is dominant and almost always only one pulse is emitted. In Fig. 6(a), there is an evident pedestal component in the case of a irradiation. It is an average chance count component derived from the imperfect triggering. This imperfect triggering occurs due to PMT noise or the residual pulses of the fluorescence as shown in Fig. 4. This component increases with input count rate.
Fig. 5. Typical signal trace of the a-PMT detecting the light of YOS(Eu) irradiated by a and b particles. The comparator output and preamp output are shown in the upper and lower columns, respectively. The vertical scale is the same for both.
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Fig. 6. The characteristics of delayed fluorescence of YOS. (a) shows the time decay curve of delayed fluorescence. The Y-axis represents the average number of counts observed per 100 ms. (b) is the probability distribution of delayed fluorescence. The value of the Y-axis is normalized by the total number of observed events.
Fig. 6(b) shows the probability distribution of multiple pulses generated in delayed fluorescence. The value of the Y-axis is the relative probability normalized by the total number of observed events. This figure represents the histogram of observed counts per event or the gate signal (Tc ). For the a particles, the average number of multiple pulses per event is about 24. For b and g; the distribution limit, such that the probability is less than 1E4, is about 18. These results support the signal trace shown in Fig. 3 quantitatively and we fixed the parameters of the DFD using these results. The two gate signals (Ts ; Tc ) are unified to one gate signal (Tc ) for the purpose of simplicity in practical use, and the gate width is set to 500 ms, according to the decay time constant. The threshold value (Cth) for multiple pulses is set to 18, according to the probability distribution limit of the b particles. Using these DFD parameters, we considered that it is possible to reject the mixed b component of the a signal output and performed the detector performance test. 3.2. Results of detector performance test 3.2.1. Position dependence of the counting efficiency Fig. 7 shows the results for position dependence of the counting efficiency. The relative a counting efficiency at the a-PMT and b efficiency at the bPMT are shown. For the a-PMT which measures the radiation light from the opaque YOS sheet, the
Fig. 7. Position dependence of a and b counting efficiency. The measurement points on the detector are indicated above the plot frame.
efficiencies at measurement position {3–0 cm} in the sensitive area L are relatively constant, but degradation caused by geometrical interrupting by the b-PMT in light collection is evident at {0– 3 cm} in the sensitive area R. For the b-PMT, the b-counting efficiency was relatively constant even at the edge position of the scintillator, because of the total internal reflection by well-polished surfaces and the transparency. 3.2.2. Intrinsic counting efficiency and mixture level for a and b particles Table 1 shows the test results of counting efficiency and mixture level for the a and b
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Table 1 Results of the detector performance tests. The typical values of conventional specification of phoswich detector are also shown Measurement items
Intrinsic a counting efficiency Intrinsic b counting efficiency Intrinsic b counting efficiency Intrinsic a counting efficiency b Mixture level of a detector: a Mixture level of b detector:
Result (%)
of a detector: of a detector: of b detector: of b detector: Ab =ðAa þAb Þ Ba =ðBa þBb Þ
Aa Aa Bb Ba
32.4 0.004 73.2 0.98 0.01 1.32
particles. In this table, the conventional specifications of ZnS(Ag)/Plastic phoswich detector are also shown. In particular, the specifications of b precedence design are the typical values for the dust radiation monitoring. The intrinsic efficiency values are represented as the unit of %/2p. As described above, a counting efficiencies (Aa ; Ba ) were obtained by the averaging of the values of measurements performed twice for area L and R as shown in Fig. 7. The definitions of mixture levels are as follows: *
*
b mixture level in the a detector is given by Ab =ðAa þ Ab Þ; where Ab is the b efficiency at the a detector and Aa ; the a efficiency at the a detector; a mixture level in the b detector is given by Ba =ðBb þ Ba Þ; where Ba is the a efficiency at the b detector and Bb ; the b efficiency at the b detector.
The b detection efficiency is 73% and sufficiently satisfies the conventional specification of the b precedence design. For the a detection efficiency, the average value is 32%. This satisfies the specification of the a precedence design. The mixture levels for both a and b are less than those in the conventional specification and it is established that drastic suppression is executed by the DFD. Thus, we concluded that this a=b detector had sufficient dust radiation monitoring performance for practical use. 3.2.3. Effectiveness of DFD The test results are shown in Fig. 8. Values measured by a signal scaler are plotted.
Conventional specification of ZnS(Ag)/plastic phoswich detector b Precedence design
a Precedence design
10
15
60
20
0.1 5.0
Fig. 8. Effectiveness of delayed fluorescence discrimination. The left Y-axis is for the measurement without DFD and the right Y-axis is for the measurement with DFD.
The case of ‘‘with DFD’’ represents the normal discriminating DFD output using the condition of {Cth=18}. In this case, one pulse is released as the DFD output, only if more than 18 multiple pulses are observed. On the contrary, the case of ‘‘without DFD’’ means using the condition of {Cth=1}. In this case, all input pulses pass through the DFD. In the case of ‘‘without DFD,’’ the measured count rate is proportional to the dose rate of 60 Co and the measured counts seemed to be derived from g: On the contrary, the count rate ‘‘with DFD’’ is not proportional to the dose rate and suppressed at a lower level, because not only g-ray but also PMT noise is rejected by the DFD. We assumed that the residual count rate (o0.05 cps) was caused by the highenergy charged particles in cosmic rays. As a result, we confirmed that the delayed fluorescence discrimination was effective for rejecting the PMT b=g noise.
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4. Discussion The delayed fluorescence discrimination method is based on the information respecting time and counts, and we infer that the performance of the DFD is dependent on count rate. Thus, we considered the dead time and the probability of exceeding Cth for the DFD. The dead time of counting gate Tc increases with count rate. In this system, the dead time can be calculated using the time width of Tc and the count rate including imperfect triggering as the nonsuffocation type. This effect reduces the live time and the a-signal throughput of the DFD. The coefficient of dead time effect (Deff ) can be represented as follows: Deff ðrÞ ¼ 1=f1 þ ðrm þ BGÞTcg
ð1Þ
where r is the true a count rate (cps), m; the correction coefficient for imperfect triggering (2.5, obtained by experiments), BG, the count rate of PMT noise (cps), and Tc; the time width of the counting gate (500 ms). The probability of exceeding Cth per Tc (Pex ) can be represented as the summation of the intrinsic component Pexi and count rate dependent component Pexd : The Pexd component is caused by the imperfect triggering and increases with the count rate. This phenomenon raises the a-signal throughput of the DFD. With regard to the Pexd caused by imperfect triggering, Fig. 9 shows the probability distribution measured using the counting gate noncorrelated to the a signals at a count rate of 490 cps. In this distribution, the signal exceeding threshold value Cth is recognized as an a signal and the probability Pexd can be obtained by calculating the section over Cth. For the evaluation of other count rate conditions, the equivalent threshold (Ceq ) is newly introduced as follows: Ceq ¼ Cth 490=r
Fig. 9. Probability distribution by the counting gate noncorrelated to the a signal. True count rate of a in YOS is 490 cps.
ð2Þ
where Cth=18, and r (cps) is the count rate to evaluate. Using the calculated Ceq ; the exceeding probability Pexd (r) for various count rate conditions was calculated using the section over Ceq of the
Fig. 10. The DFD throughput characteristics. Throughput is represented by multiplying Deff by Pex and normalized by the value at 80 cps.
distribution data at 490 cps. Moreover, Pexi was obtained using the section over Cth using the distribution data at 80 cps, indicating that the influence of imperfect triggering is negligible. Finally, the throughput (TPc ) of the DFD can be obtained by multiplying the coefficient of dead time (Deff ) by the probability of exceeding Cth (Pex ). Fig. 10 shows these results with some practical measured data (TPm ), which are normalized by the value at 80 cps. For the dust radiation monitoring, the ordinary count rate ranges from a few cps to the order of 200 cps, and the maximum deviation of calculation throughput is about 20% in this range. Furthermore, the calculation value (TPc ) matches the measured data (TPm ) well. Then, if the strict correction for the quantitative data analysis is required, the correction factor shown in Fig. 10 can be applied for the measured count rate. Finally, we conclude that there is no fatal problem for the count rate dependence of the DFD.
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5. Conclusion We have newly developed an a=b detector using wavelength and delayed fluorescence discrimination for dust radiation monitoring in nuclear power plants. For wavelength discrimination, we have adopted a two-layer scintillator consisting of the plastic scintillator (NE-111A) and YOS(Eu), which is used as the red fluor for color CRTs, and designed the detector system. For delayed fluorescence discrimination, we have proposed a simple DFD logic and used it. First, we carried out the basic feasibility test for the new discrimination methods and confirmed that the discrimination methods work effectively. Second, we verified the detector performance. The obtained performance surpassed that of the conventional specification based on the ZnS(Ag)/ plastic phoswich detector with rise time discrimination. Adopting the new delayed fluorescence discrimination technique, the mixture level was drastically improved at the first layer and the counting efficiencies for aand b particles became compatible. Thus, we have proved the suitability of the developed detector system for practical use in dust radiation monitoring at nuclear power plants. This new technique realizes the compatibility of the efficiency and mixture level in the detector design. We conclude that not only the a=b detection described in this paper, but also a=g detection using YOS single layer can be realized.
Acknowledgements We are grateful to K. Nittoh, T. Tamura, N. Naito, S. Morimoto, S. Fujita, S.Yamaga, T. Itoh, T. Sato, K. Chiba and Y. Ohara for useful
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discussion and suggestions. We are indebted to T. Takahara, A. Saito and E. Oyaizu for assistance in the trial manufacture of YOS(Eu) and various other fluors.
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a/b radiation detector using wavelength and delayed fluorescence discrimination T. Maekawa*, A. Sumita Power and Industrial Systems Research and Development Center, Toshiba Corporation, 4-1, Ukishima-cho, Kawasaki-ku, Kawasaki 210-0862, Japan Received 12 June 2000; received in revised form 13 March 2001; accepted 7 April 2001
Abstract This paper describes a novel two-layer radiation detector for a=b simultaneous counting for dust radiation monitoring in nuclear power plants. For a=b discrimination, wavelength and delayed fluorescence discrimination techniques were newly developed. To establish the wavelength discrimination, we adopted a two-layer scintillator consisting of the plastic scintillator (NE-111A) and Y2O2S(Eu) whose emission spectra are quite different. To reject the mixed b signal in the a detection layer, we used the delayed fluorescence characteristics of Y2O2S(Eu) in the signal processing. We manufactured the detector and tested its feasibility and the detection performance for dust radiation monitoring. Finally, we concluded that the performance of this new a=b detector using the new discrimination methods is suitable for dust radiation monitoring. r 2002 Elsevier Science B.V. All rights reserved. PACS: Detector scintillator: 29.40.m; Radiation detector: 07.57.k; 29.40; 85.25.p Keywords: Radiation discrimination; Wavelength; Delayed fluorescence; YOS(Eu); a/b simultaneous counting; Dust radiation monitor
1. Introduction A dust radiation monitor is an important apparatus for safe operation of nuclear power plants. Conventional monitors use b scintillation detectors because the main target of monitoring is b particles derived from artificial radionuclides. One problem concerning dust monitoring is that the concentration deviation of natural Rn daugh*Corresponding author. Tel.: 81-44-288-8173; fax: 81-44-2701809. E-mail address: [email protected] (T. Maekawa).
ter nuclides directly influences b counting rates. To suppress this influence, a particles emitted from Rn daughters should be monitored and discriminated against. The composition ratio of Rn daughter nuclides adsorbed on a filter paper varies depending on the time of adsorption. Therefore, the a=b measurement should be performed simultaneously. For particle discrimination using scintillators, various methods have been developed [1]. The basic properties of scintillators were reviewed and optimum processing systems have been developed theoretically [2]. The well-known methods are the ‘‘zero crossing method’’ [3–5], the ‘‘double
0168-9002/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 1 1 0 6 - 8
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integration method’’ [6,7], use of space charge saturation effect in photomultiplier tube (PMT) [8] and the rise time discrimination method called the ‘‘Kumahara–Kinbara method’’ [9,10]. These methods have been applied to various systems such as phoswich detectors [11–19]. To realize suppression of Rn daughter influences during the dust monitoring, the phoswich detector combined with ZnS(Ag) and a plastic scintillator is normally used with the rise time discrimination method. The two scintillators emit light of a similar wavelength spectrum and are set in layers on a photocathode of PMT. In such a method, there are two basic problems to be solved. The first problem concerns the electrical dynamic range of the pulse height. The number of photons emitted from ZnS(Ag) induced by an a particle is considerably greater than that emitted from the plastic scintillator induced by a b particle. Therefore, the gain of measurement circuits cannot satisfy output levels of both a and b particles, and should be adjusted to either a or b particle. S. Usuda et al. have reported the use of various scintillators and measurement techniques for phoswich detectors [20–24]. Use of a neutral density (ND) filter to reduce the light output of ZnS(Ag) and flatten the light output levels of both scintillators was reported [25]. If the main target to be detected is the a particle, this method is certainly effective. However, it is not always adequate for b particle detection, because a filter of this type absorbs a portion of b particle energy and degrades the b particle detection efficiency. The second problem concerns the thickness optimization of the first layer scintillator. To keep b and g sensitivity sufficiently low, the thickness of the a scintillator (the first layer) should be as thin as possible. On the other hand, it should be a finite thickness to keep the a particle within the scintillator thickness. Thus, in the design of the first layer, there is often a trade-off between a detection efficiency and b mixture level. To solve these problems, we have developed the wavelength and delayed fluorescence discrimination techniques, and realized a practical a=b particle detector using them. The wavelength discrimination method solves the dynamic range
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problem of the light output, and leads to a simplified measurement system. The delayed fluorescence discrimination method solves the trade-off problem concerning the first layer. This paper describes the principles of the new discrimination methods and how they are adopted for the practical detector system, and presents the detection performance achieved by the manufactured a=b detector. 2. Experimental setup 2.1. Detector design for wavelength discrimination Fig. 1 shows the schematic diagram of the newly developed a=b detector system using the wavelength and the delayed fluorescence discrimination methods. The two-layer scintillator consists of two kinds of scintillators whose emission wavelength spectra are entirely different. The first layer mainly detects a particles and the second layer is for the b particle detection. Two PMTs, each equipped with a color filter, and each filter having its transmission characteristics matched to the respective layer spectrum, can detect the light from the respective layers. Using this structure, simultaneous a=b counting is achieved. The bialkali photocathode has its maximum sensitivity at 400 nm (blue) range. Considering that the b particle detection is prior to the a particle detection in dust monitoring, we selected the NE111A plastic scintillator as the second layer for the b particle, which shows a relatively narrow emission spectrum of blue. Regarding selection of the first scintillator for a particle, it is necessary to satisfy the following conditions: *
*
*
the overlap region of the emission spectra of two scintillators should be minimized to avoid optical cross-talk, the effective atomic mass number should be minimized to reduce the b or g sensitivity, the emission spectrum should be sharp to match the sensitivity of the photocathode.
According to the above conditions, we selected Y2O2S(Eu) (hereinafter YOS) which is a red fluor used in color CRTs.
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Fig. 1. Schematic diagram of a=b detector. The composite light emitted from YOS(Eu) and NE-111A is detected by two PMTs, each equipped with a color filter. A conventional counting system is adopted for the b-PMT. The output of the a-PMT is connected to a scaler through the newly developed delayed fluorescence discriminator.
The sensitive area of the detector is a rectangle (26 75 mm2) and equivalent to the conventional circular filter paper (5 cm+) used in dust radiation monitors. The incident window sheet consists of an Al vaporized thin polymer film to shield the light. YOS particles (6 mm+) are fixed with polymer binder material on a transparent PET sheet (9 mm). The thickness of NE-111A is 1 mm in consideration of the need to maintain the b sensitivity and suppress the g sensitivity as much as possible. On the other hand, the thickness of the YOS sheet is designed based on a consideration of Rn daughter nuclides. The energy range of the a particles emitted from Rn daughters is from 5.5 MeV to 8 MeV. The total required thickness for full energy deposition is about 10 mg/cm2. In the case of a practical design, some energy deposition of the a particles occurs in the air and the incident window. Considering these effects, the thickness of YOS is designed to be about 5 mg/cm2. Thus, a 5.5–8 MeV a particle can reach the YOS layer and deposit its remaining energy. The NE-111A scintillation photons transmitted through the blue filter (1+ 5 mm-thickness, BFL-390, Sigma Koki) were detected by a bPMT (1+; R1924P, Hamamatsu Photonics). The
b-PMT is attached to the surface of NE-111A. The light output of the NE-111A is trapped in the scintillator by total internal reflection and transferred to the position of the b-PMT. The YOS scintillation photons transmitted through the red filter (1+ 5 mm-thickness, SCF-560, Sigma Koki) were detected by the aPMT (1+, R1924P, Hamamatsu Photonics). The light output of YOS cannot be transferred by the total internal reflection, because the YOS sheet is translucent. So, the a-PMT is distanced from the sheet with an air gap as shown in Fig. 1 and catches the radiation light emitted to the air from the YOS sheet. Fig. 2 shows the typical emission spectra of NE-111A and YOS together with the transmission characteristics of the adopted color filters. As shown in Fig. 2, the characteristics of these color filters match the respective scintillator spectrum well. 2.2. Signal processing for delayed fluorescence discrimination As the a particle does not reach the second layer due to the optimum thickness design, the light emitted from the second layer contains only the b
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Fig. 2. Emission spectra of NE-111A and YOS(Eu) and transmission characteristics of the adopted blue and red color filters. The left Y-axis represents the transparency of optical filters and the right Y-axis represents the relative intensity of emission spectra of the scintillators.
particle signal. On the contrary, the light emitted from the first layer contains not only the a signal, but also some amount of the b signal. This is an unavoidable problem of the two-layer scintillation detector. For good separation of a and b; a new method to reject the b signal within the signal of the first layer is required. Therefore, we newly proposed the delayed fluorescence discrimination for the first layer, as mentioned below. YOS is not widely used as a scintillator, but it is well known as the most popular red fluor for color CRTs. Generally, CRT fluors show delayed fluorescence (phosphorescence) characteristics depending on the density of the intense electron beam [26]. Therefore, we assumed that delayed fluorescence phenomena of YOS would differ even in the case of a and bðgÞ irradiation. To utilize the difference in delayed fluorescence characteristics, the output of the a-PMT is connected to the counter through the newly developed delayed fluorescence discriminator (DFD) as shown in Fig. 1. The normal integral output of the preamp is differentiated once again to split it into multiple pulses, and connected to the voltage comparator. On the other hand, the bPMT output is connected to a conventional counting system. Fig. 3 shows the basic timing chart of the DFD. When a radiation incidence occurs, the signal detection gate Ts is triggered. After a delay time Td ; the counting gate Tc is activated. While the former gate Ts is activated, the next trigger signal is inhibited. The time width of delay time Td is normally zero, but non-zero values are used in the
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Fig. 3. Basic timing chart of measurement for the delayed fluorescence characteristics. The signal detection gate (Ts ) is activated by the radiation incidence. The multiple pulses are counted during the active state of the counting gate (Tc ).
subsequent experiments. Furthermore, Ts and Tc can be set to various values independently for experiments. The final values of these parameters for the practical discrimination are presented below. The concept of the DFD is simple. The output of the DFD connected to the a signal scaler as shown in Fig. 1 is activated and counted as a signal, only if the number of observed multiple pulses during Tc exceeds the threshold (Cth). Thus, the b particle and the noise of the PMT signals from the a-PMT are almost entirely rejected by the optimized Cth. 2.3. Experimental items 2.3.1. Feasibility test for discrimination method We performed the following two experiments to confirm the basic feasibility of the two discrimination methods. 2.3.1.1. Pulse height distributions of two PMTs. The aim of this experiment was to confirm the discrimination effect of the color filters. In this experiment, either the NE-111A or the YOS scintillator was mounted on the detector shown in Fig. 1. The point radiation source (90Sr for b; 241 Am for a) was put on the sensing area at the center position of the target PMT. The preamp output was connected to a multi-channel analyzer (MCA) through the shaping amplifier (NAIG
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E-511). The pulse height distributions of YOS and NE-111A by b irradiation were measured using the b-PMT equipped with the blue filter, respectively. Using the a-PMT equipped with the red filter, the measurements by a irradiation were performed in the same way. 2.3.1.2. Delayed fluorescence characteristics of YOS. Using the measurement system shown in Fig. 1, we have measured the time decay curve and the probability distribution of the number of multiple pulses during the fluorescence in order to arrive at a detailed understanding of the phenomenon and optimize time parameters (Tc ; Ts ; Td ) of the DFD. The same radiation point sources of a and b are used and put on the sensing area at the center position of the a-PMT in these experiments. The output signal of the a-PMT was measured by a and b irradiation. For the measurement of time decay curve, the time parameters of the DFD were set to {Ts ¼ 1 ms, Tc ¼ 100 ms, Td ¼ 0B900 ms with 10 steps}. The number of multiple pulses generated during each 100 ms step was measured by the multiple pulses memory and scaler as shown in Fig. 1. To obtain the probability distribution, the time parameters of the DFD are set to {Ts ¼ 4 ms, Tc ¼ 500 ms, Td =0}. The frequency of the number of multiple counts generated by the radiation incident event was measured by the multiple pulses memory and scaler. The obtained data were normalized by the number of events. 2.3.2. Detector performance test After the above two measurements, the detector performance test for the dust radiation monitor was carried out as follows. *
Position dependence of the counting efficiency: The counting rates were measured at seven positions equally spaced along the detector length by the a and b signal scalers. The point radioactive source (90Sr for b; 241Am for a) was put directly on the sensitive area. Normalizing measured counting rates by the rate at the center position of the entire sensitive area, we obtained the position dependence of the relative
*
*
a counting efficiency at the a-PMT and the relative b counting efficiency at the b-PMT. Intrinsic counting efficiency and mixture level for a and b particles: The test was performed using the plate radioactive sources. A natural uranium oxide U3O8 source (26 75 mm2, 12.9 Bq/ cm2) for b and an Am-241 source (26 37.5 mm2, 183 Bq/cm2) for a were used. The source was set about 5 mm above the sensitive area and the active area of the b source is covered with the Al sheet (27 mg/cm2) to shield a and low energy b particles emitted from U3O8. Each count rate was measured by the a and b signal scaler during irradiation. For a efficiency, measurements for sensitive areas R and L were performed twice, as shown in Fig. 7, because the active area of the a plate source cannot cover the entire sensitive area at one time. Then, we obtained the count rate of a particle by averaging the two values for area R and L. Finally, we calculated the intrinsic counting efficiencies (%/2p) using the count rates and the calibration activities of the radioactive sources. Effectiveness of DFD: The detector was set under the known dose rate conditions at g (60Co) irradiation facility. The DFD output was measured by the a signal scaler. The Compton scattering electrons generated by g irradiation can be regarded as equivalent to b irradiation for the scintillator. Using the g source, bðgÞ rejection function of the DFD can be confirmed under various dose rate conditions.
3. Results of experiments 3.1. Results of feasibility test for discrimination method 3.1.1. Pulse height distributions of two PMT outputs The measurement results are shown in Fig. 4. Fig. 4(a) shows the pulse height distributions of NE-111A and YOS using the b-PMT equipped with the blue filter by b irradiation and (b) shows the results for the a-PMT equipped with the red filter by a irradiation. X-axis represents pulse
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height and it is relatively proportional to the number of photons generated per radiation incident event. Regarding these spectra, we paid attention to the maximum distributed channel. The sensitivity characteristic of R1924P (bialkali photocathode of curve number 400 K) matches the NE-111A emission spectrum. The NE-111A signal is definitely bright in the case of the b-PMT with the blue filter shown in Fig. 4(a). Even in the case of the a-PMT, the maximum pulse height of YOS is greater than that of NE-111A by the red color filter and the pulse height of NE-111A is drastically reduced, compared to (a). Thus, we con-
Fig. 4. Pulse height distribution of NE-111A and YOS. The preamp output of the b-PMT with the blue filter by b irradiation and that of the a-PMT with the red filter by a irridiation were measured.
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firmed the feasibility of wavelength discrimination whereby each target light was filtered to be dominant and unnecessary light was absorbed by the filter adopted at the respective PMT.
3.1.2. Delayed fluorescence characteristics of YOS Fig. 5 shows the comparator output at the upper column and preamp output at the lower column as the typical signal trace of YOS. In the case of the b particles, the simple exponential decay curve is observed, related to the integral time constant of the preamp (B20 ms). In particular, in the case of an a particle that has an ionizing density higher than that of a b particle, multiple pulses are emitted at a burst as the comparator output. Fig. 6 shows the results of measurement for characteristics of delayed fluorescence of YOS. Fig. 6(a) shows the time decay characteristic. The X-axis represents the time in 100 ms units after the radiation incidence. This decay curve can be represented by the two exponential decay functions. We found that the dominant component for a particles shows slow decay characteristic with a time constant of B450 ms. For b- and g-ray, fast decay component is dominant and almost always only one pulse is emitted. In Fig. 6(a), there is an evident pedestal component in the case of a irradiation. It is an average chance count component derived from the imperfect triggering. This imperfect triggering occurs due to PMT noise or the residual pulses of the fluorescence as shown in Fig. 4. This component increases with input count rate.
Fig. 5. Typical signal trace of the a-PMT detecting the light of YOS(Eu) irradiated by a and b particles. The comparator output and preamp output are shown in the upper and lower columns, respectively. The vertical scale is the same for both.
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Fig. 6. The characteristics of delayed fluorescence of YOS. (a) shows the time decay curve of delayed fluorescence. The Y-axis represents the average number of counts observed per 100 ms. (b) is the probability distribution of delayed fluorescence. The value of the Y-axis is normalized by the total number of observed events.
Fig. 6(b) shows the probability distribution of multiple pulses generated in delayed fluorescence. The value of the Y-axis is the relative probability normalized by the total number of observed events. This figure represents the histogram of observed counts per event or the gate signal (Tc ). For the a particles, the average number of multiple pulses per event is about 24. For b and g; the distribution limit, such that the probability is less than 1E4, is about 18. These results support the signal trace shown in Fig. 3 quantitatively and we fixed the parameters of the DFD using these results. The two gate signals (Ts ; Tc ) are unified to one gate signal (Tc ) for the purpose of simplicity in practical use, and the gate width is set to 500 ms, according to the decay time constant. The threshold value (Cth) for multiple pulses is set to 18, according to the probability distribution limit of the b particles. Using these DFD parameters, we considered that it is possible to reject the mixed b component of the a signal output and performed the detector performance test. 3.2. Results of detector performance test 3.2.1. Position dependence of the counting efficiency Fig. 7 shows the results for position dependence of the counting efficiency. The relative a counting efficiency at the a-PMT and b efficiency at the bPMT are shown. For the a-PMT which measures the radiation light from the opaque YOS sheet, the
Fig. 7. Position dependence of a and b counting efficiency. The measurement points on the detector are indicated above the plot frame.
efficiencies at measurement position {3–0 cm} in the sensitive area L are relatively constant, but degradation caused by geometrical interrupting by the b-PMT in light collection is evident at {0– 3 cm} in the sensitive area R. For the b-PMT, the b-counting efficiency was relatively constant even at the edge position of the scintillator, because of the total internal reflection by well-polished surfaces and the transparency. 3.2.2. Intrinsic counting efficiency and mixture level for a and b particles Table 1 shows the test results of counting efficiency and mixture level for the a and b
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Table 1 Results of the detector performance tests. The typical values of conventional specification of phoswich detector are also shown Measurement items
Intrinsic a counting efficiency Intrinsic b counting efficiency Intrinsic b counting efficiency Intrinsic a counting efficiency b Mixture level of a detector: a Mixture level of b detector:
Result (%)
of a detector: of a detector: of b detector: of b detector: Ab =ðAa þAb Þ Ba =ðBa þBb Þ
Aa Aa Bb Ba
32.4 0.004 73.2 0.98 0.01 1.32
particles. In this table, the conventional specifications of ZnS(Ag)/Plastic phoswich detector are also shown. In particular, the specifications of b precedence design are the typical values for the dust radiation monitoring. The intrinsic efficiency values are represented as the unit of %/2p. As described above, a counting efficiencies (Aa ; Ba ) were obtained by the averaging of the values of measurements performed twice for area L and R as shown in Fig. 7. The definitions of mixture levels are as follows: *
*
b mixture level in the a detector is given by Ab =ðAa þ Ab Þ; where Ab is the b efficiency at the a detector and Aa ; the a efficiency at the a detector; a mixture level in the b detector is given by Ba =ðBb þ Ba Þ; where Ba is the a efficiency at the b detector and Bb ; the b efficiency at the b detector.
The b detection efficiency is 73% and sufficiently satisfies the conventional specification of the b precedence design. For the a detection efficiency, the average value is 32%. This satisfies the specification of the a precedence design. The mixture levels for both a and b are less than those in the conventional specification and it is established that drastic suppression is executed by the DFD. Thus, we concluded that this a=b detector had sufficient dust radiation monitoring performance for practical use. 3.2.3. Effectiveness of DFD The test results are shown in Fig. 8. Values measured by a signal scaler are plotted.
Conventional specification of ZnS(Ag)/plastic phoswich detector b Precedence design
a Precedence design
10
15
60
20
0.1 5.0
Fig. 8. Effectiveness of delayed fluorescence discrimination. The left Y-axis is for the measurement without DFD and the right Y-axis is for the measurement with DFD.
The case of ‘‘with DFD’’ represents the normal discriminating DFD output using the condition of {Cth=18}. In this case, one pulse is released as the DFD output, only if more than 18 multiple pulses are observed. On the contrary, the case of ‘‘without DFD’’ means using the condition of {Cth=1}. In this case, all input pulses pass through the DFD. In the case of ‘‘without DFD,’’ the measured count rate is proportional to the dose rate of 60 Co and the measured counts seemed to be derived from g: On the contrary, the count rate ‘‘with DFD’’ is not proportional to the dose rate and suppressed at a lower level, because not only g-ray but also PMT noise is rejected by the DFD. We assumed that the residual count rate (o0.05 cps) was caused by the highenergy charged particles in cosmic rays. As a result, we confirmed that the delayed fluorescence discrimination was effective for rejecting the PMT b=g noise.
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4. Discussion The delayed fluorescence discrimination method is based on the information respecting time and counts, and we infer that the performance of the DFD is dependent on count rate. Thus, we considered the dead time and the probability of exceeding Cth for the DFD. The dead time of counting gate Tc increases with count rate. In this system, the dead time can be calculated using the time width of Tc and the count rate including imperfect triggering as the nonsuffocation type. This effect reduces the live time and the a-signal throughput of the DFD. The coefficient of dead time effect (Deff ) can be represented as follows: Deff ðrÞ ¼ 1=f1 þ ðrm þ BGÞTcg
ð1Þ
where r is the true a count rate (cps), m; the correction coefficient for imperfect triggering (2.5, obtained by experiments), BG, the count rate of PMT noise (cps), and Tc; the time width of the counting gate (500 ms). The probability of exceeding Cth per Tc (Pex ) can be represented as the summation of the intrinsic component Pexi and count rate dependent component Pexd : The Pexd component is caused by the imperfect triggering and increases with the count rate. This phenomenon raises the a-signal throughput of the DFD. With regard to the Pexd caused by imperfect triggering, Fig. 9 shows the probability distribution measured using the counting gate noncorrelated to the a signals at a count rate of 490 cps. In this distribution, the signal exceeding threshold value Cth is recognized as an a signal and the probability Pexd can be obtained by calculating the section over Cth. For the evaluation of other count rate conditions, the equivalent threshold (Ceq ) is newly introduced as follows: Ceq ¼ Cth 490=r
Fig. 9. Probability distribution by the counting gate noncorrelated to the a signal. True count rate of a in YOS is 490 cps.
ð2Þ
where Cth=18, and r (cps) is the count rate to evaluate. Using the calculated Ceq ; the exceeding probability Pexd (r) for various count rate conditions was calculated using the section over Ceq of the
Fig. 10. The DFD throughput characteristics. Throughput is represented by multiplying Deff by Pex and normalized by the value at 80 cps.
distribution data at 490 cps. Moreover, Pexi was obtained using the section over Cth using the distribution data at 80 cps, indicating that the influence of imperfect triggering is negligible. Finally, the throughput (TPc ) of the DFD can be obtained by multiplying the coefficient of dead time (Deff ) by the probability of exceeding Cth (Pex ). Fig. 10 shows these results with some practical measured data (TPm ), which are normalized by the value at 80 cps. For the dust radiation monitoring, the ordinary count rate ranges from a few cps to the order of 200 cps, and the maximum deviation of calculation throughput is about 20% in this range. Furthermore, the calculation value (TPc ) matches the measured data (TPm ) well. Then, if the strict correction for the quantitative data analysis is required, the correction factor shown in Fig. 10 can be applied for the measured count rate. Finally, we conclude that there is no fatal problem for the count rate dependence of the DFD.
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5. Conclusion We have newly developed an a=b detector using wavelength and delayed fluorescence discrimination for dust radiation monitoring in nuclear power plants. For wavelength discrimination, we have adopted a two-layer scintillator consisting of the plastic scintillator (NE-111A) and YOS(Eu), which is used as the red fluor for color CRTs, and designed the detector system. For delayed fluorescence discrimination, we have proposed a simple DFD logic and used it. First, we carried out the basic feasibility test for the new discrimination methods and confirmed that the discrimination methods work effectively. Second, we verified the detector performance. The obtained performance surpassed that of the conventional specification based on the ZnS(Ag)/ plastic phoswich detector with rise time discrimination. Adopting the new delayed fluorescence discrimination technique, the mixture level was drastically improved at the first layer and the counting efficiencies for aand b particles became compatible. Thus, we have proved the suitability of the developed detector system for practical use in dust radiation monitoring at nuclear power plants. This new technique realizes the compatibility of the efficiency and mixture level in the detector design. We conclude that not only the a=b detection described in this paper, but also a=g detection using YOS single layer can be realized.
Acknowledgements We are grateful to K. Nittoh, T. Tamura, N. Naito, S. Morimoto, S. Fujita, S.Yamaga, T. Itoh, T. Sato, K. Chiba and Y. Ohara for useful
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discussion and suggestions. We are indebted to T. Takahara, A. Saito and E. Oyaizu for assistance in the trial manufacture of YOS(Eu) and various other fluors.
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