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A position-sensitive alpha detector using a thin plastic scintillator combined with a position-sensitive photomultiplier tube
Nuclear Instruments and Methods in Physics Research A 418 (1998) 387—393
A position-sensitive alpha detector using a thin plastic scintillator combined with a position-sensitive photomultiplier tube Seiichi Yamamoto *, Takao Iida Department of Electrical Engineering, Kobe City College of Technology 8-3, Gakuen-Higashi-machi, Nishi-ku Kobe, 651-21, Japan Department of Nuclear Engineering, Nagoya University, Japan Received 10 March 1998; received in revised form 18 May 1998
Abstract A position-sensitive alpha detector was developed and tested. The alpha detector consists of a thin plastic scintillator, a position-sensitive photomultiplier tube, a position calculation circuit and a personal computer based data acquisition system. Because the thin plastic scintillator has high-detection efficiency for alpha particles while it has low-sensitivity for beta particles or gamma ray, the detector can selectively detect alpha particles with low background counts. The spatial resolution of the detector was approximately 3 mm FWHM. An autoradiographic images of plutonium distribution in the lung of an animal as well as an image of an uranium particle were successively obtained. Spatial and energy distribution of radon daughters could also be measured. We conclude that the developed position-sensitive alpha detector is useful for some applications such as plutonium detection or alpha autoradiography as well as distribution analysis of radon daughters. 1998 Elsevier Science B.V. All rights reserved. Keywords: Position-sensitive alpha detectors; Photomultiplier tube; Gamma rays; Autoradiography
1. Introduction Imaging of alpha particles is sometimes necessary in several fields. For example, in the facility where plutonium (Pu) is handled, imaging of alpha particles is used to distinguish plutonium particles from radon daughters. For another application of alpha imaging, alpha autoradiography of animals is used for estimating the distribution of alpha emitting radionuclide such as Pu or uranium (U) in
* Corresponding author. Tel.: #81 78 795 3232; fax: #81 78 795 3314; e-mail: [email protected].
the body. We are also interested in the distribution of electrostatically collected radon (Rn) daughters for a radon monitor [1]. The ZnS (Ag) autoradiography [2] or video camera system [3] has been used for alpha imaging. However, ZnS (Ag) autoradiography needs much time to obtain clear images because of its low sensitivity. In addition, one cannot obtain any energy information with these systems. A positionsensitive photomultiplier tube (PSPMT) [4] is one candidate for a component of the alpha imaging system because both the position and energy information can be obtained by using the PSPMT. It has been used for many applications
0168-9002/98/$19.00 1998 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 8 ) 0 8 8 6 - 9
388
S. Yamamoto, T. Iida/Nucl. Instr. and Meth. in Phys. Res. A 418 (1998) 387—393
such as detector modules for positron emission tomography (PET) [5—8] and small gamma cameras [9—12]. Another candidate for alpha imaging is a charge coupled device (CCD) using directory [13] or with scintillator via image intensifier [14]. We previously reported a high-resolution alpha camera using a CaF (Eu) scintillator, a tapered fiber optics plate and a PSPMT [15]. However, the 0.5 mm thick CaF (Eu) scintillator was so thick and dense that environmental beta particles orgamma ray were detected considerably in the situation of low alpha counting rate. The energy information of the camera was not useful to distinguish alpha particles from these beta particles or gamma rays because the pulse height for these beta particles or gamma rays are similar or higher than the energy of the alpha particles such as plutonium or radon daughters. The count rate due to the background beta particles or gamma rays can be decreased if a scintillator which has low detection efficiency for beta particles or gamma rays but has
high detection efficiency for alpha particles is used. Using a thinner CaF (Eu) scintillator may be one candidate, however thickness of 0.5 mm seem to be the limit when considering the mechanical strength of the CaF (Eu) scintillator. Thus, we try to use a thin plastic scintillator combined with PSPMT for imaging alpha particles while minimizing the background counts due to the environmental beta particles or gamma rays. First, we estimated the performance of the system. Then, we tried some applications to imaging alpha particles in the situation of low count rate.
2. Materials and methods Schematic diagram of the detector part of the developed position-sensitive alpha detector is shown in Fig. 1. The detector is quite simple which consists of a thin plastic scintillator and a PSPMT. They are contained in a black box which is supported by a base element.
Fig. 1. Schematic diagram of the developed position-sensitive alpha detector.
S. Yamamoto, T. Iida/Nucl. Instr. and Meth. in Phys. Res. A 418 (1998) 387—393
389
A photograph of the detector part is shown in Fig. 2. The plastic scintillator has thickness of 50 lm, thus most of the energy of target alpha particles such as 239Pu (5.1 MeV), 238U (4.2 MeV) or 218 Po (6.0 MeV) can be absorbed in the scintillator [16]. Because the thickness of the plastic scintillator is so thin in addition to relatively low stopping power for beta particles or gamma ray, energy loss of the environmental beta particles in the scintillator and the detection probability of environmental gamma rays become quite small. The plastic scintillator used was NE102A and had a diameter of 50 mm (Ouyo-Koken,Japan) optically coupled to the PSPMT by KE-420 silicon rubber (Shinetsu Chemical, Japan). The scintillation photons in the plastic scintillator are fed to the PSPMT. The PSPMT used was Hamamatsu R2486-06, which has useful diameter of 50 mm. The scintillation photons are converted to photoelectrons in the photocathode of the PSPMT and multiplied by the mesh dynodes while keeping the position information. The multiplied electrons are read out by the cross wire anodes of the PSPMT in X and ½ direction, respectively. The signals from the cross wire anodes are fed to the resistor chain to produce four signals which correspond to X>, X\, Y> and Y\.
The signals are fed to the position calculation circuit which is also used for beta camera [17]. Energy threshold was set to approximately 2.5 MeV for the pulse height of alpha particles. The calculated position signal is fed to CAMAC and K-max based data acquisition system and transferred to a personal computer (Power Macintosh). Image display and analysis are performed using the K-max software on the personal computer.
Fig. 2. Photograph of the detector part of the position-sensitive alpha detector.
Fig. 3. Image of the resolution phantom obtained by the alpha detector.
3. Results 3.1. Spatial resolution Spatial resolution is one of the most important performance for position-sensitive detector. Thus, we estimated the performance for alpha particles. A resolution phantom and a 241Am plane alpha source (5.5 MeV) were used for the measurements. The alpha source was attached on the resolution phantom and set on the input surface of the detector. An image of the alpha particle distribution which passed through the resolution phantom was acquired. The obtained image of the resolution phantom is shown in Fig. 3. The slits widths of the resolution phantom are 1, 1.5, 2 and 2.5 mm, respectively. The slits of 1.5 mm was clearly resolved which is obvious from the profile distribution perpendicular to the slits. Thus, the spatial resolution
390
S. Yamamoto, T. Iida/Nucl. Instr. and Meth. in Phys. Res. A 418 (1998) 387—393
for 5.5 MeV alpha particles was estimated to be approximately 3 mm FWHM. 3.2. Spatial linearity Spatial linearity was measured using a linearity phantom and the same 241Am alpha source as was used for spatial resolution measurement. An image of the spatial linearity phantom is shown in Fig. 4. A slight distortion was observed in the image which is probably due to the distortion of the PSPMT. 3.3. Energy resolution Energy resolution was measured using a 241Am source which has a diameter of approximately 10 mm placed on the detector surface. The energy signal of the position calculation circuit was fed to a multi-channel analyzer (MCA). The energy distribution for 241Am alpha particles (5.5 MeV) is shown in Fig. 5. Energy resolution was 15% FWHM. 3.4. Image of
Fig. 4. Image of the spatial linearity phantom obtained by the alpha detector.
239
Pu autoradiography
For an alpha autoradiography application, a sliced lung of a rat administered 239Pu particles was imaged. The measurement was done in a controlled laboratory at National Institute of Radiological Science (Japan). The sliced lung was placed on the detector surface through a thin Mylar sheet to prevent Pu contamination to the detector. Count rate of the sample was approximately 1 cps. The acquisition time was 30 min. The image obtained by the detector is shown in Fig. 6a. The distribution of 239Pu in the lung was obvious though the spatial resolution was not good enough to observe small structures. For comparison, a CR-39 film based autoradiograph of the same slice which took one day is shown Fig. 6b. 3.5. Image of alpha emitting particle An alpha emitting particle was imaged for estimating the possibility of distinguishing alpha emitting particles from Rn daughters. The alpha emitting particle was an approximately 0.3 mm diameter particle which contains U obtained from a small piece of an ore. Count rate for the measure-
Fig. 5. Energy distribution for of the alpha detector.
241
Am alpha particles (5.5 MeV)
ment was approximately 0.1 cps. The image of the alpha emitting particle is shown in Fig. 7. The hot spot corresponds to the alpha emitting particle. Pixels which contain only one count in the image are probably from alpha particles of Rn daughters. 3.6. Image and energy distribution of Rn daughters We tried to image the distribution of Rn daughters by using the developed detector. Because
S. Yamamoto, T. Iida/Nucl. Instr. and Meth. in Phys. Res. A 418 (1998) 387—393
391
Fig. 6. Image of sliced lung of a rat administered 239Pu particles obtained by the alpha detector (a) and a film based autoradiograph (b).
Fig. 7. Image of the alpha emitting particle obtained by the alpha detector.
minus high voltage is supplied on the photocathode of the PSPMT which act as an electrode of the electrostatically collection of Rn daughters, Rn daughters can be collected on the input surface of the detector. Count rate for the measurement was
Fig. 8. Image of the alpha particles from the radon daughters obtained by the alpha detector.
approximately 0.01 cps. An image of the alpha particles from the radon daughters is shown in Fig. 8. To clarify that the image contains only radon daughters, energy distribution was measured. Energy distribution obtained in the measurement is
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S. Yamamoto, T. Iida/Nucl. Instr. and Meth. in Phys. Res. A 418 (1998) 387—393
Fig. 9. Energy distribution of Rn daughters obtained by the alpha detector. Two peaks which corresponds to 218Po (6.0 MeV) and 214Po (7.7 MeV) are clearly observed.
shown in Fig. 9. Two peaks which corresponds to alpha particles of 218Po (6.0 MeV) and 214Po (7.7 MeV) were clearly obtained. Background counts due to the environmental beta particles or gamma ray can be observed at lower channels of the distribution. Energy resolution for 218Po and 214 Po were 24% and 20% FWHM, respectively. A little worse resolution compared with 241Am alpha particles is probably due to the different distribution of the source. The diameter of the 241Am source was 10 mm which occupied only a small part of the detector input surface while the Rn daughters were distributed on entire input surface and was affected more by the nonuniform response of the PSPMT.
4. Discussion and conclusion We have developed and tested the position-sensitive alpha detector. The plastic scintillator of 50 lm thickness was used for the detector. The thickness was determined to absorb most of the energy of alpha particles of targets (239Pu, 238U, 218Po) in the scintillator. If thicker scintillator is selected, the background counts due to the beta particles or gamma ray will increase. The spatial resolution of the detector was not good as the previous report [15]. That is mainly due to the relatively lower light output of the plastic scintillator for alpha particles.
In fact, NE-102A plastic scintillator has only times light output for 241Am alpha particles (5.5 MeV) to CaF (Eu) [15]. Coating a reflective material on the plastic scintillator surface may be one solution to improve the spatial resolution. However, reflector absorbs a part of the energy of the alpha particles thus decreases the energy response. Using a tapered fiber is another solution to obtain higher resolution, however the field of view becomes smaller by the magnification ratio [16]. ZnS (Ag) has relatively high light output for alpha particles and is often used for alpha detectors. However, ZnS (Ag) has no transparency so one cannot obtain energy information with it. Thus, it is quite likely that a thin plastic scintillator is a better selection for alpha imaging. Energy information was useful to determine if the detected events are really alpha particles. Though the energy resolution of the detector is not very good compared with silicon semiconductor detectors, obtained energy information was useful to confirm that the detected events were Rn daughters. The Hamamatsu PSPMT is known to have non-uniform tube gain across the PSPMT active region which was observed in the image of the spatial resolution phantom (Fig. 3). The non-uniformity was also observed in this study for energy distribution measurements. The energy resolution for 218Po or 214Po was worse than that for 241Am though the energy of former is higher than that of latter. This is because the 218Po or 214Po distributed all over the active region while the 241Am source was smaller and detected only small region of the PSPMT. It will be possible to improve the energy resolution if an energy correction scheme is applied [18]. For conclusion, it is confirmed that the developed position sensitive alpha detector is useful for rapid imaging of alpha particles such as Pu detection or brief imaging of alpha autoradiography which is performed before high resolution and long-time exposure.
Acknowledgements Authors would like to thank Dr. Shimo and Dr. Ishigure in National Institute of Radiological
S. Yamamoto, T. Iida/Nucl. Instr. and Meth. in Phys. Res. A 418 (1998) 387—393
Science for helpful discussion and corporation in some of the experiments. This work was partly supported by Japan Science and Technology Corporation. References [1] T. Iida, Y. Ikebe et al., Health Phys. 54 (2) (1988) 139—148. [2] J.J.C. Hsieh, F.P. Hungate, S.A. Wilson, Science 150 (1965) 1821—1822. [3] T. Iida, Rev. Sci. Instr. 59 (1988) 2206—2210. [4] H. Kume, T. Suzuki, T. Matsushita et al., IEEE Trans. Nucl. Sci. Ns-32 (1985) 448—452. [5] H. Uchida, M. Yamashita, M. Iida et al., IEEE Trans. Nucl. Sci. Ns-33 (1986) 889—893. [6] T. Yamashita, M. Watanabe, K. Shimizu et al., IEEE Trans. Nucl. Sci. Ns-37 (1990) 589—593. [7] M. Watanabe, H. Okada, H. Kyushima et al., IEEE Trans. Nucl. Sci. Ns-42 (1995) 655—658.
393
[8] M. Watanabe, H. Okada, K. Shimizu, T. Omura et al., IEEE Trans. Nucl. Sci. Ns-44 (3) (1997) 1277—1282. [9] C.J. Thompson, K. Murthy et al., IEEE Trans. Nucl. Sci. Ns-42 (1995) 1012—1017. [10] C.S. Levin, E.J. Hoffman, M.P. Tornai et al., IEEE Trans. Nucl. Sci. Ns-44 (1990) 1513—1520. [11] N.J. Yasillo, R.N. Beck, M. Cooper, IEEE Trans. Nucl. Sci. Ns-37 (1990) 609—615. [12] A.J. Bird et al., Nucl. Instr. and Meth. A 348 (1994) 668—672. [13] A.R. Faruqi, H.N. Andrews, Nucl. Instr. and Meth. A 392 (1997) 233—236. [14] I. Aubineau-Laniece, Y. Charon, G. Dorner et al., Nucl. Instr. and Meth. A 392 (1997) 237—243. [15] S. Yamamoto, T. Iida, I. Kanno, Nucl. Instr. and Meth. A 392 (1997) 291—294. [16] Knoll, Radiation Detection and Measurement, Wiley, New York, 1979. [17] S. Yamamoto, C. Seki, K. Kashikura et al., IEEE Trans. Nucl. Sci. Ns-44 (1997) 1538—1542. [18] J.L. Robar, C.J. Thompson, K. Murthy et al., Nucl. Instr. and Meth. A 392 (1997) 402.
A position-sensitive alpha detector using a thin plastic scintillator combined with a position-sensitive photomultiplier tube Seiichi Yamamoto *, Takao Iida Department of Electrical Engineering, Kobe City College of Technology 8-3, Gakuen-Higashi-machi, Nishi-ku Kobe, 651-21, Japan Department of Nuclear Engineering, Nagoya University, Japan Received 10 March 1998; received in revised form 18 May 1998
Abstract A position-sensitive alpha detector was developed and tested. The alpha detector consists of a thin plastic scintillator, a position-sensitive photomultiplier tube, a position calculation circuit and a personal computer based data acquisition system. Because the thin plastic scintillator has high-detection efficiency for alpha particles while it has low-sensitivity for beta particles or gamma ray, the detector can selectively detect alpha particles with low background counts. The spatial resolution of the detector was approximately 3 mm FWHM. An autoradiographic images of plutonium distribution in the lung of an animal as well as an image of an uranium particle were successively obtained. Spatial and energy distribution of radon daughters could also be measured. We conclude that the developed position-sensitive alpha detector is useful for some applications such as plutonium detection or alpha autoradiography as well as distribution analysis of radon daughters. 1998 Elsevier Science B.V. All rights reserved. Keywords: Position-sensitive alpha detectors; Photomultiplier tube; Gamma rays; Autoradiography
1. Introduction Imaging of alpha particles is sometimes necessary in several fields. For example, in the facility where plutonium (Pu) is handled, imaging of alpha particles is used to distinguish plutonium particles from radon daughters. For another application of alpha imaging, alpha autoradiography of animals is used for estimating the distribution of alpha emitting radionuclide such as Pu or uranium (U) in
* Corresponding author. Tel.: #81 78 795 3232; fax: #81 78 795 3314; e-mail: [email protected].
the body. We are also interested in the distribution of electrostatically collected radon (Rn) daughters for a radon monitor [1]. The ZnS (Ag) autoradiography [2] or video camera system [3] has been used for alpha imaging. However, ZnS (Ag) autoradiography needs much time to obtain clear images because of its low sensitivity. In addition, one cannot obtain any energy information with these systems. A positionsensitive photomultiplier tube (PSPMT) [4] is one candidate for a component of the alpha imaging system because both the position and energy information can be obtained by using the PSPMT. It has been used for many applications
0168-9002/98/$19.00 1998 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 8 ) 0 8 8 6 - 9
388
S. Yamamoto, T. Iida/Nucl. Instr. and Meth. in Phys. Res. A 418 (1998) 387—393
such as detector modules for positron emission tomography (PET) [5—8] and small gamma cameras [9—12]. Another candidate for alpha imaging is a charge coupled device (CCD) using directory [13] or with scintillator via image intensifier [14]. We previously reported a high-resolution alpha camera using a CaF (Eu) scintillator, a tapered fiber optics plate and a PSPMT [15]. However, the 0.5 mm thick CaF (Eu) scintillator was so thick and dense that environmental beta particles orgamma ray were detected considerably in the situation of low alpha counting rate. The energy information of the camera was not useful to distinguish alpha particles from these beta particles or gamma rays because the pulse height for these beta particles or gamma rays are similar or higher than the energy of the alpha particles such as plutonium or radon daughters. The count rate due to the background beta particles or gamma rays can be decreased if a scintillator which has low detection efficiency for beta particles or gamma rays but has
high detection efficiency for alpha particles is used. Using a thinner CaF (Eu) scintillator may be one candidate, however thickness of 0.5 mm seem to be the limit when considering the mechanical strength of the CaF (Eu) scintillator. Thus, we try to use a thin plastic scintillator combined with PSPMT for imaging alpha particles while minimizing the background counts due to the environmental beta particles or gamma rays. First, we estimated the performance of the system. Then, we tried some applications to imaging alpha particles in the situation of low count rate.
2. Materials and methods Schematic diagram of the detector part of the developed position-sensitive alpha detector is shown in Fig. 1. The detector is quite simple which consists of a thin plastic scintillator and a PSPMT. They are contained in a black box which is supported by a base element.
Fig. 1. Schematic diagram of the developed position-sensitive alpha detector.
S. Yamamoto, T. Iida/Nucl. Instr. and Meth. in Phys. Res. A 418 (1998) 387—393
389
A photograph of the detector part is shown in Fig. 2. The plastic scintillator has thickness of 50 lm, thus most of the energy of target alpha particles such as 239Pu (5.1 MeV), 238U (4.2 MeV) or 218 Po (6.0 MeV) can be absorbed in the scintillator [16]. Because the thickness of the plastic scintillator is so thin in addition to relatively low stopping power for beta particles or gamma ray, energy loss of the environmental beta particles in the scintillator and the detection probability of environmental gamma rays become quite small. The plastic scintillator used was NE102A and had a diameter of 50 mm (Ouyo-Koken,Japan) optically coupled to the PSPMT by KE-420 silicon rubber (Shinetsu Chemical, Japan). The scintillation photons in the plastic scintillator are fed to the PSPMT. The PSPMT used was Hamamatsu R2486-06, which has useful diameter of 50 mm. The scintillation photons are converted to photoelectrons in the photocathode of the PSPMT and multiplied by the mesh dynodes while keeping the position information. The multiplied electrons are read out by the cross wire anodes of the PSPMT in X and ½ direction, respectively. The signals from the cross wire anodes are fed to the resistor chain to produce four signals which correspond to X>, X\, Y> and Y\.
The signals are fed to the position calculation circuit which is also used for beta camera [17]. Energy threshold was set to approximately 2.5 MeV for the pulse height of alpha particles. The calculated position signal is fed to CAMAC and K-max based data acquisition system and transferred to a personal computer (Power Macintosh). Image display and analysis are performed using the K-max software on the personal computer.
Fig. 2. Photograph of the detector part of the position-sensitive alpha detector.
Fig. 3. Image of the resolution phantom obtained by the alpha detector.
3. Results 3.1. Spatial resolution Spatial resolution is one of the most important performance for position-sensitive detector. Thus, we estimated the performance for alpha particles. A resolution phantom and a 241Am plane alpha source (5.5 MeV) were used for the measurements. The alpha source was attached on the resolution phantom and set on the input surface of the detector. An image of the alpha particle distribution which passed through the resolution phantom was acquired. The obtained image of the resolution phantom is shown in Fig. 3. The slits widths of the resolution phantom are 1, 1.5, 2 and 2.5 mm, respectively. The slits of 1.5 mm was clearly resolved which is obvious from the profile distribution perpendicular to the slits. Thus, the spatial resolution
390
S. Yamamoto, T. Iida/Nucl. Instr. and Meth. in Phys. Res. A 418 (1998) 387—393
for 5.5 MeV alpha particles was estimated to be approximately 3 mm FWHM. 3.2. Spatial linearity Spatial linearity was measured using a linearity phantom and the same 241Am alpha source as was used for spatial resolution measurement. An image of the spatial linearity phantom is shown in Fig. 4. A slight distortion was observed in the image which is probably due to the distortion of the PSPMT. 3.3. Energy resolution Energy resolution was measured using a 241Am source which has a diameter of approximately 10 mm placed on the detector surface. The energy signal of the position calculation circuit was fed to a multi-channel analyzer (MCA). The energy distribution for 241Am alpha particles (5.5 MeV) is shown in Fig. 5. Energy resolution was 15% FWHM. 3.4. Image of
Fig. 4. Image of the spatial linearity phantom obtained by the alpha detector.
239
Pu autoradiography
For an alpha autoradiography application, a sliced lung of a rat administered 239Pu particles was imaged. The measurement was done in a controlled laboratory at National Institute of Radiological Science (Japan). The sliced lung was placed on the detector surface through a thin Mylar sheet to prevent Pu contamination to the detector. Count rate of the sample was approximately 1 cps. The acquisition time was 30 min. The image obtained by the detector is shown in Fig. 6a. The distribution of 239Pu in the lung was obvious though the spatial resolution was not good enough to observe small structures. For comparison, a CR-39 film based autoradiograph of the same slice which took one day is shown Fig. 6b. 3.5. Image of alpha emitting particle An alpha emitting particle was imaged for estimating the possibility of distinguishing alpha emitting particles from Rn daughters. The alpha emitting particle was an approximately 0.3 mm diameter particle which contains U obtained from a small piece of an ore. Count rate for the measure-
Fig. 5. Energy distribution for of the alpha detector.
241
Am alpha particles (5.5 MeV)
ment was approximately 0.1 cps. The image of the alpha emitting particle is shown in Fig. 7. The hot spot corresponds to the alpha emitting particle. Pixels which contain only one count in the image are probably from alpha particles of Rn daughters. 3.6. Image and energy distribution of Rn daughters We tried to image the distribution of Rn daughters by using the developed detector. Because
S. Yamamoto, T. Iida/Nucl. Instr. and Meth. in Phys. Res. A 418 (1998) 387—393
391
Fig. 6. Image of sliced lung of a rat administered 239Pu particles obtained by the alpha detector (a) and a film based autoradiograph (b).
Fig. 7. Image of the alpha emitting particle obtained by the alpha detector.
minus high voltage is supplied on the photocathode of the PSPMT which act as an electrode of the electrostatically collection of Rn daughters, Rn daughters can be collected on the input surface of the detector. Count rate for the measurement was
Fig. 8. Image of the alpha particles from the radon daughters obtained by the alpha detector.
approximately 0.01 cps. An image of the alpha particles from the radon daughters is shown in Fig. 8. To clarify that the image contains only radon daughters, energy distribution was measured. Energy distribution obtained in the measurement is
392
S. Yamamoto, T. Iida/Nucl. Instr. and Meth. in Phys. Res. A 418 (1998) 387—393
Fig. 9. Energy distribution of Rn daughters obtained by the alpha detector. Two peaks which corresponds to 218Po (6.0 MeV) and 214Po (7.7 MeV) are clearly observed.
shown in Fig. 9. Two peaks which corresponds to alpha particles of 218Po (6.0 MeV) and 214Po (7.7 MeV) were clearly obtained. Background counts due to the environmental beta particles or gamma ray can be observed at lower channels of the distribution. Energy resolution for 218Po and 214 Po were 24% and 20% FWHM, respectively. A little worse resolution compared with 241Am alpha particles is probably due to the different distribution of the source. The diameter of the 241Am source was 10 mm which occupied only a small part of the detector input surface while the Rn daughters were distributed on entire input surface and was affected more by the nonuniform response of the PSPMT.
4. Discussion and conclusion We have developed and tested the position-sensitive alpha detector. The plastic scintillator of 50 lm thickness was used for the detector. The thickness was determined to absorb most of the energy of alpha particles of targets (239Pu, 238U, 218Po) in the scintillator. If thicker scintillator is selected, the background counts due to the beta particles or gamma ray will increase. The spatial resolution of the detector was not good as the previous report [15]. That is mainly due to the relatively lower light output of the plastic scintillator for alpha particles.
In fact, NE-102A plastic scintillator has only times light output for 241Am alpha particles (5.5 MeV) to CaF (Eu) [15]. Coating a reflective material on the plastic scintillator surface may be one solution to improve the spatial resolution. However, reflector absorbs a part of the energy of the alpha particles thus decreases the energy response. Using a tapered fiber is another solution to obtain higher resolution, however the field of view becomes smaller by the magnification ratio [16]. ZnS (Ag) has relatively high light output for alpha particles and is often used for alpha detectors. However, ZnS (Ag) has no transparency so one cannot obtain energy information with it. Thus, it is quite likely that a thin plastic scintillator is a better selection for alpha imaging. Energy information was useful to determine if the detected events are really alpha particles. Though the energy resolution of the detector is not very good compared with silicon semiconductor detectors, obtained energy information was useful to confirm that the detected events were Rn daughters. The Hamamatsu PSPMT is known to have non-uniform tube gain across the PSPMT active region which was observed in the image of the spatial resolution phantom (Fig. 3). The non-uniformity was also observed in this study for energy distribution measurements. The energy resolution for 218Po or 214Po was worse than that for 241Am though the energy of former is higher than that of latter. This is because the 218Po or 214Po distributed all over the active region while the 241Am source was smaller and detected only small region of the PSPMT. It will be possible to improve the energy resolution if an energy correction scheme is applied [18]. For conclusion, it is confirmed that the developed position sensitive alpha detector is useful for rapid imaging of alpha particles such as Pu detection or brief imaging of alpha autoradiography which is performed before high resolution and long-time exposure.
Acknowledgements Authors would like to thank Dr. Shimo and Dr. Ishigure in National Institute of Radiological
S. Yamamoto, T. Iida/Nucl. Instr. and Meth. in Phys. Res. A 418 (1998) 387—393
Science for helpful discussion and corporation in some of the experiments. This work was partly supported by Japan Science and Technology Corporation. References [1] T. Iida, Y. Ikebe et al., Health Phys. 54 (2) (1988) 139—148. [2] J.J.C. Hsieh, F.P. Hungate, S.A. Wilson, Science 150 (1965) 1821—1822. [3] T. Iida, Rev. Sci. Instr. 59 (1988) 2206—2210. [4] H. Kume, T. Suzuki, T. Matsushita et al., IEEE Trans. Nucl. Sci. Ns-32 (1985) 448—452. [5] H. Uchida, M. Yamashita, M. Iida et al., IEEE Trans. Nucl. Sci. Ns-33 (1986) 889—893. [6] T. Yamashita, M. Watanabe, K. Shimizu et al., IEEE Trans. Nucl. Sci. Ns-37 (1990) 589—593. [7] M. Watanabe, H. Okada, H. Kyushima et al., IEEE Trans. Nucl. Sci. Ns-42 (1995) 655—658.
393
[8] M. Watanabe, H. Okada, K. Shimizu, T. Omura et al., IEEE Trans. Nucl. Sci. Ns-44 (3) (1997) 1277—1282. [9] C.J. Thompson, K. Murthy et al., IEEE Trans. Nucl. Sci. Ns-42 (1995) 1012—1017. [10] C.S. Levin, E.J. Hoffman, M.P. Tornai et al., IEEE Trans. Nucl. Sci. Ns-44 (1990) 1513—1520. [11] N.J. Yasillo, R.N. Beck, M. Cooper, IEEE Trans. Nucl. Sci. Ns-37 (1990) 609—615. [12] A.J. Bird et al., Nucl. Instr. and Meth. A 348 (1994) 668—672. [13] A.R. Faruqi, H.N. Andrews, Nucl. Instr. and Meth. A 392 (1997) 233—236. [14] I. Aubineau-Laniece, Y. Charon, G. Dorner et al., Nucl. Instr. and Meth. A 392 (1997) 237—243. [15] S. Yamamoto, T. Iida, I. Kanno, Nucl. Instr. and Meth. A 392 (1997) 291—294. [16] Knoll, Radiation Detection and Measurement, Wiley, New York, 1979. [17] S. Yamamoto, C. Seki, K. Kashikura et al., IEEE Trans. Nucl. Sci. Ns-44 (1997) 1538—1542. [18] J.L. Robar, C.J. Thompson, K. Murthy et al., Nucl. Instr. and Meth. A 392 (1997) 402.