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A high-sensitivity portable spectrometer for ESR dosimetry
Appl. Radiat. Isot. Vol. 47, No. 11/12,pp. 1589-1594,1996
Pergamon PII: S0969-8043(96)00129-7
Copyright© 1996ElsevierScienceLtd Printed in Great Britain. All rights reserved 0969-8043/96 $15.00+ 0.00
A High-sensitivity Portable Spectrometer for ESR Dosimetry T O S H I H I D E O K A *l, M O T O J I I K E Y A j, N A H O K O S U G A W A R A 2 and AKIO NAKANISHF ~Department of Earth and Space Science, Osaka University, Toyonaka, Osaka 560 and 2Sumitomo Special Metals Co. Ltd, Shimamoto, Osaka 618, Japan A portable ESR spectrometer using the permanent magnet, NEOMAX (Nd-B-Fe), has been developed to detect 2 x 10~°spins/0.1 mT by further improvements of the magnet, microwave and electric systems from previous spectrometer (SPIN-XX). The thermal stability of the NEOMAX was improved from 1200 ppm/°C to 35 ppm/°C. The magnetic circuit and a ceramic cavity are described with the applications to radiation dosimetry using an alanine dosimeter irradiated at CERN, a shell button for accident dosimetry and to dating of aragonitic corals. Copyright © 1996 Elsevier Science Ltd
Introduction Electron spin resonance (ESR) spectroscopy has been used in physics and chemistry for radiation dosimetry and dating. Although many researchers hope to use their own portable ESR spectrometer, the present commercial ESR spectrometers are generally large and expensive because of a heavy electromagnet with water cooling attachment. Recently, techniques of microwave and electronics have been improved dramatically. Sagawa et al. (1984) developed the most intense permanent magnet alloy, NEOMAX (Nd-BFe), with superior magnetic properties and mechanical strength. NEOMAX is used for a magnet circuit in magnetic resonance imaging (MRI) which requires intense magnetic field and high homogeneity (Sakurai et al., 1990). A small and low cost ESR spectrometer has been manufactured based on the NEOMAX and recent microwave technology (lkeya and Furusawa, 1989; Yamanaka et al., 1991). However, the spectrometer was used only to demonstrate the possibility and was useful only in the student experiments (lkeya et al., 1991). A sensitive spectrometer of this series is SPIN-XX which has the sensitivity of 5 x 10~2 spins/0.1 mT and weight of only 5 kg (Nakanishi et al., 1993). We have further developed the sensitivity dramatically down to 2.0 x 10r° spins/ 0.1 mT which is much higher than that of SPIN-XX and comparable to that of commercial ESR spectrometers (0.8 ~ 1 × 10~° spins/0.1 mT). In this paper, the performance of the new ESR spectrometer using NEOMAX is compared to that of the commercial ESR spectrometers. This new *To whom all correspondence should be addressed.
spectrometer can finally be used in various fields such as radiation dosimetry, geological dating, analysis of new materials and so on, both in laboratory and in situ field measurements. Apparatus
A new spectrometer (Sumitomo Special Metals and Nikkisou Co., Ltd, ES-10) is used throughout this work and compared with other spectrometers (JEOL FE-IX and JEOL RE-IX). Figure 1 illustrates the magnet system consisting of two NEOMAX, temperature compensation metal (MS), pole pieces, yokes and magnetic field sweep coils. The large temperature coefficient of the NEOMAX was improved dramatically from 1200 ppm/°C to 35 ppm/°C or from about 0.4 mT/°C to about 0.012 mT/°C using the temperature compensation metal shown in Fig. l(a). Hence, we are free from the temperature variation so far as the magnetic field is concerned. A photograph of the magnet system having the size of 60 x 130 × 130 mm and 6 kg in weight are shown in Fig. i(b). A conventional homodyne microwave system is adopted at the frequency of 9.4 GHz from a commercial low noise gun diode with continuous power variation up to 150 mW using a microwave attenuator and amplifier. The microwave frequency is fixed by AFC system at the resonant frequency with the accuracy of 10-5. Planar microwave devices such as isolators are linked with semi-rigid coaxial lines instead of using solid circuits of waveguides. The TE~02 mode rectangular cavity is made of ceramics coated with thin gold metal as shown in Fig. 2. The thermal expansion of the metal cavity, which leads to the shift of the resonance frequency, is thus reduced by using ceramics coated with Au.
1589
Toshihide Oka et al.
1590
Table I. Somespecificationsof a new spectrometer, ES-10 Magneticcircuit System 335 _+5 mT Sensitivity 2 x 10'° spins/0.1 mT < 100 ppm/q~ 5 x 20 mm Resolution 6 x 10-~ < 30 ppm/h Accuracy < 2% < 150 ppm/°C Sample tube ~b5mm quartz > 20 mT Detector crystal detector f = 9.4 GHz Computer built-in32 bits computer Pm.~= 150 mW Size 500(W) x 500(D)x 230(H) mm 2 mT (max.) Weight 25 kg Power source 100 V single-phasecurrent, 80 W
H0 AHIHo dHIdt ~/A~ Sweep width Microwave Modulation
The magnetic modulation at 100 kHz is applied from the outside of the cavity by the coil attached at the pole pieces. The high impedance coils allow one to remove the transformer for high current in the low impedance coils inserted in a cavity. The resonant signal is amplified at the first step by a low-noise microwave pre-amplifier and next detected through the locked-in amplifier. The system is operated by the digital circuit using a built-in 32 bit computer as specified in Table 1 and photographed in Fig. 3. The linewidth of DPPH was plotted against the position in the cavity as shown in Fig. 4 when the test sample of two DPPH dots separated for 10 mm was vertically moved. The uncertainty in the measurements is about 5%, 0.005 mT. The flat line indicates a good uniformity of static magnetic field in the effective sample length. A high homogeneity of the magnetic field is characteristic of this new permanent magnet system. Experimental results
One of the most important applications of this spectrometer is the measurement of gamma radiation dose. The ESR signal intensities of orthorhombic CO2- radicals at g = 1.9973 in the sample of eggshells irradiated by gamma rays to 5 kGy are plotted
against the sample weight in Fig. 5. The intensities increase linearly with the sample weight up to a level of about 200 mg. The ESR intensities of the commercial alanine dosimeter are plotted against gamma irradiation from 1 Gy to 2 x 105 Gy in Fig. 6. Results show a good linear correlation in the range of 10 Gy to 20 kGy and usefulness in radiation dosimetry in the intermediate dose range. The non-linearity in the low dose range is due to the presence of a signal at zero dose in this dosimeter material. Figure 7(a) shows ESR spectra of shell buttons after gamma irradiation at several different doses. These spectra were recorded with a scan range of 6 mT and a magnetic field modulation amplitude of 0.4 mT. The signals at g = 2.0007 (signal C) and g = 1.9973 (signal D) are due to isotropic and orthorhombic C O l , respectively (Ikeya, 1993). The intensities of signal C and D are plotted against the dose in Fig. 7(b). Black points and white points were obtained with ES-10 and a former commercial ESR spectrometer, FE-IX, respectively. The sensitivity of ES-10 is comparable to that of FE-IX with the specification of 10~° spins/0.1 mT but less than that within a factor 1.5 ~ 2.0 in this case. The specification of this spectrometer was thus confirmed for shell buttons.
0.2
Samp'eTube IDPPH
gE .x2 0.1
•
°
5 mm
¢._1
°~
10 mm 5 mm
0
-
'
I
,
I
0
,
Eggshell (5kGy)
I
2
Position (cm) <
)
Effective sample length
Fig. 4. The plot of linewidth of DPPH versus the position in a cavity when two DPPH dots separated by I0 ram were vertically moved to check the uniformity of magnetic field.
6
'
16o Weight
2o0 (mg)
Fig. 5. Relation between the ESR intensity and sample weight. The sample eggshells were irradiated to 5 kGy.
159
A portable ESR spectrometer
(a
NEOMAX
Sweep coil
Yok
(b)
Fig. 1. (a) Assembly of the permanent magnet system using Nd-B-Fe alloys and (b) a photograph of the magnet.
1592
Toshihide Oka et al.
Fig. 2. The ceramic cavity coated with thin gold metal.
Fig. 3. The ESR spectrometer used in this study. It is 500 × 500 × 230 (mm) in size and 25 kg in weight.
1593
A portable ESR spectrometer
4}
100
101
Sample: Holocene coral
/ o/
"5" 100
z-
o} e- 10-1 (I) t"
ffl e-"
5o n-
OC 10.2 W
10-3,
~
Alanine dosimeter ...... z
I
10"~ 100
I
101
I
I
102
I
103 104
105
Dose (Gy) Fig. 6. Relation between the ESR intensity and the irradiation dose:the alaninedosimeterirradiatedat CERN is used as a sample.
I
0
I
50 Dose (Gy)
Fig. 8. The growth of the ESR intensity of corals at Ryukyu Islands as a function of additive dose. The equivalent dose of 10 Gy was obtained. Black and white dots were obtained with RE-IX and ES-10, respectively.
(a)
(b) . . . .
13.75 Gy
100
/.~,.
signal D go=1.9973
60
03 t"(D
i
. . . .
]
,
,
O: Signal C (FE-IX) / I1: Signal D (FE-1X) ~ " O: Signal C (ES-10) , / / I--7:Signal D (ES-10)
o.
40
24.75 Gy o~ 20 w a
Shell button
gc=2.0007
1 rnT
....
lt] . . . .
a
2t]'
Dose (Gy)
Fig. 7. (a) ESR spectra of modern shell buttons at different doses; 13.75, 19.35 and 24.75 Gy. (b) The intensities of signal C and D due to orthorhombic CO2- were plotted against additive dose. Black and white dots were obtained with FE-IX and ES-10, respectively.
Another important application of this spectrometer is as a readout device for ESR dating. The E D of Holocene corals from Kikai-jima, Ryukyu Islands, Japan was obtained with ES-10 and R E - I X using same samples (lkeya and Ohmura, 1983). The white dots and black dots were obtained with ES-10 and R E - I X , respectively as shown in Fig. 8. The sensitivity for the case of coral is the factor 2 ,,, 3 less than that of R E - I X with the specification of 8 x 1 0 9 spins/0.1 mT. These results indicate that the present spectrometer (ES-10) can also be used for dating of corals and
other geological materials as well as for development and routine use as radiation dosimeters and for monitoring irradiated food.
Acknowledgements--We thank Mr Kazuo Nakagawa and Mr Makoto Tsuneda of Nikkiso Co., Ltd for supplying the spectrometer and some advice.
References lkeya M. and Furusawa M. (1989) A portable spectrometer for ESR microscopy, dosimetry and dating. Appl. Radiat. lsot. 40, 845--850.
1594
Toshihide Oka et al.
Ikeya M., Meguro K., Miyamaru H. and Ishii H. (1991) Educational Experiments on ESR Imaging with a Portable ESR Spectrometer. Appl. Magn. Res. 2, 663-673. lkeya M. and Ohmura K. 0983) Comparison of ESR ages of corals at marine terraces with '4C and 2~rh/234U ages. Earth Planet. Sci. Left. 65, 34-34. Ikeya M. (1993) New Applications of Electron Spin Resonance-Dating, Dosimetry and Microscopy. World Scientific, Singapore. Nakanishi A., Sugawara N. and Furuse A. (1993) Portable
ESR spectrometer with NEOMAX (Nd-B-Fe) permanent magnet circuit. Appl. Radiat. Isot. 44, 357-360. Sagawa M., Fujimura S., Togawa N., Yamamoto H. and Matuura Y. 0984) New material for P. M. on a base on Nd and Fe. J. Appl. Phys. 55, 2083-2087. Sakurai H., Aoki M. and Miyamoto T. (1990) Improvement of the field homogeneity with a permanent magnet assembly for MRI. J. Mag. Soc. Jap. 14, 2, 465--468. Yamanaka C., Ikeya M., Meguro K. and Nakanishi A. (1991) A portable ESR spectrometer using Nd-B--Fe permanent magnets. Nucl. Tracks 18, 279-282.
Pergamon PII: S0969-8043(96)00129-7
Copyright© 1996ElsevierScienceLtd Printed in Great Britain. All rights reserved 0969-8043/96 $15.00+ 0.00
A High-sensitivity Portable Spectrometer for ESR Dosimetry T O S H I H I D E O K A *l, M O T O J I I K E Y A j, N A H O K O S U G A W A R A 2 and AKIO NAKANISHF ~Department of Earth and Space Science, Osaka University, Toyonaka, Osaka 560 and 2Sumitomo Special Metals Co. Ltd, Shimamoto, Osaka 618, Japan A portable ESR spectrometer using the permanent magnet, NEOMAX (Nd-B-Fe), has been developed to detect 2 x 10~°spins/0.1 mT by further improvements of the magnet, microwave and electric systems from previous spectrometer (SPIN-XX). The thermal stability of the NEOMAX was improved from 1200 ppm/°C to 35 ppm/°C. The magnetic circuit and a ceramic cavity are described with the applications to radiation dosimetry using an alanine dosimeter irradiated at CERN, a shell button for accident dosimetry and to dating of aragonitic corals. Copyright © 1996 Elsevier Science Ltd
Introduction Electron spin resonance (ESR) spectroscopy has been used in physics and chemistry for radiation dosimetry and dating. Although many researchers hope to use their own portable ESR spectrometer, the present commercial ESR spectrometers are generally large and expensive because of a heavy electromagnet with water cooling attachment. Recently, techniques of microwave and electronics have been improved dramatically. Sagawa et al. (1984) developed the most intense permanent magnet alloy, NEOMAX (Nd-BFe), with superior magnetic properties and mechanical strength. NEOMAX is used for a magnet circuit in magnetic resonance imaging (MRI) which requires intense magnetic field and high homogeneity (Sakurai et al., 1990). A small and low cost ESR spectrometer has been manufactured based on the NEOMAX and recent microwave technology (lkeya and Furusawa, 1989; Yamanaka et al., 1991). However, the spectrometer was used only to demonstrate the possibility and was useful only in the student experiments (lkeya et al., 1991). A sensitive spectrometer of this series is SPIN-XX which has the sensitivity of 5 x 10~2 spins/0.1 mT and weight of only 5 kg (Nakanishi et al., 1993). We have further developed the sensitivity dramatically down to 2.0 x 10r° spins/ 0.1 mT which is much higher than that of SPIN-XX and comparable to that of commercial ESR spectrometers (0.8 ~ 1 × 10~° spins/0.1 mT). In this paper, the performance of the new ESR spectrometer using NEOMAX is compared to that of the commercial ESR spectrometers. This new *To whom all correspondence should be addressed.
spectrometer can finally be used in various fields such as radiation dosimetry, geological dating, analysis of new materials and so on, both in laboratory and in situ field measurements. Apparatus
A new spectrometer (Sumitomo Special Metals and Nikkisou Co., Ltd, ES-10) is used throughout this work and compared with other spectrometers (JEOL FE-IX and JEOL RE-IX). Figure 1 illustrates the magnet system consisting of two NEOMAX, temperature compensation metal (MS), pole pieces, yokes and magnetic field sweep coils. The large temperature coefficient of the NEOMAX was improved dramatically from 1200 ppm/°C to 35 ppm/°C or from about 0.4 mT/°C to about 0.012 mT/°C using the temperature compensation metal shown in Fig. l(a). Hence, we are free from the temperature variation so far as the magnetic field is concerned. A photograph of the magnet system having the size of 60 x 130 × 130 mm and 6 kg in weight are shown in Fig. i(b). A conventional homodyne microwave system is adopted at the frequency of 9.4 GHz from a commercial low noise gun diode with continuous power variation up to 150 mW using a microwave attenuator and amplifier. The microwave frequency is fixed by AFC system at the resonant frequency with the accuracy of 10-5. Planar microwave devices such as isolators are linked with semi-rigid coaxial lines instead of using solid circuits of waveguides. The TE~02 mode rectangular cavity is made of ceramics coated with thin gold metal as shown in Fig. 2. The thermal expansion of the metal cavity, which leads to the shift of the resonance frequency, is thus reduced by using ceramics coated with Au.
1589
Toshihide Oka et al.
1590
Table I. Somespecificationsof a new spectrometer, ES-10 Magneticcircuit System 335 _+5 mT Sensitivity 2 x 10'° spins/0.1 mT < 100 ppm/q~ 5 x 20 mm Resolution 6 x 10-~ < 30 ppm/h Accuracy < 2% < 150 ppm/°C Sample tube ~b5mm quartz > 20 mT Detector crystal detector f = 9.4 GHz Computer built-in32 bits computer Pm.~= 150 mW Size 500(W) x 500(D)x 230(H) mm 2 mT (max.) Weight 25 kg Power source 100 V single-phasecurrent, 80 W
H0 AHIHo dHIdt ~/A~ Sweep width Microwave Modulation
The magnetic modulation at 100 kHz is applied from the outside of the cavity by the coil attached at the pole pieces. The high impedance coils allow one to remove the transformer for high current in the low impedance coils inserted in a cavity. The resonant signal is amplified at the first step by a low-noise microwave pre-amplifier and next detected through the locked-in amplifier. The system is operated by the digital circuit using a built-in 32 bit computer as specified in Table 1 and photographed in Fig. 3. The linewidth of DPPH was plotted against the position in the cavity as shown in Fig. 4 when the test sample of two DPPH dots separated for 10 mm was vertically moved. The uncertainty in the measurements is about 5%, 0.005 mT. The flat line indicates a good uniformity of static magnetic field in the effective sample length. A high homogeneity of the magnetic field is characteristic of this new permanent magnet system. Experimental results
One of the most important applications of this spectrometer is the measurement of gamma radiation dose. The ESR signal intensities of orthorhombic CO2- radicals at g = 1.9973 in the sample of eggshells irradiated by gamma rays to 5 kGy are plotted
against the sample weight in Fig. 5. The intensities increase linearly with the sample weight up to a level of about 200 mg. The ESR intensities of the commercial alanine dosimeter are plotted against gamma irradiation from 1 Gy to 2 x 105 Gy in Fig. 6. Results show a good linear correlation in the range of 10 Gy to 20 kGy and usefulness in radiation dosimetry in the intermediate dose range. The non-linearity in the low dose range is due to the presence of a signal at zero dose in this dosimeter material. Figure 7(a) shows ESR spectra of shell buttons after gamma irradiation at several different doses. These spectra were recorded with a scan range of 6 mT and a magnetic field modulation amplitude of 0.4 mT. The signals at g = 2.0007 (signal C) and g = 1.9973 (signal D) are due to isotropic and orthorhombic C O l , respectively (Ikeya, 1993). The intensities of signal C and D are plotted against the dose in Fig. 7(b). Black points and white points were obtained with ES-10 and a former commercial ESR spectrometer, FE-IX, respectively. The sensitivity of ES-10 is comparable to that of FE-IX with the specification of 10~° spins/0.1 mT but less than that within a factor 1.5 ~ 2.0 in this case. The specification of this spectrometer was thus confirmed for shell buttons.
0.2
Samp'eTube IDPPH
gE .x2 0.1
•
°
5 mm
¢._1
°~
10 mm 5 mm
0
-
'
I
,
I
0
,
Eggshell (5kGy)
I
2
Position (cm) <
)
Effective sample length
Fig. 4. The plot of linewidth of DPPH versus the position in a cavity when two DPPH dots separated by I0 ram were vertically moved to check the uniformity of magnetic field.
6
'
16o Weight
2o0 (mg)
Fig. 5. Relation between the ESR intensity and sample weight. The sample eggshells were irradiated to 5 kGy.
159
A portable ESR spectrometer
(a
NEOMAX
Sweep coil
Yok
(b)
Fig. 1. (a) Assembly of the permanent magnet system using Nd-B-Fe alloys and (b) a photograph of the magnet.
1592
Toshihide Oka et al.
Fig. 2. The ceramic cavity coated with thin gold metal.
Fig. 3. The ESR spectrometer used in this study. It is 500 × 500 × 230 (mm) in size and 25 kg in weight.
1593
A portable ESR spectrometer
4}
100
101
Sample: Holocene coral
/ o/
"5" 100
z-
o} e- 10-1 (I) t"
ffl e-"
5o n-
OC 10.2 W
10-3,
~
Alanine dosimeter ...... z
I
10"~ 100
I
101
I
I
102
I
103 104
105
Dose (Gy) Fig. 6. Relation between the ESR intensity and the irradiation dose:the alaninedosimeterirradiatedat CERN is used as a sample.
I
0
I
50 Dose (Gy)
Fig. 8. The growth of the ESR intensity of corals at Ryukyu Islands as a function of additive dose. The equivalent dose of 10 Gy was obtained. Black and white dots were obtained with RE-IX and ES-10, respectively.
(a)
(b) . . . .
13.75 Gy
100
/.~,.
signal D go=1.9973
60
03 t"(D
i
. . . .
]
,
,
O: Signal C (FE-IX) / I1: Signal D (FE-1X) ~ " O: Signal C (ES-10) , / / I--7:Signal D (ES-10)
o.
40
24.75 Gy o~ 20 w a
Shell button
gc=2.0007
1 rnT
....
lt] . . . .
a
2t]'
Dose (Gy)
Fig. 7. (a) ESR spectra of modern shell buttons at different doses; 13.75, 19.35 and 24.75 Gy. (b) The intensities of signal C and D due to orthorhombic CO2- were plotted against additive dose. Black and white dots were obtained with FE-IX and ES-10, respectively.
Another important application of this spectrometer is as a readout device for ESR dating. The E D of Holocene corals from Kikai-jima, Ryukyu Islands, Japan was obtained with ES-10 and R E - I X using same samples (lkeya and Ohmura, 1983). The white dots and black dots were obtained with ES-10 and R E - I X , respectively as shown in Fig. 8. The sensitivity for the case of coral is the factor 2 ,,, 3 less than that of R E - I X with the specification of 8 x 1 0 9 spins/0.1 mT. These results indicate that the present spectrometer (ES-10) can also be used for dating of corals and
other geological materials as well as for development and routine use as radiation dosimeters and for monitoring irradiated food.
Acknowledgements--We thank Mr Kazuo Nakagawa and Mr Makoto Tsuneda of Nikkiso Co., Ltd for supplying the spectrometer and some advice.
References lkeya M. and Furusawa M. (1989) A portable spectrometer for ESR microscopy, dosimetry and dating. Appl. Radiat. lsot. 40, 845--850.
1594
Toshihide Oka et al.
Ikeya M., Meguro K., Miyamaru H. and Ishii H. (1991) Educational Experiments on ESR Imaging with a Portable ESR Spectrometer. Appl. Magn. Res. 2, 663-673. lkeya M. and Ohmura K. 0983) Comparison of ESR ages of corals at marine terraces with '4C and 2~rh/234U ages. Earth Planet. Sci. Left. 65, 34-34. Ikeya M. (1993) New Applications of Electron Spin Resonance-Dating, Dosimetry and Microscopy. World Scientific, Singapore. Nakanishi A., Sugawara N. and Furuse A. (1993) Portable
ESR spectrometer with NEOMAX (Nd-B-Fe) permanent magnet circuit. Appl. Radiat. Isot. 44, 357-360. Sagawa M., Fujimura S., Togawa N., Yamamoto H. and Matuura Y. 0984) New material for P. M. on a base on Nd and Fe. J. Appl. Phys. 55, 2083-2087. Sakurai H., Aoki M. and Miyamoto T. (1990) Improvement of the field homogeneity with a permanent magnet assembly for MRI. J. Mag. Soc. Jap. 14, 2, 465--468. Yamanaka C., Ikeya M., Meguro K. and Nakanishi A. (1991) A portable ESR spectrometer using Nd-B--Fe permanent magnets. Nucl. Tracks 18, 279-282.