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Development of neutron-sensitive glass dosimeter containing isotopically enriched boron
Radiation Measurements 46 (2011) 1484e1487
Contents lists available at ScienceDirect
Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas
Development of neutron-sensitive glass dosimeter containing isotopically enriched boron D. Maki a, b, F. Sato a, I. Murata a, Y. Kato a, Y. Tanimura c, T. Yamamoto b, T. Iida a, * a
Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka, Japan Oarai Research Center, Chiyoda Technol Corporation, 3681 Narita-cho, Oarai-machi, Higashiibaraki-gun, Ibaraki, Japan c Nuclear Science and Research Institute, Japan Atomic Energy Agency, Tokai-mura, Ibaraki, Japan b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 November 2010 Received in revised form 20 June 2011 Accepted 28 June 2011
Neutron-sensitive glass dosimeters were successfully made from reagents of NaPO3, Al(PO3)3, enriched 10 or 11 B2O3 and AgCl. The glass dosimeters had good RPL characteristics: satisfactory linearity of 0.01 e500 mGy and low variation in sensitivity. The RPL characteristics for gamma-rays were compatible with those of a commercial glass dosimeter. A pair of the 10 or 11B-containing glass dosimeters was effectively used for the thermal neutron dosimetry. In 565 keV and 5.0 MeV monoenergetic neutron calibration fields, albedo neutrons from the ISO water slab phantom were successfully detected with the glass dosimeter pairs. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Radiophotoluminescence Glass dosimeter Enriched boron Neutron dosimetry
1. Introduction Some characteristics of radiophotoluminescence (RPL) glass dosimeter systems have been studied to apply them for personal dosimetry and environmental monitoring (Piesch et al., 1986). The RPL material is silver activated phosphate glass, in which RPL centers are produced by incident ionization radiations. In dose measuring, a pulsed ultraviolet laser has been used to read the photoluminescence intensity from glass dosimeters. The photoluminescence has two different components. The short-lifetime photoluminescence (w0.3 ms) around 450 nm in wavelength is related to the pre-dose. In contrast, the photoluminescence of w4 ms lifetime around 650 nm is linked to radiation dose (RPL). It is possible to determine the dose by discriminating the RPL from the photoluminescence in the time and the wavelength. The RPL glass has excellent characteristics for dosimeter such as long-term stability of small fading effect, good dose linearity of 0.01e500 mGy and high reproducibility. There further is a capability of repeatable reading of RPL responses in an individual dosimeter. This fact means high precision dose measurement with glass dosimeters compared to others. However, the glass dosimeter has low sensitivity to neutrons (Yokota et al., 1961; Croft, 1990). Some of thermoluminescent detectors (TLDs) have thermal neutron sensitivity (Tanaka and Furuta, 1974). The basis of the * Corresponding author. Tel.: þ81 6 6879 7909; fax: þ81 6 6879 7363. E-mail address: [email protected] (T. Iida). 1350-4487/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2011.06.068
thermal neutron detection with thermoluminescence detectors (TLDs) is the production of high-energy alpha particles upon of neutron capture reactions of 6Li (940 barns) or 10B (3840 barns) nuclei. A typical neutron measurement with TLDs consists of more than two detectors containing different enriched nuclei such as a pair of 6LiF and 7LiF (Tanaka and Furuta, 1974). The 7LiF TLD is used only to record gamma-ray exposure and the 6LiF is sensitive to thermal neutron and gamma-rays. The error assessment of these reading is significant due to the neutron dose is evaluated from the deference between readings of the detectors in the pair. In fast neutron energy regions, 6Li and 10B nuclei have small cross-sections of (n, a) reactions. In fast neutron environment, an albedo neutron dosimeter of TLD pairs has been generally used by means of moderating and scattering of fast neutrons in a human body or constructional materials. This paper describes the way of synthesizing RPL glass materials with 10 or 11B enrichments for neutron dosimetry. In a preliminary experiment, thermal neutron measurements were performed with a pair of 10 or 11B-containing glass dosimeters.
2. RPL glass dosimeter 2.1. Synthesis of RPL glass material Sodium metaphosphate (NaPO3), aluminum metaphosphate (Al(PO3)3), silver chloride (AgCl) and enriched boron oxide (10 or
D. Maki et al. / Radiation Measurements 46 (2011) 1484e1487
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11
1000 60 50
Gamma-ray irradiated
40 10
30
Photoluminescence 355 nm excitation
20 1 Non-irradiated
10 0 200
300
2.2. Calibration experiment for RPL sensitivity
3.1. Neutron kerma factor Fig. 2 shows neutron kerma factors for RPL glass materials, which were estimated by Monte Carlo code, PHITS (Niita et al., 2006). The kerma factor for 10B-containing glass material is
B-containing glass dosimeters.
Dosimeter NaPO3 Al(PO3)3 B2O3 Ag Number RPL intensity Standard type (%) (%) (%) (%) of samples for gamma-ray deviation of (pieces) (1 mGy) RPL intensity (%) 10
B 11 B Non-B a
49 49 50
50 50 50
1 1 0
0.1 5 0.1 5 0.1 5
Reference result for RPL intensity.
0.96 0.95 1a
500
600
0.1 800
700
6.9 6.8 6.7
Fig. 1. Optical absorption and photoluminescence spectra of enriched glass dosimeter.
10
B-contining
extremely large in thermal neutron region owing to 10B(n, a)7Li reactions (Q value; 2.79 or 2.31 MeV). There is no large difference among the three kerma factors above 100 keV due to the domination of the neutron scattering cross-sections for P and O nuclei. Therefore, the assessment of the neutron below 100 keV is very significant for a neutron measurement with a pair of 10 or 11Bcontaining glass dosimeters. 3.2. Thermal neutron irradiation Thermal neutron irradiation tests for the glass dosimeters were performed at a thermal neutron field constructed by use of an 241 Am-Be neutron source (46 GBq) and graphite neutron moderators. The neutron source was placed in the center of the graphite cube (1 1 1 m3), which was built of the pile of graphite blocks. A 30 mm4 hole was prepared at the center of the graphite cube for the setting of the neutron source and the glass dosimeters. The glass dosimeters were set 30 cm away from the neutron source. A brass rod of 30 cm in length and 26 mm in diameter was placed between the neutron source and the irradiation samples. The insertion of the brass rod was for the shield against gamma-rays from the neutron source (Venkataraman et al., 1970). After the setting of the glass dosimeters, the hole was obstructed by a 30 mm4 graphite rod. The thermal neutron flux at the sample
10-9 Neutron Kerma Factor [Gy cm2]
3. Neutron dosimetry
10 or 11
400
Wavelength [nm]
For erasing of dose information, the dosimeters were annealed for 1 h at 400 C. Some of the glass dosimeters were exposed to the 500 mGy of 60Co gamma-rays. After the irradiation, the preheating process for the glass dosimeters was performed for 1 h at 100 C. Fig. 1 shows examples of optical absorption and photoluminescence spectra of an enriched 10B-containing glass dosimeter. The optical absorption spectra of glass dosimeters were obtained using a UVevisible spectrophotometer (Miyamoto et al., 2010). The photoluminescence spectra were analyzed with a photon spectrometer (PMA-10, Hamamatsu Photonics) and a 355 nm YAG-laser for the excitation light. The optical absorption spectrum was transparent in the visible region and the absorption coefficient was approximately 1.4 cm1 at 355 nm. Therefore, the self-absorption effect for RPL was small because of the transparency in visible range. The photoluminescence spectrum had a large peak for RPL around 650 nm in wavelength. In addition, there was not a large difference in the shapes of the RPL spectra among the synthesized glass dosimeters and a commercial dosimeter, GD-450. The dosimeters were irradiated by the 60Co gamma-ray irradiation up to the absorbed dose of 500 mGy. The dose of RPL glass dosimeters was measured with an RPL readout system with a 355 nm YAG-laser (Maki et al., in press). The RPL glass dosimeters had satisfactory linearity in the absorbed dose range from 10 mGy to 500 mGy. The standard deviation of the RPL intensity was less than 7%. It has been found from a preliminary test that the RPL sensitivity of the self-made reference sample (NaPO3eAl(PO3)3 Ag) is compatible with these characteristics of the commercial dosimeter, GD-450.
Table 1 Properties of enriched
100
RPL Intensity [a.u.]
Absorption Coefficient [cm-1]
B2O3) were mixed in an alumina crucible. Table 1 summarizes the components of the synthesized glass dosimeters and theses RPL characteristics. The weight composition of the B-containing glass material was 32% P, 51% O, 0.3% B, 11% Na, 5% Al, and 0.1% Ag. We prepared two type B2O3 containing 10B (>99%) and 11B (>99%) enrichments for neutron-sensitive glass dosimeters. The crucible was set in an electric furnace. At first water was removed from the mixture through the heat-treatment at 250 C for 15 min, and then the mixture was melted at 1200 C for 1 h. The melted mixture was poured into a preheated brass mold and was slowly cooled down to room temperature for about 10 h. The molded glass was cut into small pieces with a rotating diamond cutter and optical-polished with cerium oxide powder. The size of the glass dosimeter was 10 7 1 mm3.
10
B-contained B-contained nat B-contained Non-B-contained 11
10
-10
10-11 10-12 10-13 10-14 10-15
10-5 10-4 10-3 10-2 10-1
100
101
102
Neutron Energy [keV] Fig. 2. Neutron kerma factors for RPL glass material.
103
104
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D. Maki et al. / Radiation Measurements 46 (2011) 1484e1487
Table 2 RPL sensitivities to thermal neutrons. Neutron source
Fast neutron fluence (cm2)
Thermal neutron fluence (cm2)a
241
7.0 106
1.0 108
a b c d
Am-Be
Dosimeter type
RPL intensityb (neutron þ gamma)
Net RPL intensity for thermal neutronc
RPL sensitivity to thermal neutron (cm2)d
10
1.511 0.083 0.168 0.008 0.17 0.01
1.34 0.08
1.34 108
e
e
B B Non-B
11
En < 1 eV, MCNP simulation result for 241Am-Be neutron source and graphite neutron moderators. Reference; RPL efficiency of Non-B type dosimeter for 1 mGy gamma irradiation. Difference between the RPL intensity of the 10B glass and that of 11B glass. Net RPL intensity for thermal neutrons divided by thermal neutron fluence (1 108 n/cm2).
position was measured by means of the activation method with Au foils and Cd filters and was estimated to be 3.0 103 n/cm2/s. The neutron spectrum at the sample position was calculated by the Monte Carlo code, MCNP (Briesmeister, 2000). In the calculation, the ISO8529-1 (International Organization for Standardization, 2001) gave neutron energy spectrum data for 241Am-Be neutron source. The thermal neutron flux at the sample position was about 5 times and about 10 times larger than the epithermal and fast neutron flux, respectively. The calculation results on the neutron irradiation field were on the whole consistent with those measured with the activation method. Table 2 shows the sensitivities to thermal neutrons for 10B-, 11Band non-containing glass dosimeters. Fast and thermal neutron fluences were found to be 7.0 106 and 1.0 108 n/cm2, respectively. All the samples were set at the same position in the thermal column, and their RPL intensities were measured under the same conditions. The RPL intensity of the enriched 11B-containing dosimeter was almost equal to that of the non-B-containing dosimeter owing to small kerma factor for 11B in the neutron energy below 100 keV (Fig. 2). Therefore, the difference in the RPL intensities between the 10B- and 11B-containing dosimeters was caused by the thermal neutron dose. The net RPL intensity for thermal neutrons was determined from the difference between the RPL intensity of the 10B-containing glass and that of 11B-containing glass. The thermal neutron sensitivity was determined from the net RPL intensity divided by thermal neutron fluence. The thermal neutron sensitivity was 1.34 108 cm2, which was less than that evaluated from the absorbed dose for thermal neutron and the RPL intensity for photon (2.0 107 cm2). According to a previous report (Croft and Weaver, 1989), the difference between gamma and neutron RPL efficiencies might be caused by linear energy transfer effects. The RPL efficiency for heavy charged particles such as proton, alpha and recoils was mostly one order smaller than that for gamma-rays.
3.3. Albedo neutron measurement The glass dosimeters were irradiated in 565 keV and 5.0 MeV monoenergetic neutron calibration fields. The monoenergetic neutron calibration fields were used at the Facility of Radiation Standards (FRS) of Japan Atomic Energy Agency using a 4 MV Van de Graaff accelerator (Tanimura et al., 2004). The 7Li(p, n)7Be and 2 H(d, n)3He reactions were employed for neutron production. The fluences of 565 keV and 5.0 MeV neutrons were 2.19 107 and 4.70 107 n/cm2 at the calibrated positions, respectively. Some pairs of the glass dosimeters were placed on the front surface of an ISO water slab phantom (International Organization for Standardization, 1998). The distance between the glass dosimeter pairs and the neutron source was 50 cm. Fig. 3 shows the neutron spectra at the sample position were calculated by Monte Carlo code, MCNP-ANT (Yoshizawa et al., 2002). The neutron fluences were normalized by the number of accelerated ions, i.e., the beam current of the accelerator. In the neutron irradiation with the phantom, the neutron fluence below 100 keV was approximately two order larger than that without the phantom. For the irradiation without the phantom, the low-energy neutrons were mainly scattered and moderated in constructional materials of the FRS facility. Table 3 shows RPL intensities for albedo neutrons in the monoenergetic neutron calibration fields. The difference between the pair of glass dosimeters was obviously caused by the 10B(n, a)7Li reactions for low-energy albedo neutrons. The fluences of albedo neutrons in thermal energy region were experimentally estimated from the net RPL intensity for albedo neutron multiplied by RPL sensitivity to thermal neutron. The results had a large margin of error due to RPL intensities of 10Bcontaining glass dosimeter were close to that of 11B-containing glass dosimeter. Nevertheless, the experimental results on the albedo neutron fluences were roughly in accordance with calculated ones. The fact indicated that the pair of the enriched 10 or
Fig. 3. Neutron spectra at sample positions in 565 keV and 5.0 MeV monoenergetic neutron calibration fields.
D. Maki et al. / Radiation Measurements 46 (2011) 1484e1487
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Table 3 RPL intensities for albedo neutrons in monoenergetic neutron calibration fields. Neutron source
Fast neutron fluence (cm2)
Calculated albedo neutron fluence (cm2)a
565 keV
2.19 107
3.15 106
4.70 107
5.37 106
5.0 MeV a b c d
Dosimeter type
RPL intensityb (photon þ neutron)
10
0.752 0.696 1.43 1.35
B B 10 B 11 B 11
0.022 0.019 0.03 0.02
Net RPL intensity for albedo neutronc
Experimental albedo neutron fluence (cm2)d
0.056 0.032
4 2 106
0.08 0.03
6 2 106
En < 1 eV, MCNP-ANT simulation results. Reference; RPL efficiency of Non-B type dosimeter for 1 mGy gamma irradiation. Difference between the RPL intensity of the 10B glass and that of 11B glass. Net RPL intensity multiplied by RPL sensitivity to thermal neutron.
11 B-containing glass dosimeters could be effectively used for the albedo neutron dosimetry.
were performed under a collaborative research program of Osaka University and Japan Atomic Energy Agency.
4. Conclusions References 10 or 11
B-containing glass dosimeters for neutron Enriched dosimetry were synthesized from reagents of NaPO3, Al(PO3)3, AgCl and isotopically enriched 10 or 11B2O3. The all glass dosimeters were irradiated by 60Co gamma-rays for calibration experiments. The dose of RPL glass dosimeters was measured with an RPL readout system with a YAG-laser. The photoluminescence spectra of the glass dosimeters had a large peak for radiophotoluminescence (RPL) around 650 nm in wavelength. The optical absorption spectrum was transparent in the visible region. The glass dosimeters had satisfactory linearity of 0.01e500 mGy and low variation in sensitivity. Thus, the RPL characteristics for photon detection were compatible with these characteristics of a commercial glass dosimeter. Thermal neutron irradiation tests for glass dosimeters were performed at a thermal neutron field constructed by use of an 241 Am-Be neutron source and graphite neutron moderators. A pair of 10 or 11B-containing glass dosimeters was set in the thermal neutron filed. The net RPL intensity for thermal neutrons was determined from difference between the RPL intensity of the 10Bcontaining glass and that of 11B-containing glass. The glass dosimeter pair could be effectively used for the evaluation of thermal neutron fluence. In 565 keV and 5.0 MeV monoenergetic neutron calibration fields, pairs of 10 or 11B-containing glass dosimeters was set on an ISO water slab phantom. The albedo neutron doses from a water slab phantom were evaluated with that the glass dosimeter pairs. Acknowledgments The authors sincerely thank S. Hisakado and T. Nagai of Osaka University for their valuable suggestions in preparing phosphate glass samples. Experiments for monoenergetic neutron irradiations
Briesmeister, J.F. (Ed.), 2000. MCNP-A General Monte Carlo N-Particle Transport Code Version 4C-Manual. LA-13709-M. Los Alamos National Laboratory, Los Alamos, NM. Croft, S., Weaver, D.R., 1989. The fast neutron sensitivities of SEI and Toshiba FDP10-7 radiophotoluminescent glass dosimeters. Radiat. Prot. Dosim. 27, 251e260. Croft, S., 1990. The thermal neutron sensitivities of SEI and TOSHIBA FD-P107 radiophotoluminescent glass dosimeters. Nucl. Instr. Meth. A 286, 270e278. International Organization for Standardization, 1998. Reference Neutron Radiations e Part3: Calibration of Area and Personal Dosimeters and Determination of Response as a Function of Energy and Angle of Incidence. International Standard ISO 8529-3. ISO, Geneva. International Organization for Standardization, 2001. Reference Neutron Radiations - Part1: Characteristics and Methods of Production. International Standard ISO 8529-1. ISO, Geneva. Maki, D., Sato, F., Kato, Y., Yamamoto, T., Iida, T. Improvement of a dose reading system for radiophotoluminescence glass dosimeters. Radioisotopes, in press. Miyamoto, Y., Yamamoto, T., Kinoshita, K., Koyama, S., Takei, Y., Nanto, H., Shimotsuma, Y., Sakakura, M., Miura, K., Hirao, K., 2010. Emission mechanism of radiophotoluminescence in Ag-doped phosphate glass. Radiat. Meas. 45, 546e549. Niita, K., Sato, T., Iwase, H., Nose, H., Nakashima, H., Sihver, L., 2006. PHITS-a particle and heavy ion transport code system. Radiat. Meas. 41, 1080e1090. Piesch, E., Burgkhardt, B., Fischer, M., Rober, H.G., Ugi, S., 1986. Properties of radiophotoluminescent glass dosimeter systems using pulsed laser UV excitation. Radiat. Prot. Dosim. 17, 293e297. Tanaka, S., Furuta, Y., 1974. Estimation of gamma-ray exposure in mixed gammaneutron fields by 6LiF and 7LiF thermoluminescence dosimeters in pair use. Nucl. Instr. Meth. 117, 93e97. Tanimura, Y., Yoshizawa, M., Saegusa, J., Fujii, K., Shimizu, S., Yoshida, M., Shibata, Y., Uritani, A., Kudo, K., 2004. Construction of 144, 565 KeV and 5.0 MeV monoenergetic neutron calibration fields at JAERI. Radiat. Prot. Dosim. 110, 85e89. Venkataraman, G., Dayashankar, Jayakar, J.S., 1970. The gamma to neutron ratios for Pu-Be and Am-Be neutron sources. Nucl. Instr. Meth. 82, 49e50. Yoshizawa, M., Sugita, T., Saegusa, J., Tanimura, Y., Yoshida, M., 2002. A Monte Carlo Program for estimating characteristics of neutron calibration fields using a pelletron accelerator. J. Nucl. Sci. Technol. Suppl. 2, 1240e1243. Yokota, R., Nakajima, S., Sakai, E., 1961. High sensitivity silver-activated phosphate glass for the simultaneous measurement of thermal neutron, gamma and/or beta-rays. Health Phys. 5, 219e224.
Contents lists available at ScienceDirect
Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas
Development of neutron-sensitive glass dosimeter containing isotopically enriched boron D. Maki a, b, F. Sato a, I. Murata a, Y. Kato a, Y. Tanimura c, T. Yamamoto b, T. Iida a, * a
Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka, Japan Oarai Research Center, Chiyoda Technol Corporation, 3681 Narita-cho, Oarai-machi, Higashiibaraki-gun, Ibaraki, Japan c Nuclear Science and Research Institute, Japan Atomic Energy Agency, Tokai-mura, Ibaraki, Japan b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 November 2010 Received in revised form 20 June 2011 Accepted 28 June 2011
Neutron-sensitive glass dosimeters were successfully made from reagents of NaPO3, Al(PO3)3, enriched 10 or 11 B2O3 and AgCl. The glass dosimeters had good RPL characteristics: satisfactory linearity of 0.01 e500 mGy and low variation in sensitivity. The RPL characteristics for gamma-rays were compatible with those of a commercial glass dosimeter. A pair of the 10 or 11B-containing glass dosimeters was effectively used for the thermal neutron dosimetry. In 565 keV and 5.0 MeV monoenergetic neutron calibration fields, albedo neutrons from the ISO water slab phantom were successfully detected with the glass dosimeter pairs. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Radiophotoluminescence Glass dosimeter Enriched boron Neutron dosimetry
1. Introduction Some characteristics of radiophotoluminescence (RPL) glass dosimeter systems have been studied to apply them for personal dosimetry and environmental monitoring (Piesch et al., 1986). The RPL material is silver activated phosphate glass, in which RPL centers are produced by incident ionization radiations. In dose measuring, a pulsed ultraviolet laser has been used to read the photoluminescence intensity from glass dosimeters. The photoluminescence has two different components. The short-lifetime photoluminescence (w0.3 ms) around 450 nm in wavelength is related to the pre-dose. In contrast, the photoluminescence of w4 ms lifetime around 650 nm is linked to radiation dose (RPL). It is possible to determine the dose by discriminating the RPL from the photoluminescence in the time and the wavelength. The RPL glass has excellent characteristics for dosimeter such as long-term stability of small fading effect, good dose linearity of 0.01e500 mGy and high reproducibility. There further is a capability of repeatable reading of RPL responses in an individual dosimeter. This fact means high precision dose measurement with glass dosimeters compared to others. However, the glass dosimeter has low sensitivity to neutrons (Yokota et al., 1961; Croft, 1990). Some of thermoluminescent detectors (TLDs) have thermal neutron sensitivity (Tanaka and Furuta, 1974). The basis of the * Corresponding author. Tel.: þ81 6 6879 7909; fax: þ81 6 6879 7363. E-mail address: [email protected] (T. Iida). 1350-4487/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2011.06.068
thermal neutron detection with thermoluminescence detectors (TLDs) is the production of high-energy alpha particles upon of neutron capture reactions of 6Li (940 barns) or 10B (3840 barns) nuclei. A typical neutron measurement with TLDs consists of more than two detectors containing different enriched nuclei such as a pair of 6LiF and 7LiF (Tanaka and Furuta, 1974). The 7LiF TLD is used only to record gamma-ray exposure and the 6LiF is sensitive to thermal neutron and gamma-rays. The error assessment of these reading is significant due to the neutron dose is evaluated from the deference between readings of the detectors in the pair. In fast neutron energy regions, 6Li and 10B nuclei have small cross-sections of (n, a) reactions. In fast neutron environment, an albedo neutron dosimeter of TLD pairs has been generally used by means of moderating and scattering of fast neutrons in a human body or constructional materials. This paper describes the way of synthesizing RPL glass materials with 10 or 11B enrichments for neutron dosimetry. In a preliminary experiment, thermal neutron measurements were performed with a pair of 10 or 11B-containing glass dosimeters.
2. RPL glass dosimeter 2.1. Synthesis of RPL glass material Sodium metaphosphate (NaPO3), aluminum metaphosphate (Al(PO3)3), silver chloride (AgCl) and enriched boron oxide (10 or
D. Maki et al. / Radiation Measurements 46 (2011) 1484e1487
1485
11
1000 60 50
Gamma-ray irradiated
40 10
30
Photoluminescence 355 nm excitation
20 1 Non-irradiated
10 0 200
300
2.2. Calibration experiment for RPL sensitivity
3.1. Neutron kerma factor Fig. 2 shows neutron kerma factors for RPL glass materials, which were estimated by Monte Carlo code, PHITS (Niita et al., 2006). The kerma factor for 10B-containing glass material is
B-containing glass dosimeters.
Dosimeter NaPO3 Al(PO3)3 B2O3 Ag Number RPL intensity Standard type (%) (%) (%) (%) of samples for gamma-ray deviation of (pieces) (1 mGy) RPL intensity (%) 10
B 11 B Non-B a
49 49 50
50 50 50
1 1 0
0.1 5 0.1 5 0.1 5
Reference result for RPL intensity.
0.96 0.95 1a
500
600
0.1 800
700
6.9 6.8 6.7
Fig. 1. Optical absorption and photoluminescence spectra of enriched glass dosimeter.
10
B-contining
extremely large in thermal neutron region owing to 10B(n, a)7Li reactions (Q value; 2.79 or 2.31 MeV). There is no large difference among the three kerma factors above 100 keV due to the domination of the neutron scattering cross-sections for P and O nuclei. Therefore, the assessment of the neutron below 100 keV is very significant for a neutron measurement with a pair of 10 or 11Bcontaining glass dosimeters. 3.2. Thermal neutron irradiation Thermal neutron irradiation tests for the glass dosimeters were performed at a thermal neutron field constructed by use of an 241 Am-Be neutron source (46 GBq) and graphite neutron moderators. The neutron source was placed in the center of the graphite cube (1 1 1 m3), which was built of the pile of graphite blocks. A 30 mm4 hole was prepared at the center of the graphite cube for the setting of the neutron source and the glass dosimeters. The glass dosimeters were set 30 cm away from the neutron source. A brass rod of 30 cm in length and 26 mm in diameter was placed between the neutron source and the irradiation samples. The insertion of the brass rod was for the shield against gamma-rays from the neutron source (Venkataraman et al., 1970). After the setting of the glass dosimeters, the hole was obstructed by a 30 mm4 graphite rod. The thermal neutron flux at the sample
10-9 Neutron Kerma Factor [Gy cm2]
3. Neutron dosimetry
10 or 11
400
Wavelength [nm]
For erasing of dose information, the dosimeters were annealed for 1 h at 400 C. Some of the glass dosimeters were exposed to the 500 mGy of 60Co gamma-rays. After the irradiation, the preheating process for the glass dosimeters was performed for 1 h at 100 C. Fig. 1 shows examples of optical absorption and photoluminescence spectra of an enriched 10B-containing glass dosimeter. The optical absorption spectra of glass dosimeters were obtained using a UVevisible spectrophotometer (Miyamoto et al., 2010). The photoluminescence spectra were analyzed with a photon spectrometer (PMA-10, Hamamatsu Photonics) and a 355 nm YAG-laser for the excitation light. The optical absorption spectrum was transparent in the visible region and the absorption coefficient was approximately 1.4 cm1 at 355 nm. Therefore, the self-absorption effect for RPL was small because of the transparency in visible range. The photoluminescence spectrum had a large peak for RPL around 650 nm in wavelength. In addition, there was not a large difference in the shapes of the RPL spectra among the synthesized glass dosimeters and a commercial dosimeter, GD-450. The dosimeters were irradiated by the 60Co gamma-ray irradiation up to the absorbed dose of 500 mGy. The dose of RPL glass dosimeters was measured with an RPL readout system with a 355 nm YAG-laser (Maki et al., in press). The RPL glass dosimeters had satisfactory linearity in the absorbed dose range from 10 mGy to 500 mGy. The standard deviation of the RPL intensity was less than 7%. It has been found from a preliminary test that the RPL sensitivity of the self-made reference sample (NaPO3eAl(PO3)3 Ag) is compatible with these characteristics of the commercial dosimeter, GD-450.
Table 1 Properties of enriched
100
RPL Intensity [a.u.]
Absorption Coefficient [cm-1]
B2O3) were mixed in an alumina crucible. Table 1 summarizes the components of the synthesized glass dosimeters and theses RPL characteristics. The weight composition of the B-containing glass material was 32% P, 51% O, 0.3% B, 11% Na, 5% Al, and 0.1% Ag. We prepared two type B2O3 containing 10B (>99%) and 11B (>99%) enrichments for neutron-sensitive glass dosimeters. The crucible was set in an electric furnace. At first water was removed from the mixture through the heat-treatment at 250 C for 15 min, and then the mixture was melted at 1200 C for 1 h. The melted mixture was poured into a preheated brass mold and was slowly cooled down to room temperature for about 10 h. The molded glass was cut into small pieces with a rotating diamond cutter and optical-polished with cerium oxide powder. The size of the glass dosimeter was 10 7 1 mm3.
10
B-contained B-contained nat B-contained Non-B-contained 11
10
-10
10-11 10-12 10-13 10-14 10-15
10-5 10-4 10-3 10-2 10-1
100
101
102
Neutron Energy [keV] Fig. 2. Neutron kerma factors for RPL glass material.
103
104
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D. Maki et al. / Radiation Measurements 46 (2011) 1484e1487
Table 2 RPL sensitivities to thermal neutrons. Neutron source
Fast neutron fluence (cm2)
Thermal neutron fluence (cm2)a
241
7.0 106
1.0 108
a b c d
Am-Be
Dosimeter type
RPL intensityb (neutron þ gamma)
Net RPL intensity for thermal neutronc
RPL sensitivity to thermal neutron (cm2)d
10
1.511 0.083 0.168 0.008 0.17 0.01
1.34 0.08
1.34 108
e
e
B B Non-B
11
En < 1 eV, MCNP simulation result for 241Am-Be neutron source and graphite neutron moderators. Reference; RPL efficiency of Non-B type dosimeter for 1 mGy gamma irradiation. Difference between the RPL intensity of the 10B glass and that of 11B glass. Net RPL intensity for thermal neutrons divided by thermal neutron fluence (1 108 n/cm2).
position was measured by means of the activation method with Au foils and Cd filters and was estimated to be 3.0 103 n/cm2/s. The neutron spectrum at the sample position was calculated by the Monte Carlo code, MCNP (Briesmeister, 2000). In the calculation, the ISO8529-1 (International Organization for Standardization, 2001) gave neutron energy spectrum data for 241Am-Be neutron source. The thermal neutron flux at the sample position was about 5 times and about 10 times larger than the epithermal and fast neutron flux, respectively. The calculation results on the neutron irradiation field were on the whole consistent with those measured with the activation method. Table 2 shows the sensitivities to thermal neutrons for 10B-, 11Band non-containing glass dosimeters. Fast and thermal neutron fluences were found to be 7.0 106 and 1.0 108 n/cm2, respectively. All the samples were set at the same position in the thermal column, and their RPL intensities were measured under the same conditions. The RPL intensity of the enriched 11B-containing dosimeter was almost equal to that of the non-B-containing dosimeter owing to small kerma factor for 11B in the neutron energy below 100 keV (Fig. 2). Therefore, the difference in the RPL intensities between the 10B- and 11B-containing dosimeters was caused by the thermal neutron dose. The net RPL intensity for thermal neutrons was determined from the difference between the RPL intensity of the 10B-containing glass and that of 11B-containing glass. The thermal neutron sensitivity was determined from the net RPL intensity divided by thermal neutron fluence. The thermal neutron sensitivity was 1.34 108 cm2, which was less than that evaluated from the absorbed dose for thermal neutron and the RPL intensity for photon (2.0 107 cm2). According to a previous report (Croft and Weaver, 1989), the difference between gamma and neutron RPL efficiencies might be caused by linear energy transfer effects. The RPL efficiency for heavy charged particles such as proton, alpha and recoils was mostly one order smaller than that for gamma-rays.
3.3. Albedo neutron measurement The glass dosimeters were irradiated in 565 keV and 5.0 MeV monoenergetic neutron calibration fields. The monoenergetic neutron calibration fields were used at the Facility of Radiation Standards (FRS) of Japan Atomic Energy Agency using a 4 MV Van de Graaff accelerator (Tanimura et al., 2004). The 7Li(p, n)7Be and 2 H(d, n)3He reactions were employed for neutron production. The fluences of 565 keV and 5.0 MeV neutrons were 2.19 107 and 4.70 107 n/cm2 at the calibrated positions, respectively. Some pairs of the glass dosimeters were placed on the front surface of an ISO water slab phantom (International Organization for Standardization, 1998). The distance between the glass dosimeter pairs and the neutron source was 50 cm. Fig. 3 shows the neutron spectra at the sample position were calculated by Monte Carlo code, MCNP-ANT (Yoshizawa et al., 2002). The neutron fluences were normalized by the number of accelerated ions, i.e., the beam current of the accelerator. In the neutron irradiation with the phantom, the neutron fluence below 100 keV was approximately two order larger than that without the phantom. For the irradiation without the phantom, the low-energy neutrons were mainly scattered and moderated in constructional materials of the FRS facility. Table 3 shows RPL intensities for albedo neutrons in the monoenergetic neutron calibration fields. The difference between the pair of glass dosimeters was obviously caused by the 10B(n, a)7Li reactions for low-energy albedo neutrons. The fluences of albedo neutrons in thermal energy region were experimentally estimated from the net RPL intensity for albedo neutron multiplied by RPL sensitivity to thermal neutron. The results had a large margin of error due to RPL intensities of 10Bcontaining glass dosimeter were close to that of 11B-containing glass dosimeter. Nevertheless, the experimental results on the albedo neutron fluences were roughly in accordance with calculated ones. The fact indicated that the pair of the enriched 10 or
Fig. 3. Neutron spectra at sample positions in 565 keV and 5.0 MeV monoenergetic neutron calibration fields.
D. Maki et al. / Radiation Measurements 46 (2011) 1484e1487
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Table 3 RPL intensities for albedo neutrons in monoenergetic neutron calibration fields. Neutron source
Fast neutron fluence (cm2)
Calculated albedo neutron fluence (cm2)a
565 keV
2.19 107
3.15 106
4.70 107
5.37 106
5.0 MeV a b c d
Dosimeter type
RPL intensityb (photon þ neutron)
10
0.752 0.696 1.43 1.35
B B 10 B 11 B 11
0.022 0.019 0.03 0.02
Net RPL intensity for albedo neutronc
Experimental albedo neutron fluence (cm2)d
0.056 0.032
4 2 106
0.08 0.03
6 2 106
En < 1 eV, MCNP-ANT simulation results. Reference; RPL efficiency of Non-B type dosimeter for 1 mGy gamma irradiation. Difference between the RPL intensity of the 10B glass and that of 11B glass. Net RPL intensity multiplied by RPL sensitivity to thermal neutron.
11 B-containing glass dosimeters could be effectively used for the albedo neutron dosimetry.
were performed under a collaborative research program of Osaka University and Japan Atomic Energy Agency.
4. Conclusions References 10 or 11
B-containing glass dosimeters for neutron Enriched dosimetry were synthesized from reagents of NaPO3, Al(PO3)3, AgCl and isotopically enriched 10 or 11B2O3. The all glass dosimeters were irradiated by 60Co gamma-rays for calibration experiments. The dose of RPL glass dosimeters was measured with an RPL readout system with a YAG-laser. The photoluminescence spectra of the glass dosimeters had a large peak for radiophotoluminescence (RPL) around 650 nm in wavelength. The optical absorption spectrum was transparent in the visible region. The glass dosimeters had satisfactory linearity of 0.01e500 mGy and low variation in sensitivity. Thus, the RPL characteristics for photon detection were compatible with these characteristics of a commercial glass dosimeter. Thermal neutron irradiation tests for glass dosimeters were performed at a thermal neutron field constructed by use of an 241 Am-Be neutron source and graphite neutron moderators. A pair of 10 or 11B-containing glass dosimeters was set in the thermal neutron filed. The net RPL intensity for thermal neutrons was determined from difference between the RPL intensity of the 10Bcontaining glass and that of 11B-containing glass. The glass dosimeter pair could be effectively used for the evaluation of thermal neutron fluence. In 565 keV and 5.0 MeV monoenergetic neutron calibration fields, pairs of 10 or 11B-containing glass dosimeters was set on an ISO water slab phantom. The albedo neutron doses from a water slab phantom were evaluated with that the glass dosimeter pairs. Acknowledgments The authors sincerely thank S. Hisakado and T. Nagai of Osaka University for their valuable suggestions in preparing phosphate glass samples. Experiments for monoenergetic neutron irradiations
Briesmeister, J.F. (Ed.), 2000. MCNP-A General Monte Carlo N-Particle Transport Code Version 4C-Manual. LA-13709-M. Los Alamos National Laboratory, Los Alamos, NM. Croft, S., Weaver, D.R., 1989. The fast neutron sensitivities of SEI and Toshiba FDP10-7 radiophotoluminescent glass dosimeters. Radiat. Prot. Dosim. 27, 251e260. Croft, S., 1990. The thermal neutron sensitivities of SEI and TOSHIBA FD-P107 radiophotoluminescent glass dosimeters. Nucl. Instr. Meth. A 286, 270e278. International Organization for Standardization, 1998. Reference Neutron Radiations e Part3: Calibration of Area and Personal Dosimeters and Determination of Response as a Function of Energy and Angle of Incidence. International Standard ISO 8529-3. ISO, Geneva. International Organization for Standardization, 2001. Reference Neutron Radiations - Part1: Characteristics and Methods of Production. International Standard ISO 8529-1. ISO, Geneva. Maki, D., Sato, F., Kato, Y., Yamamoto, T., Iida, T. Improvement of a dose reading system for radiophotoluminescence glass dosimeters. Radioisotopes, in press. Miyamoto, Y., Yamamoto, T., Kinoshita, K., Koyama, S., Takei, Y., Nanto, H., Shimotsuma, Y., Sakakura, M., Miura, K., Hirao, K., 2010. Emission mechanism of radiophotoluminescence in Ag-doped phosphate glass. Radiat. Meas. 45, 546e549. Niita, K., Sato, T., Iwase, H., Nose, H., Nakashima, H., Sihver, L., 2006. PHITS-a particle and heavy ion transport code system. Radiat. Meas. 41, 1080e1090. Piesch, E., Burgkhardt, B., Fischer, M., Rober, H.G., Ugi, S., 1986. Properties of radiophotoluminescent glass dosimeter systems using pulsed laser UV excitation. Radiat. Prot. Dosim. 17, 293e297. Tanaka, S., Furuta, Y., 1974. Estimation of gamma-ray exposure in mixed gammaneutron fields by 6LiF and 7LiF thermoluminescence dosimeters in pair use. Nucl. Instr. Meth. 117, 93e97. Tanimura, Y., Yoshizawa, M., Saegusa, J., Fujii, K., Shimizu, S., Yoshida, M., Shibata, Y., Uritani, A., Kudo, K., 2004. Construction of 144, 565 KeV and 5.0 MeV monoenergetic neutron calibration fields at JAERI. Radiat. Prot. Dosim. 110, 85e89. Venkataraman, G., Dayashankar, Jayakar, J.S., 1970. The gamma to neutron ratios for Pu-Be and Am-Be neutron sources. Nucl. Instr. Meth. 82, 49e50. Yoshizawa, M., Sugita, T., Saegusa, J., Tanimura, Y., Yoshida, M., 2002. A Monte Carlo Program for estimating characteristics of neutron calibration fields using a pelletron accelerator. J. Nucl. Sci. Technol. Suppl. 2, 1240e1243. Yokota, R., Nakajima, S., Sakai, E., 1961. High sensitivity silver-activated phosphate glass for the simultaneous measurement of thermal neutron, gamma and/or beta-rays. Health Phys. 5, 219e224.