- Email: [email protected]
A broad range YAP–plastic phoswich dosimeter
Nuclear Instruments and Methods in Physics Research B 134 (1998) 413±417
A broad range YAP±plastic phoswich dosimeter A.A. Fyodorov 1, W.P. Trower *, R.F. Zuevsky
1
World Physics Technologies Incorporated, 1105 Highland Circle, Blacksburg, VA 24061, USA Received 3 January 1995; received in revised form 12 September 1997
Abstract We describe a YAP±plastic phoswich detector, its scintillation properties and its potential as a dosimeter. Elsevier Science B.V. All rights reserved.
1. Introduction Cerium-doped Yttrium Aluminum Perovskite (YAP), YAlOa:Ce3 , is a new, fast, bright, and durable scintillator [1], which has already found a variety of applications in detectors for a particles [2], c rays, and X rays [3]. We have constructed a YAP-based c- and X-ray dosimeter with special shielding and an algorithm for dose-energy dependence compensation [4]. Its energy range is 20± 3000 keV, measured exposure dose power range is 0.005±1000 mR/h, measured dose power is 0.01 mR/h in 10 s, with an accuracy of 7%, and measured energy linearity of 10%. The detector sensitivity, S(Ec ), of scintillator x is the Photomultiplier Tube (PMT) anode current, IA , per exposure dose power, P:
* Corresponding author. Tel.: 540 953 2249; fax: 540 953 2249; e-mail: [email protected]. 1 Permanent address: Institute for Nuclear Problems, Belarus State University, Minsk, Belarus.
Ó 1998
Sx Ec IA =P C Ec f1 ÿ eÿ lx Ec t gflx Ec =lx Ec tlair Ec g; where C(Ec ) is an energy dependent compensation coecient, t is the scintillator thickness (cm), lx (Ec ) is the scintillator linear attenuation coecient (cmÿl ), and the mass absorption coecients (cm2 / g) for the scintillator and air are lx (Ec ) and lair (Ec ), respectively. C(Ec ) can be calculated from the measured IA , or equivalently, found from the measured pulse height to produce an energy independent S(Ec ). The large dose energy dependence for 20 < Ec < 250 keV made it impossible to reconstruct values of lx (Ec ), lx (Ec ), and lair (Ec ) for Compton scattered events in this region. For Ec > 250 keV, the energy deposited in the scintillator is between 20 and 250 keV, and produces erroneous mass absorption and linear attenuation coecients. Above 250 keV, Sx (Ec ) is substantially energy independent so assigning events to photoabsorption and Compton scattering is not required. Shielding the scintillator with copper containing an array of holes results in Sx (Ec ) being
0168-583X/98/$19.00 Ó 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 8 - 5 8 3 X ( 9 7 ) 0 0 5 3 5 - 1
414
A.A. Fyodorov et al. / Nucl. Instr. and Meth. in Phys. Res. B 134 (1998) 413±417
underestimated between 20 and 60 keV and overestimated in the 60±250 keV region. The energy dependence of Compton events can, therefore, be compensated. Our success encouraged us to extend our dosimeter energy and dose ranges by combining YAP and plastic scintillators in a so-called phoswich detector. YAP was chosen for its relatively high stopping power and fast luminescent decay, 28 ns, while plastic has low stopping power similar to that of air and decays in a few nanoseconds. The yttrium K-edge at 17 keV de®nes the YAP c-ray energy dose limit, while the plastic is limited only by the PMT dark current, or by the energy noise equivalent of the plastic±PMT system. The typical NaI:Tl-PMT noise equivalent is 1.2±1.5 keV, and plastic is 30% as bright as sodium iodide, so the PMT±plastic energy noise equivalent is 4±5 keV. However, copper cathodes of X-ray tubes have a K-edge at 8 keV, which de®nes the limit in these applications. In summary, our YAP±plastic phoswich detector should (i) extend both our dosimeter dose and energy ranges, (ii) be useful as a low-energy X-ray dosimeter, and (iii) measure c-ray doses in mixed radiation ®elds when the plastic is used as a charged particle anti-coincidence shield. 2. Results and discussion
With these optimized coecients from each of the 25 energy intervals, we reconstructed the experimental sensitivity curve, shown in Fig. 1, with the calculated SYAP (Ec ) curve. Our plastic scintillator, Polyvinyltoluene, has a light output of 60% relative to anthracene, a 3.3ns decay time, and an emission maximum at 434 nm [6]. The YAP has its emission peak at 360 nm and does not absorb NC3 H6 light [1]. Between the 25-mm diameter, 5-mm thick, BC-412 disk and the Hamamatsu R5113 PMT face, we placed a 25mm diameter, 15-mm thick, YAP disk whose measured attenuation of 440-nm light is less than 10%. With a 24l Am c-ray source, we found the plastic-R5113 energy-noise equivalent to be 6.3 keV, which can be reduced below 5 keV with optimized light collection and pulse shaping. An 24l Am pulse height spectrum in BC-412 is shown in Fig. 2(a), while in Fig. 2(b) the 137 Cs YAP±plastic phoswich signals processed with a Canberra 2160A pulse shape discriminator are seen. Fig. 3 shows our phoswich detector construction. To determine the dependence of dosimetric sensitivities on plastic scintillator thickness, the YAP was placed alternately inside 5- and 0.3-mm thick plastic shields. The resulting dosimetric sensitivity curves, seen in Fig. 4, are energy independent from 100 to 3000 keV consistent with the similarity of the air and plastic mass absorption coecients seen in Fig. 5. However, for 60±80
We veri®ed our calculated dosimetric sensitivity by repeating our YAP dosimeter sensitivity measurements with high statistics spectra from ®ve sources with individual activity uncertainties from 1.5% to 3.5% at the 99% con®dence level and whose c-ray lines were: 26.35, 59.54 (241 Am); 122.06, 136.5, 692.0 (57 Co); 31.81, 32.19, 36.4, 661.7 (137 Cs); 834.8 (54 Mn); and 1173, 1332 keV (60 Co) [5]. We obtained the dose from these spectra using compensation coecients in 25 energy intervals from our previously measured YAP±copper shield sensitivity curve. The dierence between the measured and calculated dose using source activity, exposure rate constant, and stando distance was less than 5%. We recomputed the compensation coecients which reduced the measured-to-calculated dierence to less than 0.1%.
Fig. 1. Experimental and calculated YAP-based dosimeter sensitivities.
A.A. Fyodorov et al. / Nucl. Instr. and Meth. in Phys. Res. B 134 (1998) 413±417
415
Fig. 3. YAP-plastic phoswich detector design.
Fig. 2. c-ray pulse height spectrum of (a) 24l Am in plastic scintillator and (b) 137 Cs in YAP-plastic phoswich.
keV c rays the photoabsorption in plastic and in air diverge, since the atomic number of plastic is less than that of air. Also at 20 keV the 5-mm thick plastic exhibits the so-called ``surface eect,'' where soft c rays are absorbed only in the detector surface layer reducing the detector's eective volume. This eect can be suppressed by decreasing the detector thickness. For a 0.1-mm thick plastic, the calculated sensitivity is almost energy independent from 5 to 40 keV and even the 0.3-mm plastic sensitivity is acceptably energy independent. However, the thinner the scintillator, the lower its sensitivity, since the count-rate-to-dose-power ratio is reduced. Placing the plastic inside the copper shield had little eect on the sensitivity energy dependence between 5 and 20 keV but the response is
Fig. 4. Sensitivity of plastic and YAP detectors.
1.44 and 0.18 ´ 106 counts/R for 5- and 0.3-mm thick plastic, respectively, exposed to 10 keV X rays. Fig. 6 shows the photoeciency energy dependence for 0.3- and 5-mm thick plastic. From 5 to 20 keV, the 5-mm scintillator has a signi®cant photoeciency, so it is possible to use a compensation coecient to correct for the dosimetric sensitivity energy dependence. However, for Ec greater than 100 keV, c rays are not photoabsorbed in the scintillator and the energy deposited by Compton scattering falls below 100 keV resulting in an erroneous correction coecient. c rays that scatter in
416
A.A. Fyodorov et al. / Nucl. Instr. and Meth. in Phys. Res. B 134 (1998) 413±417
the plastic and are absorbed and registered in the YAP can be rejected in this so-called anti-Compton shielding mode. If the scattered c-ray energy exceeds 100 keV, the 25-mm diameter, 15-mm thick YAP crystal is too small to be an eective absorber. Further, c rays with energy from some tens to some hundreds of keV scatter with high probability at angles greater than 90° and so may not even traverse the YAP.
3. Conclusions
Fig. 5. YAP, plastic, and air (a) linear attenuation and (b) mass absorption coecients [7].
It has not proven possible to completely solve the response linearization problem for plastic as we did in our YAP-based dosimeter. We can, however, identify three promising YAP±plastic phoswich dosimeter applications. 1. X-ray dose measurements. With all the dose power concentrated at low energies, the plastic measurements can be used with or without linearization depending on the plastic thickness. Since the maximum count rate for a plastic-based detector is 107 s, the limiting measurable dose will be 20 and 200 R/hr for 5- and 0.3-mm thick plastic, respectively. Since low-energy X rays are preferentially absorbed in plastic, the fraction reaching the YAP will increase the average PMT anode current and degrade the resolution of the plastic. 2. Measure doses of c and X rays with energy distributions similar to the natural background. For c rays with energy from 5 to 20 keV the YAP data can be complimented by plastic measurements. This requires YAP±plastic intercalibration to relate the plastic-measured c rays with energy between 20 and 3000 keV. 3. Separate the charge particle and c ray dose measured in mixed radiation ®elds. Plastic up to 5-mm thick does not signi®cantly distort YAP dosimetric sensitivity at low energies and so can be used as an active shield.
Acknowledgements Fig. 6. Photoabsorption with energy for 0.3-mm and 5-mm thick plastic.
This work was supported in part by the US Nuclear Regulatory Commission.
A.A. Fyodorov et al. / Nucl. Instr. and Meth. in Phys. Res. B 134 (1998) 413±417
References [1] W.P. Trower, in: M.J. Weber, P. Lecoq, R.C. Ruchti, C. Woody, W.M. Yen, and R-y. Zhu, (Eds.), Scintillators and Phosphor Materials, Material Research Society, Pittsburgh, 1994, pp. 131. [2] V.G. Baryshevsky, M.V. Korzhik, V.I. Moroz, V.B. Pavlenko, A.A. Fyodorov, S.A. Smirnova, O.A. Egorycheva, V.A. Kachanov, Nucl. Instr. and Meth. B 58 (1991) 291.
417
[3] M.V. Korzhik, O.V. Misevich, A.A. Fyodorov, Nucl. Instr. and Meth. B 72 (1992) 499. [4] A.A. Fyodorov, W.P. Trower, R.F. Zuevsky, IEEE Nucl. Symp. Conf. Rec. NSS35-04, 1994. [5] D.C. Kocher, Radioactive Decay Data Tables, Tech. Infor. Center, USDOE, 1981, pp. 75, 77, 78, 143, 211. [6] C.R. Hurlbut, Am. Nucl. Soc. Mtg., 1985. [7] PHOTOCOEF code, ACI, Grafton MA 01519, 1993.
A broad range YAP±plastic phoswich dosimeter A.A. Fyodorov 1, W.P. Trower *, R.F. Zuevsky
1
World Physics Technologies Incorporated, 1105 Highland Circle, Blacksburg, VA 24061, USA Received 3 January 1995; received in revised form 12 September 1997
Abstract We describe a YAP±plastic phoswich detector, its scintillation properties and its potential as a dosimeter. Elsevier Science B.V. All rights reserved.
1. Introduction Cerium-doped Yttrium Aluminum Perovskite (YAP), YAlOa:Ce3 , is a new, fast, bright, and durable scintillator [1], which has already found a variety of applications in detectors for a particles [2], c rays, and X rays [3]. We have constructed a YAP-based c- and X-ray dosimeter with special shielding and an algorithm for dose-energy dependence compensation [4]. Its energy range is 20± 3000 keV, measured exposure dose power range is 0.005±1000 mR/h, measured dose power is 0.01 mR/h in 10 s, with an accuracy of 7%, and measured energy linearity of 10%. The detector sensitivity, S(Ec ), of scintillator x is the Photomultiplier Tube (PMT) anode current, IA , per exposure dose power, P:
* Corresponding author. Tel.: 540 953 2249; fax: 540 953 2249; e-mail: [email protected]. 1 Permanent address: Institute for Nuclear Problems, Belarus State University, Minsk, Belarus.
Ó 1998
Sx Ec IA =P C Ec f1 ÿ eÿ lx Ec t gflx Ec =lx Ec tlair Ec g; where C(Ec ) is an energy dependent compensation coecient, t is the scintillator thickness (cm), lx (Ec ) is the scintillator linear attenuation coecient (cmÿl ), and the mass absorption coecients (cm2 / g) for the scintillator and air are lx (Ec ) and lair (Ec ), respectively. C(Ec ) can be calculated from the measured IA , or equivalently, found from the measured pulse height to produce an energy independent S(Ec ). The large dose energy dependence for 20 < Ec < 250 keV made it impossible to reconstruct values of lx (Ec ), lx (Ec ), and lair (Ec ) for Compton scattered events in this region. For Ec > 250 keV, the energy deposited in the scintillator is between 20 and 250 keV, and produces erroneous mass absorption and linear attenuation coecients. Above 250 keV, Sx (Ec ) is substantially energy independent so assigning events to photoabsorption and Compton scattering is not required. Shielding the scintillator with copper containing an array of holes results in Sx (Ec ) being
0168-583X/98/$19.00 Ó 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 8 - 5 8 3 X ( 9 7 ) 0 0 5 3 5 - 1
414
A.A. Fyodorov et al. / Nucl. Instr. and Meth. in Phys. Res. B 134 (1998) 413±417
underestimated between 20 and 60 keV and overestimated in the 60±250 keV region. The energy dependence of Compton events can, therefore, be compensated. Our success encouraged us to extend our dosimeter energy and dose ranges by combining YAP and plastic scintillators in a so-called phoswich detector. YAP was chosen for its relatively high stopping power and fast luminescent decay, 28 ns, while plastic has low stopping power similar to that of air and decays in a few nanoseconds. The yttrium K-edge at 17 keV de®nes the YAP c-ray energy dose limit, while the plastic is limited only by the PMT dark current, or by the energy noise equivalent of the plastic±PMT system. The typical NaI:Tl-PMT noise equivalent is 1.2±1.5 keV, and plastic is 30% as bright as sodium iodide, so the PMT±plastic energy noise equivalent is 4±5 keV. However, copper cathodes of X-ray tubes have a K-edge at 8 keV, which de®nes the limit in these applications. In summary, our YAP±plastic phoswich detector should (i) extend both our dosimeter dose and energy ranges, (ii) be useful as a low-energy X-ray dosimeter, and (iii) measure c-ray doses in mixed radiation ®elds when the plastic is used as a charged particle anti-coincidence shield. 2. Results and discussion
With these optimized coecients from each of the 25 energy intervals, we reconstructed the experimental sensitivity curve, shown in Fig. 1, with the calculated SYAP (Ec ) curve. Our plastic scintillator, Polyvinyltoluene, has a light output of 60% relative to anthracene, a 3.3ns decay time, and an emission maximum at 434 nm [6]. The YAP has its emission peak at 360 nm and does not absorb NC3 H6 light [1]. Between the 25-mm diameter, 5-mm thick, BC-412 disk and the Hamamatsu R5113 PMT face, we placed a 25mm diameter, 15-mm thick, YAP disk whose measured attenuation of 440-nm light is less than 10%. With a 24l Am c-ray source, we found the plastic-R5113 energy-noise equivalent to be 6.3 keV, which can be reduced below 5 keV with optimized light collection and pulse shaping. An 24l Am pulse height spectrum in BC-412 is shown in Fig. 2(a), while in Fig. 2(b) the 137 Cs YAP±plastic phoswich signals processed with a Canberra 2160A pulse shape discriminator are seen. Fig. 3 shows our phoswich detector construction. To determine the dependence of dosimetric sensitivities on plastic scintillator thickness, the YAP was placed alternately inside 5- and 0.3-mm thick plastic shields. The resulting dosimetric sensitivity curves, seen in Fig. 4, are energy independent from 100 to 3000 keV consistent with the similarity of the air and plastic mass absorption coecients seen in Fig. 5. However, for 60±80
We veri®ed our calculated dosimetric sensitivity by repeating our YAP dosimeter sensitivity measurements with high statistics spectra from ®ve sources with individual activity uncertainties from 1.5% to 3.5% at the 99% con®dence level and whose c-ray lines were: 26.35, 59.54 (241 Am); 122.06, 136.5, 692.0 (57 Co); 31.81, 32.19, 36.4, 661.7 (137 Cs); 834.8 (54 Mn); and 1173, 1332 keV (60 Co) [5]. We obtained the dose from these spectra using compensation coecients in 25 energy intervals from our previously measured YAP±copper shield sensitivity curve. The dierence between the measured and calculated dose using source activity, exposure rate constant, and stando distance was less than 5%. We recomputed the compensation coecients which reduced the measured-to-calculated dierence to less than 0.1%.
Fig. 1. Experimental and calculated YAP-based dosimeter sensitivities.
A.A. Fyodorov et al. / Nucl. Instr. and Meth. in Phys. Res. B 134 (1998) 413±417
415
Fig. 3. YAP-plastic phoswich detector design.
Fig. 2. c-ray pulse height spectrum of (a) 24l Am in plastic scintillator and (b) 137 Cs in YAP-plastic phoswich.
keV c rays the photoabsorption in plastic and in air diverge, since the atomic number of plastic is less than that of air. Also at 20 keV the 5-mm thick plastic exhibits the so-called ``surface eect,'' where soft c rays are absorbed only in the detector surface layer reducing the detector's eective volume. This eect can be suppressed by decreasing the detector thickness. For a 0.1-mm thick plastic, the calculated sensitivity is almost energy independent from 5 to 40 keV and even the 0.3-mm plastic sensitivity is acceptably energy independent. However, the thinner the scintillator, the lower its sensitivity, since the count-rate-to-dose-power ratio is reduced. Placing the plastic inside the copper shield had little eect on the sensitivity energy dependence between 5 and 20 keV but the response is
Fig. 4. Sensitivity of plastic and YAP detectors.
1.44 and 0.18 ´ 106 counts/R for 5- and 0.3-mm thick plastic, respectively, exposed to 10 keV X rays. Fig. 6 shows the photoeciency energy dependence for 0.3- and 5-mm thick plastic. From 5 to 20 keV, the 5-mm scintillator has a signi®cant photoeciency, so it is possible to use a compensation coecient to correct for the dosimetric sensitivity energy dependence. However, for Ec greater than 100 keV, c rays are not photoabsorbed in the scintillator and the energy deposited by Compton scattering falls below 100 keV resulting in an erroneous correction coecient. c rays that scatter in
416
A.A. Fyodorov et al. / Nucl. Instr. and Meth. in Phys. Res. B 134 (1998) 413±417
the plastic and are absorbed and registered in the YAP can be rejected in this so-called anti-Compton shielding mode. If the scattered c-ray energy exceeds 100 keV, the 25-mm diameter, 15-mm thick YAP crystal is too small to be an eective absorber. Further, c rays with energy from some tens to some hundreds of keV scatter with high probability at angles greater than 90° and so may not even traverse the YAP.
3. Conclusions
Fig. 5. YAP, plastic, and air (a) linear attenuation and (b) mass absorption coecients [7].
It has not proven possible to completely solve the response linearization problem for plastic as we did in our YAP-based dosimeter. We can, however, identify three promising YAP±plastic phoswich dosimeter applications. 1. X-ray dose measurements. With all the dose power concentrated at low energies, the plastic measurements can be used with or without linearization depending on the plastic thickness. Since the maximum count rate for a plastic-based detector is 107 s, the limiting measurable dose will be 20 and 200 R/hr for 5- and 0.3-mm thick plastic, respectively. Since low-energy X rays are preferentially absorbed in plastic, the fraction reaching the YAP will increase the average PMT anode current and degrade the resolution of the plastic. 2. Measure doses of c and X rays with energy distributions similar to the natural background. For c rays with energy from 5 to 20 keV the YAP data can be complimented by plastic measurements. This requires YAP±plastic intercalibration to relate the plastic-measured c rays with energy between 20 and 3000 keV. 3. Separate the charge particle and c ray dose measured in mixed radiation ®elds. Plastic up to 5-mm thick does not signi®cantly distort YAP dosimetric sensitivity at low energies and so can be used as an active shield.
Acknowledgements Fig. 6. Photoabsorption with energy for 0.3-mm and 5-mm thick plastic.
This work was supported in part by the US Nuclear Regulatory Commission.
A.A. Fyodorov et al. / Nucl. Instr. and Meth. in Phys. Res. B 134 (1998) 413±417
References [1] W.P. Trower, in: M.J. Weber, P. Lecoq, R.C. Ruchti, C. Woody, W.M. Yen, and R-y. Zhu, (Eds.), Scintillators and Phosphor Materials, Material Research Society, Pittsburgh, 1994, pp. 131. [2] V.G. Baryshevsky, M.V. Korzhik, V.I. Moroz, V.B. Pavlenko, A.A. Fyodorov, S.A. Smirnova, O.A. Egorycheva, V.A. Kachanov, Nucl. Instr. and Meth. B 58 (1991) 291.
417
[3] M.V. Korzhik, O.V. Misevich, A.A. Fyodorov, Nucl. Instr. and Meth. B 72 (1992) 499. [4] A.A. Fyodorov, W.P. Trower, R.F. Zuevsky, IEEE Nucl. Symp. Conf. Rec. NSS35-04, 1994. [5] D.C. Kocher, Radioactive Decay Data Tables, Tech. Infor. Center, USDOE, 1981, pp. 75, 77, 78, 143, 211. [6] C.R. Hurlbut, Am. Nucl. Soc. Mtg., 1985. [7] PHOTOCOEF code, ACI, Grafton MA 01519, 1993.