- Email: [email protected]
Characterization measurement of a thick CdTe detector for BNCT-SPECT – Detection efficiency and energy resolution
Applied Radiation and Isotopes ∎ (∎∎∎∎) ∎∎∎–∎∎∎
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Characterization measurement of a thick CdTe detector for BNCT-SPECT – Detection efficiency and energy resolution Isao Murata a,n, Soichiro Nakamura a, Masanobu Manabe a, Hiroyuki Miyamaru b, Itsuro Kato c a Division of Electrical, Electronic and Information Engineering, Graduate School of Engineering, Osaka University, Yamada-oka 2-1, Suita, Osaka 565-0871, Japan b Radiation Research Center, Osaka Prefecture University, Gakuen-cho 1-1, Nakaku, Sakai, Osaka 599-8531, Japan c Department of Oral and Maxillofacial Surgery II, Graduate School of Dentistry, Osaka University, Yamada-oka 1-8, Suita, Osaka 565-0871, Japan
H I G H L I G H T S
BNCT-SPECT is developed with CdTe device to estimate therapy effect of BNCT. By design calculations, CdTe dimensions are determined to be 1.5 2 30 mm3. Collimator length is 10 cm with 2 mm diameter hole. Producing the crystal, efficiency and energy resolution were measured. Excellent agreement was obtained between measurement and calculation. Discrimination of 478 keV and 511 keV was confirmed in the spectrum.
art ic l e i nf o
Keywords: BNCT SPECT CdTe Gamma-ray 10 B(n,α)7Li 478 keV
a b s t r a c t Author's group is carrying out development of BNCT-SPECT with CdTe device, which monitors the therapy effect of BNCT in real-time. From the design calculations, the dimensions were fixed to 1.5 2 30 mm3. For the collimator it was confirmed that it would have a good spatial resolution and simultaneously the number of counts would be acceptably large. After producing the CdTe crystal, the characterization measurement was carried out. For the detection efficiency an excellent agreement between calculation and measurement was obtained. Also, the detector has a very good energy resolution so that gamma-rays of 478 keV and 511 keV could be distinguished in the spectrum. & 2014 Elsevier Ltd. All rights reserved.
1. Introduction Boron neutron capture therapy (BNCT) is known to be a very effective cancer therapy, which becomes popular worldwide. BNCT can destroy only tumor cells by charged-particles produced via 10 B(n,α)7Li reaction. The effect to normal tissues could be suppressed substantially, if 10B could be concentrated only in the tumor. The treatment effect of BNCT is surely large, however, it is difficult to know exactly how large it is. The effect (boron component of the local dose) is normally estimated by making a product of the 10B concentration and the neutron flux intensity. The distribution of 10B concentration can be estimated by using positron emission tomography (PET) just before an actual BNCT (Imahori et al., 1998). However, the 10B concentration is changing
n
Corresponding author. E-mail address: [email protected] (I. Murata).
gradually, meaning the estimated concentration would just show a reference value. On the other hand, the thermal neutron flux intensity can normally be estimated by simulation with a calculation code system like JCDS developed by JAEA (Kumada et al., 2007). Under these predicted conditions, the BNCT treatment planning can be performed. However, in reality, after starting irradiation with help of supplemented means like activation foils, small neutron detectors and so on, the ending time of irradiation is finally determined. However, it was reported that by this procedure the convalescence did not follow the expectation made before BNCT. Also, there were different convalescences observed at two Japanese available BNCT facilities of KUR and JRR-4 even with an equal protocol of BNCT (Kato, 2008). This fact indicates that the current irradiation scheduling can give just a reference plan for BNCT. The key issue is that it is not possible to know the real effect during irradiation in real time. For this problem, several studies were carried out so far with semiconductor detectors or scintillators (Verbakel et al., 1997;
http://dx.doi.org/10.1016/j.apradiso.2014.01.023 0969-8043 & 2014 Elsevier Ltd. All rights reserved.
Please cite this article as: Murata, I., et al., Characterization measurement of a thick CdTe detector for BNCT-SPECT – Detection efficiency and energy resolution. Appl. Radiat. Isotopes (2014), http://dx.doi.org/10.1016/j.apradiso.2014.01.023i
I. Murata et al. / Applied Radiation and Isotopes ∎ (∎∎∎∎) ∎∎∎–∎∎∎
2
Munck af Rosenschöld et al., 2006; Minsky et al., 2009; Kobayashi et al., 2000). However, no detection systems have been completed for practical use in BNCT. Author's group hence started a research to develop a new SPECT system to monitor the BNCT effect in real time, which is named BNCT-SPECT (Murata et al., 2011b). The principle to monitor the 3-dimensional treatment effect of BNCT in real time is as follows: measuring 478 keV gamma-rays emitted from the residual excited nuclide of 7Li, the 3-dimensional intensity distribution of 10B(n,α)7Li reaction is estimated by solving the so-called inverse problem. The obtained distribution becomes the treatment effect of BNCT directly. For BNCT-SPECT, we chose a CdTe detector as an elemental detection device with the following reasons: good energy resolution is expected with a Ge semiconductor detector. However, it is difficult to downsize to keep a good spatial resolution. Scintillation detector is a good candidate because it is used in PET. However, this is available for coincidence detection of two annihilation gamma-rays. In case of single photon (478 keV gamma-ray), background is too large to extract a signal of interest. In addition, energy resolution is too poor to distinguish 478 keV and 511 keV gamma-rays, compared to semiconductor detectors. We selected CdTe because it can be downsized easily and it has good energy resolution and at the same time it has high detection efficiency. In this study, based on the previous our results (Nakamura et al., 2012; Murata et al., 2011a; Mukai et al., 2010) more precise design calculations were carried out to determine the dimensions of the CdTe crystal and the collimator together for BNCT-SPECT. With the result, a CdTe detector was produced and its characterization measurement was conducted. Also, discrimination performance for 478 keV and 511 keV was confirmed considering the Doppler effect.
In BNCT, the following nuclear reaction is utilized. B þn-α þ 7 Li þ 2:79 MeV -α þ 7 Lin þ 2:31 MeV þ γð478 keVÞ 7
ð1Þ 7
(1) 478 keV peak net count: 41000 counts/irradiation. (2) Spatial resolution: 2 mm. (3) Energy resolution: o33 keV( ¼511 keV–478 keV).
3. Design calculation To determine basic design quantities (CdTe dimensions and collimator length and its diameter) meeting the design goal ((1) and (2) in Section 2), more precise transport calculations than before (Nakamura et al., 2012; Murata et al., 2011a; Mukai et al., 2010) were carried out using a three dimensional model taking into account the detector, collimator and gamma-ray source. 3.1. CdTe dimensions The three dimensional model is shown in Fig. 1. The source is treated as a volume source. To meet the target figure of spatial resolution, the collimator diameter is set to 2 mm. Three dimensional transport calculations were performed by varying the crystal dimensions and collimator length using the three dimensional N-particle Monte Carlo transport calculation code MCNP5. Pulse height tally (F8) was used to estimate pulse height spectrum at the detector. Other calculation conditions are as follows:
10B concentration: 10 several tens ppm. Thermal neutron intensity: 109 n/s/cm2 at the tumor. Detector position: 30 cm from the center of the tumor. Irradiation time: 1 h.
2. Design goal of BNCT-SPECT
10
Finally BNCT-SPECT should have a good energy resolution so that the signal to noise (S/N) ratio for the net photopeak count can be improved and discrimination can be done for adjacent gammaray of annihilation gamma-ray of 511 keV. We thus set our design goal as follows:
94% of the produced Li are in an excited state, Li*, from which 478 keV gamma-ray is emitted simultaneously. This gamma-ray is measured in the present study. If 3-dimensional intensity distribution of the gamma-ray emission is measured, it is equivalent to the reaction rate distribution of 10B(n,α)7Li reaction, and is the 3-dimensional treatment effect of BNCT. The measurement could be performed by a well known SPECT system. However, this is not so straightforward because a lot of background radiations exist as in the following: (1) 2223 keV gamma-rays produced by 1H(n,γ)2H reaction, (2) direct signals to the detector due to neutrons produced by scattering of incident neutrons, (3) other capture gamma-rays than 2223 keV, and (4) annihilation gamma-rays. These should be shielded with a suitably designed collimator. However, it means the distance increases between the detector and gamma-ray source, resulting in decrease of the number of counts. Also, an enough spatial resolution less than several mm is required by doctors in hospitals. To realize it, the collimator diameter should be small. But this leads also to decrease the count rate. In BNCT-SPECT, contrary to this deterioration a good statistical accuracy should be achieved being less than several percentages. We have to design CdTe crystal dimensions to keep balance for the above contradictory problem. Incidentally, for the practical design of the collimator with shields for the array-type CdTe detector was discussed by Manabe et al. (2012). In the present study, taking into account only the collimator, the design calculation was carried out for Manabe's practical design.
Table 1 shows the calculation results. At present, the length of CdTe crystal is limited up to around 40 mm due to production limit of CdTe wafer. Considering low production yield of 40 mm thick crystal, in the table the result for 30 mm is shown. Also, the entrance surface length is 0.15–0.25 mm according to the design goal of the collimator diameter of 2 mm. It is confirmed from the
1~2.5mm
~30cm 20~40mm
Collimator
1~2mm
5 ~ 15 cm
CdTe detector
Tumor
Fig. 1. 3-Dimensional model to calculate the pulse height spectrum.
Table 1 478 keV gamma-ray count rate (counts/h). CdTe entrance surface (cm2)
0.1 0.2
Collimator length (cm)
5
CdTe length (cm) 2 3.5 4
2.2E3 2.4E3 2.7E3
Collimator diameter: 2 mm and
0.15 0.2
0.2 0.25
10
10
15
10
15
5.7E2 6.8E2 6.9E2
8.6E2 1.1E3 1.1E3
4.0E2 4.8E2 5.0E2
1.3E3 1.6E3 1.6E3
5.8E2 7.1E2 7.4E2
10
B: 10 ppm.
Please cite this article as: Murata, I., et al., Characterization measurement of a thick CdTe detector for BNCT-SPECT – Detection efficiency and energy resolution. Appl. Radiat. Isotopes (2014), http://dx.doi.org/10.1016/j.apradiso.2014.01.023i
I. Murata et al. / Applied Radiation and Isotopes ∎ (∎∎∎∎) ∎∎∎–∎∎∎
3
3.2. Spatial resolution From the result in the previous section, the relation of the collimator length and CdTe crystal dimensions was made clear. In this section, for the crystal of 0.15 0.2 30 mm3, the spatial resolution was examined by changing the collimator length. Other conditions are the same as the previous section, except the source condition, i.e., a ring-shape source was used. The calculation model is shown in Fig. 2. The calculation results are shown in Figs. 3 and 4. Fig. 3 shows viewing area from the detector through the collimator. X-axis means the radius of the ring-shape source. Y-axis is the ratio of the photopeak count per source neutron. The spatial resolution is defined as a full width at half maximum (FWHM). Fig. 4 shows the spatial resolution as a function of collimator length. From the figure it is found that the collimator length should be longer. However, the number of counts becomes fewer in that case from the result of the previous section. Finally, in the present study the collimator length is set to 10 cm in the design. As for the collimator diameter we fixed 2 mm as in Section 3.1. However, calculations for other cases show that the spatial resolution can be improved if decreasing the collimator diameter. However, the number of counts of course decreases as a result. If increasing it, the number of counts is improved, but the spatial resolution is not acceptable. In conclusion, the balancing point is 2 mm in diameter.
Spatial resolution (cm)
table that the number of counts is more than 1000 per hour if the collimator length is kept to be less than 10 cm.
100
9% 5% 3.3 % ln y=a + b ln x a=1.27730494e+00 b=-8.70725436e-01 1.80803963e-02 |r|=9.98953862e-01
-1
10
5 101 15 Collimator length (cm)
100
Fig. 4. Spatial resolution evaluated from Fig. 3.
Gamma-ray SHV
4. Characterization measurement
CdTe detector
From the results obtained in Section 3, a large crystal of 1.5 2 30 mm3 was produced, which could be utilized for BNCT-SPECT as an elemental detection device. Fig. 5 shows the produced CdTe crystal and the detector. Tests carried out include efficiency measurement and energy resolution measurement. For the efficiency measurement, comparison between experiment and theoretical calculation is carried out to confirm applicability to
30 mm
~30cm 30mm
Collimator
r
1.5mm CdTe detector
5 ~ 15 cm
CdTe crystal (1.5 x 2 x 30 mm3)
Ring source
Fig. 2. 3-Dimensional model to calculate the spatial resolution. Fig. 5. Produced CdTe crystal and detector.
design of the whole detector system. For the energy resolution measurement, it is checked whether the energy resolution value would be less than 33 keV which was listed up in the design goal as (3) in Section 2.
[ 10 ] 10
Photopeak counts / Source neutron
-7
Coll. length : 15 cm : 10 cm : 5 cm
8
4.1. Detector efficiency
6 FWHM 4 2 0 -1
r -0.5 0 0.5 Distance from the collimator center (cm)
Fig. 3. Viewing area through collimator from the detector.
1
With standard sources, 241Am, 133Ba, 22Na and 137Cs, photopeak efficiencies were measured. The result is described in Fig. 6. The numerical simulation was carried out three-dimensionally by MCNP5. Pulse height tally (F8) was used to estimate the pulse height spectrum to be measured by the detector and extract the photopeak count. The experimental and calculated efficiencies are shown in Fig. 6. Excellent agreement is seen between experiment and calculation. By fitting the values with an appropriate curve the efficiency at 478 keV is evaluated as 7.4 10 6 75% in experiment and 7.1 10 6 in calculation. Both results are agreed within the
Please cite this article as: Murata, I., et al., Characterization measurement of a thick CdTe detector for BNCT-SPECT – Detection efficiency and energy resolution. Appl. Radiat. Isotopes (2014), http://dx.doi.org/10.1016/j.apradiso.2014.01.023i
I. Murata et al. / Applied Radiation and Isotopes ∎ (∎∎∎∎) ∎∎∎–∎∎∎
4
broadening for these two gamma-rays was measured to confirm whether discrimination of these two gamma-rays could really be possible.
10-4 : Cal. by MCNP5 : Experiment
Absolute efficiency
241Am
Fit. of exp. data
133Ba
-5
5.1. Experimental 22Na
10
137Cs ln y=a + b x a=-9.77410397e+00 b=-4.26513369e-03 1.46559843e-01 |r|=9.88316930e-01
10-6
200 600 400 Gamma-ray energy (keV)
0
800
5.2. Results and discussion
Fig. 6. Measured detector efficiencies.
Energy resolution (keV)
50 : Experiment
10
137Cs 22 133Ba Na
241Am
5 Fit. of exp. data
1 101
ln y=a + b ln x a=-2.84006566e-01 b=5.12679184e-01 4.45636364e-02 |r|=9.95773764e-01
102 Gamma-ray energy (keV)
The experimental arrangement is shown in Fig. 8. At the center of a graphite column of 1 1 1 m3 an AmBe neutron source was set, the intensity of which is 2.4 106 n/s. On the surface of the column an HpGe detector was arranged shielded with lead bricks. And as shown in the figure, boric acid of 50 g was put in front of the HpGe detector and gamma-ray spectrum was measured.
The obtained pulse height spectrum is described in Fig. 9. Two peaks corresponding to 478 keV and 511 keV are clearly recognized. A peak existing between the two peaks is caused by an HpGe detector itself, which would not be seen in the real case with the CdTe detector. Gamma-rays of 511 keV could be created via pair annihilation process of positron produced by high energy gamma-rays from the AmBe source. In the 478 keV peak, Doppler broadening is clearly observed. The FWHM is 11.5 keV. At 511 keV, no significant broadening is seen. Since the energy resolution of HpGe is quite well, 11.5 keV is conservatively and directly added to the result in Section 4.2. Finally the evaluated average energy resolution considering Doppler broadening and error is 31.4 keV,
AmBe neutron source inside the graphite column
103
Fig. 7. Measured energy resolution.
Lead shield
experimental error. It was confirmed that MCNP5 could predict the detector efficiency of CdTe, though one should consider even electron transport precisely, which is essential if estimating the efficiency of a very small CdTe crystal. By this result, Manabe et al. (2012) conducted the practical collimator and shield of array-type CdTe detector for the BNCT-SPECT.
HpGe detector
4.2. Energy resolution
5. Doppler broadening From the result in the previous section discrimination of 478 keV and 511 keV was expected to be realized. However, these peak shapes would be affected by Doppler effect because these gamma-rays are emitted just after nuclear reactions. Because the value at 511 keV is known to be small, there would be no serious problem in case of the CdTe detector. For 478 keV the Doppler broadening might be larger. In this section, the Doppler
Boric acid (50g) Fig. 8. Experimental setup photo of Doppler broadening measurement.
20000
Counts / channel
With the same experimental setup energy resolution was measured. The energy resolution is defined as FWHM value at photopeak. The measured result is shown in Fig. 7. Since the energy resolution is known to be described as a straight line in a log–log representation, the values at 478 keV and 511 keV were estimated by fitting the measured data. The results were 17.8 keV 710% at 478 keV and 18.4 keV 710% at 511 keV. Average energy resolution of both is confirmed to be less than 33 keV which is the difference between both peak energies.
Detector : HpGe Time : 811 sec
511keV 478keV 10000
0 100
11.5keV Measured pulse height spectrum 150
200
250
Channel Fig. 9. Pulse height spectrum around 478 keV measured by HpGe detector.
Please cite this article as: Murata, I., et al., Characterization measurement of a thick CdTe detector for BNCT-SPECT – Detection efficiency and energy resolution. Appl. Radiat. Isotopes (2014), http://dx.doi.org/10.1016/j.apradiso.2014.01.023i
I. Murata et al. / Applied Radiation and Isotopes ∎ (∎∎∎∎) ∎∎∎–∎∎∎
which is less than 33 keV, meaning that discrimination of both gamma-rays is possible in the measured spectrum. 6. Conclusion Aiming at BNCT-SPECT optimized dimensions of an elemental CdTe element were examined by design calculations, which are 1.5 2 30 mm3. Also, the collimator length and diameter were determined to be 10 cm and 2 mm, respectively. In this design, the spatial resolution was confirmed to be several mm and the number of counts was more than 1000 per hour. Producing a CdTe crystal and performing the test measurement, for the detection efficiency an excellent agreement was obtained to show that MCNP has a very good ability to design the detection system with a CdTe crystal accurately. Also, it was found that the detector had a very good energy resolution so that gamma-rays of 478 keV and 511 keV could be distinguished in the spectrum. Acknowledgments This study is being conducted in part under the support of Grants-in-Aid for Scientific Research by JSPS, Scientific Research (B) (Contract No. 22360405).
5
References Imahori, Y., et al., 1998. Fluorine-18-labeled fluoro-boronophenylalanine PET in patients with glioma. J. Nucl. Med. 39, 325–333. Kato, I., 2008. Private communication. Osaka University. Kobayashi, T., et al., 2000. A noninvasive dose estimation system for clinical BNCT based on PG-SPECT – conceptual study and fundamental experiments using HPGe and CdTe semiconductor detectors. Med. Phys. 27 (9), 2124–2132. Kumada, K., et al., 2007. Development of JCDS, a computational dosimetry system at JAEA for boron neutron capture therapy. J. Phys. Conf. Ser., 74. Manabe, M. et al., 2012. Collimator design for array-type CdTe detector for BNCTSPECT. In: Proceedings of the 15th International Congress on Neutron Capture Therapy, Sept. 10–14, Tsukuba, Japan, 6B-05. Minsky, D.M., et al., 2009. Experimental feasibility studies on a SPECT tomograph for BNCT dosimetry. Appl. Radiat. Isot. 67 (7–8), S179–S182. Mukai, T., et al., 2010. Development of CdTe Detector for BNCT-SPECT. JAEA-Conf. 2010-005, 75–80. Munck af Rosenschöld, P., et al., 2006. Prompt gamma tomography during BNCT – a feasibility study. J. Instrum. 1, 1–10. Murata, I., et al., 2011a. Development of a thick CdTe detector for BNCT-SPECT. Appl. Radiat. Isot. 69, 1706–1709. Murata, I., et al., 2011b. Feasibility study on BNCT-SPECT using a CdTe detector. Prog. Nucl. Sci. Technol. 1, 267–270. Nakamura, S., et al., 2012. Characterization test of CdTe detector element designed and developed for BNCT-SPECT. JAEA-Conf. 2012-001, 165–170. Verbakel, W., et al., 1997. γ-Ray telescope for on-line measurements of low boron concentrations in a head phantom for BNCT. Nucl. Instrum. Methods Phys. Res. Sect. A 394 (1–2), 163–172.
Please cite this article as: Murata, I., et al., Characterization measurement of a thick CdTe detector for BNCT-SPECT – Detection efficiency and energy resolution. Appl. Radiat. Isotopes (2014), http://dx.doi.org/10.1016/j.apradiso.2014.01.023i
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Characterization measurement of a thick CdTe detector for BNCT-SPECT – Detection efficiency and energy resolution Isao Murata a,n, Soichiro Nakamura a, Masanobu Manabe a, Hiroyuki Miyamaru b, Itsuro Kato c a Division of Electrical, Electronic and Information Engineering, Graduate School of Engineering, Osaka University, Yamada-oka 2-1, Suita, Osaka 565-0871, Japan b Radiation Research Center, Osaka Prefecture University, Gakuen-cho 1-1, Nakaku, Sakai, Osaka 599-8531, Japan c Department of Oral and Maxillofacial Surgery II, Graduate School of Dentistry, Osaka University, Yamada-oka 1-8, Suita, Osaka 565-0871, Japan
H I G H L I G H T S
BNCT-SPECT is developed with CdTe device to estimate therapy effect of BNCT. By design calculations, CdTe dimensions are determined to be 1.5 2 30 mm3. Collimator length is 10 cm with 2 mm diameter hole. Producing the crystal, efficiency and energy resolution were measured. Excellent agreement was obtained between measurement and calculation. Discrimination of 478 keV and 511 keV was confirmed in the spectrum.
art ic l e i nf o
Keywords: BNCT SPECT CdTe Gamma-ray 10 B(n,α)7Li 478 keV
a b s t r a c t Author's group is carrying out development of BNCT-SPECT with CdTe device, which monitors the therapy effect of BNCT in real-time. From the design calculations, the dimensions were fixed to 1.5 2 30 mm3. For the collimator it was confirmed that it would have a good spatial resolution and simultaneously the number of counts would be acceptably large. After producing the CdTe crystal, the characterization measurement was carried out. For the detection efficiency an excellent agreement between calculation and measurement was obtained. Also, the detector has a very good energy resolution so that gamma-rays of 478 keV and 511 keV could be distinguished in the spectrum. & 2014 Elsevier Ltd. All rights reserved.
1. Introduction Boron neutron capture therapy (BNCT) is known to be a very effective cancer therapy, which becomes popular worldwide. BNCT can destroy only tumor cells by charged-particles produced via 10 B(n,α)7Li reaction. The effect to normal tissues could be suppressed substantially, if 10B could be concentrated only in the tumor. The treatment effect of BNCT is surely large, however, it is difficult to know exactly how large it is. The effect (boron component of the local dose) is normally estimated by making a product of the 10B concentration and the neutron flux intensity. The distribution of 10B concentration can be estimated by using positron emission tomography (PET) just before an actual BNCT (Imahori et al., 1998). However, the 10B concentration is changing
n
Corresponding author. E-mail address: [email protected] (I. Murata).
gradually, meaning the estimated concentration would just show a reference value. On the other hand, the thermal neutron flux intensity can normally be estimated by simulation with a calculation code system like JCDS developed by JAEA (Kumada et al., 2007). Under these predicted conditions, the BNCT treatment planning can be performed. However, in reality, after starting irradiation with help of supplemented means like activation foils, small neutron detectors and so on, the ending time of irradiation is finally determined. However, it was reported that by this procedure the convalescence did not follow the expectation made before BNCT. Also, there were different convalescences observed at two Japanese available BNCT facilities of KUR and JRR-4 even with an equal protocol of BNCT (Kato, 2008). This fact indicates that the current irradiation scheduling can give just a reference plan for BNCT. The key issue is that it is not possible to know the real effect during irradiation in real time. For this problem, several studies were carried out so far with semiconductor detectors or scintillators (Verbakel et al., 1997;
http://dx.doi.org/10.1016/j.apradiso.2014.01.023 0969-8043 & 2014 Elsevier Ltd. All rights reserved.
Please cite this article as: Murata, I., et al., Characterization measurement of a thick CdTe detector for BNCT-SPECT – Detection efficiency and energy resolution. Appl. Radiat. Isotopes (2014), http://dx.doi.org/10.1016/j.apradiso.2014.01.023i
I. Murata et al. / Applied Radiation and Isotopes ∎ (∎∎∎∎) ∎∎∎–∎∎∎
2
Munck af Rosenschöld et al., 2006; Minsky et al., 2009; Kobayashi et al., 2000). However, no detection systems have been completed for practical use in BNCT. Author's group hence started a research to develop a new SPECT system to monitor the BNCT effect in real time, which is named BNCT-SPECT (Murata et al., 2011b). The principle to monitor the 3-dimensional treatment effect of BNCT in real time is as follows: measuring 478 keV gamma-rays emitted from the residual excited nuclide of 7Li, the 3-dimensional intensity distribution of 10B(n,α)7Li reaction is estimated by solving the so-called inverse problem. The obtained distribution becomes the treatment effect of BNCT directly. For BNCT-SPECT, we chose a CdTe detector as an elemental detection device with the following reasons: good energy resolution is expected with a Ge semiconductor detector. However, it is difficult to downsize to keep a good spatial resolution. Scintillation detector is a good candidate because it is used in PET. However, this is available for coincidence detection of two annihilation gamma-rays. In case of single photon (478 keV gamma-ray), background is too large to extract a signal of interest. In addition, energy resolution is too poor to distinguish 478 keV and 511 keV gamma-rays, compared to semiconductor detectors. We selected CdTe because it can be downsized easily and it has good energy resolution and at the same time it has high detection efficiency. In this study, based on the previous our results (Nakamura et al., 2012; Murata et al., 2011a; Mukai et al., 2010) more precise design calculations were carried out to determine the dimensions of the CdTe crystal and the collimator together for BNCT-SPECT. With the result, a CdTe detector was produced and its characterization measurement was conducted. Also, discrimination performance for 478 keV and 511 keV was confirmed considering the Doppler effect.
In BNCT, the following nuclear reaction is utilized. B þn-α þ 7 Li þ 2:79 MeV -α þ 7 Lin þ 2:31 MeV þ γð478 keVÞ 7
ð1Þ 7
(1) 478 keV peak net count: 41000 counts/irradiation. (2) Spatial resolution: 2 mm. (3) Energy resolution: o33 keV( ¼511 keV–478 keV).
3. Design calculation To determine basic design quantities (CdTe dimensions and collimator length and its diameter) meeting the design goal ((1) and (2) in Section 2), more precise transport calculations than before (Nakamura et al., 2012; Murata et al., 2011a; Mukai et al., 2010) were carried out using a three dimensional model taking into account the detector, collimator and gamma-ray source. 3.1. CdTe dimensions The three dimensional model is shown in Fig. 1. The source is treated as a volume source. To meet the target figure of spatial resolution, the collimator diameter is set to 2 mm. Three dimensional transport calculations were performed by varying the crystal dimensions and collimator length using the three dimensional N-particle Monte Carlo transport calculation code MCNP5. Pulse height tally (F8) was used to estimate pulse height spectrum at the detector. Other calculation conditions are as follows:
10B concentration: 10 several tens ppm. Thermal neutron intensity: 109 n/s/cm2 at the tumor. Detector position: 30 cm from the center of the tumor. Irradiation time: 1 h.
2. Design goal of BNCT-SPECT
10
Finally BNCT-SPECT should have a good energy resolution so that the signal to noise (S/N) ratio for the net photopeak count can be improved and discrimination can be done for adjacent gammaray of annihilation gamma-ray of 511 keV. We thus set our design goal as follows:
94% of the produced Li are in an excited state, Li*, from which 478 keV gamma-ray is emitted simultaneously. This gamma-ray is measured in the present study. If 3-dimensional intensity distribution of the gamma-ray emission is measured, it is equivalent to the reaction rate distribution of 10B(n,α)7Li reaction, and is the 3-dimensional treatment effect of BNCT. The measurement could be performed by a well known SPECT system. However, this is not so straightforward because a lot of background radiations exist as in the following: (1) 2223 keV gamma-rays produced by 1H(n,γ)2H reaction, (2) direct signals to the detector due to neutrons produced by scattering of incident neutrons, (3) other capture gamma-rays than 2223 keV, and (4) annihilation gamma-rays. These should be shielded with a suitably designed collimator. However, it means the distance increases between the detector and gamma-ray source, resulting in decrease of the number of counts. Also, an enough spatial resolution less than several mm is required by doctors in hospitals. To realize it, the collimator diameter should be small. But this leads also to decrease the count rate. In BNCT-SPECT, contrary to this deterioration a good statistical accuracy should be achieved being less than several percentages. We have to design CdTe crystal dimensions to keep balance for the above contradictory problem. Incidentally, for the practical design of the collimator with shields for the array-type CdTe detector was discussed by Manabe et al. (2012). In the present study, taking into account only the collimator, the design calculation was carried out for Manabe's practical design.
Table 1 shows the calculation results. At present, the length of CdTe crystal is limited up to around 40 mm due to production limit of CdTe wafer. Considering low production yield of 40 mm thick crystal, in the table the result for 30 mm is shown. Also, the entrance surface length is 0.15–0.25 mm according to the design goal of the collimator diameter of 2 mm. It is confirmed from the
1~2.5mm
~30cm 20~40mm
Collimator
1~2mm
5 ~ 15 cm
CdTe detector
Tumor
Fig. 1. 3-Dimensional model to calculate the pulse height spectrum.
Table 1 478 keV gamma-ray count rate (counts/h). CdTe entrance surface (cm2)
0.1 0.2
Collimator length (cm)
5
CdTe length (cm) 2 3.5 4
2.2E3 2.4E3 2.7E3
Collimator diameter: 2 mm and
0.15 0.2
0.2 0.25
10
10
15
10
15
5.7E2 6.8E2 6.9E2
8.6E2 1.1E3 1.1E3
4.0E2 4.8E2 5.0E2
1.3E3 1.6E3 1.6E3
5.8E2 7.1E2 7.4E2
10
B: 10 ppm.
Please cite this article as: Murata, I., et al., Characterization measurement of a thick CdTe detector for BNCT-SPECT – Detection efficiency and energy resolution. Appl. Radiat. Isotopes (2014), http://dx.doi.org/10.1016/j.apradiso.2014.01.023i
I. Murata et al. / Applied Radiation and Isotopes ∎ (∎∎∎∎) ∎∎∎–∎∎∎
3
3.2. Spatial resolution From the result in the previous section, the relation of the collimator length and CdTe crystal dimensions was made clear. In this section, for the crystal of 0.15 0.2 30 mm3, the spatial resolution was examined by changing the collimator length. Other conditions are the same as the previous section, except the source condition, i.e., a ring-shape source was used. The calculation model is shown in Fig. 2. The calculation results are shown in Figs. 3 and 4. Fig. 3 shows viewing area from the detector through the collimator. X-axis means the radius of the ring-shape source. Y-axis is the ratio of the photopeak count per source neutron. The spatial resolution is defined as a full width at half maximum (FWHM). Fig. 4 shows the spatial resolution as a function of collimator length. From the figure it is found that the collimator length should be longer. However, the number of counts becomes fewer in that case from the result of the previous section. Finally, in the present study the collimator length is set to 10 cm in the design. As for the collimator diameter we fixed 2 mm as in Section 3.1. However, calculations for other cases show that the spatial resolution can be improved if decreasing the collimator diameter. However, the number of counts of course decreases as a result. If increasing it, the number of counts is improved, but the spatial resolution is not acceptable. In conclusion, the balancing point is 2 mm in diameter.
Spatial resolution (cm)
table that the number of counts is more than 1000 per hour if the collimator length is kept to be less than 10 cm.
100
9% 5% 3.3 % ln y=a + b ln x a=1.27730494e+00 b=-8.70725436e-01 1.80803963e-02 |r|=9.98953862e-01
-1
10
5 101 15 Collimator length (cm)
100
Fig. 4. Spatial resolution evaluated from Fig. 3.
Gamma-ray SHV
4. Characterization measurement
CdTe detector
From the results obtained in Section 3, a large crystal of 1.5 2 30 mm3 was produced, which could be utilized for BNCT-SPECT as an elemental detection device. Fig. 5 shows the produced CdTe crystal and the detector. Tests carried out include efficiency measurement and energy resolution measurement. For the efficiency measurement, comparison between experiment and theoretical calculation is carried out to confirm applicability to
30 mm
~30cm 30mm
Collimator
r
1.5mm CdTe detector
5 ~ 15 cm
CdTe crystal (1.5 x 2 x 30 mm3)
Ring source
Fig. 2. 3-Dimensional model to calculate the spatial resolution. Fig. 5. Produced CdTe crystal and detector.
design of the whole detector system. For the energy resolution measurement, it is checked whether the energy resolution value would be less than 33 keV which was listed up in the design goal as (3) in Section 2.
[ 10 ] 10
Photopeak counts / Source neutron
-7
Coll. length : 15 cm : 10 cm : 5 cm
8
4.1. Detector efficiency
6 FWHM 4 2 0 -1
r -0.5 0 0.5 Distance from the collimator center (cm)
Fig. 3. Viewing area through collimator from the detector.
1
With standard sources, 241Am, 133Ba, 22Na and 137Cs, photopeak efficiencies were measured. The result is described in Fig. 6. The numerical simulation was carried out three-dimensionally by MCNP5. Pulse height tally (F8) was used to estimate the pulse height spectrum to be measured by the detector and extract the photopeak count. The experimental and calculated efficiencies are shown in Fig. 6. Excellent agreement is seen between experiment and calculation. By fitting the values with an appropriate curve the efficiency at 478 keV is evaluated as 7.4 10 6 75% in experiment and 7.1 10 6 in calculation. Both results are agreed within the
Please cite this article as: Murata, I., et al., Characterization measurement of a thick CdTe detector for BNCT-SPECT – Detection efficiency and energy resolution. Appl. Radiat. Isotopes (2014), http://dx.doi.org/10.1016/j.apradiso.2014.01.023i
I. Murata et al. / Applied Radiation and Isotopes ∎ (∎∎∎∎) ∎∎∎–∎∎∎
4
broadening for these two gamma-rays was measured to confirm whether discrimination of these two gamma-rays could really be possible.
10-4 : Cal. by MCNP5 : Experiment
Absolute efficiency
241Am
Fit. of exp. data
133Ba
-5
5.1. Experimental 22Na
10
137Cs ln y=a + b x a=-9.77410397e+00 b=-4.26513369e-03 1.46559843e-01 |r|=9.88316930e-01
10-6
200 600 400 Gamma-ray energy (keV)
0
800
5.2. Results and discussion
Fig. 6. Measured detector efficiencies.
Energy resolution (keV)
50 : Experiment
10
137Cs 22 133Ba Na
241Am
5 Fit. of exp. data
1 101
ln y=a + b ln x a=-2.84006566e-01 b=5.12679184e-01 4.45636364e-02 |r|=9.95773764e-01
102 Gamma-ray energy (keV)
The experimental arrangement is shown in Fig. 8. At the center of a graphite column of 1 1 1 m3 an AmBe neutron source was set, the intensity of which is 2.4 106 n/s. On the surface of the column an HpGe detector was arranged shielded with lead bricks. And as shown in the figure, boric acid of 50 g was put in front of the HpGe detector and gamma-ray spectrum was measured.
The obtained pulse height spectrum is described in Fig. 9. Two peaks corresponding to 478 keV and 511 keV are clearly recognized. A peak existing between the two peaks is caused by an HpGe detector itself, which would not be seen in the real case with the CdTe detector. Gamma-rays of 511 keV could be created via pair annihilation process of positron produced by high energy gamma-rays from the AmBe source. In the 478 keV peak, Doppler broadening is clearly observed. The FWHM is 11.5 keV. At 511 keV, no significant broadening is seen. Since the energy resolution of HpGe is quite well, 11.5 keV is conservatively and directly added to the result in Section 4.2. Finally the evaluated average energy resolution considering Doppler broadening and error is 31.4 keV,
AmBe neutron source inside the graphite column
103
Fig. 7. Measured energy resolution.
Lead shield
experimental error. It was confirmed that MCNP5 could predict the detector efficiency of CdTe, though one should consider even electron transport precisely, which is essential if estimating the efficiency of a very small CdTe crystal. By this result, Manabe et al. (2012) conducted the practical collimator and shield of array-type CdTe detector for the BNCT-SPECT.
HpGe detector
4.2. Energy resolution
5. Doppler broadening From the result in the previous section discrimination of 478 keV and 511 keV was expected to be realized. However, these peak shapes would be affected by Doppler effect because these gamma-rays are emitted just after nuclear reactions. Because the value at 511 keV is known to be small, there would be no serious problem in case of the CdTe detector. For 478 keV the Doppler broadening might be larger. In this section, the Doppler
Boric acid (50g) Fig. 8. Experimental setup photo of Doppler broadening measurement.
20000
Counts / channel
With the same experimental setup energy resolution was measured. The energy resolution is defined as FWHM value at photopeak. The measured result is shown in Fig. 7. Since the energy resolution is known to be described as a straight line in a log–log representation, the values at 478 keV and 511 keV were estimated by fitting the measured data. The results were 17.8 keV 710% at 478 keV and 18.4 keV 710% at 511 keV. Average energy resolution of both is confirmed to be less than 33 keV which is the difference between both peak energies.
Detector : HpGe Time : 811 sec
511keV 478keV 10000
0 100
11.5keV Measured pulse height spectrum 150
200
250
Channel Fig. 9. Pulse height spectrum around 478 keV measured by HpGe detector.
Please cite this article as: Murata, I., et al., Characterization measurement of a thick CdTe detector for BNCT-SPECT – Detection efficiency and energy resolution. Appl. Radiat. Isotopes (2014), http://dx.doi.org/10.1016/j.apradiso.2014.01.023i
I. Murata et al. / Applied Radiation and Isotopes ∎ (∎∎∎∎) ∎∎∎–∎∎∎
which is less than 33 keV, meaning that discrimination of both gamma-rays is possible in the measured spectrum. 6. Conclusion Aiming at BNCT-SPECT optimized dimensions of an elemental CdTe element were examined by design calculations, which are 1.5 2 30 mm3. Also, the collimator length and diameter were determined to be 10 cm and 2 mm, respectively. In this design, the spatial resolution was confirmed to be several mm and the number of counts was more than 1000 per hour. Producing a CdTe crystal and performing the test measurement, for the detection efficiency an excellent agreement was obtained to show that MCNP has a very good ability to design the detection system with a CdTe crystal accurately. Also, it was found that the detector had a very good energy resolution so that gamma-rays of 478 keV and 511 keV could be distinguished in the spectrum. Acknowledgments This study is being conducted in part under the support of Grants-in-Aid for Scientific Research by JSPS, Scientific Research (B) (Contract No. 22360405).
5
References Imahori, Y., et al., 1998. Fluorine-18-labeled fluoro-boronophenylalanine PET in patients with glioma. J. Nucl. Med. 39, 325–333. Kato, I., 2008. Private communication. Osaka University. Kobayashi, T., et al., 2000. A noninvasive dose estimation system for clinical BNCT based on PG-SPECT – conceptual study and fundamental experiments using HPGe and CdTe semiconductor detectors. Med. Phys. 27 (9), 2124–2132. Kumada, K., et al., 2007. Development of JCDS, a computational dosimetry system at JAEA for boron neutron capture therapy. J. Phys. Conf. Ser., 74. Manabe, M. et al., 2012. Collimator design for array-type CdTe detector for BNCTSPECT. In: Proceedings of the 15th International Congress on Neutron Capture Therapy, Sept. 10–14, Tsukuba, Japan, 6B-05. Minsky, D.M., et al., 2009. Experimental feasibility studies on a SPECT tomograph for BNCT dosimetry. Appl. Radiat. Isot. 67 (7–8), S179–S182. Mukai, T., et al., 2010. Development of CdTe Detector for BNCT-SPECT. JAEA-Conf. 2010-005, 75–80. Munck af Rosenschöld, P., et al., 2006. Prompt gamma tomography during BNCT – a feasibility study. J. Instrum. 1, 1–10. Murata, I., et al., 2011a. Development of a thick CdTe detector for BNCT-SPECT. Appl. Radiat. Isot. 69, 1706–1709. Murata, I., et al., 2011b. Feasibility study on BNCT-SPECT using a CdTe detector. Prog. Nucl. Sci. Technol. 1, 267–270. Nakamura, S., et al., 2012. Characterization test of CdTe detector element designed and developed for BNCT-SPECT. JAEA-Conf. 2012-001, 165–170. Verbakel, W., et al., 1997. γ-Ray telescope for on-line measurements of low boron concentrations in a head phantom for BNCT. Nucl. Instrum. Methods Phys. Res. Sect. A 394 (1–2), 163–172.
Please cite this article as: Murata, I., et al., Characterization measurement of a thick CdTe detector for BNCT-SPECT – Detection efficiency and energy resolution. Appl. Radiat. Isotopes (2014), http://dx.doi.org/10.1016/j.apradiso.2014.01.023i