- Email: [email protected]
Detector characterization and first coincidence tests of a Compton telescope based on LaBr3 crystals and SiPMs
Nuclear Instruments and Methods in Physics Research A 695 (2012) 105–108
Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Detector characterization and first coincidence tests of a Compton telescope based on LaBr3 crystals and SiPMs G. Llosa´ a,n, J. Barrio a, J. Cabello a, A. Crespo a, C. Lacasta a, M. Rafecas a,b, S. Callier c, C. de La Taille c, L. Raux c a
Instituto de Fı´sica Corpuscular (IFIC-CSIC/UVEG), Valencia, Spain ´mica, Molecular y Nuclear, Universitat de Vale ncia, Valencia, Spain Departamento de Fı´sica Ato c Laboratoire de l’Acce´le´rateur Line´aire, Orsay, France b
a r t i c l e i n f o
abstract
Available online 26 November 2011
A Compton telescope for dose monitoring in hadron therapy consisting of several layers of continuous LaBr3 crystals coupled to silicon photomultiplier (SiPM) arrays is under development within the ENVISION project. In order to test the possibility of employing such detectors for the telescope, a detector head consisting of a continuous 16 mm 18 mm 5 mm LaBr3 crystal coupled to a SiPM array has been assembled and characterized, employing the SPIROC1 ASIC as readout electronics. The best energy resolution obtained at 511 keV is 6.5% FWHM and the timing resolution is 3.1 ns FWHM. A position determination method for continuous crystals is being tested, with promising results. In addition, the detector has been operated in time coincidence with a second detector layer, to determine the coincidence capabilities of the system. The first tests are satisfactory, and encourage the development of larger detectors that will compose the telescope prototype. & 2011 Elsevier B.V. All rights reserved.
Keywords: Hadron therapy Compton imaging LaBr3 Continuous crystal SiPM MPPC G-APD
1. Introduction
2. Detector description
In hadron therapy, the detection of gamma rays emitted by the tissue nuclei excited during treatment is gaining interest as a method for dose monitoring. A Compton telescope is an appropriate option for this purpose [1,2]. Prompt gammas are emitted in a continuous spectrum with energies up to about 15 MeV, and their emission can be correlated with the Bragg peak [3]. In the framework of the European ENVISION project, we are developing a Compton telescope based on continuous LaBr3 crystals coupled to silicon photomultiplier (SiPM) arrays [4,5]. The high light yield of LaBr3 ensures a very good energy resolution and a fast timing response, which are essential for this application. SiPMs allow to stack several detector layers, and to maintain the fast time response. The materials selected can achieve the requested requirements, while the detector assembly and operation is relatively simple. The first characterization tests of the detector and readout electronics have been carried out. Also, in order to test the coincidence capabilities of the system, the LaBr3 detector has been operated in coincidence with a second detector.
2.1. Detectors
n
Corresponding author. E-mail address: gabriela.llosa@ific.uv.es (G. Llosa´).
0168-9002/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2011.11.041
The detector under test consists of a continuous LaBr3:Ce crystal1 of 16 mm 18 mm 5 mm size, surrounded with highly reflective material, and encapsulated in an aluminum housing. A 1 mm thick optical guide covers the face that is coupled to the photodetector, a 2D array of silicon photomultipliers (Multi-Pixel Photon Counter, MPPC) from Hamamatsu Photonics. The array composed of 16 (4 4) elements of 3 mm 3 mm size, with 3600 microcells of 50 mm 50 mm size. The pitch is 4.05 mm 4.5 mm. The MPPC array is connected to a custom made printed circuit board (PCB) for mechanical support and bias. The MPPC outputs are connected to the ASIC through a flat cable. A common bias voltage is applied to the 16 photodetector elements in the MPPC array. A plastic holder fixes the crystal to the MPPC array and the PCB. In this board, the signal from each MPPC element is split, and part of the signals from all MPPC elements are driven to a common output. This output has been employed in some of the timing resolution tests. For the uniformity tests, a pixellated crystal array is coupled one-to-one to the MPPC array. The array composed of 16 LYSO
1
s
BrilLanCe 380 from Saint-Gobain Crystals.
106
´ et al. / Nuclear Instruments and Methods in Physics Research A 695 (2012) 105–108 G. Llosa
crystals of 3 mm 3 mm 15 mm size, separated with white epoxy resin.
Peak position vs channel 1000
2.2. Electronics
800 peak position
The SPIROC1 ASIC developed at the Linear Accelerator Laboratory (LAL, Orsay) [6] is employed as readout electronics. The ASIC has 36 channels, each one hosting a slow shaper with adjustable shaping time (from 50 to 100 ns), and a fast shaper with 15 ns shaping time, followed by a level discriminator to generate the trigger signal per channel. The ASIC also provides the overall trigger signal given by the logical OR of the triggers from all channels. Each channel has an input Digital-to-Analog Converter (DAC) that allows us to modify the bias voltage applied to each MPPC element up to 5 V in 20 mV steps. The ASIC is connected to s a test board with a FPGA Altera Cyclone that controls the ASIC operation and the communication with the computer through a USB port. An on board ADC digitizes the ASIC data when an external HOLD signal is provided. A LabView program is employed to control the data acquisition.
600 400 200 0 0
2
4
6 8 10 Channel number
12
14
16
Fig. 1. Position of the 22Na 511 keV photopeak for each channel before (dashed histogram) and after (solid histogram) applying the DAC corrections to the bias voltage of each channel.
2.3. Coincidence system
LYSO Na-22 spectra with crystal array In order to test the coincidence capabilities of the system, a set-up has been mounted with two detectors. The first detector is the LaBr3 detector described in Section 2.1. The second detector consists of a continuous 12 mm 12 mm 5 mm LYSO crystal coupled to another MPPC array similar to the one described in Section 2.1. The LaBr3 detector is placed on top of the second detector at 23 mm distance, and the radioactive source is on top of both, at the center of the first detector. Each detector is connected to one SPIROC1 board. The trigger provided by each ASIC is connected to a coincidence unit, and the coincidence output signal is sent to a timing unit to generate the HOLD signal that is led to the ADCs in the two SPIROC1 boards to digitize the data. The LabView program has been modified to operate two SPIROC1 boards simultaneously and save the data from the two detectors.
400 350 300 250 200 150 100 50 0 0
500
1000 1500 ADC counts
2000
2500
3. Detector characterization tests 3.1. Detector uniformity The variations in the response of the photodetector elements in the matrix can result in a degradation of the detector performance. Each of the photodetector elements has a different operating voltage recommended by the manufacturer, ranging from 71.08 V to 71.12 V. The bias applied to all elements is the average of the operating voltages of the 16 elements, 71.10 V. Thus, differences arise in the response from one element to another. The differences in the response of the photodetector elements have been tested by coupling a pixellated crystal array described in Section 2.1 to the MPPC array. Data have been taken with a 22 Na source. An energy spectrum is obtained for each detector channel, and the 511 keV photopeak is fitted with a Gaussian function. The input DACs in the ASIC have been employed to compensate for the differences among the detector elements. A small increment or decrement of the common bias voltage has been applied to each channel by means of the DAC, and 22Na energy spectra have been acquired again. Fig. 1 shows the difference in the photopeak position for each channel before (dashed histogram) and after (solid histogram) applying the corrections voltage variations.
Fig. 2. 22Na 511 keV energy spectrum obtained with the data from all channels before (dashed histogram) and after (solid histogram) applying the DAC corrections to the bias voltage of each channel.
In order to show the effect that these corrections can have in the energy spectra, Fig. 2 shows an energy spectrum produced by including data from all channels without applying the DAC voltage corrections (dashed histogram) and applying them (solid histogram). The linearity in the photodetector response has not been checked in this case, and the spectra shown can present some saturation effects that are more pronounced at high energies. Nevertheless, the result of our test is clearly visible: the 511 keV photopeak of the corrected spectrum is much narrower than the non-corrected one. 3.2. Energy resolution The energy resolution at 511 keV has been measured with a PMT Hamamatsu R6236 and a multichannel analyzer, and the result obtained is 5.8% FWHM, relatively far from the expected value (around 4% FWHM). The result obtained with the LaBr3 crystal coupled to the MPPC array, without applying any corrections and raising the voltage to 72.1 V, is 7% FWHM at 511 keV [4]. Applying corrections to the data from each channel, calculating the correction factors from
´ et al. / Nuclear Instruments and Methods in Physics Research A 695 (2012) 105–108 G. Llosa
the photopeak positions previously obtained for each channel with the crystal array, the energy resolution obtained is 6.5% FWHM at 511 keV, as shown in Fig. 3. The linearity of the detector response has been previously tested up to 1275 keV [4] confirming the absence of saturation effects, and thus the validity of the energy resolution tests. It should be noted that the gaps between the photodetector elements result in a loss of active area of 50% with respect to an ideal photodetector with no gaps, and thus in a significant degradation of the energy resolution. 3.3. Timing resolution
107
Timing resolution
χ2 / ndf ndf == 56.86 56.86/ /20 20
1000
977.1 ± 11.6
Constant
800
Mean
6.953e-09 ± 1.331e-11
Sigma
1.336e-09 ± 1.297e-11
600 400 200
The timing resolution of the LaBr3 detector has been measured in coincidence with a 1 mm 1 mm 10 mm LYSO crystal coupled to a 1 mm 1 mm MPPC. The time difference between the trigger signal of the detector, provided by the SPIROC1 board, and the 1 mm 1 mm MPPC output, is measured with the oscilloscope. A timing resolution of 7.2 ns FWHM has been obtained [4]. The trigger signal of the detector is given by the OR of the 16 channels connected, with a common threshold value set for all channels. We are investigating the effect that the differences from one channel to another might have in the timing resolution. The timing resolution has also been measured with the same set-up, but employing the sum output of the detector PCB, described in 2.1, as a trigger signal. In this case, the result obtained is significantly better, 3.1 ns FWHM, as shown in Fig. 4.
×10-9
0 -15
-10
-5
0 5 10 15 Time difference (s)
20
25
30
Fig. 4. Time difference distribution. The timing resolution is 3.1 ns FWHM.
3.4. Position determination The precise determination of the interaction position is one of the main challenges in the use of continuous crystals. We are testing a method [7] that estimates the interaction position in ðx,y,zÞ using an analytical model based on the angle subtended by the interaction position with the photodetector elements. This method has the advantage that no previous calibration is required. In order to test the method, data have been taken in different detector positions. The 511 keV photons of a 22Na source are electronically collimated by requiring the coincidence of the detector events with a 1 mm 1 mm 10 mm crystal coupled to a 1 mm 1 mm MPPC, placed at 3 cm from the detector. Fig. 5 shows a distribution of reconstructed positions for one of the
140
χ2 / ndf = 13.76 13.76 // 99 127.2 ± 5.9
Constant
120
Mean
1.472e+04 ± 16
Sigma
100
408.5 ± 14.0
80 60 40 20 0 0
5000
10000 15000 20000 Energy (ADC sum)
25000
30000
Fig. 3. 22Na energy spectrum. The energy resolution is 6.5% FWHM, applying corrections factors in the data analysis.
Fig. 5. Distribution of reconstructed positions for a given position close to the center of the detector. The resolution obtained by fitting a Gaussian function to a profile through the maximum of the distribution is 1.6 mm FWHM.
positions tested, close to the center of the detector. The resolution obtained by fitting a gaussian function to a profile through the maximum of the distribution is 1.6 mm FWHM.
4. Coincidence tests The LaBr3 detector has been successfully operated in time coincidence with a second detector. Data have been taken both with 137Cs and with 22Na sources, and in both cases the results show the expected behavior. The photopeak events corresponding to full absorption of the gamma-ray are greatly suppressed in the coincidence energy spectra of the two detectors, and the few remaining events correspond to noise. In the first (LaBr3) detector, only low energy events corresponding to forward-scattered photons are recorded. In the second detector, the energies recorded are limited by the threshold, and they correspond both to scatter–scatter and scatter–absorption events. If the energies recorded in the two detectors are summed, the peak corresponding to the scatter–absorption events is recovered. Fig. 6 shows a reference 137Cs energy spectrum (dashed histogram), and a coincidence energy spectrum (solid histogram), where only low energy events are recorded in the LaBr3 detector. Fig. 7 shows the sum of the energies acquired in the two detectors for 137Cs, where the 662 keV peak corresponding to the scatter–absorption events
´ et al. / Nuclear Instruments and Methods in Physics Research A 695 (2012) 105–108 G. Llosa
108
detectors, as explained in Section 3.3. In this case, the timing resolution is significantly improved, up to 2 ns FWHM.
LaBr3 Cs-137 spectra 300 250
5. Conclusions and future work
200 150 100 50
0
200
400 600 Energy (keV)
800
1000
Fig. 6. 137Cs energy spectra: reference spectrum (dashed histogram) and coincidence spectrum (solid histogram) showing only forward-scattered events with low energy.
Sum Energy spectrum Cs137 250
The energy resolution obtained with the LaBr3 detector is worse than expected, both employing the MPPC array, which has a significant dead area, and a PMT. We are in contact with the providers to try to find out the reason of this result. The timing resolution obtained with the trigger generated by the electronics is far from our aim of 1 ns FWHM, and this effect is being investigated. An alternative trigger based on the sum of the outputs of all channels has also been tested, yielding significantly improved results, with a timing resolution of 3.1 ns FWHM. Position determination tests in continuous crystals are also being performed. The resolution obtained is 1.6 mm FWHM, and the results are very promising. The first coincidence tests have been successfully carried out. Our efforts are aimed at reducing the noise and improving the performance. Future plans include further improvement of the detector performance, mainly in terms of timing resolution, position determination and operation in time coincidence. The development of larger detectors that will compose the telescope prototype is also ongoing. The detectors composed of a LaBr3 crystal coupled to four MPPC arrays, and the VATA64HDR16 ASICs from Gamma Medica-IDEAS is being tested for this purpose.
200
150
Acknowledgments
100
Part of this work has been carried out within the ENVISION project, that is co-funded by the European Commission under FP7 Grant Agreement No. 241851.
50 References
0 0
200
400
600
800 1000 1200 1400 1600 1800 Energy (KeV)
Fig. 7. Sum of the energies measured in the two detectors with the The 662 keV peak is reconstructed for scatter–absorption events.
137
Cs source.
can be seen. The scatter–scatter events, which have lower energy, are not acquired due to the high threshold on the second detector. The timing resolution obtained with the trigger signals from the two detectors provided by the SPIROC1 boards is 10 ns FWHM. The measurement has also been carried out employing the signals coming from the sum outputs in the PCBs of both
[1] D. Dauvergne, M. Battaglia, G. Montarou, E. Testa. New methods of real-time control imaging for ion therapy, in: NIRS-ETOILE Joint Symposium on Carbon Ion Therapy, Lyon, France, 2009. [2] M.-H. Richard, et al. Design study of a Compton camera for prompt gamma imaging During Ion Beam Therapy, in: IEEE 2009 NSS-MIC Conference Record HT3-6, 2009, pp. P4172–4175. [3] E. Testa, et al., Applied Physics Letters 93 (2008) 093506. [4] G. Llosa´, et al., Nuclear Instruments and Methods in Physics Research Section A 648 (2011) S96. [5] J. Gillam et al., A Compton imaging algorithm for on-line monitoring in hadron therapy, in: Medical Imaging 2011: Physics of Medical Imaging, 2011, vol. 7961, pp. 79611O, number 1. [6] M. Bouchel, et al., SPIROC (SiPM Integrated Read-Out Chip): dedicated very front-end electronics for an ILC prototype hadronic calorimeter with SiPM read-out, in: 2007 NSS MIC Conference Record, 2007, pp. 1857–1860. [7] Z. Li, et al., Physics in Medicine and Biology 55 (21) (2010) 6515.
Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Detector characterization and first coincidence tests of a Compton telescope based on LaBr3 crystals and SiPMs G. Llosa´ a,n, J. Barrio a, J. Cabello a, A. Crespo a, C. Lacasta a, M. Rafecas a,b, S. Callier c, C. de La Taille c, L. Raux c a
Instituto de Fı´sica Corpuscular (IFIC-CSIC/UVEG), Valencia, Spain ´mica, Molecular y Nuclear, Universitat de Vale ncia, Valencia, Spain Departamento de Fı´sica Ato c Laboratoire de l’Acce´le´rateur Line´aire, Orsay, France b
a r t i c l e i n f o
abstract
Available online 26 November 2011
A Compton telescope for dose monitoring in hadron therapy consisting of several layers of continuous LaBr3 crystals coupled to silicon photomultiplier (SiPM) arrays is under development within the ENVISION project. In order to test the possibility of employing such detectors for the telescope, a detector head consisting of a continuous 16 mm 18 mm 5 mm LaBr3 crystal coupled to a SiPM array has been assembled and characterized, employing the SPIROC1 ASIC as readout electronics. The best energy resolution obtained at 511 keV is 6.5% FWHM and the timing resolution is 3.1 ns FWHM. A position determination method for continuous crystals is being tested, with promising results. In addition, the detector has been operated in time coincidence with a second detector layer, to determine the coincidence capabilities of the system. The first tests are satisfactory, and encourage the development of larger detectors that will compose the telescope prototype. & 2011 Elsevier B.V. All rights reserved.
Keywords: Hadron therapy Compton imaging LaBr3 Continuous crystal SiPM MPPC G-APD
1. Introduction
2. Detector description
In hadron therapy, the detection of gamma rays emitted by the tissue nuclei excited during treatment is gaining interest as a method for dose monitoring. A Compton telescope is an appropriate option for this purpose [1,2]. Prompt gammas are emitted in a continuous spectrum with energies up to about 15 MeV, and their emission can be correlated with the Bragg peak [3]. In the framework of the European ENVISION project, we are developing a Compton telescope based on continuous LaBr3 crystals coupled to silicon photomultiplier (SiPM) arrays [4,5]. The high light yield of LaBr3 ensures a very good energy resolution and a fast timing response, which are essential for this application. SiPMs allow to stack several detector layers, and to maintain the fast time response. The materials selected can achieve the requested requirements, while the detector assembly and operation is relatively simple. The first characterization tests of the detector and readout electronics have been carried out. Also, in order to test the coincidence capabilities of the system, the LaBr3 detector has been operated in coincidence with a second detector.
2.1. Detectors
n
Corresponding author. E-mail address: gabriela.llosa@ific.uv.es (G. Llosa´).
0168-9002/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2011.11.041
The detector under test consists of a continuous LaBr3:Ce crystal1 of 16 mm 18 mm 5 mm size, surrounded with highly reflective material, and encapsulated in an aluminum housing. A 1 mm thick optical guide covers the face that is coupled to the photodetector, a 2D array of silicon photomultipliers (Multi-Pixel Photon Counter, MPPC) from Hamamatsu Photonics. The array composed of 16 (4 4) elements of 3 mm 3 mm size, with 3600 microcells of 50 mm 50 mm size. The pitch is 4.05 mm 4.5 mm. The MPPC array is connected to a custom made printed circuit board (PCB) for mechanical support and bias. The MPPC outputs are connected to the ASIC through a flat cable. A common bias voltage is applied to the 16 photodetector elements in the MPPC array. A plastic holder fixes the crystal to the MPPC array and the PCB. In this board, the signal from each MPPC element is split, and part of the signals from all MPPC elements are driven to a common output. This output has been employed in some of the timing resolution tests. For the uniformity tests, a pixellated crystal array is coupled one-to-one to the MPPC array. The array composed of 16 LYSO
1
s
BrilLanCe 380 from Saint-Gobain Crystals.
106
´ et al. / Nuclear Instruments and Methods in Physics Research A 695 (2012) 105–108 G. Llosa
crystals of 3 mm 3 mm 15 mm size, separated with white epoxy resin.
Peak position vs channel 1000
2.2. Electronics
800 peak position
The SPIROC1 ASIC developed at the Linear Accelerator Laboratory (LAL, Orsay) [6] is employed as readout electronics. The ASIC has 36 channels, each one hosting a slow shaper with adjustable shaping time (from 50 to 100 ns), and a fast shaper with 15 ns shaping time, followed by a level discriminator to generate the trigger signal per channel. The ASIC also provides the overall trigger signal given by the logical OR of the triggers from all channels. Each channel has an input Digital-to-Analog Converter (DAC) that allows us to modify the bias voltage applied to each MPPC element up to 5 V in 20 mV steps. The ASIC is connected to s a test board with a FPGA Altera Cyclone that controls the ASIC operation and the communication with the computer through a USB port. An on board ADC digitizes the ASIC data when an external HOLD signal is provided. A LabView program is employed to control the data acquisition.
600 400 200 0 0
2
4
6 8 10 Channel number
12
14
16
Fig. 1. Position of the 22Na 511 keV photopeak for each channel before (dashed histogram) and after (solid histogram) applying the DAC corrections to the bias voltage of each channel.
2.3. Coincidence system
LYSO Na-22 spectra with crystal array In order to test the coincidence capabilities of the system, a set-up has been mounted with two detectors. The first detector is the LaBr3 detector described in Section 2.1. The second detector consists of a continuous 12 mm 12 mm 5 mm LYSO crystal coupled to another MPPC array similar to the one described in Section 2.1. The LaBr3 detector is placed on top of the second detector at 23 mm distance, and the radioactive source is on top of both, at the center of the first detector. Each detector is connected to one SPIROC1 board. The trigger provided by each ASIC is connected to a coincidence unit, and the coincidence output signal is sent to a timing unit to generate the HOLD signal that is led to the ADCs in the two SPIROC1 boards to digitize the data. The LabView program has been modified to operate two SPIROC1 boards simultaneously and save the data from the two detectors.
400 350 300 250 200 150 100 50 0 0
500
1000 1500 ADC counts
2000
2500
3. Detector characterization tests 3.1. Detector uniformity The variations in the response of the photodetector elements in the matrix can result in a degradation of the detector performance. Each of the photodetector elements has a different operating voltage recommended by the manufacturer, ranging from 71.08 V to 71.12 V. The bias applied to all elements is the average of the operating voltages of the 16 elements, 71.10 V. Thus, differences arise in the response from one element to another. The differences in the response of the photodetector elements have been tested by coupling a pixellated crystal array described in Section 2.1 to the MPPC array. Data have been taken with a 22 Na source. An energy spectrum is obtained for each detector channel, and the 511 keV photopeak is fitted with a Gaussian function. The input DACs in the ASIC have been employed to compensate for the differences among the detector elements. A small increment or decrement of the common bias voltage has been applied to each channel by means of the DAC, and 22Na energy spectra have been acquired again. Fig. 1 shows the difference in the photopeak position for each channel before (dashed histogram) and after (solid histogram) applying the corrections voltage variations.
Fig. 2. 22Na 511 keV energy spectrum obtained with the data from all channels before (dashed histogram) and after (solid histogram) applying the DAC corrections to the bias voltage of each channel.
In order to show the effect that these corrections can have in the energy spectra, Fig. 2 shows an energy spectrum produced by including data from all channels without applying the DAC voltage corrections (dashed histogram) and applying them (solid histogram). The linearity in the photodetector response has not been checked in this case, and the spectra shown can present some saturation effects that are more pronounced at high energies. Nevertheless, the result of our test is clearly visible: the 511 keV photopeak of the corrected spectrum is much narrower than the non-corrected one. 3.2. Energy resolution The energy resolution at 511 keV has been measured with a PMT Hamamatsu R6236 and a multichannel analyzer, and the result obtained is 5.8% FWHM, relatively far from the expected value (around 4% FWHM). The result obtained with the LaBr3 crystal coupled to the MPPC array, without applying any corrections and raising the voltage to 72.1 V, is 7% FWHM at 511 keV [4]. Applying corrections to the data from each channel, calculating the correction factors from
´ et al. / Nuclear Instruments and Methods in Physics Research A 695 (2012) 105–108 G. Llosa
the photopeak positions previously obtained for each channel with the crystal array, the energy resolution obtained is 6.5% FWHM at 511 keV, as shown in Fig. 3. The linearity of the detector response has been previously tested up to 1275 keV [4] confirming the absence of saturation effects, and thus the validity of the energy resolution tests. It should be noted that the gaps between the photodetector elements result in a loss of active area of 50% with respect to an ideal photodetector with no gaps, and thus in a significant degradation of the energy resolution. 3.3. Timing resolution
107
Timing resolution
χ2 / ndf ndf == 56.86 56.86/ /20 20
1000
977.1 ± 11.6
Constant
800
Mean
6.953e-09 ± 1.331e-11
Sigma
1.336e-09 ± 1.297e-11
600 400 200
The timing resolution of the LaBr3 detector has been measured in coincidence with a 1 mm 1 mm 10 mm LYSO crystal coupled to a 1 mm 1 mm MPPC. The time difference between the trigger signal of the detector, provided by the SPIROC1 board, and the 1 mm 1 mm MPPC output, is measured with the oscilloscope. A timing resolution of 7.2 ns FWHM has been obtained [4]. The trigger signal of the detector is given by the OR of the 16 channels connected, with a common threshold value set for all channels. We are investigating the effect that the differences from one channel to another might have in the timing resolution. The timing resolution has also been measured with the same set-up, but employing the sum output of the detector PCB, described in 2.1, as a trigger signal. In this case, the result obtained is significantly better, 3.1 ns FWHM, as shown in Fig. 4.
×10-9
0 -15
-10
-5
0 5 10 15 Time difference (s)
20
25
30
Fig. 4. Time difference distribution. The timing resolution is 3.1 ns FWHM.
3.4. Position determination The precise determination of the interaction position is one of the main challenges in the use of continuous crystals. We are testing a method [7] that estimates the interaction position in ðx,y,zÞ using an analytical model based on the angle subtended by the interaction position with the photodetector elements. This method has the advantage that no previous calibration is required. In order to test the method, data have been taken in different detector positions. The 511 keV photons of a 22Na source are electronically collimated by requiring the coincidence of the detector events with a 1 mm 1 mm 10 mm crystal coupled to a 1 mm 1 mm MPPC, placed at 3 cm from the detector. Fig. 5 shows a distribution of reconstructed positions for one of the
140
χ2 / ndf = 13.76 13.76 // 99 127.2 ± 5.9
Constant
120
Mean
1.472e+04 ± 16
Sigma
100
408.5 ± 14.0
80 60 40 20 0 0
5000
10000 15000 20000 Energy (ADC sum)
25000
30000
Fig. 3. 22Na energy spectrum. The energy resolution is 6.5% FWHM, applying corrections factors in the data analysis.
Fig. 5. Distribution of reconstructed positions for a given position close to the center of the detector. The resolution obtained by fitting a Gaussian function to a profile through the maximum of the distribution is 1.6 mm FWHM.
positions tested, close to the center of the detector. The resolution obtained by fitting a gaussian function to a profile through the maximum of the distribution is 1.6 mm FWHM.
4. Coincidence tests The LaBr3 detector has been successfully operated in time coincidence with a second detector. Data have been taken both with 137Cs and with 22Na sources, and in both cases the results show the expected behavior. The photopeak events corresponding to full absorption of the gamma-ray are greatly suppressed in the coincidence energy spectra of the two detectors, and the few remaining events correspond to noise. In the first (LaBr3) detector, only low energy events corresponding to forward-scattered photons are recorded. In the second detector, the energies recorded are limited by the threshold, and they correspond both to scatter–scatter and scatter–absorption events. If the energies recorded in the two detectors are summed, the peak corresponding to the scatter–absorption events is recovered. Fig. 6 shows a reference 137Cs energy spectrum (dashed histogram), and a coincidence energy spectrum (solid histogram), where only low energy events are recorded in the LaBr3 detector. Fig. 7 shows the sum of the energies acquired in the two detectors for 137Cs, where the 662 keV peak corresponding to the scatter–absorption events
´ et al. / Nuclear Instruments and Methods in Physics Research A 695 (2012) 105–108 G. Llosa
108
detectors, as explained in Section 3.3. In this case, the timing resolution is significantly improved, up to 2 ns FWHM.
LaBr3 Cs-137 spectra 300 250
5. Conclusions and future work
200 150 100 50
0
200
400 600 Energy (keV)
800
1000
Fig. 6. 137Cs energy spectra: reference spectrum (dashed histogram) and coincidence spectrum (solid histogram) showing only forward-scattered events with low energy.
Sum Energy spectrum Cs137 250
The energy resolution obtained with the LaBr3 detector is worse than expected, both employing the MPPC array, which has a significant dead area, and a PMT. We are in contact with the providers to try to find out the reason of this result. The timing resolution obtained with the trigger generated by the electronics is far from our aim of 1 ns FWHM, and this effect is being investigated. An alternative trigger based on the sum of the outputs of all channels has also been tested, yielding significantly improved results, with a timing resolution of 3.1 ns FWHM. Position determination tests in continuous crystals are also being performed. The resolution obtained is 1.6 mm FWHM, and the results are very promising. The first coincidence tests have been successfully carried out. Our efforts are aimed at reducing the noise and improving the performance. Future plans include further improvement of the detector performance, mainly in terms of timing resolution, position determination and operation in time coincidence. The development of larger detectors that will compose the telescope prototype is also ongoing. The detectors composed of a LaBr3 crystal coupled to four MPPC arrays, and the VATA64HDR16 ASICs from Gamma Medica-IDEAS is being tested for this purpose.
200
150
Acknowledgments
100
Part of this work has been carried out within the ENVISION project, that is co-funded by the European Commission under FP7 Grant Agreement No. 241851.
50 References
0 0
200
400
600
800 1000 1200 1400 1600 1800 Energy (KeV)
Fig. 7. Sum of the energies measured in the two detectors with the The 662 keV peak is reconstructed for scatter–absorption events.
137
Cs source.
can be seen. The scatter–scatter events, which have lower energy, are not acquired due to the high threshold on the second detector. The timing resolution obtained with the trigger signals from the two detectors provided by the SPIROC1 boards is 10 ns FWHM. The measurement has also been carried out employing the signals coming from the sum outputs in the PCBs of both
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