- Email: [email protected]
Design of the Prototype Readout System of BGO Calorimeter in the Space Dark Matter Detector of Purple Mountain Observatory
CHINESE ASTRONOMY AND ASTROPHYSICS
ELSEVIER
Chinese Astronomy and Astrophysics 36 (2012) 318–326
Design of the Prototype Readout System of BGO Calorimeter in the Space Dark Matter Detector of Purple Mountain Observatory† GUO Jian-hua1,2 1 2
CAI Ming-sheng1,2
HU Yi-ming1,2
CHANG Jin1,2
Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008
Key Laboratory of Dark Matter and Space Astronomy, Chinese Academy of Sciences, Nanjing 210008
Abstract A space dark matter detector is proposed by the Key Laboratory of Dark Matter And Space Astronomy of Chinese Academy of Sciences for detecting the high-energy electrons and Gamma particles produced by the annihilation of dark matter in space. The whole detector is mainly composed of the BGO (bismuth germanium oxide) high-energy image calorimeter and scintillation hodoscope. The energy range of the detector will cover the high-energy electrons and Gamma particles of 10 Gev∼10 TeV, in which the energies of highenergy particles are mainly deposited in the BGO calorimeter. For verifying the scheme of the detector, we have designed a prototype readout system for the BGO calorimeter of the space dark matter detector, and made a preliminary test on it. Key words: instrumentation: detectors—dark matter—cosmic rays
1. INTRODUCTION Since Zwicky[1,2] proposed a possible astronomical evidence for the existence of dark matter in the 30s of the 20th century, more and more pieces of evidence indicate the real existence of dark matter. In the universe, the part known for us occupies only a proportion of under 4%, dark matter and dark energy are almost full of the whole universe. However, besides the gravitational effect, human being knows dark matter very little. What is the nature of dark †
Supported by Intellectual Innovation Project of Chinese Academy of Sciences Received 2011–03–29; revised version 2011–06–07 A translation of Acta Astron. Sin. Vol. 53, No. 1, pp. 72–79, 2012 [email protected]
0275-1062/11/$-see front front mattermatter © 2012 Elsevier All rights reserved. c 2012 B.V. 0275-1062/01/$-see Elsevier Science B. V. All rights reserved. doi:10.1016/j.chinastron.2012.07.008 PII:
GUO Jian-hua et al. / Chinese Astronomy and Astrophysics 36 (2012) 318–326
319
matter, and what kinds of fundamental particles it consists of? All these basic problems have not yet been solved. In the past several ten years, physicists have developed a super-symmetry theory. This theory predicts that each kind of common particle possesses a kind of undetected largemass “super-symmetrical companion” particle[1−2] . At present, the particles predicted by the super-symmetry theory are the dominant candidates of dark matter particles. These particles possess both mass and weakly interacting force, but they do not participate in any electromagnetic interaction, and they are called the WIMPs (Weakly Interacting Massive Particles). Since the dark matter particles do not participate in any electromagnetic interaction, so, in order to detect them, the present experiments should be promoted to extremes, as a result, so far people do not understand that what kinds of particles the dark matter particles are. Now, the methods of dark matter detection are mainly the direct detection, accelerator detection, and indirect detection. The direct detection, for example to detect the collisions between dark matter particles and common materials, verifies the existence of dark matter particles by observing the thermal or other kind signals produced by these collisions. The indirect detection observes mainly the stable secondary particles, such as the Gamma-particles and electrons that produced by the decay or annihilation of dark matter particles. Now, multiple experiments for the direct and indirect detections of dark matter particles have been made in the world. And indebted to the efforts of scientists in many years, a new progress has been achieved in the indirect observation of dark matter particles since 2008. At first, by analyzing the 10 yr observational data, the American Antarctic balloon experiment ATIC (Advanced Thin Ionization Calorimeter) discovered that between 300 Gev and 800 GeV the energy spectrum of cosmic high-energy electrons has an abnormal excess relative to the theoretical spectrum of cosmic rays[3]. Successively, the European space experiment PAMELA (Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics) observed the similar result[4] . The abnormality of high-energy electron spectrum is considered as a possible signal from the dark matter particles, and received extensive attentions. In order to measure more accurately the energy spectra of cosmic high-energy electrons and Gamma-particles, on the basis of the China-US cooperated ATIC experiment, the Key Laboratory of Dark Matter And Space Astronomy of Chinese Academy of Sciences proposed a experimental project of space dark matter detection[5] , which purports to detect the highenergy electrons and Gamma-particles produced possibly by the annihilation of dark matter particles. This paper is an introduction to the readout design of the BGO calorimeter in the space dark matter detector, as well as the preliminary test result.
2. SYSTEM FRAMEWORK OF THE DARK MATTER DETECTOR The space dark matter detector proposed by the Key Laboratory of Dark Matter And Space Astronomy of Chinese Academy of Sciences is mainly composed of two sub-detectors[5] : the scintillation hodoscope on the top of the detector and the BGO calorimeter beneath, the structure of the detector is shown as Fig.1. In order to discriminate accurately the incident particles, such as proton, electron, Gamma-particle etc., it is necessary to reconstruct the
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track of the interaction between the incident particle and the detector. Hence, each subdetector system has the functions of both position sense and energy measurement.
Fig. 1 Structure of the space dark matter detector
The scintillation hodoscope on the top of the detector has 6 layers, which are vertically placed with the interleaved X, Y directions, each piece of plastic scintillator has the dimension of 1 cm×1 cm and the length of 1 m. The BGO calorimeter under the scintillation hodoscope has 12 layers, which are placed with the interleaved X, Y directions, and the area of each layer is 0.6 m×0.6 m. Restricted by the fabrication technology, the required 0.6m length of BGO crystal is realized by joining two pieces of 0.3m-long BGO crystals together in the BGO calorimeter. The size of each piece of BGO crystal is 2.5 cm×2.5 cm, and each layer has 24×2 pieces of BGO crystals. The predicted energy measurement range of the whole detector is 10 GeV∼10 TeV, the energy resolution is 1.5%@800 GeV, and the discriminability between Gamma-particles and electrons is better than 1%. As the major part of incident particle energies are deposited in the BGO image calorimeter, so its ability of energy detection plays a key role for the performance of the whole detector.
3. DESIGN OF THE BGO CALORIMETER 3.1 Structure of the BGO Calorimeter The 12-layer BGO image calorimeter is fixed by a framework structure, its exterior is shown as Fig.2, in which PMT indicates the photomultiplier tube. When the particle energy is deposited in the BGO crystal, the BGO crystal has the deposited energy convert into the fluorescence with corresponding strength, then it is converted into an electric signal by the photomultiplier tube (R5610A, Hamamatsu) on a terminal of the BGO crystal, and sent to the electronic readout system for measurements. There are totally 576 pieces of PMTs responsible for the readout of optical signals of the whole BGO calorimeter. Fig.3 shows the sealed package of a single BGO crystal and PMT.
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3.2 Readout Electronics of the BGO Calorimeter In order to make the whole detector system be able to measure the high-energy particles (mainly the Gamma-particles and electrons) with energies in the range of 10 GeV∼10 TeV, it is required that a single BGO detector element can measure the particle energies in the range from 10 MeV to 2 TeV (the result of GEANT4 simulations). Hence, for each BGO crystal, the dynamic range of 105 ∼106 is required. Considered that relying on only the anode or single dynode of the photomultiplier tube can hardly achieve such a large dynamic range, a technique similar to the ATIC experiment is adopted, namely to broaden the dynamic range by using the signals taken from 3 different dynodes. These 3 dynodes are respectively the 1st, 4th and 7th dynodes. After the fluorescence of the BGO crystal entered the cathode of a photomultiplier tube, the charge signal is amplified by each of the dynodes, and finally reaches the anode. As the signals taken from the different dynodes are related by 3 fixed multiples, so the signals taken from the 3 dynodes with different gains can be used for the measurements of high-energy, medium-energy, and low-energy regions, respectively. By measuring the signals of 3 dynodes, the dynamic range of the detector is raised for 103 times relative to the single-route measurement. Thus, if only the single-route dynode signal can reach the dynamic range of 103 , then by combining the 3-route signals, the dynamic range of the whole BGO detector will easily reach 106 .
Fig. 2 The structure figure of the BGO calorimeter
The signals taken from the photomultiplier tube are sent to the electronic readout system to make processing. The electronic measurement system of the BGO calorimeter mainly consists of two parts: the FEE (front-end electronics) board, and the sub-detector data acquisition system. The FEE board uses a charge-sensitive amplifier to collect the charge signals of the PMT, makes the shaping amplification (slowly shaping), peak holding, and digitalization, then the digitalized signals are sent to the sub-detector data acquisition system. At the same time, by the fast-shaping amplifier and discriminator in the FEE board,
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the hitting signal of a particle is sent to the triggering system as the basis of the 1st-stage judgment. The structure of the FEE board is shown in Fig. 4. In this figure, the symbol “dy” indicates the dynode of the PMT, FPGA is the programmable gate array, ADC the analog-digital converter, RAM the random access memory, and Mux the multiplexer.
Fig. 3 Single BGO detector element
Fig. 4 Readout electronics of the BGO calorimeter
Considered that the FEE board is closely placed upon the BGO detector, for the sake of its area, the FEE board is divided into two parts: the FAE (front-end analog electronics) subboard and FCB (front-end control board) sub-board. They are responsible for the processing of analog signals and the transmission of digital control signals, respectively. The two subboards are overlapped together to compose the FEE board through a connector, the details can be seen in Fig.5, in which PCB means the printed circuit board. The sub-detector data acquisition system (sub DAQ) takes mainly the responsibility for receiving the commands from the upper-layer main data acquisition system and trigger system, starting the data sampling of front-end circuits, and receiving the data coming from the FEE board, then packing the data, and transmitting them to the upper-layer main data acquisition system by through the data bus. One sub-detector data acquisition board takes the responsibility for managing 6 FEE boards (corresponding to the readout of all the PMTs on the one side of the BGO calorimeter). 4. TEST RESULTS OF THE BGO DETECTOR For verifying the performance of the BGO detector, we have made a test on it. The preliminary test of the whole system is divided into two steps: testing the performance of front-end
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Fig. 5 Picture of the PCB of front-end electronics
electronics by using a signal generator, and testing the detector by using cosmic-ray and radiative sources as signal sources. 4.1 Test Result Obtained by Using A Signal Generator Before the FEE board is connected with PMTs, the performances (noise, dynamic range, linearity, etc.) of front-end electronics should be tested. The test on the frontend electronics is made by using a signal generator. The pulse signal generated by the signal generator is coupled to the input terminal of front-end electronics by through a 10 pF capacitor, to simulate the output charge signal of the PMT[6] . The signal strength of the generator is adjusted continually to scan the entire dynamic range of front-end electronics. Fig.6 shows the results of the noise and linearity tests on the circuits, in which the symbol “Bin” indicates the channel of the ADC, and each channel corresponds to 0.68 fC. From this figure we can find that the noise of each channel (reduced to the amount of charge at the input terminal) is about 1.2∼1.4 fC (rms). And the test result indicates that the linearity of front-end electronics allows the maximum amount of charge reaching +13 pC (the nonlinearity is 1%), but for the +14 pC input the nonlinearity has been rather large (2%). Hence, as shown by the test result, the dynamic range of single-route readout electronics is about 104 , satisfying completely the requirement of the detector.
Fig. 6 Base-line noises of 32 channels (a) and linearity test result (b) of the FEE
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4.2 Test Results of Cosmic-ray and Radiative Sources A fixed multiple-relation exists between the signals of two different dynodes, for the 3 dynodes used by the dark matter detector, the multiple between the signals of two neighboring dynodes is about 30∼40 (+800 V high voltage). In order to extend conveniently the dynamic range of energy measurement of the whole detector to 106 by combining the 3 dynode signals of different multiples, the energy measurement ranges of two neighboring dynodes should be properly overlapped, the size of the overlapped part is related with the difference of multiples. In order to test the linear relation between the multiples of different dynodes, as well as the noise and other performances, we have performed tests on the single BGO detector element by using the cosmic-ray and radiative sources, respectively. Based on the simulation of GEANT 4, in order that the whole detector can detect the electrons of about 10 TeV, the single detector element should be able to detect the electron energy of about 2 TeV. Although by using the signals from the 3 dynodes of each PMT in the BGO calorimeter, the dynamic range of the whole detector can reach 106 , but if the photons produced by the 2 TeV electrons are directly input to a PMT, then they will much exceed the acceptable range of the PMT cathode. Hence, in practical applications, only if the corresponding fluorescence intensity of the particles whose energies need to be measured is adjusted to the measurable range of the detector by means of optical attenuation or other methods, would the detector be able to detect the particles in the required energy range. But for the laboratory test, we can hardly obtain such a high-energy radiation source, in order to make the performance test of the detector by using the radiative and cosmic-ray sources, in the laboratory test the input optical signal is directly coupled to the PMT, without any optical attenuation. In this way, the photon number produced in the BGO crystal by the particle of 1 MeV is equivalent to that produced by the particle of about 40 MeV in practical situations. Thus, the detector can be tested by using the particle sources obtainable in laboratory. Fig.7 is a schematic diagram of the test bench of cosmic-ray and radiative source tests, the radiative source uses mainly the Gamma radiation of Co-60. In the figure, HV is the high voltage of the PMT, GND expresses the ground of the high voltage. The data sampling is realized by means of self-triggering, and the triggering signal is produced by the 7th dynode.
Fig. 7 Schematic diagram of the test bench
In the experiment, a random signal is used to acquire the base-line signal of the dynode, the preliminary test result shows that the charge noise of the single-route dynode base-line is
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6 fC (rms). Because that the charge measurement range of electronics is 0∼13 pC, hence the maximum measurable signal-to-noise ratio of the single-route dynode measurement will be 1.3×103/6, about 2000, satisfying the requirement of the detector. Fig.8 shows the energy spectra for the 7th dynode obtained respectively from the self-triggering tests of radiative and cosmic-ray sources, from this figure we can find that the detection system can separate very well the Gamma-ray spectral region of Co-60 from the noise, the fitted Gamma-ray spectrum of Co-60 is fallen in the channel 497, the energies of minimum-ionization particles of cosmic-ray Moun are deposited in the vicinity of the channel 9157, and the front-end electronics gets saturated about at the channel 3×104 .
Fig. 8 Gamma-ray energy spectrum of Co-60 (a) and the cosmic-ray energy spectrum of Moun (b)
Fig.9 shows the multiple-relation between the signals of the 7th and 4th dynodes (Fig.9(a)), and that between the signals of the 4th and 1st dynodes (Fig.9(b)), obtained by the long-duration cosmic-ray test. From this figure it is calculated that the multiple between the signals of the 7th and 1st dynodes is 1200, and in combination with the signalto-noise ratio of single-route dynode measurement calculated above, we can conclude that the dynamic range of measurement can be extended to about 106 by combining the signals of 3 dynodes.
Fig. 9 The multiple between the signals of the 7th and 4th dynodes (a) and that between the signals of the 4th and 1st dynodes (b)
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5. CONCLUSION Above we have described the prototype design and test results of the BGO calorimeter in the dark matter detector, the test results indicate that the dynamic range of the whole detector can be effectively extended by means of multi-dynode readout to satisfy the requirement of 106 dynamic range. Besides, for a more overall result, we have to make some more detailed tests. References 1
Feng J. L. arXiv:1003.0904, astro-ph. CO, 2010
2
Gianfranco B., Dan H., Joseph S., PhR, 2005, 405, 279
3
Chang J., Adams Jr J. H., Ahn H. S., et al., Nature, 2008, 456, 362
4
Adriani O., Barbarino G. C., Bazilevskaja G. A., et al., Nature, 2009, 458, 607
5
Chang J., Wu J., Proceedings of the 31st ICRC, 2009
6
Wang Zhi-ying, Lou Bin-qiao, Zhu Jun-jie, et al., Principle of Nuclear Electronic Techniques, Beijing: Atomic Energy Press, 1989, 8-11
ELSEVIER
Chinese Astronomy and Astrophysics 36 (2012) 318–326
Design of the Prototype Readout System of BGO Calorimeter in the Space Dark Matter Detector of Purple Mountain Observatory† GUO Jian-hua1,2 1 2
CAI Ming-sheng1,2
HU Yi-ming1,2
CHANG Jin1,2
Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008
Key Laboratory of Dark Matter and Space Astronomy, Chinese Academy of Sciences, Nanjing 210008
Abstract A space dark matter detector is proposed by the Key Laboratory of Dark Matter And Space Astronomy of Chinese Academy of Sciences for detecting the high-energy electrons and Gamma particles produced by the annihilation of dark matter in space. The whole detector is mainly composed of the BGO (bismuth germanium oxide) high-energy image calorimeter and scintillation hodoscope. The energy range of the detector will cover the high-energy electrons and Gamma particles of 10 Gev∼10 TeV, in which the energies of highenergy particles are mainly deposited in the BGO calorimeter. For verifying the scheme of the detector, we have designed a prototype readout system for the BGO calorimeter of the space dark matter detector, and made a preliminary test on it. Key words: instrumentation: detectors—dark matter—cosmic rays
1. INTRODUCTION Since Zwicky[1,2] proposed a possible astronomical evidence for the existence of dark matter in the 30s of the 20th century, more and more pieces of evidence indicate the real existence of dark matter. In the universe, the part known for us occupies only a proportion of under 4%, dark matter and dark energy are almost full of the whole universe. However, besides the gravitational effect, human being knows dark matter very little. What is the nature of dark †
Supported by Intellectual Innovation Project of Chinese Academy of Sciences Received 2011–03–29; revised version 2011–06–07 A translation of Acta Astron. Sin. Vol. 53, No. 1, pp. 72–79, 2012 [email protected]
0275-1062/11/$-see front front mattermatter © 2012 Elsevier All rights reserved. c 2012 B.V. 0275-1062/01/$-see Elsevier Science B. V. All rights reserved. doi:10.1016/j.chinastron.2012.07.008 PII:
GUO Jian-hua et al. / Chinese Astronomy and Astrophysics 36 (2012) 318–326
319
matter, and what kinds of fundamental particles it consists of? All these basic problems have not yet been solved. In the past several ten years, physicists have developed a super-symmetry theory. This theory predicts that each kind of common particle possesses a kind of undetected largemass “super-symmetrical companion” particle[1−2] . At present, the particles predicted by the super-symmetry theory are the dominant candidates of dark matter particles. These particles possess both mass and weakly interacting force, but they do not participate in any electromagnetic interaction, and they are called the WIMPs (Weakly Interacting Massive Particles). Since the dark matter particles do not participate in any electromagnetic interaction, so, in order to detect them, the present experiments should be promoted to extremes, as a result, so far people do not understand that what kinds of particles the dark matter particles are. Now, the methods of dark matter detection are mainly the direct detection, accelerator detection, and indirect detection. The direct detection, for example to detect the collisions between dark matter particles and common materials, verifies the existence of dark matter particles by observing the thermal or other kind signals produced by these collisions. The indirect detection observes mainly the stable secondary particles, such as the Gamma-particles and electrons that produced by the decay or annihilation of dark matter particles. Now, multiple experiments for the direct and indirect detections of dark matter particles have been made in the world. And indebted to the efforts of scientists in many years, a new progress has been achieved in the indirect observation of dark matter particles since 2008. At first, by analyzing the 10 yr observational data, the American Antarctic balloon experiment ATIC (Advanced Thin Ionization Calorimeter) discovered that between 300 Gev and 800 GeV the energy spectrum of cosmic high-energy electrons has an abnormal excess relative to the theoretical spectrum of cosmic rays[3]. Successively, the European space experiment PAMELA (Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics) observed the similar result[4] . The abnormality of high-energy electron spectrum is considered as a possible signal from the dark matter particles, and received extensive attentions. In order to measure more accurately the energy spectra of cosmic high-energy electrons and Gamma-particles, on the basis of the China-US cooperated ATIC experiment, the Key Laboratory of Dark Matter And Space Astronomy of Chinese Academy of Sciences proposed a experimental project of space dark matter detection[5] , which purports to detect the highenergy electrons and Gamma-particles produced possibly by the annihilation of dark matter particles. This paper is an introduction to the readout design of the BGO calorimeter in the space dark matter detector, as well as the preliminary test result.
2. SYSTEM FRAMEWORK OF THE DARK MATTER DETECTOR The space dark matter detector proposed by the Key Laboratory of Dark Matter And Space Astronomy of Chinese Academy of Sciences is mainly composed of two sub-detectors[5] : the scintillation hodoscope on the top of the detector and the BGO calorimeter beneath, the structure of the detector is shown as Fig.1. In order to discriminate accurately the incident particles, such as proton, electron, Gamma-particle etc., it is necessary to reconstruct the
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track of the interaction between the incident particle and the detector. Hence, each subdetector system has the functions of both position sense and energy measurement.
Fig. 1 Structure of the space dark matter detector
The scintillation hodoscope on the top of the detector has 6 layers, which are vertically placed with the interleaved X, Y directions, each piece of plastic scintillator has the dimension of 1 cm×1 cm and the length of 1 m. The BGO calorimeter under the scintillation hodoscope has 12 layers, which are placed with the interleaved X, Y directions, and the area of each layer is 0.6 m×0.6 m. Restricted by the fabrication technology, the required 0.6m length of BGO crystal is realized by joining two pieces of 0.3m-long BGO crystals together in the BGO calorimeter. The size of each piece of BGO crystal is 2.5 cm×2.5 cm, and each layer has 24×2 pieces of BGO crystals. The predicted energy measurement range of the whole detector is 10 GeV∼10 TeV, the energy resolution is 1.5%@800 GeV, and the discriminability between Gamma-particles and electrons is better than 1%. As the major part of incident particle energies are deposited in the BGO image calorimeter, so its ability of energy detection plays a key role for the performance of the whole detector.
3. DESIGN OF THE BGO CALORIMETER 3.1 Structure of the BGO Calorimeter The 12-layer BGO image calorimeter is fixed by a framework structure, its exterior is shown as Fig.2, in which PMT indicates the photomultiplier tube. When the particle energy is deposited in the BGO crystal, the BGO crystal has the deposited energy convert into the fluorescence with corresponding strength, then it is converted into an electric signal by the photomultiplier tube (R5610A, Hamamatsu) on a terminal of the BGO crystal, and sent to the electronic readout system for measurements. There are totally 576 pieces of PMTs responsible for the readout of optical signals of the whole BGO calorimeter. Fig.3 shows the sealed package of a single BGO crystal and PMT.
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3.2 Readout Electronics of the BGO Calorimeter In order to make the whole detector system be able to measure the high-energy particles (mainly the Gamma-particles and electrons) with energies in the range of 10 GeV∼10 TeV, it is required that a single BGO detector element can measure the particle energies in the range from 10 MeV to 2 TeV (the result of GEANT4 simulations). Hence, for each BGO crystal, the dynamic range of 105 ∼106 is required. Considered that relying on only the anode or single dynode of the photomultiplier tube can hardly achieve such a large dynamic range, a technique similar to the ATIC experiment is adopted, namely to broaden the dynamic range by using the signals taken from 3 different dynodes. These 3 dynodes are respectively the 1st, 4th and 7th dynodes. After the fluorescence of the BGO crystal entered the cathode of a photomultiplier tube, the charge signal is amplified by each of the dynodes, and finally reaches the anode. As the signals taken from the different dynodes are related by 3 fixed multiples, so the signals taken from the 3 dynodes with different gains can be used for the measurements of high-energy, medium-energy, and low-energy regions, respectively. By measuring the signals of 3 dynodes, the dynamic range of the detector is raised for 103 times relative to the single-route measurement. Thus, if only the single-route dynode signal can reach the dynamic range of 103 , then by combining the 3-route signals, the dynamic range of the whole BGO detector will easily reach 106 .
Fig. 2 The structure figure of the BGO calorimeter
The signals taken from the photomultiplier tube are sent to the electronic readout system to make processing. The electronic measurement system of the BGO calorimeter mainly consists of two parts: the FEE (front-end electronics) board, and the sub-detector data acquisition system. The FEE board uses a charge-sensitive amplifier to collect the charge signals of the PMT, makes the shaping amplification (slowly shaping), peak holding, and digitalization, then the digitalized signals are sent to the sub-detector data acquisition system. At the same time, by the fast-shaping amplifier and discriminator in the FEE board,
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the hitting signal of a particle is sent to the triggering system as the basis of the 1st-stage judgment. The structure of the FEE board is shown in Fig. 4. In this figure, the symbol “dy” indicates the dynode of the PMT, FPGA is the programmable gate array, ADC the analog-digital converter, RAM the random access memory, and Mux the multiplexer.
Fig. 3 Single BGO detector element
Fig. 4 Readout electronics of the BGO calorimeter
Considered that the FEE board is closely placed upon the BGO detector, for the sake of its area, the FEE board is divided into two parts: the FAE (front-end analog electronics) subboard and FCB (front-end control board) sub-board. They are responsible for the processing of analog signals and the transmission of digital control signals, respectively. The two subboards are overlapped together to compose the FEE board through a connector, the details can be seen in Fig.5, in which PCB means the printed circuit board. The sub-detector data acquisition system (sub DAQ) takes mainly the responsibility for receiving the commands from the upper-layer main data acquisition system and trigger system, starting the data sampling of front-end circuits, and receiving the data coming from the FEE board, then packing the data, and transmitting them to the upper-layer main data acquisition system by through the data bus. One sub-detector data acquisition board takes the responsibility for managing 6 FEE boards (corresponding to the readout of all the PMTs on the one side of the BGO calorimeter). 4. TEST RESULTS OF THE BGO DETECTOR For verifying the performance of the BGO detector, we have made a test on it. The preliminary test of the whole system is divided into two steps: testing the performance of front-end
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Fig. 5 Picture of the PCB of front-end electronics
electronics by using a signal generator, and testing the detector by using cosmic-ray and radiative sources as signal sources. 4.1 Test Result Obtained by Using A Signal Generator Before the FEE board is connected with PMTs, the performances (noise, dynamic range, linearity, etc.) of front-end electronics should be tested. The test on the frontend electronics is made by using a signal generator. The pulse signal generated by the signal generator is coupled to the input terminal of front-end electronics by through a 10 pF capacitor, to simulate the output charge signal of the PMT[6] . The signal strength of the generator is adjusted continually to scan the entire dynamic range of front-end electronics. Fig.6 shows the results of the noise and linearity tests on the circuits, in which the symbol “Bin” indicates the channel of the ADC, and each channel corresponds to 0.68 fC. From this figure we can find that the noise of each channel (reduced to the amount of charge at the input terminal) is about 1.2∼1.4 fC (rms). And the test result indicates that the linearity of front-end electronics allows the maximum amount of charge reaching +13 pC (the nonlinearity is 1%), but for the +14 pC input the nonlinearity has been rather large (2%). Hence, as shown by the test result, the dynamic range of single-route readout electronics is about 104 , satisfying completely the requirement of the detector.
Fig. 6 Base-line noises of 32 channels (a) and linearity test result (b) of the FEE
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4.2 Test Results of Cosmic-ray and Radiative Sources A fixed multiple-relation exists between the signals of two different dynodes, for the 3 dynodes used by the dark matter detector, the multiple between the signals of two neighboring dynodes is about 30∼40 (+800 V high voltage). In order to extend conveniently the dynamic range of energy measurement of the whole detector to 106 by combining the 3 dynode signals of different multiples, the energy measurement ranges of two neighboring dynodes should be properly overlapped, the size of the overlapped part is related with the difference of multiples. In order to test the linear relation between the multiples of different dynodes, as well as the noise and other performances, we have performed tests on the single BGO detector element by using the cosmic-ray and radiative sources, respectively. Based on the simulation of GEANT 4, in order that the whole detector can detect the electrons of about 10 TeV, the single detector element should be able to detect the electron energy of about 2 TeV. Although by using the signals from the 3 dynodes of each PMT in the BGO calorimeter, the dynamic range of the whole detector can reach 106 , but if the photons produced by the 2 TeV electrons are directly input to a PMT, then they will much exceed the acceptable range of the PMT cathode. Hence, in practical applications, only if the corresponding fluorescence intensity of the particles whose energies need to be measured is adjusted to the measurable range of the detector by means of optical attenuation or other methods, would the detector be able to detect the particles in the required energy range. But for the laboratory test, we can hardly obtain such a high-energy radiation source, in order to make the performance test of the detector by using the radiative and cosmic-ray sources, in the laboratory test the input optical signal is directly coupled to the PMT, without any optical attenuation. In this way, the photon number produced in the BGO crystal by the particle of 1 MeV is equivalent to that produced by the particle of about 40 MeV in practical situations. Thus, the detector can be tested by using the particle sources obtainable in laboratory. Fig.7 is a schematic diagram of the test bench of cosmic-ray and radiative source tests, the radiative source uses mainly the Gamma radiation of Co-60. In the figure, HV is the high voltage of the PMT, GND expresses the ground of the high voltage. The data sampling is realized by means of self-triggering, and the triggering signal is produced by the 7th dynode.
Fig. 7 Schematic diagram of the test bench
In the experiment, a random signal is used to acquire the base-line signal of the dynode, the preliminary test result shows that the charge noise of the single-route dynode base-line is
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6 fC (rms). Because that the charge measurement range of electronics is 0∼13 pC, hence the maximum measurable signal-to-noise ratio of the single-route dynode measurement will be 1.3×103/6, about 2000, satisfying the requirement of the detector. Fig.8 shows the energy spectra for the 7th dynode obtained respectively from the self-triggering tests of radiative and cosmic-ray sources, from this figure we can find that the detection system can separate very well the Gamma-ray spectral region of Co-60 from the noise, the fitted Gamma-ray spectrum of Co-60 is fallen in the channel 497, the energies of minimum-ionization particles of cosmic-ray Moun are deposited in the vicinity of the channel 9157, and the front-end electronics gets saturated about at the channel 3×104 .
Fig. 8 Gamma-ray energy spectrum of Co-60 (a) and the cosmic-ray energy spectrum of Moun (b)
Fig.9 shows the multiple-relation between the signals of the 7th and 4th dynodes (Fig.9(a)), and that between the signals of the 4th and 1st dynodes (Fig.9(b)), obtained by the long-duration cosmic-ray test. From this figure it is calculated that the multiple between the signals of the 7th and 1st dynodes is 1200, and in combination with the signalto-noise ratio of single-route dynode measurement calculated above, we can conclude that the dynamic range of measurement can be extended to about 106 by combining the signals of 3 dynodes.
Fig. 9 The multiple between the signals of the 7th and 4th dynodes (a) and that between the signals of the 4th and 1st dynodes (b)
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5. CONCLUSION Above we have described the prototype design and test results of the BGO calorimeter in the dark matter detector, the test results indicate that the dynamic range of the whole detector can be effectively extended by means of multi-dynode readout to satisfy the requirement of 106 dynamic range. Besides, for a more overall result, we have to make some more detailed tests. References 1
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