Characterization of Silicon Photomultiplier (SiPM) for Silicon Geiger Hybrid Tube (SiGHT) to Improve Dark Matter Experiments

Transcription

1 Characterization of Silicon Photomultiplier (SiPM) for Silicon Geiger Hybrid Tube (SiGHT) to Improve Dark Matter Experiments Phuoc Quach Department of Physics and Astronomy, University of California, Los Angeles, 475 Portola Plaza, Los Angles, CA 90095, USA 8/23/2013 Abstract For the purpose of direct dark matter detection experiments, highly sensitive photodetectors are needed. The UCLA dark matter team is testing and developing a novel hybrid photodector called Silicon Geiger Hybrid Tube (SiGHT) that can potentially replace the currently used photomultiplier tube. The main component of SiGHT is the silicon photomultiplier. The characterization of the SiPM is being discussed here. Data were taken for five Sensl SiPM type MicroSM X13-NE. With the amplifier, the average gains at mininum operating voltage is Introduction Dark matter is a different type of matter that has been postulated through collective evidence from cosmology and astrophysics. The first evidence for dark matter was discovered by astronomer Fritz Zwicky in 1933[2]. When observing the Coma Cluster, he noticed that the velocity of the galaxies was moving too fast for them to be held in a cluster by the luminous matter. He deduced that there has to be missing mass. More evidences emerged later on including the Bullet Cluster, the Big Bang nucleosynthesis (BBN), distant supernovae, the cosmic microwave background (CMB), and others supporting the existence of dark matter [2]. The two types of dark matter are hot (particles that are nearly massless and can move close to the speed of light) and cold (non-relativistic particles). Interested in how large scale structures of the universe were formed, more concentration is given to cold dark matter [8]. Of the four fundamental forces of the universe, dark matter interacts gravitationally and there has been unconfirmed hypotheses that it may interact with other unknown forces. There are a number of hypothesized candidates for what cold dark matter (CDM) is and three different techniques for detecting them (directly, indirectly, and accelerator). Some 1

2 potential candidates are Axions (particle), Weakly Interacting Massive Particles (WIMPs), MACHO (massive compact halo objects such as black hole), and the list goes on as perhaps more possible candidates are added while existing ones can be eliminated from experiments. WIMPs are weakly interacting massive particles that are slow moving, massive, subatomic particles unlike the ones in Standard Model particles [8].Different candidates of CDM require different detection techniques. Within each detection technique, there are different experiments and materials used to perform them. 1.1 Direct Detection Direct detection techniques attempt to detect CDM when the CDM particle interacts with known matter. When the energy of the CDM particle is deposited onto the atom of known matter, there are three ways of detecting this interaction: phonon (thermal energy), scintillation (light), and ionization [6]. Liquid xenon, argon, or neon are all scintillators, that is, when a particle deposits energy in the liquid, the atoms emit light in the form of UV photons [8].This scintillation light can be observed and characterized in order to determine the amount of energy deposited [8]. Figure 1 has the various experiments and which ways to reveal the interaction between CDM particle with ordinary matter if or when it happens. Figure 1: Three ways of detecting if CDM particles interact with ordinary matter. Some experiments attempt to detect one evidence of this interaction while others try to detect two [6]. 1.2 Noble Liquid TPCs XENON100 and Darkside50 are two operational direct detection dark matter search programs located in Gran Sasso Mountain, Italy, that the UCLA team is directly involved 2

3 with. Both experiments utilize the time projection chambers (TPCs). Figure 2 shows the basic parts of the TPC system. XENON100 uses liquid xenon as a target material that can interact with cold dark matter particle while Darkside50 uses liquid Argon. They are both dual-phase (both gas and liquid phases exist at the same time) TPC noble liquid dark matter experiments that target WIMPs as the candidate for CDM. Both TPCs detect the interaction between WIMPs and Standard Model particles via ionization and scintillation. This interaction results in low energy deposited in the TPCs. In order to detect low energy deposition signal from the CDM particles, both experiments need highly sensitive photodetector with low radioactivity. Radioactivity in the TPCs causes background events which needs to be minimize [8]. Figure 2: This shows how WIMPs are detected in the time projection chamber with a 3D model of XENON100 [8]. 1.3 Principles of PMT Presently, photomultiplier tubes (PMT) are being used in both experiments. The PMT comes in different sizes and shapes. However, its functions and components are the same throughout. The PMTs used are from the Hamamatsu Photonics company. Figure 3 shows the components of the PMT. When a photon hits the photocathode, which is a thin layer of deposited metal with low work function, there is a certain percent chance that an electron will be ejected from the photocathode material. The probability that an incident photon will eject an electron is called the quantum efficiency (QE) and this phenomenon is called the photoelectric effect. There is a probability (collection efficiency) that the ejected electron will hit the first dynode, which is composed of a certain type of metal that allows more electron emissions. The electrons accelerates to the first dynode and through all the dynodes by an electric field which is controlled by applying voltages to the focusing eletrode and all dynodes [5]. The anode reads out the signal. The total number of electrons at the anode for every initial photoelectron is called gain [5]. 3

4 2 SIGHT Figure 3: Components of the photomultiplier tubes [5]. The PMT metallic shield (cover) and dynodes accounts for about 68% gamma ray background in XENON100 [8]. This background can create signals mimicing that of DM interactions. The base also cause about 7% background events to the experiment [8]. Reducing this high level of radiation produced by the PMT is important for the dark matter experiments. Motivated by the need to have a photodetector with an intrinsic radioactivity significantly lower than the current generation PMTs, the UCLA dark matter team is testing and developing a novel hybrid photodetector called Silicon Geiger Hybrid Tube (SiGHT) that can potentially replace the current PMT [8]. The design of SIGHT will be as figure 5 shows. The cover is synthetic fused silica which is ultra low radioactivity [7]. When a photon hits the photocathode, an electron will be ejected. The electric field applied in the areas between the photocathode and the Silicon PM accelerates the ejected electron onto the SiPM where electron multiplication takes place. The SiPM has the same function as the dynode in the PMT. To explain the SiPM, an explanation of the avalanche photodiode (APD) is needed. An APD is a semiconductor device that can detect photons. Operated in Geiger-mode, the APD gives very large signals. The silicon photomultiplier (SiPM) is an array of several thousand APDs microcells operated in Geiger-mode. Figure 4: Sensl SiPM type MicroSM X13-NE [1]. Zoomed SiPM with four Giegermode APD [3]. 4

5 Figure 5: This is a rough drawing of SiGHT with some components labeled. 2.1 Characterizing SiPM Gains Data were taken for five Sensl SiPM (MicroSM X13-NE). The breakdown voltage is the least amount of reverse voltage required to make the diode conduct in reverse. In this case the Sensl defined breakdown voltage as point at which dark current = 100nA (3mm devices) [1]. The ability of the SiPM to count single photons is tested by shooting light from an LED at it. The SiPM is mounted onto a circuit board with an amplifier (in order to amplify the signal for better read out). The board is mounted inside a black box (a box lining with black cloth material to keep light out) a certain distance away from a LED light source with a filter on it that can filter enough light to allow only one photon through. The V Br was found for each SiPM. Added 0.1 volt to each breakdown voltage and started to record signal from single photons from this point on. Data were taken from V Br + 0.1V to 4.5 volts above this point with increment of 0.5V at each data point for each SiPM. 5

6 Typical breakdown voltage (VBr ) Bias range (above VBr Active Area (Approx.) No. of microcells Pixel Fill Factor Excess Noise Factor Cross-talk Microcell Recovery Time Dark Current - Typical Dark Current - Max Gain PDE at 500 nm 27.5 V ± 0.5 V 1V 5V 3 3 mm % % 130 ns 3.8 µa 8 µa % Table 1: SiPM information given by Sensl for MicroSM X13-NE. From excess noice down, data were taken at VBr + 2V and 21 degree C [1]. Figure 6: Inside of the black box: the wires are used to connet the LED, amplifier, and SiPM. Figure 7: The board holds the amplifier and the SiPM with capacitors. The other side of the amplifier is also shown. 6

7 2.2 Polishing Quartz Cylinders Quartz is used for the prototype of SIGHT. Figure 8 shows what the prototype will look like. For good sealing, both ends of the cylinder need to be polished. A lapping machine and grits of 120, 400, 800, and 1800 were used. Seven cylinders are needed to be polished for the prototype of SiGHT. The seven disks need to be cleaned and weighted because three thin films of copper, chromium, and indium will be deposited onto the disk. Figure 8: Quartz prototype of SiGHT. There are a top and bottom disk with indium ring for sealing. Figure 9: Polished and unpolished end of the cylinders are shown. 7

8 3 Data Figure 10 shows the area of the pulses from the SiPM, with the peaks representing 0,1, 2, and ect. photons. Then a Guassian is fit to the one photon peak and the gains is extracted. The bias voltage and the mean (average) gain for each SiPM is shown in table 3 and 4. Then a graph of gain vs. bias voltage is generated for each SiPM. The amplifier is used so the gains of these SiPM is with the amplifier. SiPM Number Breakdown (volts) Sensl-SiPM Sensl-SiPM Sensl-SiPM Sensl-SiPM Sensl-SiPM Table 2: This is the V Br of each SiPM in volts. Figure 10: Histogram of event/bin vs. gains (number of electrons) are displayed with the red fitted curve for one photon count. The display window shows the average gain is 43.95x10 6 ± 0.07 electrons for the fitted red cuve for one photon count. 8

9 Bias Voltage (V) Gain SiPM 1 ( 10 6 ) Table 3: This shows the voltage applied to the SiPM and the gains at these voltages. Voltage (V) Gain SiPM 2 ( 10 6 ) Gain SiPM3 ( 10 6 ) Gain SiPM 4 ( 10 6 ) Gain SiPM5 ( 10 6 ) Table 4: Bias voltage and gains for SiPM2, 3, 4, and 5. Figure 11: Graph for SiPM 5. Bias voltage applied to the SiPM vs. the gain in number of electrons is shown. 9

10 4 Discussion Four SiPMs breakdown voltages were at 27.4V and one was at 27.3V. The average gain of the five SiPM at thisv Br was at X10 6. From these data, the gains of the SiPM vs. bias voltage applied to the SiPM were understood. There were high level of noise due to the electronic components of the setup and dark count due to the nature of the SiPM. The noise issue can be addressed through changing out the electronic components. The dark count is due to the thermal energy, which gives the same signal as photon but there is no photon hit, so lowering the temperature should reduce the dark count. The next step in characterizing the SiPM is to take data for gain with electron bombardment, lower the dark count rate, and test it under cryogenic temperature. SiGHT will be iin or close to the target material in the Time Projection Chamber, as shown by Figure 2, therefore it should be able to work under cryogenic temperature. Data will be taken for the SiPM in a small vacuum chamber that was submerged under liquid nitrogen. This is still in progress. 5 Acknowledgement It has been an honor and privilege to work with Dr. Hanguo Wang on his dark matter research team. I have gained a variety of knowledge; from what it means to work as a team on a research project to what it takes to becoming a physics experimentalist. Dr. Emilija Pantic has been a role model for me as an admirable talented female physics experimentalist who is not afraid to tackle any problems or handle any devices. Dr. Artin Teymourian has patiently explained in great details of the operations of devices used as well as keeping the big picture in mind for me while encouraging me every step of the way. Yixiong Meng always welcomes my question with such enthusiasm. Alden Fan has guided me in the right direction for my project. William Huang has been there to help me with works and keeping track of everything. They have been a wonderful team to work with. I cannot thank them enough for everything they have taught and given me. I would like to thank Franoise Queval for giving me this extraordinary experience to learn and grow. I would like to thank the REU Program for providing us with preparation for my future. Lastly but not least, I would like to thank the National Science Foundation for their funding of the research project and our program. References [1] Fast Silicon Photomultiplier Detectors. Sensl [2] J.L. Feng. Dark Matter Candidates from Particle Physics and Methods of Detection. Annual Review of Astronomy and Astrophysics

11 [3] A. Knsken. Simulation of a detector prototype with direct SiPM read-out and comparison with measurements Physics Institute. [4] N. Otte. The Silicon Photomultiplier - A new device for High Energy Physics, Astroparticle Physics, Industrial and Medical Applications Max-Planck-Institut. [5] Photomultiplier Tubes: Basics and Applications. Hamamatsu Photonics K. K. 3rd [6] T. Saab. An Introduction to Dark Matter Direct Detection Searches & Techniques. (2012), arxiv: v1. [7] A. Teymourian, et al., Characterization of the QUartz Photon Intensifying Detector (QUPID) for Noble Liquid Detectors (2011), arxiv: v2. [8] A. Teymourian, et al., Development and Characterization of the QUartz Photon Intensifying Detector (QUPID), and Applications in Future Dark Matter and Neutrinoless Double Beta Decay UCLA. 11