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PMT and HV Control PMT and HV control M. Nomachi PMT Photo multiplier is used to detect weak light. Q1: What does “weak” mean? Scintillation light generated by radiation detector, such as Plastic scintillator, is very weak. PMT is used to detect such weak light. CHAPTER 2PMT BASIC PRINCIPLES OF 1)-5) PHOTOMULTIPLIERYou can learn PMT with “PMT handbook” TUBES from Hamamatsu, which is at http://www.hamamatsu.com/resources/pdf/etd/PMT_handbook_v3aE.pdf Figures on this text is copied from the handbook. A photomultiplier tube is a vacuum tube consisting of an input window, a photocathode, focusing electrodes, an electron multiplier and an anode usu- ally sealed into an evacuated glass tube. Figure 2-1 shows the schematic construction of a photomultiplier tube. FOCUSING ELECTRODE SECONDARY ELECTRON LAST DYNODE STEM PIN VACUUM (~10P-4 ) DIRECTION e - OF LIGHT FACEPLATE STEM ELECTRON MULTIPLIER ANODE (DYNODES) PHOTOCATHODE THBV3_0201EA Figure 2-1: Construction of a photomultiplier tube Light which enters a photomultiplier tube is detected and produces an output signal through the following processes. (1) Light passes through the input window. (2) Light excites the electrons in the photocathode so that photoelec- trons are emitted into the vacuum (external photoelectric effect). (3) Photoelectrons are accelerated and focused by the focusing elec- trode onto the first dynode where they are multiplied by means of secondary electron emission. This secondary emission is repeated at each of the successive dynodes. (4) The multiplied secondary electrons emitted from the last dynode are finally collected by the anode. This chapter describes the principles of photoelectron emission, electron tra- jectory, and the design and function of electron multipliers. The electron multi- pliers used for photomultiplier tubes are classified into two types: normal dis- crete dynodes consisting of multiple stages and continuous dynodes such as mi- crochannel plates. Since both types of dynodes differ considerably in operating principle, photomultiplier tubes using microchannel plates (MCP-PMTs) are separately described in Chapter 10. Furthermore, electron multipliers for vari- ous particle beams and ion detectors are discussed in Chapter 12. © 2007 HAMAMATSU PHOTONICS K. K. Photoelectric effect 1921 Nobel prize If metal electrodes are exposed to light, electrical sparks between them occur more readily. For this "photoelectric effect" to occur, the light waves must be above a certain frequency, however. According to physics theory, the light's intensity should be critical. In one of several epoch-making studies beginning in 1905, Albert Einstein explained that light consists of quanta - "packets" with fixed energies corresponding to certain frequencies. One such light quantum, a photon, must have a certain minimum frequency before it can liberate an electron. https://www.nobelprize.org/nobel_prizes/physics/laureates/1921/einstein-facts.html PMT is a device which can observe single photon. CHAPTER 2 BASIC PRINCIPLES OF PHOTOMULTIPLIER TUBES 1)-5) A photomultiplier tube is a vacuum tube consisting of an input window, a photocathode, focusing electrodes, an electron multiplier and an anode usu- ally sealed into an evacuated glass tube. Figure 2-1 shows the schematic construction of a photomultiplier tube. photoelectric effect FOCUSING ELECTRODE SECONDARY ELECTRON LAST DYNODE STEM PIN VACUUM (~10P-4 ) DIRECTIONPhoton electrone - OF LIGHT FACEPLATE STEM ELECTRON MULTIPLIER ANODE (DYNODES) PHOTOCATHODE THBV3_0201EA Figure 2-1: Construction of a photomultiplier tube Light which enters a photomultiplier tube is detected and produces an output signal through the following processes. (1) Light passes through the input window. (2) Light excites the electrons in the photocathode so that photoelec- trons are emitted into the vacuum (external photoelectric effect). (3) Photoelectrons are accelerated and focused by the focusing elec- trode onto the first dynode where they are multiplied by means of secondary electron emission. This secondary emission is repeated at each of the successive dynodes. (4) The multiplied secondary electrons emitted from the last dynode are finally collected by the anode. This chapter describes the principles of photoelectron emission, electron tra- jectory, and the design and function of electron multipliers. The electron multi- pliers used for photomultiplier tubes are classified into two types: normal dis- crete dynodes consisting of multiple stages and continuous dynodes such as mi- crochannel plates. Since both types of dynodes differ considerably in operating principle, photomultiplier tubes using microchannel plates (MCP-PMTs) are separately described in Chapter 10. Furthermore, electron multipliers for vari- ous particle beams and ion detectors are discussed in Chapter 12. © 2007 HAMAMATSU PHOTONICS K. K. Multiplication at Dynode Photo cathode 1) Electric field accelerate an electron. Electric potential Q: How much energy is obtained? Dynode 2) The electron kicks out Electric potential several electrons in a dynode. Dynode 46 CHAPTER 4 CHARACTERISTICS OF PHOTOMULTIPLIER TUBES 100 46 CHAPTER 4 CHARACTERISTICS OF PHOTOMULTIPLIER TUBES 80 100 60 80 60 40 40 20 20 RELATIVE COLLECTION EFFICIENCY (%) 0 40 50 100 150 RELATIVE COLLECTION EFFICIENCY (%) 0 PHOTOCATHODE40 50 TO FIRST DYNODE VOLTAGE (V)100 150 THBV3_0412EA Figure 4-12: Collection efficiency vs. photocathode-to-first dynode voltage 46 CHAPTER 4 CHARACTERISTICS OF PHOTOMULTIPLIERPHOTOCATHODE TUBES TO FIRST DYNODE VOLTAGE (V) Figure 4-12 shows that about 100 volts should be applied between the cathode and the first dynode. The THBV3_0412EA collection efficiency100 influences energy resolution, detection efficiency and signal-to-noise ratio in scintil- lation counting.Figure The detection 4-12: efficiency Collection is the ratio efficiencyof the detected signalvs. photocathode-to-firstto the input signal of a photo- dynode voltage multiplier tube. 80In photon counting this is expressed as the product of the photocathode quantum efficiency and the collection efficiency. Figure 4-1260 shows that about 100 volts should be applied between the cathode and the first dynode. The 2.3 Electron Multiplier (Dynode Section) 17 (2)collection Gain (current efficiency40 amplification) influences energy resolution, detection efficiency and signal-to-noise ratio in scintil- PHOTOCATHODE lationSecondary counting. emission The ratio detectionδ is a function efficiencyof the interstageF is voltage the ratioof8 dynodes of theE, and detected is given by thesignal to the input signal of a photo- 20 6 following equation: PHOTO- 1 multiplier tube. In photon counting thisELECTRONS is expressed4 7 as the product of the photocathode quantum efficiency k 5 RELATIVE COLLECTION EFFICIENCY (%) INCIDENT LIGHT δ = a·E0 ··························································································· (Eq. 4-3) 150 40 50 100 3 and the collection efficiency. 2 Where a is a constant and k is determined by the structure and material of the dynode and has a value PHOTOCATHODE TO FIRST DYNODE VOLTAGE (V) 1 to 7 = DYNODES 8 = ANODE THBV3_0412EA from 0.7 to 0.8. F = FOCUSING ELECTRODE Figure 4-12: Collection efficiency vs. photocathode-to-first dynode voltage THBV3_0204EA Figure 2-4: Box-and-grid type (2) GainTheFigure photoelectron 4-12 shows(current that about current 100 volts should amplification)Ik emitted be applied betweenfrom thethe cathode photocathode and the first dynode. strikes The the first dynode where secondary collection efficiency influences energy resolution, detection efficiency and signal-to-noiseF ratio in scintil- 4.2 Basic Characteristics of Dynodes1 47 electrons Idl are released. At this point, theINCIDENT secondary emission ratio δ1 at11 the first dynode is given by LIGHT 3579 lation counting. The detection efficiency is the ratio of the detected signal to thePHOTO- input signal of a photo- Multiplication at DynodeELECTRONS multiplier tube. In photon counting this is expressed as the product of the photocathode quantum efficiency 10 Id1 2 4 6 8 Secondaryand the collection = efficiency. emission ratio δ is a function of the interstage voltage of dynodes E, and is given by the δ 1 ····························································································1 to 10 = DYNODES (Eq. 4-4) IK 11 = ANODE where α is the collection efficiency. F = FOCUSING ELECTRODE following(2) Gain (current equation: amplification) THBV3_0205EA The number of secondary electron is These electrons are multiplied in a cascade processFigure from 2-5: Linear-focusedthe first type dynode → second dynode → .... The productSecondary emissionof α, δratio1, δ 2is .....a functionδ n isof thecalled interstage the voltage gain of µdynodes(current E, and isamplification), given by the and is given by the following thefollowing n-th dynode.equation: The secondaryk emission ratio δn of n-th stage is given by = a·E ···························································································2.3 Electron Multiplier (Dynode Section) (Eq. 4-3) equation: δk δ = a·E ··························································································· (Eq. 4-3) Idn As stated above, the potential distribution and electrode structure of a photomultiplier tube is designed to Where aδ isn a= constant and k is determined by the structureprovide and optimum material performance. of the Photoelectrons dynode and emitted has afrom value the photocathode are multiplied by the first dyn- µ = α·δ1·δ2 ····························································································
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