Techniques for Nuclear and Particle Physics Experiments By

Techniques for Nuclear and Particle Physics Experiments By W. R. Leo Chapter Eight: The Photomultiplier Tube (PMT)

Basic PMT Structure 1. 2. 3. 4. Photocathode Electron Optical Input System Electron Multiplier Anode

The Photocathode converts incident photons to photoelectrons Emitted electron energy given by Einstein’s photoelectric affect: Must reach minimum frequency for equation to be applicable

Photocathode Values 1. Quantum Efficiency: 2. Radiant Cathode Sensitivity:

Photocathode Values Or: For Units in Amperes/Watts Or: Luminous Cathode Sensitivity (Not Recommended)

Photocathode Values Energy Loss given by Escape Depth Most materials η(λ): 0. 1% Semiconductors η(λ): 10%-30% Negative Electron Affinity Metals η(λ): ≤ 80%

Electron Optical Input System Two electrodes guide electrons to first dynode using an electric field Focusing electrode on the sides of the PMT Accelerating electrode by first dynode Two requirements: 1) As efficient as possible 2) Uniform time from cathode to dynode

Electron Multiplier Section Secondary emission electrodes (dynodes) Each has secondary emission factor δ Like photocathode, but with incident electrons and E-field Dynode material requirements: 1) High δ 2) Stability of emission even with current 3) Low thermionic emission § Use 10 -14 stages with total Gain ≈ 10^7

Electron Multiplier Section § Dynode Configurations: a) Venetian Blind b) Box and Grid c) Linear focused d) Side-On Configuration

Electron Multiplier Section e) Microchannel Plate

Electron Multiplier Section: The Single Electron Spectrum Fluctuations created by variable nature of secondary emissions, variations in δ, different electron transit times Plotting many multiplier responses to single electron give total gain fluctuations Linear focused have lower fluctuations Venetian blind have higher fluctuations

Operating Parameters Gain of dynode determined by voltage: Assuming voltage divided equally: Gives Min voltage

Operating Parameters § By minimizing the function for minimum V: We find: Although minimum voltage is ideal for minimal noise, this is not typical due to need for a smaller transit time § Gain vs Supply Voltage:

Voltage Dividers Series of resistors regulate each voltage Variable resistors used for fine adjustment Bleeder current must be much greater than anode current: Bleeder current maintained 100 times anode current for 1% linearity In pulse mode, decoupling capacitors or Zener diodes are used

Voltage Dividers Dynodes high voltage must be negative relative to photocathode If positive, photocathode should be grounded, minimizing noise but also complicating anode setup If negative, anode can be grounded and coupled with other detector electronics, but cathode must then be well insulated

Linearity Current must be transferred entirely from each dynode for proportionality Total current saturation depends on voltage Initial formation of space charge at electrodes is swept away at increased voltage High resistance of photocathode can allow large currents of photoelectrons to change potential; important to use sufficient voltage

Pulse Shape PMT can be considered current generator in parallel with a resistor and capacitor Assuming input is exponentially decaying light:

Pulse Shape Then gives equation of form: Which, solved for V(t), gives solution: For τ<< scintillator decay constant, decay time is accurately produced: Current mode For τ>>scintillator decay constant, amp and decay time both heightened: Voltage mode

Time Response Two main factors affect time resolution : 1) Fluctuations in electron transit time 2) Fluctuations due to statistical noise The electron optical input system accelerates central electrons much faster. Cathode or field can be fixed. Transit time spread: if we have

Noise: Dark Current Dark current arises from: 1) Thermionic emission 2) Leakage currents 3) Radioactive contamination 4) Ionization phenomena 5) Light Phenomena § Thermionic dark current noise given by: lowering temperature lowers thermal noise

Noise: Dark Currents Leakage currents lowered by a reduced atmosphere Only small current from radioactive materials Gas ions can be accelerated toward dynodes, also small amount of current (Afterpulsing) Dark currents create no more than a few nanoamperes

Noise: Statistical Number of photoelectrons and secondary electrons fluctuate with time: shot noise or Schottky Effect Physical limit of photocathode determines fluctuations in emitted photoelectrons For PMT under constant illumination, rms deviation emitted photoelectrons given by: Extent of total deviation best measured by single electron spectrum

Environmental Factors: Light and B-Fields Ambient light, even without high voltage, increases dark current over time Magnetic fields interfere with Anode current and path of electrons in electron optical input section Have least influence when oriented parallel to axis of PMT, and PMT is shielded with mu-metal and iron screen

Environmental Factors: Temperature Small contribution to dark current Cathode sensitivity: variation of 0. 5%/degree between 25 and 50 degrees Surface materials of dynodes can be affected and can vary gain by a few tenths of a percent per degree Kelvin, although varies between PMTs

Gain Stability Two types of gain change: 1) Drift- Variation over time under constant illumination level 2) Shift- Sudden current shift drastically changes gain § Several methods of measuring PM output peaks from the same source at different time frequencies can be used to find drift and shift