techniques for nuclear and particle physics experiments by w.r. leo chapter eight:
TRANSCRIPT
The Photomultiplier Tube (PMT)
Techniques for Nuclear and Particle Physics Experiments
By W.R. Leo Chapter Eight:
Basic PMT Structure
1. Photocathode2. Electron Optical Input System3. Electron Multiplier4. Anode
The Photocathode
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 possible2) 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 δ2) Stability of emission even
with current3) Low thermionic emission
Use 10-14 stages with total Gain ≈ 10^7
Use negative electron affinity metals
Electron Multiplier Section
Dynode Configurations:a) Venetian Blindb) Box and Gridc) Linear focusedd) 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 DividersSeries of resistors regulate each
voltageVariable resistors used for fine
adjustmentBleeder 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 ShapeThen 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 ResponseTwo main factors affect time
resolution :1) Fluctuations in electron transit time2) 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 CurrentDark current arises from:
1) Thermionic emission2) Leakage currents3) Radioactive contamination4) Ionization phenomena5) 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: StatisticalNumber 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 currentCathode 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 level2) 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