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