Download TechniquesChap8

Techniques for Nuclear and Particle Physics Experiments
By W.R. Leo
Chapter Eight:
1.
2.
3.
4.
Photocathode
Electron Optical Input System
Electron Multiplier
Anode
Photocathode converts incident photons to
photoelectrons
Emitted electron energy given by Einstein’s
photoelectric affect:
Must reach minimum frequency for equation to
be applicable
1. Quantum Efficiency:
2.Radiant Cathode Sensitivity:
Or:
For Units in Amperes/Watts
Or: Luminous Cathode Sensitivity
(Not Recommended)
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Energy Loss given by Escape Depth
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Most materials η(λ): 0.1%
Semiconductors η(λ): 10%-30%
Negative Electron Affinity Metals η(λ): ≤80%
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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
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
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 Dynode Configurations:
a) Venetian Blind
b) Box and Grid
c) Linear focused
d) Side-On Configuration
e) Microchannel Plate
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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
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Gain of dynode determined by voltage:
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Assuming voltage divided equally:
Gives Min voltage
 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:
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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
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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
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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
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PMT can be considered current generator in
parallel with a resistor and capacitor
Assuming input is exponentially decaying
light:
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Then gives equation of form:
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Which, solved for V(t), gives solution:
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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
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Two main factors affect time resolution :
1) Fluctuations in electron transit time
2) Fluctuations due to statistical noise
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The electron optical input system accelerates
central electrons much faster. Cathode or
field can be fixed.
Transit time spread: if we have
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:
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lowering temperature lowers thermal noise
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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
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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
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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
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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
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
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