Click to Enlarge
Click for Logarithmic Plot
This graph shows typical radiant sensitivity curves for our PMT modules. The PMT2101(/M) and PMT2102 provide the highest radiant sensitivity, but are also the most susceptible to degradation from high intensity illumination. The PMM01, PMM02, PMT1001(/M), PMT1002, PMT2101(/M) and PMT2102 feature built-in transimpedance amplifiers for simpler integration with setups. The PMM01, PMM02, PMT1001(/M), and PMT2101(/M) also offer internal SM1 threads and four 4-40 taps for direct compatibility with our SM1 lens tubes and 30 mm cage system.

Features

• Bialkali, Multialkali, and GaAsP Detectors for Wavelengths from 185 nm to 920 nm
• Typical Radiant Sensitivities from 51 mA/W to >176 mA/W (See Graph to the Right)
• Various Mounting Features (See Selection Guide Tab)
  
  • SM1 (1.035"-40) or C-Mount (1.00"-32) Threads for Lens Tubes or Filters
  • 8-32 or 1/4"-20 (M6) Tapped Holes for Post Mounting
  • 1/4" (M6) Counterbored Slots for Table Mounting
  • Four 4-40 Taps for 30 mm Cage System
• Some Models Offer Built-In Transimpedance Amplifiers
• PMTSS2 Modules are Designed for Building Expandable Multi-Channel Imaging Setups
• Control Software for 64-Bit Windows^® 7 or 10 (See Software Tab)

Photomultiplier tubes (PMTs) are used to detect faint optical signals from weakly emitting sources. Compared to avalanche photodetectors (APDs), they offer significantly larger active areas, making them ideal for capturing signals that may be diverging due to scatter or nonlinear optical effects. These PMTs are controlled with an included software package designed for use with 64-Bit Windows^® 7 or 10, which provides a GUI for adjusting gain, output offset voltage, and bandwidth. This software package includes .NET, C++, and LabVIEW drivers. 

Thorlabs offers PMTs utilizing alkali or GaAsP detector elements, which offer sensitivity in the UV, visible, and NIR. Their performance can be quantified by their quantum efficiency (in %), which describes how many incident photons are converted to photoelectrons. The quantum efficiency is closely related to the radiant sensitivity (in mA/W), as shown in the equation in footnote b below. Radiant sensitivity curves for Thorlabs' PMTs are given in the graph to the right.

In order to generate high internal gain, the electrons initially generated at the PMT's photocathode are accelerated by a high-voltage potential, on the order of kV. This potential causes the electrons to strike several dynodes that generate secondary electrons. The photocurrent that has accumulated after the final dynode stage represents the amplified signal and is collected by an anode. (See the PMT Tutorial tab for more details.) All of our PMT modules feature built-in high-voltage circuitry that eliminates the need for a standalone high-voltage source, reducing the package's footprint and risk of electric shock.

The photocurrent collected at the anode is typically sent into a transimpedance amplifier that converts the current into a voltage and provides additional amplification. Our PMM01, PMM02, PMT1001(/M), PMT1002, PMT2101(/M), and PMT2102 modules include a built-in transimpedance amplifier, allowing them to be connected directly to lab equipment that requires voltage signals. In contrast, the PMTSS, PMTSS2, and PMTSS2-SCM have no built-in amplifier. They are compatible with our TIA60 Transimpedance Amplifier, as well as other transimpedance amplifiers, which are available separately.

PMT Comparisona	Standalone					Multi-Channel
Item #	PMM01	PMM02	PMTSS	PMT1001(/M) PMT1002	PMT2101(/M) PMT2102	PMTSS2 PMTSS2-SCM
Photocathode Material	Bialkali	Multialkali	Multialkali	Multialkali	GaAsP	Multialkali
Wavelength Range	280 - 630 nm	300 - 800 nm	185 - 900 nm	230 - 920 nm	300 - 720 nm	185 - 900 nm
Radiant Sensitivityb (Typ.)	80 mA/W (at 400 nm)	51 mA/W (at 420 nm)	105 mA/W (at 450 nm)	78 mA/W (at 630 nm)	176 mA/W (at 550 nm)	105 mA/W (at 450 nm)
Photocathode Active Area	Ø22 mm	Ø21 mm	3.7 mm x 13.0 mm (H x V)	Ø8 mm	Ø5 mm	3.7 mm x 13.0 mm (H x V)
Transimpedance Amplifier			Not Included (Compatible with TIA60)			Not Included (Compatible with TIA60)
Complete specifications are available in the Specs tab. The graph above compares the radiant sensitivity curves of all our PMTs. Radiant sensitivity (RS) is related to quantum efficiency (QE) by the following expression:

Standalone Alkali PMTs

Item #	PMM01	PMM02	PMTSS	PMT1001(/M)	PMT1002
Detector Specifications
Wavelength Range	280 - 630 nm	300 - 800 nm	185 - 900 nm	230 - 920 nm
Peak Wavelength (λp)	400 nm	420 nm	450 nm	630 nm
Radiant Sensitivity at λpa (Typ.)	80 mA/W	51 mA/W	105 mA/W	78 mA/W
Quantum Efficiency at λpa (Calculated from Radiant Sensitivity)	25%	15%	>28%	>15%
PMT Gain (Max)	7.1 x 106	5.1 x 105	>1.0 x 107 See Graphs Tab for PMT Gain vs. Control Voltage	>3.0 × 106 See Graphs Tab for PMT Gain vs. Control Voltage
HV Control Voltage (Max)	0 V to +1.8 Vb	0 V to +1.25 Vb	+0.25 V to +1.00 V (Recommended) +0.25 V to +1.20 V (Maxc)	+0.50 V to +1.10 V Software Controlled
Control Voltage Connector	2.5 mm Mono Jack		M8 x 1 Power Connector	USB Mini
PMT Voltage	0 V to -1800 Vb,d	0 V to -1250 Vb,d	+250 V to +1000 V (Recommended) +250 V to +1200 V (Maxc)	+500 V to +1100 V
Photocathode Active Area	Ø22 mm	Ø21 mm	3.7 mm x 13.0 mm (H x V)	Ø8 mm
Dark Currente	0.3 - 3 nA (at 20 °C)	3 nA (Typ.) 20 nA (Max) (After 30-Minute Storage in Darkness)	2 nA (Typ.) 10 nA (Max)	10 nA (Typ.) 100 nA (Max)
Dark Count Ratee	100 s-1 (at 20 °C)	-	-	-
Warm-Up Time Before Applying Control Voltagee	<10 s		30 to 60 Minutes	30 to 60 Minutes
Anode Currentf	100 µA (Max)g		10 µAh	100 µAh
Rise and Fall Time	15 µs		1.4 ns	0.57 ns (Rise)i
Photocathode Type	Bialkali	Multialkali	Multialkali	Multialkali
Photocathode Geometry	Head On		Side On	Head On
Window	Borosilicate, Plano-Concave (n = 1.49)		UV-Transmitting Glass (n = 1.48)	Borosilicate, Flat Window (n = 1.487)
Transimpedance Amplifier Specifications
Transimpedance Gain	High Z: 1 x 106 V/A 50 Ω: 5 x 105 V/A		No Amplifier Included	11000 +1000 / -500 V/A
Amplifier Bandwidth (at 6 dB)j	DC to 20 kHz		N/A	Software Configurable DC to 80 MHz, 2.5 MHz, or 250 kHz
Amplifier Noise (Typ.)	2 mV (RMS)		N/A	5.8 pA / √Hz (Total Input Noise at DC to 80 MHz Bandwidth) 6.5 pA / √Hz (Input Current Noise at 1 MHz, Cin = 4 pF)
Amplifier Offset (Typ.)	1 mV		N/A	±103 µV / °Ck
Output Signall
Output Voltage	0 - 10 V (High Z) 0 - 5 V (50 Ω)		N/A	±1.5 V (50 Ω)
Output Current	N/A		10 µA (Max)	N/A
Connector	SMA		BNC	SMA
Physical Specifications
Module Dimensions	3.65" x 1.60" x 2.46" (92.8 mm x 40.6 mm x 62.5 mm)		5.20" x 1.26" x 2.50" (132.0 x 32.1 x 63.5 mm)	3.43” x 1.60” x 2.10” (87.2 mm x 40.6 mm x 53.5 mm)	3.32" x 1.35" x 1.95" (84.4 mm x 34.3 mm x 49.6 mm)
Power Input	+12 V Pin: 40 mA Max, +12 V to +15 V -12 V Pin: 10 mA Max, -12 V to -15 V		15 VDC, 7 mA Max	5 VDC +4.5 V to +5.5 V 350 mA Typ., 500 mA Max
Included Power Supply	±12 VDC (100/120/230 VAC, 50 or 60 Hz, Switchable)m		-	-
Operating Temperature	5 to 55 °C		15 to 40 °C	5 to 50 °C
Storage Temperature	-40 to 55 °C		-20 to 50 °C	-20 to 50 °C
Module Weight	200 g (0.44 lbs)	200 g (0.44 lbs)	0.3 kg (0.66 lbs)	300 g (0.66 lbs)
Mounting Options
Internal SM1 Threads	(In Front of Window)	(In Front of Window)	-	(In Front of Window)	-
Internal C-Mount Threads	-	-	(In Front of Window)	-	(In Front of Window)
Mounting Holes	Three 8-32 Taps (8-32 to M4 Adapter Included)	Three 8-32 Taps (8-32 to M4 Adapter Included)	-	1/4"-20 (M6) Tap	-
30 mm Cage System	(Four 4-40 Taps)	(Four 4-40 Taps)	-	(Four 4-40 Taps)	-

• Radiant Sensitivity (RS) is related to quantum efficiency (QE) by the following expression:
  
• Do not exceed the maximum output voltage of the PMT.
• Although the PMT may be operated up to a maximum control voltage of +1.20 V, operation beyond the recommended control voltage range may decrease the PMT's operational lifetime.
• This is a calculated value using the following formula: PMT Voltage = -1000 × HV Control.
• The dark current rate and dark count specifications are valid when the PMT has been turned on in the dark, no control voltage has been applied during the specified warm-up time, and no signal has been incident during the specified warm-up time.
• The PMT must be shielded from ambient light and the control voltage must be carefully chosen so that unexpected signal spikes do not cause the anode current to be exceeded.
• Exceeding the maximum anode current will irreparably damage the PMT.
• Beyond this value, the output signal exhibits nonlinear behavior.
• The fall time is not specified for these PMTs.
• The bandwidth decreases as the output signal magnitude increases.
• DC Voltage Drift
• The output signal should be kept below the maximum to prevent saturation. ND filters can be mounted using the SM1 or C-Mount threads in front of the window to attenuate the optical signal before it reaches the PMT.
• A replacement power supply is available below.

All specifications are valid at 25 °C unless otherwise stated.

GaAsP PMTs

Item #	PMT2101(/M)	PMT2102
Detector Specifications
Wavelength Range	300 - 720 nm
Peak Wavelength (λp)	580 nm
Radiant Sensitivitya (Typ.)	176 mA/W at 550 nm
	108 mA/W at 420 nm
Quantum Efficiencya (Calculated from Radiant Sensitivity)	39% at 550 nm (Typical)b
	32% at 420 nm (Typical)b
PMT Gain (Max)	>1.0 × 106
Control Voltage	+0.50 V to +0.80 V (Recommended) +0.5 V to +1.0 V (Maxc) Software Controlled
Control Voltage Connector	USB Mini
PMT Voltage	+500 V to +800 V (Recommended) +500 V to +1000 V (Max)
Photocathode Active Area	Ø5 mm
Dark Count Rated, e	6000 s-1 (at 25 °C, Typical) 18000 s-1 (at 25 °C, Maximum)
Warm-Up Time Before Applying Control Voltagee	30 to 60 minutes (at 25 °C)
Anode Currentf	500 µA (Max)
Rise Timeg	1.00 ns
Photocathode Type	GaAsP
Photocathode Geometry	Head On
Window	Borosilicate, Flat Window
Transimpedance Amplifier Specifications
Transimpedance Gain	11000 +1000 / -500 V/A
Amplifier Bandwidth (at 6 dB)h,i	Software Configurable DC to 80 MHz, 2.5 MHz, or 250 kHz
Amplifier Noisei (Typ.)	5.8 pA / √Hz (Total Input Noise at DC to 80 MHz Bandwidth) 6.5 pA / √Hz (Input Current Noise at 1 MHz, Cin = 4 pF)
Amplifier DC Offset Drift (Typ.)	±103 µV / °C
Maximum Inputj	±500 μA
Output Signalk
Output Voltage	±1.5 V (50 Ω)
Output Current	N/A
Connector	SMA
Physical Specifications
Module Dimensions	3.43” × 1.60” × 2.10” (87.2 × 40.6 × 53.5 mm)	3.32" × 1.35" × 1.95" (84.4 × 34.3 × 49.6 mm
Power Input	5 VDC (+4.5 V to +5.5 V) 350 mA Typ., 500 mA Max
Operating Temperature	5 to 35 °C
Storage Temperature	-20 to 50 °C
Module Weight	300 g (0.66 lb)
Mounting Options
Internal SM1 Threads	(In Front of Window)	-
Internal C-Mount Threads	-	(In Front of Window)
Mounting Holes	1/4"-20 (M6) Tap	-
30 mm Cage System	(Four 4-40 Taps)	-

• Radiant Sensitivity (RS) is related to quantum efficiency (QE) by the following expression:
  
• The quantum efficiency can vary from PMT to PMT.
• Although the PMT may be operated up to a maximum control voltage of +1.0 V, operation beyond the recommended control voltage range may decrease the PMT's operational lifetime.
• Measured after 30 min of storage in darkness.
• The dark count specification is valid when the PMT has been turned on in the dark, no control voltage has been applied during the specified warm-up time, and no signal has been incident during the specified warm-up time.
• The PMT must be shielded from ambient light and the control voltage must be carefully chosen so that unexpected signal spikes do not cause the anode current to be exceeded. Beyond this value, the output signal exhibits nonlinear behavior.
• Measured at +0.8 V Control Voltage, 25 °C
• The bandwidth decreases as the output signal magnitude increases.
• Amplifier bandwidth and equivalent input current noise are typical values, which depend on the source capacitance. To achieve the best possible bandwidth and noise performance, reduce the source capacitance by using short cables at the input of the amplifier.
• Operation above this specification is likely to permanently damage the amplifier.
• The output signal should be kept below the maximum to prevent saturation. ND filters can be mounted using the SM1 or C-Mount threads in front of the window to attenuate the optical signal before it reaches the PMT.

All specifications are valid at 25 °C unless otherwise stated.

Multi-Channel PMTs

Item #	PMTSS2	PMTSS2-SCM
Detector Specifications
Wavelength Range	185 - 900 nm
Peak Wavelength (λp)	450 nm
Radiant Sensitivity at λpa (Typ.)	105 mA/W
Quantum Efficiency at λpa (Calculated from Radiant Sensitivity)	>28%
PMT Gain (Max)	>1.0 × 107 See Graphs Tab for PMT Gain vs. Control Voltage
Control Voltage	+0.25 V to +1.00 V (Recommended) +0.25 V to +1.20 V (Maxb)
Control Voltage Connector	M8 x 1 Power Connector
PMT Voltage	+250 V to +1000 V (Recommended) +250 V to +1200 V (Maxb)
Photocathode Active Area	3.7 mm × 13.0 mm (H × V)
Dark Currentc	2 nA (Typ.) 10 nA (Max)
Dark Count Ratec	-
Warm-Up Time Before Applying Control Voltagec	30 to 60 Minutes
Anode Currentd	10 µA (Max)
Rise and Fall Time	1.4 ns
Photocathode Type	Multialkali
Photocathode Geometry	Side On
Window	UV-Transmitting Glass (n = 1.48)
Transimpedance Amplifier Specifications
Transimpedance Gain	No Amplifier Included
Amplifier Bandwidth (at 6 dB)e	N/A
Amplifier Noise (Typ.)	N/A
Amplifier Offset (Typ.)	N/A
Output Signalf
Output Voltage	N/A
Output Current	10 µA (Max)
Connector	BNC
Physical Specifications
Module Dimensions	10.18" × 8.09" × 3.40" (258.5 × 205.5 × 86.4 mm)	8.09" × 2.43" × 3.40" (205.5 × 61.8 × 86.4 mm)
Power Input	15 VDC, 7 mA Max
Operating Temperature	15 to 40 °C
Storage Temperature	-20 to 50 °C
Module Weight	1.4 kg (3.08 lb)	0.9 kg (1.98 lb)
Mounting Options
Internal SM1 Threads	(On Input Port and Filter Cube)	(On Input Port and Filter Cube)
Internal C-Mount Threads	(In Front of Windowg)	(In Front of Windowg)
Mounting Holes	1/4" (M6) Counterbored Slots	1/4" (M6) Counterbored Slots
30 mm Cage System	-	-

• Radiant Sensitivity (RS) is related to quantum efficiency (QE) by the following expression:
  
• Although the PMT may be operated up to a maximum control voltage of +1.20 V, operation beyond the recommended control voltage range may decrease the PMT's operational lifetime.
• The dark current rate and dark count specifications are valid when the PMT has been turned on in the dark, no control voltage has been applied during the specified warm-up time, and no signal has been incident during the specified warm-up time.
• The PMT must be shielded from ambient light and the control voltage must be carefully chosen so that unexpected signal spikes do not cause the anode current to be exceeded. Beyond this value, the output signal exhibits nonlinear behavior.
• The bandwidth decreases as the output signal magnitude increases.
• The output signal should be kept below the maximum to prevent saturation. ND filters can be mounted using the SM1 or C-Mount threads in front of the window to attenuate the optical signal before it reaches the PMT.
• These C-Mount threads are not accessible unless the PMT module is detached from the filter cube assembly.

All specifications are valid at 25 °C unless otherwise stated.

Standalone Alkali PMTs

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PMM01 and PMM02 Mechanical Drawing

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PMTSS Mechanical Drawing

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PMT1001(/M) Mechanical Drawing

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PMT1002 Mechanical Drawing

GaAsP PMTs

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PMT2101(/M) Mechanical Drawing

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PMT2102 Mechanical Drawing

PMTs for Multi-Channel Detection

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PMTSS2 Mechanical Drawing


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PMTSS2-SCM Mechanical Drawing

PMM01 and PMM02 Modules

These PMT modules ship with a Y-cable for control voltage and power (6-pin Hirose to 2.5 mm mono jack and 3-pin Lumberg power connector). This cable is pictured in the Shipping List tab.

PMT1001(/M), PMT1002, PMT2101(/M), and PMT2102 Modules

These PMT modules ship with a SMA to SMB cable and a Mini USB to USB A cable. These cables are pictured in the Shipping List tab.

PMTSS, PMTSS2, and PMTSS2-SCM Modules

These PMT modules ship with a male M8 x 1 connector with colored wire leads. This connector is pictured in the Shipping List tab.

Output Signal

BNC Female

Output Current: 10 µA (Max)

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PMM01 and PMM02 Contents

PMM01 and PMM02

Item #'s PMM01 and PMM02 consist of:

• PMT Module
• SM1RR Retaining Ring
• AS4M8E 8-32 to M4 Thread Adapter
• Y-Cable for Control Voltage and Power (6-pin Hirose to 2.5 mm Mono Jack and 3-pin Lumberg
  Power Connector)
• 2.5 mm Mono Plug
• ±12 V Power Supply with Region Specific Power Cord

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PMT1001(/M) Contents

PMT1001(/M)

Item # PMT1001(/M) consists of:

• PMT Module
• USB 2.0 Cable (A to Mini)
• SMA to SMB Cable
• SM1 End Cap

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PMT1002 Contents

PMT1002

Item # PMT1002 consists of:

• PMT Module
• USB 2.0 Cable (A to Mini)
• SMA to SMB Cable
• C-Mount Lens Cap (Replacement Item # CMCP2)

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PMTSS Contents

PMTSS

Item # PMTSS consists of:

• PMT Module
• Male M8 x 1 Connector with Colored Wire Leads
• M8 x 1 Extension Cord
• C-Mount Cap (Replacement Item # CMCP2)

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PMT2101(/M) Contents

PMT2101(/M)

Item # PMT2101(/M) consists of:

• PMT Module
• USB 2.0 Cable (A to Mini)
• SMA to SMB Cable
• SM1 End Cap

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PMT2102 Contents

PMT2102

Item # PMT2102 consists of:

• PMT Module
• USB 2.0 Cable (A to Mini)
• SMA to SMB Cable
• C-Mount Lens Cap (Replacement Item # CMCP2)

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PMTSS2 Contents

PMTSS2

Item # PMTSS2 consists of:

• Two Detachable PMT Modules
• DFM1T1 Filter Cube Top
• Mechanical Housing with SMA905-Terminated Fiber Collimation Assembly and Male Dovetail for
  PMTSS2-SCM Add-On Module
• Two Male M8 x 1 Connectors with Colored Wire Leads
• Two M8 x 1 Extension Cords
• Ø910 µm Core, 0.22 NA, SMA905-Terminated Fiber Patch Cable for 250 - 1200 nm

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PMTSS2-SCM Contents

PMTSS2-SCM

Item # PMTSS2-SCM consists of:

• Detachable PMT Module
• DFM1T1 Filter Cube Top
• Mechanical Housing with Female Dovetail for the PMTSS2 Module and Male Dovetail for Additional
  PMTSS2-SCM Modules
• Male M8 x 1 Connector with Colored Wire Leads
• M8 x 1 Extension Cord

The PMT1001(/M), PMT1002, PMT2101(/M), and PMT2102 standalone PMTs can be controlled using Thorlabs' software. The software download page below offers a link to the GUI interface as well as a LabVIEW™ software development kit (SDK) to allow these PMTs to be controlled using custom imaging software. The most recent firmware for these PMTs is available as well.

Software

Version 4.0 (June 7, 2019)

This software package for the PMT1001(/M), PMT1002, PMT2101(/M), and PMT2102 contains the installation files for the GUI interface, SDK, and most recent firmware. The software is compatible with Windows^® 7 or 10 (64-bit) systems.

Choosing a Photomultiplier Tube for Your Application

Introduction
Since the first commercial photomultiplier tube (PMT) was developed in the early 1940s, it has remained the detector of choice for experiments requiring fast response times and high sensitivity. Today, the PMT is a staple for research in many fields including analytical chemistry, particle physics, medical imaging, industrial process control, astronomy, and atomic and molecular physics. This tutorial provides introductory material for the principle of operation and key specifications to consider when choosing a PMT for a given application. 

Basic Principle of Operation
Photomultiplier Tubes (PMTs) are sensitive, high-gain devices that provide a current output that is proportional to the incident light. The PMT consists of a glass vacuum tube that houses a photoemissive material called a photocathode, 8 - 14 secondary emitting electrodes called dynodes, and a collection electrode called an anode. If a photon with sufficiently high energy (i.e. more energy than the binding energy of the photocathode material) is incident on the photocathode, it is absorbed, and an electron is released in accordance with the photoelectric effect. Since the first dynode is maintained at a higher potential than the cathode (thereby creating a potential difference between these two elements), the ejected electron will accelerate toward the dynode and crash into it, releasing secondary electrons. Typically, 3 - 5 secondary electrons are released during this process. Each of these 3 - 5 electrons is then in turn accelerated toward and crashes into the second dynode, thereby releasing 3 - 5 more electrons. This process continues through the entire dynode chain providing an electron gain of 3 - 5. Typically, each dynode is maintained at a potential that is 100 - 200 V higher than the previous one. At the end of the dynode chain, the electrons are collected by the anode and a current pulse is output. However, to read that pulse, the current usually needs to be converted to a voltage; the simplest way to do this is to connect a low load resistance across the anode and ground. Thorlabs' PMM01 and PMM02 modules use a transimpedance amplifier (TIA) to convert the nA- or µA-scale current output by the anode to a voltage in the mV or V range, respectively. In comparison, our PMTSS, PMTSS2, and PMTSS2-SCM modules do not include a TIA.

For example, if a PMT consists of 8 dynodes as shown in the figure below and each electron is able to produce 4 secondary electrons, the total current amplification after traveling through the dynode chain will be 4⁸ ≈ 66,000. Each photoelectron for this example PMT produces a charge avalanche at the anode of Q = 4⁸e. The corresponding voltage pulse is V = Q/C = 4⁸e /C where C is the capacitance of the anode (including connections). If the capacitance is 5 pF, the output voltage pulse will be 2.1 mV.

Spectral Response
When choosing a PMT for a given application, the photocathode material should be matched to the intended application. Generally, the long-wavelength cutoff is determined by the photocathode, while the window material determines the short-wavelength cutoff. PMTs are manufactured for wavelengths from the deep UV through the infrared. However, since the photocathode is responsible for converting incident photons into electrons, the efficiency with which it does this for the wavelength of interest is of utmost importance. There are a variety of materials used for photocathodes, each with a different work function and each intended for use in a different spectral range.

Quantum Efficiency (QE) is a specification that is usually expressed as a percentage and is associated with the PMTs ability to convert incident photons into detectable electrons. For instance, a QE value of 20% means that one in every five photons that strike the photocathode will produce a photoelectron. For photon counting, it is desirable to have a PMT with a high QE value. Since QE is dependent upon wavelength, it is important to choose a PMT with the best quantum efficiency over the wavelength range of interest. It should be noted that photocathodes for the visible portion of the electromagnetic spectrum typically have QE values that are less than 30%.

The QE of a PMT can be quickly calculated from its spectral response plot (see the Graphs tab) by using the following equation:

where S is the radiant sensitivity in units of A/W and λ is the wavelength in nm.

Geometries
PMTs are available primarily with two different geometries: head-on (i.e. the photocathode is located at either end of the vacuum tube) and side-on (i.e. the photocathode element is located on the side of the vacuum tube). Head-on PMTs have semitransparent photocathodes and are characterized by large collection surfaces, better spatial uniformity, and better performance in the blue and green spectral regions. For applications requiring a wide spectral response, such as spectroscopy, the head-on geometry is preferable. In contrast, side-on PMTs have opaque photocathodes and are preferable for applications in the UV and IR. This configuration tends to be less expensive than head-on and is widely used in spectrometers and for applications requiring efficient optical coupling and high QE such as scintillation counting.

The 8 - 14 secondary emitting electrodes (i.e., dynodes) are often arranged in one of two configurations: linear or circular. Linear dynode arrays (such as the one shown in the figure above) are popular due to their fast time response, good time resolution, and excellent pulse linearity. The circular cage-type array is found on all side-on PMTs and some head-on PMTs. This configuration is compact and offers fast response times.

Gain
PMTs are unique because they are capable of amplifying very weak signals produced by photocathodes to detectable levels above the readout circuitry noise without introducing substantial noise. In a PMT, the dynodes are responsible for producing this amplification, which is referred to as gain. Gain is highly dependent on the voltage being applied. PMTs can operate well above the manufacturer's stated high voltage recommendation, yielding gains that are 10 - 100 times above spec; this generally has no detrimental affects to the PMT if the anode current is kept well below the rated value.

Dark Current
Ideally, all of the signal produced by a photocathode would be due to current generated by light incident on the tube. However, in reality, PMTs will produce currents regardless of whether light is present. The signal that results in the absence of light is known as dark current, and it effectively degrades the signal-to-noise ratio of the PMT. Dark current is due mainly to the thermionic emission of electrons from the photocathode and first few dynodes but with far smaller contributions from cosmic rays and radioactive decay. In general, tubes designed for use in the red part of the spectrum will exhibit more dark current than others due to the lower binding energy of red-sensitive photocathodes. If it is assumed that the primary source of dark current is thermionic emission from the photocathode, the dark count rate is given by:

Since thermionic emission depends highly on the photocathode's temperature and work function, cooling a PMT will greatly reduce dark current counts. Offered in configurations with our Confocal and Bergamo microscope systems, a PMT equipped with a thermoelectric cooler can be cooled from 20 °C to 0 °C, reducing the dark current by a factor of ~10. When using a thermoelectric cooler, care should be taken to avoid condensation at the window since this moisture will reduce the amount of light incident on the photocathode. In addition, excessive cooling should be avoided, as it can actually have adverse effects. These effects include signal reduction or voltage drops across the cathode, since the resistance of the cathode film is inversely proportional to the temperature.

Rise Time
For experiments demanding high time resolution, short rise times are a must. Anode pulse rise time is the most commonly specified time response characteristic for a PMT and is defined as the time required for the output of the PMT to rise from 10% to 90% of its peak amplitude when the photocathode is fully illuminated. Our PMTs' rise times are listed in the Specs tab.

Ultimately, the pulse rise time is determined by the spread in transit times for the different electrons. These times vary for several reasons. First, the initial velocities of secondary electrons will vary because they are released from different depths within the dynode material. Some electrons will have no initial energy when leaving the dynode whereas other will have a non-zero initial energy; hence, the latter arrive at the next dynode in a shorter time period. In addition to the variation in initial ejected electron speed, transit time spread is also caused by electron path length variations. Due to these effects, the rise time of an anode pulse will decrease with increasing voltage as V^-1/2.

Other Considerations
There are several other important considerations. First, choose the electronics that will be used with the PMT carefully. Small changes in the high voltage applied across the cathode and anode can dramatically change the output. Second, the lab environment can also affect the performance of the PMT. Changes in temperature and humidity as well as the presence of vibrations all negatively affect tube operation. Finally, the tube's housing is of importance; not only does it shield the tube from external and extraneous light, but it can also reduce the effects of external magnetic fields. Magnetic fields of a few gauss can greatly reduce the gain, but these adverse affects can be minimized by creating a magnetic shield from a high permeability material.