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Foundations of Photodetection
Photomultiplier Tubes (PMTs) have been a key photodetection technology for decades. Despite the advent of silicon photomultipliers (SiPMs), which offer advantages in certain applications, PMTs remain indispensable in scenarios requiring high speed, low noise, and high gain. Their continued relevance is shown by their critical role in neutrino physics experiments, such as Super-Kamiokande, where PMTs provide large photosensitive areas with minimal noise — an advantage over any alternative photodetectors.
Advantages of Photomultiplier Tubes
PMTs are known for their incredible versatility. Their adaptability arises from a wide range of available configurations, including various shapes, sizes, spectral response ranges, dynode structures, and readout methods. This allows them to be utilized in many applications ranging from medical devices to environmental monitors to particle physics experiments.
Hamamatsu Photonics has been manufacturing phototubes for over 70 years. During this time, the company has pushed the boundaries of light detection by enhancing PMT technology in key areas:
- Spectral response range: Devices sensitive from the vacuum ultraviolet (VUV) to the near-infrared (NIR) regions have been developed through the use of numerous photocathodes and window materials.
- Size: From compact, TO-8 package devices to 20” tubes, Hamamatsu offers PMTs with a photosensitive area in a shape and size to suit a variety of application requirements.
- Gain structure: Hamamatsu has developed PMTs with various gain structures, including classic dynodes (such as box-and-grid and circular-stage types), metal channels, microchannel plates (MCPs), and semiconductors (avalanche diodes). Each offers its own unique advantage to light detection.
- Accessories: A suite of accessories, including high-voltage power supplies, housings, and plug-and-play modules, supports PMT integration into various systems.
Hamamatsu experts in the PMT manufacturing process. It can be done by hand so that even a single product can be tailored according to the request.
Hamamatsu’s experience and constant engagement in the research and development of PMT technology are the reasons that we continue to be the world-leading manufacturer of these devices.
Essential Components for PMT Operation
At its core, a PMT consists of a vacuum tube with an input window and photocathode for light collection, a gain structure for the multiplication of the photoelectrons, and an anode to collect the photoelectrons generated. Several accessories enhance PMT functionality:
Key Accessories
1) High Voltage Power Supply (HVPS):
- Required to achieve electron multiplication, a high voltage (typically between 500 and 2000 volts) across the gain structure (i.e., dynodes, metal channels, MCP, etc.).
- A stable HVPS is critical as the applied voltage affects the gain and signal output of the device.
- Hamamatsu offers several HVPS options, ranging from benchtop to board-level devices, with a variety of voltage outputs.
2) Amplifier:
- Converts high-impedance current signals from the PMT into low-impedance voltage signals.
- Hamamatsu amplifiers offer excellent output linearity and a range of bandwidths and current-to-voltage conversion factors.
3) Voltage Divider Circuits:
- Used with dynode structures to distribute HVPS output to each electrode.
- Hamamatsu provides socket assemblies with a built-in voltage divider circuit and optional socket assemblies that include an amplifier and/or HVPS:
i. D-type: built-in voltage divider circuit
ii. DA-type: built-in voltage divider circuit + amplifier
iii. DP-type: built-in voltage divider circuit + HVPS
iv. DAP-type: built-in voltage divider circuit + amplifier + HVPS
4) Thermoelectric Coolers:
- Reduce thermal noise by lowering PMT temperature, improving the signal-to-noise ratio (SNR).
5) Magnetic shielding:
- Protects PMTs from external magnetic fields that can cause changes in the PMT output.
PMT Modules
PMT modules offer simple, “plug-and-play” PMT options. They integrate essential components such as the PMT, HVPS, and voltage divider circuit, requiring only a low-voltage input (e.g., 5 or 15 volts).
PMT composition
Fig.1: Block diagram of a photomultiplier module
Hamamatsu also offers modules with additional features:
- Voltage Output Modules: Include an integrated amplifier for easy signal processing.
- Photon Counting Heads: Include an integrated photon counting circuit, which produces a digital output signal. This signal can be measured by an external counter for photon counting measurements in very low-light-level applications.
- Gated PMT Modules: Feature an electrical gate function (electronic shutter) for timed control over when the PMT is sensitive to light. These are useful for eliminating signals from excitation light or for high-speed time-resolved measurements.
- Cooler-Integrated Modules: Reduce noise and improve SNR.
Images of various PMT types
PMT accessories
Comparison of PMT Technologies
While the standard PMT with dynode structures remains widely used, alternative designs such as Microchannel plate PMTs (MCP-PMTs) and hybrid photodetectors (HPDs) offer unique benefits:
| Key benefits | Main applications | More information | |
|---|---|---|---|
| MCP-PMTs | Utilize microchannel plates for electron multiplication, providing robustness in magnetic fields and superior timing response characteristics (rise time, fall time, transit time spread). | Particularly useful in calorimetry and time-of-flight (TOF) experiments. | Microchannel plate PMTs (MCP-PMT) |
| HPDs | Use a unique approach for signal amplification. They have a better S/N ratio in the case of very weak light detection. | Advantageous in laser microscopy, time-correlated single-photon counting (TCSPC), LiDAR, and High Energy Physics applications in general. | Hybrid photodetectors (HPDs) |
| PMTs | Retain superiority in specific applications. Custom solutions, though costly, will meet specialized needs. | Dark matter detection. Lower radioactivity value applications or oil-well logging (high-temperature PMTs). | Photomultiplier tubes (PMTs) |
Future Directions
Hamamatsu is committed to advancing PMT technology to address emerging challenges and applications. Current development priorities include:
- Enhancing the lifetime characteristics of MCP-PMTs.
- Reducing operating voltages and expanding module options for HPDs.
By continually innovating and refining its products, Hamamatsu ensures PMTs remain at the forefront of photodetection technology, meeting the needs of both current and future applications.
Further readings and resources
Hamamatsu Photonics, Photomultiplier Tubes: Basics and Applications, 4th ed., Hamamatsu Photonics K.K., Japan, 2017. [Online]. Available: https://www.hamamatsu.com/content/dam/hamamatsu-photonics/sites/documents/99_SALES_LIBRARY/etd/PMT_handbook_v4E.pdf
Hamamatsu Photonics, PMT Selection Guide, Hamamatsu Photonics K.K., Japan, 2021. [Online]. Available: https://www.hamamatsu.com/content/dam/hamamatsu-photonics/sites/documents/99_SALES_LIBRARY/etd/PMT_TPMZ0002E.pdf
Hamamatsu Photonics, PMT Modules Selection Guide, Hamamatsu Photonics K.K., Japan, 2021. [Online]. Available: https://www.hamamatsu.com/content/dam/hamamatsu-photonics/sites/documents/99_SALES_LIBRARY/etd/PMTmodules_TPMO1113E.pdf
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