Apr-2025

A comparative analysis of vacuum PMT and SiPM

Radiometric measurements have become indispensable tools across various industrial applications, serving critical roles as level, density, and concentration measurements.

Bernhard Kreft
Berthold Technologies

Article Summary

The fundamental setup of a radiometric measurement, comprising a gamma radiation source, typically Cs-137 or Co-60 and a radiation detector, is shown in Figure 1. These components are normally mounted on opposite sides of a vessel or a pipe. The gamma count rate (usually provided as counts per second, cps) measured by the detector is directly proportional to the absorption of gamma radiation by the product inside the vessel, hence, the measured process value is dependent on the level of the product between source and detector as well as its density.

The detectors employed in radiometric measurements can differ widely in construction, physical properties, and performance capabilities. One prevailing measurement principle that is widely adopted in industry is the scintillation technology. This principle encapsulates a threefold process involving a scintillator, a photomultiplier, and processing electronics as primary components as shown in Figure 2.

The scintillator is responsible for converting incident gamma radiation into visible light flashes, while a photomultiplier is employed for detecting the light signals and transforming them into electrical signals.

For the last step of the signal generation, processing electronics are used that facilitate further amplification and smoothing.
Within the area of industrial radiometric measurements, two photomultiplying techniques have taken center stage: vacuum photomultiplier tubes (vacuum PMT), representing the current industrial standard, and silicon photomultipliers (SiPM), which have been on the market for approximately two decades. This article aims to explain distinctions between these two techniques, shedding light on their respective advantages and applications.

Vacuum-Photomultiplier Tubes (vacuum PMT)
A vacuum photomultiplier (Figure 3), the standard for industrial radiometric applications, is designed to amplify weak light signals through a cascade of electron multiplication within an evacuated glass tube. Its key components include the photocathode, focusing electrode, dynodes, and anode, each playing a vital role in the photomultiplier‘s function.

Photocathode
The process begins with the photocathode, a photosensitive material typically made of cesiumantimony or other similar compounds. When exposed to incident photons, the photocathode releases electrons through the photoelectric effect. These photoelectrons are the initial carriers of the incoming light signal.

Focusing Electrode
Following the release of photoelectrons, a focusing electrode strategically placed within the photomultiplier helps direct and concentrate these electrons towards the next component, the dynodes. This electrode ensures that the electrons maintain a controlled trajectory, enhancing the efficiency of the entire multiplication process.

Dynodes
The dynodes are a series of metal electrodes arranged in a cascading fashion within the evacuated glass tube. Each dynode is biased at a slightly higher voltage than the previous one. As the photoelectrons approach the first dynode, they undergo a process of secondary electron emission, releasing additional electrons. These secondary electrons are then accelerated towards the next dynode, resulting in a continuous cascade of electron multiplication. The cascade effect occurs through a combination of electron multiplication mechanisms, such as secondary emission and electron impact ionisation. This process ensures that the original signal is significantly amplified as it progresses through the dynode chain.

Anode
At the end of the dynode chain, the now-amplified electron signal reaches the anode. The anode is maintained at a high positive voltage relative to the dynodes, creating an electrostatic field that accelerates the electrons towards it. As the electrons strike the anode, they generate a measurable current, providing an output signal that is proportional to the intensity of the incoming light.

In summary, the vacuum photomultiplier efficiently converts weak light signals into measurable electrical currents, making it an indispensable tool for industrial radiometry and various other applications, including spectroscopy, medical imaging, or even detecting of particles in high energy physics.¹

Silicon-Photomultiplier (SiPM)
The silicon photomultiplier (SiPM) is a cutting-edge device that revolutionises light detection by employing an array of avalanche photodiodes (APDs) organised in microcells. This innovative design provides excellent photon sensitivity and single-photon resolution, making SiPMs valuable in various applications, from medical imaging to high-energy physics.²

Avalanche Photo Diode (APD)
The fundamental building block of a silicon photomultiplier is the avalanche photodiode. A schematic representation of such an APD is shown in Figure 4 on the left side. These diodes can undergo avalanche breakdown when exposed to incident photons, leading to the generation of a significant number of charge carriers. The APD is created through the process of diode doping, where the semiconductor materials are intentionally impregnated with certain dopants to alter their electrical properties. As a result of this doping, the APD establishes a robust electrical field within its structure. This field is crucial for the functioning of the device and is spatially differentiated into an absorption and an adjacent multiplication zone. When a photon strikes the APD, it is absorbed by the silicon material in the absorption zone. The energy from the absorbed photon triggers the initial charge carrier generation. The released photoelectrons undergo an avalanche multiplication process within the multiplication zone. Due to the high electric field across the diode, the charge carriers gain sufficient energy to ionise other atoms in the silicon lattice, leading to a cascade effect known as avalanche breakdown. This process results in the exponential amplification of charge carriers, creating a detectable signal.

To preserve the energy information, SiPMs consist of an array of microcells as shown in Figure 5, each containing an individual avalanche photodiode. These microcells are typically very small, often on the order of tens of micrometers, allowing for high-density packing of APDs on a single silicon substrate. The small size also minimises the probability of simultaneous events on one microcell. To ensure the SiPM operates within a controlled range, a quenching resistor is employed. This resistor helps limit the duration of the avalanche breakdown, preventing excessive charge buildup and ensuring a rapid reset of the microcell for subsequent photon detections. The microcell arrangement enables the SiPM to achieve high photon detection efficiency and excellent temporal resolution. The individual signals from each microcell are read out and processed. The output is a digital signal proportional to the number of photons detected. The high density of microcells allows SiPMs to provide excellent spatial resolution and sensitivity, making them particularly useful in applications demanding precise detection of low-intensity light signals. The signals from all the microcells are summed, providing a collective output that corresponds to the total photon flux incident on the SiPM. This summation process allows SiPMs to operate over a wide dynamic range, accommodating both low and high-intensity light conditions.