With the continuous expansion of the application fields of photomultiplier tubes (PMTs), the requirements for PMTs are becoming increasingly strict. This requires a large number of parameters to characterize its unique performance. A type of PMT cannot perform at its best for any application. Therefore, understanding the characteristics of PMT is necessary to more accurately select the product model that is suitable for its application.

Basic performance

For PMT, the spectral response range, anode and cathode sensitivity, and dark current (noise) are all basic properties of photomultiplier tubes. The spectral response range can be understood as the range in which PMT can detect the wavelength of light signals. For example, a day blind photomultiplier tube can detect light signals in the ultraviolet band, a double alkali photomultiplier tube can detect light signals in the visible band, and a multi alkali photomultiplier tube can detect light signals in the infrared band or even a wider range. (For more information, please refer to "Fundamentals of Photomultiplier Tube Part 3: Basic Selection Methods")


Figure 1. Spectral response range

Light sensitivity is a characteristic that reflects the sensitivity of a photomultiplier tube to light signals under a specific light source and high voltage. Cathode sensitivity characterizes the strength of the photoelectric surface's ability to convert light signals into photoelectrons; The anode sensitivity reflects the strength of the output signal ability of the doubled photoelectrons when they reach the anode. The ratio of anode sensitivity to cathode sensitivity is the amplification factor (also known as gain) of a photomultiplier tube.
Dark current refers to the current value output by PMT in the absence of light incidence. The size of general dark current (noise) is not only related to the product itself, but also closely related to the temperature, humidity, high voltage, and light avoidance during the use of PMT.
Therefore, for each photomultiplier tube, sensitivity, gain, and dark current are all related to the operating voltage of the PMT.

Time characteristics

A photomultiplier tube is a photoelectric detection device with a very fast time response. The time characteristics are mainly determined by the PMT doubling structure. It is also related to the working voltage. If the high voltage of the power supply is increased to enhance the electric field strength and accelerate the electronic flight speed, it can also achieve the effect of shortening the PMT time response.
The time characteristics of PMT are usually characterized by three indicators: rise time, transition time, and dispersion of transition time:
(1) The rise time is the time when the output pulse height value reaches 90% from 10%;
(2) The transition time is the time from the incident light entering the photocathode to the appearance of the output pulse;
(3) Transit time dispersion refers to the fluctuation of the transit time of all individual photoelectron pulses when irradiating the photocathode surface.


Figure 2. Electron transit time

Uniformity

The sensitivity of a photomultiplier tube will vary with the illuminated position of the photocathode. The inconsistency of photoelectric surface sensitivity is usually characterized by cathode uniformity. In the application process, whether using a point light source or a surface light source, the light signal is usually focused on the geometric center of the photoelectric surface. Generally speaking, the uniformity of end window PMT is better than that of side window PMT.


Figure 3. Uniformity

Energy resolution

In the application of scintillation counting method, energy resolution is the ability of PMT to distinguish different energy peaks. Energy resolution can be approximately understood as the minimum accuracy of resolution. The smaller the energy resolution, the stronger the ability to distinguish different energy peaks.


Figure 4. Energy resolution, which is the ratio of the peak's half width a to the peak amplitude b, expressed as a percentage.

The factors that affect energy resolution are not only related to the collection efficiency and quantum efficiency of the photocathode of the photomultiplier tube, but also closely related to the luminous efficiency and intrinsic resolution of the scintillator. Therefore, when mentioning the energy resolution of PMT, it is important to clearly specify the scintillator and radiation source type that will be used for testing.

Stability and lifespan

The output variation characteristics of photomultiplier tubes over time are called drift characteristics or lifetime characteristics. The phenomenon of deterioration caused by direct effects such as voltage, current, and temperature is called fatigue.
Stability can also be understood as the short-term variation characteristics of PMT, which refers to the degree of stability of the output under working conditions. Normally, a photomultiplier tube can obtain a relatively stable signal output after 30 minutes of operation. Therefore, for applications with high stability requirements or when comparing test data, "30 minute preheating" is very necessary. (Tips: If PMT needs to continue working after a short power outage, there is no need to preheat again.)


Figure 5. Example of CR332 stability test data

The output variation characteristics of PMT over a long period of time from 103 to 104 hours are called lifetime characteristics. The service life of PMT is directly related to the detection light intensity and high voltage. According to long-term continuous testing data, under extreme usage conditions (ambient temperature: 25 ℃, working voltage of 1000V, anode output current of 100 μ A), the service life of conventional PMT exceeds 1000 hours. If the output current is much less than 100 μ A in actual application, or if it does not work continuously for a long time, the service life of PMT can be greatly extended.


Figure 6. Long term variation characteristics (lifespan)

There are no two identical leaves in the world, nor are there two photomultiplier tubes with identical performance. Therefore, a deep understanding of the working principle and performance of photomultiplier tubes is necessary to transform their individuality into commonality.