Photomultiplier tube (PMT) is an extremely sensitive photodetector widely used in scientific research and industrial fields to detect weak light signals. However, in practical use, there may be a problem of continuous decrease in photon quantity. The following are some possible reasons that may lead to a decrease in photon quantity in photomultiplier tubes, and corresponding solutions.

1. Aging of photocathode

The photocathode is a crucial part of a photomultiplier tube that converts light signals into electrical signals. Over time, photocathode materials may age due to continuous light exposure or environmental factors, resulting in a decrease in their photoelectron emission efficiency. To solve this problem, it is necessary to regularly inspect and replace the photocathode.

2. Decreased performance of the multiplier pole

The multiplier electrode in a photomultiplier tube is responsible for amplifying primary photoelectrons through secondary electron emission effects. If the performance of the doubling pole material decreases, such as due to pollution or physical damage, the amplification efficiency will decrease, resulting in a decrease in photon quantity. Regular cleaning and maintenance are essential for doubling the pole.

3. Environmental factors

Environmental conditions, such as temperature, humidity, and magnetic field, can all affect the performance of photomultiplier tubes. High temperature may cause an increase in hot electron emission, while high humidity may cause leakage, thereby increasing dark current. A strong magnetic field may interfere with the trajectory of electrons and reduce gain. Therefore, controlling laboratory environmental conditions is crucial for maintaining the performance of photomultiplier tubes.

4. Light path pollution

If the incident window or optical path of the photomultiplier tube is contaminated, such as dust, oil, etc., it will reduce the transmittance of the light signal, resulting in a decrease in the amount of photons detected. It is necessary to regularly clean the incident window and check the cleanliness of the optical path.

5. Power and voltage issues

The photomultiplier tube requires a stable power supply and precise voltage to ensure its normal operation. The fluctuation or instability of voltage may lead to unstable gain, thereby affecting the detection efficiency of photons. Using a stable power supply and precise voltage regulation equipment can solve this problem.

6. Fatigue of photomultiplier tubes

Long term or high-intensity light irradiation may cause fatigue of photomultiplier tubes, which is a temporary decrease in sensitivity. Usually, after stopping irradiation for a period of time, sensitivity will partially recover. Avoiding prolonged high-intensity exposure or using appropriate light attenuators can reduce fatigue.

7. Uneven surface of photocathode

If the sensitivity of the photocathode surface is uneven, it may lead to a decrease in the efficiency of light signal detection in some areas. This may be due to material defects or manufacturing process issues. In this case, it may be necessary to replace the photomultiplier tube.

8. Increased dark current

Dark current refers to the current output by a photomultiplier tube even when there is no light signal input. The increase of dark current will reduce the signal-to-noise ratio, thereby affecting the detection of photons. Dark current can be reduced by lowering ambient temperature, reducing thermal radiation, and using dark current compensation techniques.

9. Changes in spectral characteristics

The spectral characteristics of photomultiplier tubes determine their ability to respond to light signals of different wavelengths. If the spectral characteristics change, it may lead to a decrease in the detection efficiency of specific wavelength light signals. Regular calibration and testing of the spectral response characteristics of photomultiplier tubes is necessary.

10. Stability issues of photomultiplier tubes

The stability issues of photomultiplier tubes may be caused by various factors, including poor electrode welding, loose structure, and poor contact of cathode shrapnel. These issues can all lead to unstable signal output, thereby affecting the detection efficiency of photons. Regular inspection and maintenance of the mechanical structure and electrical connections of photomultiplier tubes can improve their stability.

The continuous decrease in photon quantity in photomultiplier tubes is a complex problem that may be caused by multiple factors. To ensure the high performance and long-term stability of photomultiplier tubes, regular maintenance and calibration are required, while paying attention to environmental conditions control and power management. Through these measures, the problem of photon reduction can be minimized to the greatest extent possible, ensuring the reliability and accuracy of photomultiplier tubes in various applications.