Photocathode technology for photomultiplier tubes

A photomultiplier tube is a vacuum tube composed of a light incidence window, a photocathode, a multiplier stage, and an anode. Photons are irradiated onto the photocathode through a light window, and the photocathode undergoes a photoelectric effect to produce photoelectrons, which are then accelerated and aggregated, entering the doubling system. At the doubling pole, electrons undergo doubling through secondary emission, which repeats on each doubling pole, resulting in a doubling of the number of electron clusters received by the anode by 10 ^ 6 to 10 ^ 7 times, or even more.

The end window photomultiplier tube forms a photocathode directly on the inner surface of the light incident window. Due to the easy coupling of scintillators to the light incident window, it is often used for radiation measurement.

Photocathode

Different photocathode materials can result in different spectral response characteristics in the sensing range of PMT. If combined with the incident window material, the overall sensing range of PMT can be appropriately modulated. For photocathode materials, due to the most active physical properties of alkali metals, almost all of them contain alkali metal elements. How were these photocathodes discovered, recognized, and subsequently used? Next, let's take a closer look at the PMT photocathode technology.

Alkaline photocathode

Compared with other photodetectors, photomultiplier tubes have excellent signal-to-noise ratio characteristics due to their low noise electron multiplier. In order to further improve the signal-to-noise ratio and achieve higher sensitivity, the quantum efficiency of the photocathode also needs to be further improved. Figure 2 shows the relationship between quantum efficiency and the wavelength of typical photocathodes currently used.

In 1951, American Sommer (left in Figure 3) invented the photocathode treatment process, which produced a multi alkali photocathode by reacting a layer of Sb with Na, K, and Cs. This type of photocathode has high sensitivity in the wide spectral range of ultraviolet to 850nm, and is used in spectrophotometers and fluorescence measurements in the fields of biology and genetics.

The dual base cathode is manufactured by reacting Sb with K and Cs, and has high sensitivity around 400nm. The PMT using this dual alkali photocathode is widely used in radiation measurement of scintillation counting, as this spectral response characteristic is well matched with the emission wavelength of NaI scintillators. By the way, this dual alkali photocathode was also invented by Sommer in 1963. After Sommer invented this photocathode, later professionals further improved these two types of photocathodes in practice, making them the most widely used PMT photocathodes today. The working principle of a photocathode can be described using an energy band model. According to the energy band theory, new semiconductor photocathodes and highly sensitive double alkali photocathodes have been developed, which has also opened up the way to enhance the sensitivity of photocathodes and extend the spectral response range.