Photomultiplier Tube Basis IV: The Basic Usage

Photomultiplier Tube (PMT) converts optical signals into electrical signals, and also need cooperate with the appropriate accessories and circuits, such as the socket, voltage divider and high-voltage power supply, in order to make the electrical signal output and then be processed.

In order to facilitate the installation and removal, users can solder the voltage divider circuit with the socket, thus PMT can be directly pluged in the socket assembly. Common socket shown in Fig. 1, can be selected according to the installation and space. In addition, there are several different patterns of finished PMT pin parts, such as hard wire, soft wire and with base, as shown in Fig. 2, the PMT of hard wire and with tube base can directly use the socket, the PMT of soft wire is usually soldered with the voltage divider directly.

Fig. 1 The socket of different shape and size

Fig. 2 PMT with different pin patterns

The movement of electrons in the PMT is determined by the electric field. The electrons can be multiplied and output from the anode by supplying high voltage between the poles and this high voltage is supplied by a stable high-voltage power supply (usually from 1kV to 2kV). Due to the large gain of the PMT, it is very sensitive to the voltage variations of the high-voltage power supply. In this case, in order to ensure the stability of the PMT output current within 1%, the stability of the high-voltage power supply must be better than 0.1%.

In addition, the power supply needs to use a voltage divider to distribute the high voltage to each dynode and provide a suitable voltage gradient across dynodes. Fig. 3 is a schematic of PMT pin layout and the orientation is facing the PMT pin, where "K" is the cathode, "DY is the dynode", "P" is the anode (signal output), "IC" is internally shorted (vacant in use here). The pin layouts of different PMTs are different, and the voltage divider circuit design should match the pin layout. Fig. 4 is a schematic diagram of the PMT voltage divider, which uses several resistors (100k ~ 1MΩ) between the cathode, dynodes and anode to distribute the required voltage between the stages ( recommended voltage distribution ratio differs in PMTs, can be consulted in product brochure). If the output is a pulse signal, the latter stages should add capacitors. The details of the voltage divider circuit design will be described in later chapters.

Fig. 3 PMT pin layout

Fig. 4 voltage divider design diagram

PMTs with voltage divider and high voltage can output current signal from the anode. The design of the related circuits varies according to the intensity of incident light. As shown in Fig. 5, a) The DC mode is to detect the DC components of the PMT through an amplifier and a low pass filter, this method is widely used in detecting strong light; b) The AC mode is to take the AC signal of the PMT output through capacitor, and then transfer it to DC signal. This method is generally applicable for detecting faint light, in this case, the AC components of the output signal suppress the DC component and dominate; c) The photon counting mode is to amplify the PMT output pulse first, select it by the pulse amplitude discriminator, and then count the pulses whose amplitude is above a threshold voltage. This method is suitable for observing the discrete pulses output from the PMT, therefore, it is a very effective method in faint light detection, also called single photon detection.

Fig. 5 PMT output signal processing methods