Transmission Electron Microscope Operating Characteristics
1.Stability
The stability of a photomultiplier tube is determined by a number of factors such as the characteristics of the device itself, the operating state and the environmental conditions. There are many cases where the output of the tube is unstable during operation, mainly:
a. Poor welding of electrodes in the tube, loose structure, poor contact of cathode shrapnel, inter-pole tip discharge, fire jumping and other jumping instability phenomena caused by the signal is large and small.
b. Continuity and fatigue instability caused by too large anode output current.
c. The effect of environmental conditions on stability. The tube sensitivity decreases when the ambient temperature rises.
d.Humid environment causes leakage between pins, resulting in increased dark current and instability.
e. Environmental electromagnetic field interference causes instability.
2.Limit working voltage
Limit operating voltage is the upper limit of the voltage allowed to be applied to the tube. Higher than this voltage, the tube generates discharge or even breakdown.
Applications
Due to the high gain and short response time of the photomultiplier tube, and because its output current is proportional to the number of incident photons, it is widely used in astrophotometry and astro-spectrophotometry. Its advantages are: high measurement accuracy, the ability to measure relatively faint objects, and the ability to measure rapid changes in astronomical luminosity. In astronomical photometry, the more widely used is the antimony cesium photocathode multiplier tube, such as RCA1p21. The great quantum efficiency of this photomultiplier tube is around 4200Å, which is about 20%. There is also a photomultiplier with a double alkali photocathode, such as GDB-53, which has a signal-to-noise ratio one order of magnitude larger than that of RCA1p21, with a very low dark current. In order to observe the near-infrared region, commonly used multi-base photocathode and gallium arsenide cathode photomultiplier tube, the latter quantum efficiency up to 50%.
Common photomultiplier tubes can only measure one piece of information at a time, i.e., the number of channels is 1. matrix. Because the number of channels is limited by the fine metal wire at the end of the anode, only hundreds of channels are possible.
