1. Spherical cathode structure:
In order to fully avoid the tip effect and make the photoelectron emission more stable and uniform, it is necessary to fix the operating area on the cathode, because the ultraviolet tube relies on photoelectron emission and gas multiplication to complete the conversion of the optical signal into an electrical signal and amplify it, generally in the area close to the electrode, the optical emission utilization rate is the highest.
Regardless of the angle from which the photons radiate onto the hemispherical cathode, the discharge region is always at the hemispherical apex close to the anode. Due to the small effective area of the cathode, the operating current of the tube is generally less than 0.3mA, but its viewing angle is wider and the viewing angle sensitivity is relatively uniform, especially suitable for the place of fire warning, and the sensitivity can also be improved by the method of aggregation.
The anode is made into a hemispherical reflective surface in the tube, such as a United States ultraviolet photosensitive tube that can withstand high temperature of 540 °C. The received ultraviolet radiation is reflected to the cathode in the center, improving the sensitivity of the tube, because the far-ultraviolet radiation has the effect of linear propagation and reflection like visible light.
2. Filamentous electrode structure:
The electrode of this kind of tube is generally composed of two or more symmetrical metal wires, which is a structural form of ultraviolet tube in the early stage, mostly using tungsten wire or platinum wire with high purity, and the parallel lines that are closely spaced are the operation area. Since the ultraviolet tube completely depends on the photoelectron emission effect on the electrode surface, and then uses gas multiplication to obtain a stronger signal, its spectral echo scale depends on the work of the cathode data.
In the process of photoelectron emission, the shorter the wavelength of the photon, the higher the energy, and even if the amount is small, it can provoke the electron to overcome the escape work and fly out of the cathode surface. Even if there are a large number of photons with low energy, they cannot arouse the electrons on the surface of the cathode. In the ultraviolet tube, the purity of the cathode data is very high, otherwise it will affect the scale of the spectrum and lose its value, the symmetrical filamentous structure is selected for the convenience of process processing, as far as possible to avoid the pollution of other substances to the electrode.
The characteristics of this kind of tube are that it can operate in the AC state, the operating current is large, the use of the circuit is simple, and the impurities on the surface of the electrode can be removed by appropriate process treatment, but the sensitivity of the viewing angle fluctuates greatly, and the operation area is prone to uneven emission.
3. Flat plate cathode structure
The sensitivity of the ultraviolet tube depends on the number of photons received by the cathode with far ultraviolet radiation, the larger the area of the cathode, the higher the probability of reception, so that more electrons on the cathode escape, under the action of the electric field of the external high voltage is accelerated and collides with the gas molecules in the tube to ionize the gas molecules, and the electrons that occur after ionization collide with the gas molecules, so that the cyclic movement will eventually discharge the gas in the tube. The timing of this avalanche discharge depends on the photoelectron emission effect on the cathode. In order to improve sensitivity, in recent years, a flat-shaped cathode structure ultraviolet tube has been developed and developed.
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