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Photoemissive Detectors
Published in Antoni Rogalski, Zbigniew Bielecki, Detection of Optical Signals, 2022
Antoni Rogalski, Zbigniew Bielecki
Photodetectors based on photoemission usually take the form of vacuum tubes called phototubes. Electrons are emitted from the surface of a cathode and travel to an electrode (anode), which is maintained at a higher electric potential (Figure 4.59). As a result of the electron transport between the cathode and anode, an electric current proportional to the photon flux incident on the photocathode is created in the circuit. The photoemitted electrons may also impact other specially placed metal or semiconductor surfaces in the tube, called dynodes, from which a cascade of electrons is emitted by the process of secondary emission. A photomultiplier tube (PMT) generally contains between ten and fifteen dynodes and approximately 100 V is maintained between successive dynode plates. The result is an amplification of the generated electric current by a factor as high as 109. This device illustrated in Figure 6.7, is known as a photomultiplier tube. To meet market demands, different photomultiplier tubes have been developed. Hamamatsu has developed hundreds of different photomultiplier tubes and their representative selection is shown in Figure 6.8 [20].
Ultraviolet, Visible, Near-Infrared Spectrophotometers
Published in Grinberg Nelu, Rodriguez Sonia, Ewing’s Analytical Instrumentation Handbook, Fourth Edition, 2019
A vacuum phototube consists of a concave cathode made from (or coated with) a photoemissive material that emits electrons when irradiated with photons. The electrons from the cathode negatively charged by the photons, cross the vacuum to the positive anode and into the amplifier. The number of electrons ejected is directly proportional to the power of the incident radiation. The sensitivity of a photoemissive cathode to various wavelengths depends upon the composition of the emitting surface. For example, a red-sensitive surface is coated with a sequence of metals typically Na/K/Cs/Sb, whereas NIR sensitivity is obtained from coatings made with Ga/In/As. Generally, phototubes have a small dark current resulting from thermally induced electron emission.
Phase (x, T) and (p, T) diagrams of TlIn(S1−xSex)2 polycrystal in the compositional range 0 ≤ x ≤ 0.15
Published in Phase Transitions, 2019
P. P. Guranich, R. R. Rosul, O. O. Gomonnai, V. M. Rubish, A. V. Gomonnai, A. G. Slivka, P. Huranych
TlIn(SSe) polycrystals under investigation were obtained from the melt of a stoichiometric mixture of the initial TlInS and TlInSe components. For the characterisation of the TlIn(SSe) polycrystals, X-ray diffraction (XRD) studies were carried out at room temperature using a conventional Bragg–Brentano technique with a DRON-4 diffractometer and Cu Ka radiation. Scanning electron microscopy (SEM) studies combined with energy-dispersive X-ray spectroscopy (EDX) were performed using a SEM JEOL 7000F microscope. Raman measurements were performed at room temperature on a LOMO DFS-24 double grating monochromator with a FEU-136 phototube and a photon counting system, the excitation being provided by a He–Ne laser (632.8 nm) and Ar laser ( nm). The instrumental width did not exceed 1 cm−1. Dielectric permittivity studies of mm3 samples were carried out at the frequency of 1 MHz using an automated setup with an AC bridge with temperature variation rate of 0.01–0.02 K/s. Contacts were made of silver paste. The sample temperature was measured by a copper-constantane thermocouple. Pressure studies were performed in the hydrostatic pressure range up to 750 MPa. The pressure was measured with the accuracy of 1 MPa. A high-pressure chamber with petrol being used as the working liquid.