Phy433_manual_ch4_rev09 - 4 Scintillation Detectors 4.1...

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4 Scintillation Detectors 4.1 Photomultipliers See W. R. Leo, Chapt. 8; also Knoll, Ch. 9 The essential elements of a photomultiplier tube (PM) are: (a) Front window, usually borosilicate glass but sometimes quartz (b) Light sensitive cathode, which for the better tubes is now a bialkali (K-Cs-Sb), coated on the inside of the front window. Photoelectrons are emitted by the cathode and collected at the first dynode. (c) Focusing electrodes (d) Secondary emitting dynodes usually made of copper-beryllium; most tubes have 10 to 14 stages with each dynode more positive than the previous one by about 150 volts. An external resistive divider chain located in the base to which the tube is connected sets these voltages. (e) Anode on which the electrons are finally collected and a signal developed. It is often built into a 50-ohm transmission line structure. Some important tube parameters are: (a) Spectral response. This depends on front window and cathode material; it is usually given in ma/watt; peak sensitivity is usually at about 400 nm. (b) Quantum efficiency is defined as the ratio of the number of emitted photoelectrons to the number of incident photons. This is an important tube parameter for counting applications; it reaches a peak of about 30% for tubes with bialkali cathodes but is only about 20% for tubes with standard cathodes (Cs-Sb). (c) Secondary electron yield. The number of secondary electrons emitted by each dynode depends on the energy of the incident electron and hence on the voltage between dynodes. A typical yield might be about 5 secondary electrons varying as a power (less than one) of the applied voltage. The overall gain of the tube is then d N where d is the secondary yield and N is the number of stages. For a ten stage tube this might be 5 10
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or 10 7 . The variation of gain with the voltage goes as a high power of the (but less than N) of V. (d) Dark current, noise. The cathode emits thermionic electrons at a low rate even in the absence of illumination. This rate depends, of course, on the cathode area and increases with temperature. The dark current may be very high if the tube has recently been exposed to light even with the high voltage off; it usually recovers. Tubes with bialkali cathodes have unusually low noise rates. There is a large variation in noise rate from tube to tube. Of course the noise rate above some voltage threshold will increase with high voltage. (e) Linearity. For very high voltages or for high light levels the gain of the phototube will begin to saturate due to space charge buildup near the anode and the response will no longer be linear with input pulse height. This should be avoided if linearity is required. Increasing the voltage between stages at the anode end of the tube helps. Capacitors are always placed across the resistors of the divider chain for the last few dynodes. These capacitors supply charge for pulses that draw more current than is
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Phy433_manual_ch4_rev09 - 4 Scintillation Detectors 4.1...

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