Editing Photomultiplier tube (section)

==Usage considerations==
Photomultiplier tubes typically utilize 1000 to 2000 [[volts]] to accelerate electrons within the chain of dynodes. (See Figure near top of article.) The most negative voltage is connected to the cathode, and the most positive voltage is connected to the anode. Negative high-voltage supplies (with the positive terminal grounded) are often preferred, because this configuration enables the [[photocurrent]] to be measured at the low voltage side of the circuit for amplification by subsequent electronic circuits operating at low voltage. However, with the photocathode at high voltage, leakage currents sometimes result in unwanted "dark current" pulses that may affect the operation. Voltages are distributed to the dynodes by a resistive [[voltage divider]], although variations such as active designs (with [[transistors]] or [[diode]]s) are possible. The divider design, which influences frequency response or [[rise time]], can be selected to suit varying applications. Some instruments that use photomultipliers have provisions to vary the anode voltage to control the gain of the system.

While powered (energized), photomultipliers must be shielded from ambient light to prevent their destruction through overexcitation. In some applications this protection is accomplished mechanically by electrical interlocks or shutters that protect the tube when the photomultiplier compartment is opened. Another option is to add overcurrent protection in the external circuit, so that when the measured anode current exceeds a safe limit, the high voltage is reduced.

If used in a location with strong [[magnetic field]]s, which can curve electron paths, steer the electrons away from the dynodes and cause loss of gain, photomultipliers are usually magnetically shielded by a layer of soft iron or [[mu-metal]]. This magnetic shield is often maintained at cathode potential. When this is the case, the external shield must also be electrically insulated because of the high voltage on it. Photomultipliers with large distances between the photocathode and the first dynode are especially sensitive to magnetic fields.<ref name="pmt"/>