A small amount of current flows in a photomultiplier tube even when operated in a completely dark state. This output current is called the dark current and ideally it should be kept as small as possible because photomultiplier tubes are used for detecting minute amounts of light and current.
Causes of dark current:
(a) Thermionic emission current from the photocathode and dynodes.
(b) Leakage current (ohmic leakage) between the anode and other electrodes inside the tube and/or anode lead and other wiring.
(c) Photocurrent produced by scintillation from glass envelope or electrode supports.
(d) Field emission current.
(e) Ionization current from residual gasses.
(f) Noise current caused by cosmic rays, radiation from radioisotopes contained in the glass envelope and gamma rays.
Voltage divider circuits
For photomultiplier tube operation, a high voltage from 500 to 3000 volts is usually applied across the cathode and anode.
In practice, an interstage voltage for each electrode is supplied by using voltage-dividing resistors (100 Kohms to 1 Megohms) connected between the anode and cathode. Sometimes Zener diodes are used with voltage-dividing resistors.
For fun, we modified Syd Levine's 106PMT1 schematic by adding a transistor and by doing minimal re-wiring.
The purpose of the changes is to have an output near ground and allow DC coupling.
Likewise, we modified Syd Levine's 106PMT3A schematic by adding a transistor and some re-wiring.
Again, the purpose of the changes is to have an output near ground and allow DC coupling.
In this case, note that the capacitors C1, C2, C3 and C4 could be replaced with suitable-valued Codatron® high voltage zeners.
As shown below, this circuit utilizes a Cockcroft-Walton voltage multiplier circuit in which an array of diodes is connected in series. Along each side of the alternate connection points, capacitors are connected in series. This circuit provides equally-incremented voltage potentials derived from the constant Vin source, assuring good linearity for both DC and pulsed currents yet with low power consumption, making it suitable for use in compact circuits.
Photomultiplier tube operation stability depends on the total stability of the power supply characteristics including drift, ripple, temperature dependence, input regulation and load regulation. The power supply must provide high stability which is at least 10 times as stable as the output stability required of the photomultiplier.
The following table gives a guide for selecting a high-voltage power supply for good linearity.
(1) Line regulation +-0.1 % or less;
(2) Load regulation +-0.2 % or less;
(3) Ripple noise 0.05% or less;
(4) Temperature coefficient +-0.05 %/°C or less.
Oil well logging
Oil well logging is used to locate an oil deposit and determine its size. This technique makes use of photo-multiplier tubes as detectors for density well logging using radiation, neutron well logging and natural gamma-ray-spectrum well logging.
A probe containing a neutron or gamma ray source is lowered into a well as it is being drilled. The radiation or the neutrons that are scattered by the rock surrounding the well are detected by a scintillator/ photomultiplier. The amount of scattered radiation detected is indicative of the density of the rock that surrounds the well. The scattered neutrons indicate the porosity of the rock which is required to ascertain if the oil can be removed. Naturally occurring gamma rays are detected to locate shale which indicates the presence of oil or gas.