Application of Keithley Picoammeter in Photomultiplier Tube Light Measurement

Overview

Applications that measure light with photomultiplier tubes usually require the use of a picoammeter because the currents to be measured are very small.

A photomultiplier tube (PMT) is a device that converts light into an electric current. A photomultiplier tube has a light-sensitive cathode that emits electrons in proportion to the number of photons that hit it. These electrons are accelerated and hit the next stage, causing the emission of 3 to 6 secondary electrons. This process continues for 6 to 14 stages (called dynodes), depending on the model of the tube. Total gains of 1 million times or more are usually achieved.

Specific operation process:

The voltage of each successive photomultiplier tube electrode is made higher than the voltage of the previous electrode, so that the electrons are accelerated. The easiest way to do this is to add a voltage across the entire photomultiplier tube, and then obtain the voltage supplied to each multiplier tube electrode from each tap of a voltage divider, as shown in Figure 1.

Figure 1. Voltage source for a photomultiplier tube.

The voltage applied to each photomultiplier tube electrode depends on the design of the PMT and is determined by the model of each tube.

The total resistance of the PMT electrode resistors should be such that the current flowing through this series of resistors is at least 100 times greater than the PMT anode current to be measured:

Most photomultiplier tubes require an anode to cathode potential between 1000V and 3000V. Since the anode is the readout point, it is usually operated at a potential close to ground, while the cathode is at a high negative potential. Keithley's Model 248 High Voltage Power Supply can provide up to 5000V for this application.

The anode current of most photomultiplier tubes ranges from picoamps to 100mA. Picoammeters are often used as a readout of the anode current because of their high sensitivity. Picoammeters have a very low input voltage drop (voltage drop across the input terminals), which places the anode at practically ground potential. Figure 2 shows a typical configuration using a Keithley Model 6485 Picoammeter. If the voltage required by the PMT does not exceed 1000V, the Model 6517B Electrometer Voltage Source can provide a convenient solution because it can measure current and provide voltages up to ± 1000V.

When connected this way, the current read by the Keithley picoammeter is negative. Sometimes, it is necessary to measure a positive current. In this case, a simple rearrangement of the circuit and the use of an additional power supply can produce a positive current. The circuit configuration for measuring the positive PMT current is shown in Figure 3. The picoammeter reads a current at the last dynode electrode, which is equal to the anode current minus the current flowing through the previous dynode electrode. In effect, the PMT gain is slightly sacrificed when making this measurement.

Figure 2. Basic connections for a photomultiplier tube.

Figure 3. Reading positive polarity PMT current.

A small current usually flows in the PMT even when the cathode is not illuminated. This phenomenon is called "dark current" and is of no concern in most applications. In other cases, it can be subtracted from the reading using the REL (zero) function, or simply eliminated using the built-in zero suppression feature if the instrument has this feature.