Photo Multiplier Tubes

Photomultiplier tubes (or PMTs) are photodetectors which provide extremely high sensitivity and ultra-fast response. A typical PMT consists of a photoemissive cathode (photocathode) followed by focusing electrodes, an electron multiplier and an electron collector (anode) in a vacuum tube.

When light enters the tube and strikes the photocathode, the photocathode emits photoelectrons into the vacuum. These photoelectrons are then directed by the focusing electrode voltages towards the electron multiplier where electrons are multiplied by the process of secondary emission. The multiplied electrons are collected by the anode as an output signal. Because of secondary-emission multiplication, photomultiplier tubes provide extremely high sensitivity and exceptionally low noise among the photosensitive devices currently used to detect radiant energy in the ultraviolet, visible, and near infrared regions. A photon striking the photocathode would usually yield the emission of a single electron but the multiplier can create a final output of one million electrons for each electron emitted. This is the gain of the PMT and it is enormous!

The electron multiplier section consists of nine (or more, some PMTs use up to 19) electrodes called dynodes. Each dynode is charged with about 100 volts more positive charge than the previous dynode in the chain. As electrons are emitted from a previous dynode they are focussed to the next dynode by means of this increasing positive voltage. The electrons strike that dynode and are multiplied and the cascade of emitted electrons continues to grow at each dynode. Finally the stream of electrons, which began perhaps as a single electron, is collected by an anode where it appears as an electrical current.

The entire PMT is powered by a source of about 1000 Volts. The photocathode being the most negative electrode, each dynode is succession is more positive than the last. The potential difference required is easily derived using a resistive voltage divider consisting of a chain of one Megaohm resistors in series. The final dynode has a potential of 1000 volts positive relative to the photocathode. Finally the electron stream is collected by the anode.

PMTs generally have a photocathode in either a side-on or a head-on configuration. The side-on type receives incident light through the side of the glass bulb, while in the head-on type, it is received through the end of the glass bulb. Most spectrophotometers use the side-on type which employs an opaque photocathode (called a reflection-mode photocathode) and a circular-cage structure electron multiplier which has good sensitivity and high gain at a relatively low supply voltage.

Typical PMTs

Our 1m spectrophotometer uses an industry-standard PMT of the side-on type. This device resembles an old radio vacuum tube with 11 pins on it's base. We can use either a type 931A, 1P21, or 1P28 type PMT. These tubes all have a gain of about 10 million and run off 1000 volts (maximumrated voltage is 1250 V). The photocathodes are made of Sb-Cs and have glass windows giving a spectral response of 300nm to 650nm. The 1P28 tube has a quartz window extending it's range into the UV from 185nm to 650nm (The 1P28 is the currently installed PMT in the McPherson unit). 931A Photomultiplier Tube

A 931A PMT with resistor divider chain attached. The resistor is almost always constructed right at the base of the tube. When installed on the unit it is housed in an aluminum box for electrical safety as well as protection from electrical noise. It must also be sealed quite well against stray light (black silicone sealant works well for this purpose) since it's extremely high sensitivity would amplify even the smallest amount light. Note that the photocathodes in these tubes may be damaged by exposure to intense light sources - for this reason the exit shutter on the unit is closed when not in use.

A 1000 Volt regulated power supply for PMTs. In this case, a PowerDesigns 2K-10 power supply is used. This supply will produce up to 2000 volts at 10mA. Be sure to keep the voltage below 1250 when using any of the PMTs in our lab (1000 volts is the nominal voltage we use). As well, be sure to turn on the supply five minutes prior to use to allow it to stabilize. Tight regulation of voltage is required since the gain of a PMT is proportional to dynode voltage.

With notes and diagrams taken from Photomultiplier Tubes by Hamamatsu (Bulletin TPMO0003E03). The 1P28 in our unit is manufactured by Hamamatsu.