Side-On Photomultipliers

In the side-on photomultiplier tube design, photons impact an internal photocathode and eject electrons from the front face (as opposed to the rear side as in the end-on designs). These ejected photoelectrons have trajectories angled at the first dynode, which in turn emits a larger quantity of electrons angled at the second dynode (and so on). Incident light is detected through the curved side of the envelope in side-on photomultipliers. Due to their high performance ratings and low cost, side-on photomultipliers are the most widely used tubes for general photometric applications, such as spectrophotometry, fluorimetry, and confocal microscopy.

The tutorial initializes with a cutaway drawing of a side-on photomultiplier appearing in the window, and having the photocathode oriented to the left and the anode nested within the last dynode. Virtual photons, represented by small yellow spheres, incident on the photocathode are converted into photoelectrons that move through the dynode chain. As they pass from one dynode to the next, the electrons multiply when they impact the surface of the walls, and eventually are absorbed by the anode. In order to operate the tutorial, use the Gain slider to increase or decrease the number of electrons created each time an electron collides with a dynode, and the Tutorial Speed slider to regulate the speed of the electrons passing through the photomultiplier. The tutorial can be halted at any point by clicking on the Stop button, which reverts to a Play button for resumption of the tutorial action. When the Stop button is activated, the photomultiplier drawing is converted into a new illustration that schematically diagrams photoelectron flow through the photomultiplier tube.

The spectral response, quantum efficiency, sensitivity, and dark current of a photomultiplier tube are determined by the composition of the photocathode. The best photocathodes capable of responding to visible light are less than 30 percent quantum efficient, meaning that 70 percent of the photons impacting on the photocathode do not produce a photoelectron and are therefore not detected. Photocathode thickness is an important variable that must be monitored to ensure the proper response from absorbed photons. If the photocathode is too thick, more photons will be absorbed but fewer electrons will be emitted from the back surface. However, if the photocathode is too thin, too many photons will pass through without being absorbed. The photomultiplier used in this tutorial is a side-on design, which uses an opaque and relatively thick photocathode of precise thickness as well as composition. Photoelectrons are ejected from the front face of the photocathode and angled toward the first dynode. Due to differences in design, side-on photomultiplier tubes often demonstrate higher quantum efficiencies than end-on tubes having photocathodes of similar composition.

In optical microscopy, the incident illumination is dispersed over a substantial portion of the photocathode, although the sensitivity is rarely uniform over the entire surface. In side-on designs, the upper half of the photocathode is typically 20 to 30 percent more sensitive than the lower half. The number of dynodes present in photomultipliers also varies by design. End-on tubes can have a series of up to 14 electrodes, whereas the side-on designs are usually limited to no more than 9 dynodes. For this reason, side-on photomultipliers do not achieve the high electron gains exhibited by end-on tubes.