Photomultiplier tubes (PMTs) have long been recognized as the detector of choice for very weak light detection. The high gain of the PMT, in excess of 10 ×106, makes it a useful detector for applications such as LIDAR, bioluminescence, fluorescence, and chemiluminescence. However, a requirement for higher sensitivity has always existed within the detector community. Now Hamamatsu scientists have incorporated III-V semiconductor photocathodes of GaAsP(Cs) and GaAs(Cs) offering high quantum efficiency and low dark noise for applications requiring the detection of very weak light signals with high signal-to-noise ratios.

Conventional PMTs typically have quantum efficiencies (the ratio of detected electrons to incident photons expressed as a percentage) in the vicinity of 20 percent in the visible light range to a few percent in the red and near-infrared. GaAsP(Cs) and GaAs(Cs) semitransparent photocathodes have been used in other devices, such as image intensifiers, and the Electron Tube Center felt that incorporating them into PMTs would represent a significant improvement in the performance. Photomultiplier tubes employing these cathodes have quantum efficiencies in the visible of up to 50 percent and over 15 percent in the near infrared.

Spectral Response Curves for III-V compound photocathodes.

Photocathodes work by the process called the photoelectric effect, described by Einstein in the early part of the century:

E = hc - Ebλ

where E = energy imparted to the electron, h = Planck's constant, c = the speed of light, λ= wavelength of light, and Eb = binding energy of the electron to the atom.

When a photon enters the photocathode material and the energy (hc/λ) of the photon exceeds the binding energy of the electron, the latter will be excited from the valence band to the conduction band of the photocathode. The electron must then be emitted from the photocathode into the vacuum of the PMT. In conventional photocathodes, the process of emission is hampered by a characteristic known as electron affinity (EA), which represents an energy barrier to the vacuum level. Therefore, to excite an electron into the vacuum, the incident photon must have an energy of Eb + EA. The advantage of III-V photocathodes over conventional cathodes is that it is possible to produce materials with negative electron affinity, when they are properly activated with cesium. With negative electron affinity materials, the barrier to the vacuum is completely removed. In fact, the negative electron affinity actually improves the probability that the electron will escape to the vacuum.

In the past, III-V compound cathodes were available for PMTs only in a reflective mode, when the electron is emitted from the same side of the cathode on which light is incident. This configuration makes electron collection difficult, resulting in reflective-mode PMTs that are large and bulky. The latest III-V compound cathodes use transmission-mode photocathodes. In this configuration, the electron is emitted from the side opposite to the light input, allowing the production of a very compact PMT. This compactness makes for very small cooled photon-counting PMT modules.

In photon counting, a photon enters the tube and is converted to an electron that is amplified by the tube's gain. The result is a pulse with a specific amplitude. All pulses coming from the tube with this amplitude are counted as photons. The photocathode can also, however, emit "dark pulses" because of thermionic emission from the photocathode. The number of dark pulses depends on the Ebof the photocathode. The GaAsP(Cs) has about 5,000 per second, while the GaAs(Cs) has about 15,000 per second at room temperature. Therefore, for most applications, cooling is required to reduce these dark pulses. The dark counts are then reduced to 80 per second for GaAsP(Cs) and 100 for GaAs(Cs) at 0 °C.

The limiting factor in using these PMTs in all current applications is the maximum allowable photocathode current of 2 picoamps. To get long operating life from the tubes, it is prudent to use them only for photon counting or very weak analog signal detection.

Future improvements will include increasing the sensitivity, further reducing the dark pulses, and increasing the maximum photocathode current. The Hamamatsu Electron Tube Center is constantly working to raise photomultiplier technology. The incorporation of III-V compound photocathodes into very compact PMTs is a significant step forward.

This work was done at the Hamamatsu Photonics Electron Tube Center.For further information, contact Hamamatsu at 800-524-0504 or visit usa.hamamatsu.com. The author of this brief is Earl Hergert, Hamamatsu engineer.