Brown F. Williams

Current status of negative electron affinity devices

The introduction of electron emitters utilizing negative electron affinity has greatly improved the performance of many conventional light-sensing devices. The unique properties of such emitters have also made possible devices which were heretofore not feasible. Since their arrival as laboratory curiosities about five years ago, these emitters have had a large impact in the area of low-light-level detection, particularly scintillation counting. Recent advances in materials technology and surface activation processes have brought negative electron affinity photocathodes to the market place for use as detectors for both the Nd and GaAs lasers. In both cases, the detectors are more than an order of magntiude more sensitive to the laser light than previous photocathodes, and the dark currents (thermionic emission from the cathodes which can be limiting in low-light-level use) are several orders of magnitude less. Several other applications of negative electron affinity are presently under development which may further affect photocathodes, photomultipliers, imaging devices, and even the time-honored thermionic cathode. The operating principles of this type of electron emitter, the present state of the art and its effect on device performance, and the possible developments in the near future are discussed.

NEW HIGH‐GAIN DYNODE FOR PHOTOMULTIPLIERS

A new photomultiplier is described in which the recently reported GaP(Cs) material is used for the first dynode to obtain high secondary emission gain. Gain factors of 20 to 40 have been measured resulting in greatly improved performance of the new photomultiplier at very low light levels.

GaAs1−xPx AS A NEW HIGH QUANTUM YIELD PHOTOEMISSIVE MATERIAL FOR THE VISIBLE SPECTRUM

A new photocathode having high quantum yield for visible and ultraviolet radiation is reported. The cathode consists of a cesium‐activated strongly p‐type doped GaAs1−xPx alloy.

Mobile Si ions in Fe‐doped LiNbO3 crystals

A mobile ionic species fixes thick‐phase holograms in Fe‐doped LiNbO3 crystals at temperatures between 100 and 200 °C. Results are given which show that Si ions are mobile at 200 °C in these crystals.