Alternative Position Sensitive Photomultipliers

Alternative Position Sensitive Photomultipliers

DOI: 10.4018/978-1-5225-0242-5.ch008
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Abstract

In this chapter, alternative position sensitive vacuum and solid state detectors are described. We start with a description of the vacuum photomultiplier tubes and how they have evolved into solid state detectors. These devices are promising, but still fall short compared to gaseous detectors.
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1. Introduction

In the previous chapters various position sensitive gaseous photomultipliers, capable of efficiently detecting single photons, have been described in great detail. What is most significant of these developments? To answer this question it will be useful to look deeper into alternative position sensitive photodetectors.

The first photodetectors sensitive to single photons was the vacuum photomultiplier tube (PMT) developed in the 1930ies. It was a tremendous success from the beginning. PMTs are still the most widespread detectors sensitive to single photons. They are used practically in every kind of experimental studies including high-energy physics, astronomy, space research, archeology, medicine, geology, biology, art, metallurgy, chemistry, agriculture, etc. Among these applications, physics experiments, particularly high energy physics and astroparticle physics experiments, are the most common users of PMTs. The developments of PMTs are mainly driven by the needs in physics experiments.

L.A. Kubetsky was the first who proposed the new method to amplify weak photocurrents by dozens of dynodes which amplified incoming electrons a thousand times without using traditional electronic tubes. His USSR Author’s certificate #24040, has the priority date 4 August 1930 (Lubsandorzhiev, 2006).The device consisted of a source of primary photoelectrons (a photocathode) and a consecutive system of secondary electron emitters (dynodes) with a certain electron multiplication coefficient ksec each. The principle of this new electron multiplier was based on the secondary electron emission effect discovered in 1902 by L. Austin and H. Starke (Bruining, 1954). The physics behind this phenomenon is as follows: if an electron with a high kinetic energy hit a surface, it creates energetic electrons inside the material and some of them, especially those located close to the surface, may escape the surface if their kinetic energy is exceeding the affinity or the work function of the material. This process is illustrated by Figure 1.

Figure 1.

The secondary electron emission effect; a high energy primary electron hitting a surface will create several hot electrons inside the material. Some of them with high enough kinetic energy, especially those located close to the surface can escape into the vacuum. As a result one primary electron can create ksec electrons.

As in the case of photocathodes, the most efficient secondary electron emitters are semiconductors with small affinity, such as alkali antimonite. In metals the hot electrons rapidly lose their excess energy in collisions with other electrons and thus cannot efficiently escape into the vacuum (typically ksec<1 in metals). In dielectrics and semiconductors with high affinity only a small fraction of the electrons will have high enough energy to cross the surface barrier. In dielectric materials the primary electrons will not be neutralized inside the material, but will cause the surface to charge up. For the best secondary emitting materials, such as Sb3Cs and bialkali, the coefficient ksec reaches values of 10 and 20, respectively, for primary electrons with kinetic energies close to 1kV (Hakamata,2007).

In the device proposed by Kubetsky the voltage applied to each subsequent dynode was higher compared to the preceding dynode. Photoelectrons created from the photocathode were accelerated to the first emitter knocking out secondary electrons from its surface. These electrons are in turn accelerated to the next emitter where they knock out new secondary electrons and so forth. The device can in principle have arbitrary number of emitters. The electron flux from the last emitter is collected on a collector or anode. The total gain of the device will amount to:Nsec= ksecz, (1) where z is the number of emitters.

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