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Chapter 3. Blazing My Own Trail

Secondary electron multipliers (pp. 54ff)

(incl. VEU-1, VEU-6, VEU-7)

When an incident particle or a photon knocks out an electron from a surface, a secondary electron multiplier can pick it up and convert it into an avalanche of millions of electrons. Electronics can then reliably register such bursts of electrons and thus count individual incident particles.

In the mid-1930s, Vladimir Zworykin of the Radio Corporation of America and John R. Pierce of Bell Telephone Laboratories developed the first discrete dynode electron multipliers in the United States for use in various applications. [4] Leonid Kubetsky had built conceptually similar devices for his laboratory experiments even earlier in the Soviet Union. [5]

An electron entering a secondary electron multiplier is accelerated by a static electric field and hits the first dynode, producing a few secondary electrons. These electrons, in turn, are further accelerated and hit the next dynode, producing more secondary electrons. The number of electrons thus increases exponentially. After a dozen or more multiplication stages, one obtains a burst of many thousands or millions of electrons at the output of the device. Such multidynode electron multipliers could be rather bulky, for example, one inch in diameter and four inches long as illustrated by VEU-1 manufactured in the Soviet Union (Fig. 3.1). Here VEU stands for vtorichnyi elektronnyi umnozhitel’ in Russian, or secondary electron multiplier.

Nuclear physics and other fields have widely used SEMs for decades. In a major advancement of the technique in the late 1950s and early 1960s, George W. Goodrich and William C. Wiley of the Research Laboratories Division of the Bendix Corporation showed that efficient electron multiplication could be achieved in channels. [6] This breakthrough finding led to a channel electron multiplier, or CEM, that is basically a tube made of highly resistive but slightly conducting glass.

A high voltage is applied across the ends of such a tube where it creates an electric field E in the axial direction (Fig. 3.2). An incoming energetic electron, ion, neutral atom, or ultraviolet or X-ray photon knocks out the first electron at the channel entrance. The axial electric field then accelerates the electron to an energy of 50-150 eV and it collides with a channel wall, producing secondary electrons. The field then accelerates these electrons until they collide with the wall. The process repeats itself many times, resulting in an avalanche of many thousands or millions of electrons exiting the CEM.

CEMs are electrostatic devices where the total applied electric field, rather than its gradient, determines the acceleration of electrons. Consequently, the multiplication properties of a channel electron multiplier depend on the length-to-diameter ratio of the channel rather than on its absolute size. This feature opens a way for miniaturization, particularly to combining millions of small straight channel multipliers into the so-called microchannel plates (Fig. 3.3). An early CEM patent by Goodrich and Wiley envisioned "a plurality of [channel] multipliers ... arranged to intensify a light image." [7]

Originally developed for image intensifiers in night-vision devices, [8] MCPs not only enabled reliable detection and counting of individual particles and photons [9] but also provided the basis for position-sensitive detectors. Such latter devices determine a position, the coordinates, of the impact of each individual registered particle at the detector’s sensitive surface.

Various types of secondary electron multipliers, including CEMs and MCPs, would be widely used in physics laboratories across the world. By 1973, the Soviet industry had begun producing compact open-type (that is operating in a vacuum) channel electron multipliers. [Our] laboratory ... widely used such CEMs with a 10-mm diameter funnel and spiral channel, VEU-6 (Fig. 3.4), for particle detection ...

Fig. 3.1. Typical Venetian-blind discrete dynode secondary electron multiplier VEU-1 for operation in a vacuum. Particles enter the multiplier at the left end; the dynodes are in the middle. Photograph courtesy of Yu. V. Gott.

Fig. 3.2. Schematic of a channel electron multiplier, CEM, detecting and counting individual energetic particles and photons in a vacuum.

Fig. 3.3. Schematic of a microchannel plate with millions of small identical straight channels in a hexagonal pattern, each acting as independent channel electron multipliers (left). An MCP converts incoming particles into localized bursts of electrons at its exit where electronics amplify and count the bursts as detected individual events. Typically, two or three MCPs are mounted in series to provide the desired electron multiplication.

The photograph on the right shows (arrow) such a detector. MCP sensitive areas vary from one inch to a few inches in diameter; diameters of microchannels could be from 6-20 μ. (1 μ = 10^(–3) mm = 10^(–6) m is one micron.) An MCP detector converts an incoming particle into an electron avalanche locally, at a place where it hits the sensitive surface of the front MCP. This feature opens a way for building detectors that not only register individual particles but also determine positions of the electron avalanches at the MCP exit and thus the positions (coordinates) of the corresponding impact points of the registered particles on the sensitive surface. Such sensors are known as position-sensitive detectors, PSDs.

MCPs also form the basis for passive night-vision devices relying on starlight illumination. A photocathode near the MCP entrance converts an incoming photon into a photoelectron that, after acceleration, enters the MCP and produces an electron avalanche at the plate exit. Then, the electrons of the avalanche are further accelerated and strike a phosphor screen, with a resulting flash of light. Such night-vision systems operate in a vacuum in sealed enclosures and amplify image brightness by a factor of ten thousand and more.

Photograph courtesy of Mike Gruntman.

Fig. 3.4. Commercially available channel electron multiplier with funnel VEU-6 capable of detecting and counting individual energetic particles and photons in a vacuum. The funnel increases the sensitive area of the detector. The curved spiral channel efficiently suppresses ion feedback. Photographs courtesy of Yu. V. Gott.

Fig. 5.1. Commercially available two-MCP detector VEU-7. Photograph courtesy of Yu.V. Gott.

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