Channel electron multiplier
A channel electron multiplier (KEV) or channeltron ( English channel electron multiplier , CEM) generates an electron avalanche from a primary particle ( electron , photon or ion ) through secondary electron emission in a vacuum . It works on the same principle as the secondary electron multiplier, but with KEVs the pipe wall is designed as a continuously distributed dynode by means of a suitable resistance coating . In this way, reinforcements of 10 3 to 10 5 , with special arrangements 10 8 can be achieved. The avalanche, which is easy to detect, allows the primary particles to be counted using the KEV.
construction
The KEV consists z. B. from an insulating glass tube, the inner surface of which is covered with a high-resistance layer. For example lead glass doped with bismuth oxide. The resistance between the cathode at the open end of the tube and the end terminated by the anode is around 10 8 Ω , the ratio of the tube length to the inner diameter is typically 70. The operating voltage of the order of 3 kV generates an electric field along the tube axis ; When the primary particle hits the area close to the cathode, several secondary electrons are generally produced , which are accelerated by this field and, after they hit the high-resistance layer, generate tertiary electrons, which in turn are accelerated, etc. until the resulting avalanche hits the anode.
Within a KEV, positive ions are accelerated in the opposite direction to the electrons in the direction of the cathode. If they hit there near the cathode, the secondary electrons generated there would also trigger electron avalanches of about the same strength and thus generate a false signal. This effect, known as ion reaction, is effectively prevented in KEV with circular or helically curved glass tubes: Because of the much larger masses compared to the electrons, the ion paths form a much larger angle with the electric field lines running parallel to the tube axis; the ion paths are short; the energy of the ion remains small and hardly any secondary electrons are generated. In addition, the distance from the point of impact of the ion to the anode is shorter and an avalanche effect is therefore significantly smaller.
The dark count of a KEV, which is also dependent on the vacuum conditions, is generally considerably smaller than an avalanche per second. With a suitable choice of the operating voltage and the ratio of the tube length to the inner diameter, a narrow pulse height distribution of the electron avalanches can be achieved. For counting rates above approx. 10 4 s −1 , the pulse height decreases noticeably because the capacitor formed by the KEV, emptied by the preceding avalanche, lacks the time to fully charge. Very high counting rates can make the KEV unusable, probably due to overheating of the high-resistance layer.
The disadvantage of the KEVs compared to the SEVs is that the continuously running dynodes cannot be wired individually. Particularly with high pulse loads, the last dynodes of the SEVs are often protected from a voltage breakdown by capacitor circuits. This is not possible with the KEVs, so that in this case relatively long dead times (up to 15 µs) occur until the voltage has built up again.
The specific designs of KEVs can be very different. There are, for example, designs with an entrance funnel and a spiral-shaped sewer pipe. The latter improves the emission geometry and minimizes harmful echoes from returning ions that can be knocked out of the walls. Alternatively, compact sinusoidally corrugated KEVs ( Ceratron ) embedded in ceramics and other designs are often used today. The microchannel plate (MCP) can be described as a further development . It consists of many microchannels with a diameter of a few micrometers, each of which works like a channeltron.
See also
Individual evidence
- ↑ a b Jörg Hoffmann: Measuring non-electrical quantities: Fundamentals of practice . Springer-Verlag, 2013, ISBN 978-3-662-01173-7 , pp. 132 ( limited preview in Google Book search).
- ↑ Nuclear Physics: An Introduction . Springer-Verlag, 2013, ISBN 978-3-662-08061-0 , pp. 153 ( limited preview in Google Book search).
- ↑ a b Ingolf Volker Hertel: Atoms, Molecules and Optical Physics 2 - Molecules and Scattering Physics . Springer, ISBN 978-3-642-11973-6 .