Secondary electron multiplier

from Wikipedia, the free encyclopedia
Secondary electron multiplier for the detection of electrons. On the left is the entrance opening; the many plates to the right are holding plates for the dynodes (for installation in a vacuum ).

A secondary electron multiplier (SEV) is an electron tube in which the smallest electron currents or even single electrons with high time resolution can be amplified by many orders of magnitude up to measurable sizes through secondary electron emission.

They are used in photomultipliers , image intensifiers (residual light intensifiers ) and detectors for elementary particles. Because of the sequence of numerous electrodes of the same type at gradually decreasing voltage levels, the tube is also called a cascade tube.

functionality

When it hits a metal surface or semiconductors or other poorly conductive substances, if there is sufficient energy, an electron generates several so-called secondary electrons , which leave the body in the surrounding vacuum. A shallow angle of incidence and a low work function enhance this effect. Coatings such as B. a monolayer adsorbate (z. B. water) change the work function and thus the multiplication factor.

In this way, several free electrons of lower energy can arise from one free electron and a multiplication by the factor δ of 3 ... 10 takes place. This factor is extremely sensitive to the acceleration voltage between the electrodes and must therefore be very well stabilized. The ejected electrons are accelerated by an electric field to the next electrode ( called dynode ) in order to generate further secondary electrons there. In this way, a current gain of δ n is achieved with a number of n dynodes connected in series. With seven dynodes and a total voltage of 1500 V one can achieve gains of about 10 7 ( see also: photomultiplier ).

The chain of dynodes with increasing potential can also be replaced by poorly conducting channels, along which the potential increases and inside which new secondary electrons are constantly generated ( see also: channel electron multiplier , microchannel plate ).

Designs

Parallel, discrete multipliers are also offered in a common version.
The flight times of the electrons limit the reaction speed, so small designs are more suitable for high time resolution.

Electronics and special features

Fast microchannel plate electronics ( MCP electronics) with time resolution in the sub-ns range for spatially resolved measurement

Discrete (with dynodes working) secondary electron multiplier dynode need of rising to dynode voltage (typ. 200 V voltage difference between the dynodes). These voltages are generated from the total voltage (e.g. 2 kV) with a voltage divider.
The last electrode (anode, most positive potential) is usually at ground potential via a working resistor in order to simplify the subsequent evaluation electronics, especially for analog measurements.

Depending on the application, the evaluation consists of a current measurement (measurement of the smallest electron currents or radiation intensities), a time integration of the current (measurement of the smallest amounts of electrons or light) or a very fast semiconductor amplifier with the possibility of determining the pulse height (counting individual electrons, events and their Energy).
Such a pulse amplifier has a discriminator and a crossover to distinguish the "real" pulses from noise and fluctuations in the DC voltage source.

In order to achieve a high time resolution down to the ns range, the dynode spacing must be as small as possible (typically a few mm) in order to reduce runtime effects. The shape of the dynodes or additional grids reduce the time dispersion caused by electrons flying in different directions.

In the case of microchannel plates (MCP), the overall structure is particularly elegantly simplified: microscopically fine channels made of poorly conductive material form both voltage dividers and, inside, the surface for triggering the secondary electrons. A number of adjacent channels also enable spatial resolution.
The location can be achieved by sucking off and imaging the emerging secondary electrons ( image converter tube ) or by striking a transverse delay line or a waveguide, with the times at which the pulse arrives at both ends of the line.

The detection of individual electrons is also possible with microchannel plates (MCP).

Two microchannel plates can also be connected in series, with the gain at the end of the second plate being saturated.

To detect and identify charged particles, one must be able to adjust the potential of the anterior microchannel plate .

The secondary electron process has a lot of noise, on the one hand because the channels can be hit differently by a particle, on the other hand due to thermal effects. In order to obtain single pulses of the same height, one uses microchannel plates with such fine and long channels that the gain at the end of one channel in the second plate is saturated due to the space charge.
In this way, pulses of a defined height are obtained with a flank that is in a fixed temporal relationship to the time of arrival of the particle.

application