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ELECTRON-BEAM AMPLITUDE ANALYZER
In a number of problems of experimental and applied physics there arises the need to analyze automatically current pulses delivered by ionization chambers and proportional counters, according to the magnitude of their amplitudes.
In cases where it is necessary to count the number of pulses whose amplitudes are greater (or less) than a specified value, use is usually made of very simple analyzers built on vacuum tubes or thyratrons, which give at the output current pulses of the same magnitude, irrespective of the intensity of the pulses arriving at their input, provided only that their magnitude is greater (or less) than the specified response threshold.
In the same cases, when pulses are distributed according to the magnitude of their energy over several steps, differential amplitude analyzers are usually employed, consisting of several parallel-operating differential relay cells of the trigger type. Each of these responds to pulses whose intensity lies within specified limits.
Increasing the number of resolution steps (with a simultaneous narrowing of the width of each of these steps), necessary for a more detailed study of particle-energy spectra, leads to a disproportionate increase in the number of cells and thereby to a substantial complication of the circuit, the manufacture and adjustment of which present considerable difficulties. Thus, even with ten selection steps the number of vacuum tubes reaches 150.^{1,2}
The need to create simple and reliably operating amplitude analyzers of high resolving power has compelled a search for other principles of selection.
One of the most promising principles of amplitude selection of pulses has proved to be the method of electron-beam commutation,^3 developed】【。
from the one described earlier for decoding signals in telemechanical and automatic installations and known as amplitude cathode selection.
A significant distinction of this method is the use of a low-inertia current-distribution element, in which various circuits are directly closed by a beam of electrons deflected by sorting pulses applied to the deflection plates of an electron commutator.
The mechanism by which various circuits are closed by the electron-beam commutator is explained by its schematic diagram, shown in Fig. 1.
Fig. 1. Electron-beam commutator: 1 — electron guns, 2 — deflection plates, 3 — lamellae commutated by the electron beam, 4 — counters of pulses arriving at the lamellae.
The operation of this circuit is essentially as follows. Current pulses arrive after amplification at the deflection plates of an electron-beam tube, causing the electron beam to be deflected onto one of the commutator lamellae³. At the same time, a current pulse enters the circuit of this lamella and is counted by the corresponding pulse counter.
Thus, current pulses arrive in the circuits of the lamellae, the total number of which corresponds to the number of current pulses in the ionization chamber, while the distribution of pulses among the lamellae corresponds to the distribution of pulses by intensity.
On its way to a given lamella the electron beam passes over the remaining lamellae, producing in them considerably shorter current pulses. From the standpoint of improving the reliability of operation of the circuit, it is advisable to eliminate these parasitic pulses. For this purpose it is recommended to switch on the electron beam only for the time during which it closes the end lamella. Cutting off the electron beam for the duration of the amplitude value of the current pulse is carried out with the aid of a special auxiliary device.
When choosing the design of electron-beam commutators that make it possible to ensure the most reliable operation at minimum dimensions, commutators with flat beams² appear to be the most expedient. A characteristic feature of these commutators³,⁴ is the use of electron beams with a small angle of divergence, produced by the longitudinal acceleration of electrons in a uniform field; a filamentary directly heated cathode is located in the slit-like aperture of a plane control electrode (Fig. 2). In a plane anode, arranged parallel to the control electrode, a slit is made, oriented parallel to the slit of the control electrode. In one of the designs of a commutator of this type, with the distance between the anode and the control electrode equal to 10 mm, the dimensions of the slit in the anode 0.5×5 mm, and an anode voltage of about 600 V, the beam current proved to be on the order of a milliampere.
The dimensions of the transverse section of the electron beam on a screen located at a distance of 50 mm from the anode were approximately 3×8 mm.
The visible dimensions of the spot on the screen proved to depend only slightly on fluctuations of the anode voltage over fairly wide limits. Further reduction of the width of the electron beam is accomplished by means of one or two slit diaphragms with the same opening as in the anode, placed after it and parallel to it.
The decrease in the width of the electron beam during diaphragm limiting, which leads to an inevitable decrease in the beam intensity, can be quite readily compensated by secondary-electron amplification of the beam current at the lamella.
Opening and closing of the electron beam of the commutator is carried out either by means of a flat control electrode located near the cathode, or by means of a special electrode placed immediately behind the anode in the direction of beam travel.
Fig. 2.
The use of small-sized and reliable electron-beam commutators with flat, low-divergence beams made it possible to create relatively simple distribution devices, which proved to be more reliable, stable, and much simpler in design than multi-channel differential analyzers with conventional electron tubes.
In Watkins’s cited work,2 the method of electron-beam amplitude selection was used (without proper references) in a pulse analyzer for the automatic counting of particles of various energies, in order to obtain spectra of fast particles. This setup proved to be much simpler in design, more reliable in operation, and at the same time considerably more sensitive than analyzers of the energy spectra of fast particles with differential amplitude analyzers based on ordinary electron tubes.
Thus, in particular, with ten resolution steps the width of one step is equal to \(1\ \text{V}\), which gives the analyzer twice the sensitivity in comparison with the setup of Freundlich, Hinks, and Ozerov.1
The electron-beam commutator used in Watkins’s setup has a double diaphragm to limit the width of the flat electron beam, considerably reducing its intensity. Therefore, secondary-electron amplification is used to amplify the current pulses in the lamella circuit of the latter.
Watkins’s setup is assembled from standard units manufactured by industry. Only the electron-beam commutator was specially manufactured for this setup.
There is clearly a possibility of significantly simplifying the entire setup by using more suitable parts and units for it.
L. A. Goncharskii
References
- Freundlich, Hinks, Ozerov, Rev. Sci. Instr. 18, 90 (1947).
- D. A. Watkins, Rev. Sci. Instr. 20, 425 (1949).
- L. A. Goncharskii, Avtomatika i telemekhanika 1, 27 (1936).
- L. A. Goncharskii, ZhTF 8, 540 (1938).