EQUIPMENT FOR STUDYING THE INTERACTION OF FAST ELECTRONS WITH MATTER

V. M. Antonov, V. V. Evstigneev, B. A. Kononov

Introduction

Clarifying the regularities of the passage of electrons through matter is necessary for solid-state physics, radiation chemistry, and other questions of an applied nature. A large number of experimental and theoretical works have been devoted to the study of the interaction of electrons with matter. However, up to the present time little attention has been paid to the processes of interaction of electrons with matter with allowance for the chemical composition of the latter, and also to allowance for multiple scattering, which leads to an increase in losses in thick layers.

The use of thick specimens for study leads to large energy losses, consisting of a larger number of elementary processes that, in combination, form a loss-distribution function of the type shown in Fig. 1. When working with thick specimens it is necessary to use electrons of high energy. However, in doing so one must bear the following in mind: excessively high electron energies lead to large radiation losses, while excessively low energies lead to significant collision losses.

Fig. 1. Spectrum of electron energy losses during passage through a layer of matter.
$\Delta E_p$ — most probable losses, $\Delta E$ — mean losses.

The most convenient range for studying the influence of chemical composition on the mechanism of interaction of electrons with matter is the range

energies of 2–10 MeV [1], since in this region the losses due to bremsstrahlung are small, and the effect of increasing the effective thickness of the sample due to multiple scattering is small. As a source of electrons it is desirable to use a device that would generate practically monoenergetic electrons. Such a device may be a betatron with an extracted electron beam, having a narrow electron energy spectrum [2, 3].

The present paper describes apparatus developed by us that makes it possible to carry out physical investigations using the electron radiation of a betatron. The use of a circuit for stabilizing and monitoring the electron energy makes it possible to conduct experiments with an accuracy of tenths of a percent.

Design of a 10 MeV betatron for physical investigations

For carrying out investigations with the extracted electron beam of a betatron, a good access to the accelerator chamber is an important and necessary factor. For these purposes, a 10 MeV U-shaped betatron has been developed and built (Fig. 2). The betatron poles have overhangs and a height twice as large as that of the poles of ordinary betatrons of the same energy, which makes it possible to make the magnetizing winding with a diameter not exceeding the outer diameter of the pole overhangs. Thus, the dimensions of the poles together with the magnetizing windings are equal to the width of the betatron yoke, and, consequently, the exit window of the accelerator chamber is completely free and convenient for placing various irradiated samples or physical instruments directly at the exit window.

Fig. 2. Design of a 10 MeV betatron for physical investigations.

Extraction of the electron beam is carried out by an electrostatic method [2]. The extraction efficiency is 60%. The average current of the extracted electron beam of this betatron is \(10^{-8}\) A. Monitoring of the intensity of the electron beam is carried out by a transmission ionization chamber operating with a standard “Kaktus” microammeter. The betatron is equipped with a circuit for stabilizing and monitoring the energy of the electrons extracted from the betatron; its block diagram is presented in Fig. 3. The circuit provides smooth regulation of the energy and its maintenance with an accuracy better than 0.1%. The betatron is mounted on a movable stand and can rotate about a vertical axis.

Fig. 3. Block diagram for stabilization and monitoring of the energy of electrons extracted from the betatron.

Focusing and Alignment of the Betatron Electron Beam

The electron beam brought out of the accelerator chamber directly into the atmosphere has the following parameters at the chamber window.

1. Beam current \(10\text{--}8\ a\).
2. Maximum beam divergence in the horizontal plane \(5^\circ\) and in the vertical plane \(1^\circ\).
3. Beam size horizontally \(10\ \mathrm{mm}\) and vertically \(6\ \mathrm{mm}\).
4. Beam dimensions at a distance of \(50\ \mathrm{cm}\) from the window: \(90 \times 50\ \mathrm{mm}^2\).

Taking into account the large divergence, a device has been developed that makes it possible to impart the required dimensions to the electron beam at various distances from the betatron (Fig. 4).

Fig. 4. Schematic of the experimental setup.
1 — betatron, 2 — deflector, 3 — magnetic lenses, 4 — magnetic correctors, 5 — entrance slit of the spectrometer, 6 — magnet of the beta spectrometer, 7 — exit slit of the spectrometer, 8 — double ionization chamber.

Focusing of the electron beam is carried out by magnetic quadrupole lenses. The design of the betatron magnet makes it possible to place them directly at the exit window of the accelerating chamber, which ensures focusing of the electron beam into a spot of size \(5 \times 10\ \mathrm{mm}^2\) at a distance of two meters from the betatron. A system of four magnetic correctors makes it possible to shift the electron beam parallel to the axis of the latter by a distance of \(\pm 30\ \mathrm{mm}\) and provides smooth adjustment of the beam position relative to the entrance slit of the beta spectrometer.

The magnetic lenses and correctors are powered by a storage battery with a capacity of 1000 ampere-hours.

Magnetic Beta Spectrometer

The high intensity of electrons in the focused beam makes it possible in a number of cases to use a magnetic beta spectrometer without double focusing. We use a sector-type magnetic beta spectrometer with flat pole pieces (Fig. 5), having the following basic parameters: deflection angle \(90^\circ\); working ra-

radius 150 mm, interpole distance 20 mm, magnetic-field strength 3500 oersteds. The median plane of the magnetic field of the spectrometer is horizontal.

The magnetic circuit of the spectrometer is made of St-3 steel. The spectrometer is powered by a storage battery with a capacity of 1000 ampere-hours. The magnetic field in the working gap was measured by means of a ballistic galvanometer and measuring coils that rotate through 180°.

Fig. 5. Construction of the magnetic beta spectrometer.

Fig. 6. Magnetic characteristic of the beta spectrometer.

The degree of inhomogeneity of the magnetic field in the working gap, measured by the differential method, does not exceed 0.05%. The magnetizing winding is divided into two equal sections, which can be connected in series or in parallel depending on the power source. The magnetic characteristic of the spectrometer is shown in Fig. 6. It is seen that the magnetic characteristic in the energy range under study, 2–10 MeV, is an almost linear function.

A vacuum down to \(10^{-6}\) mm Hg is maintained in the chamber of the spectrometer. The spectrometer diaphragms are located at a distance of 150 mm from the edges of the magnet sector. With a slit width of 0.1 mm for the entrance and exit diaphragms, the resolving power of the spectrometer is 0.05%.

In carrying out certain experiments, for example in studying electron backscattering, the intensity of the backscattered beam may be small. In addition, when studying the energy losses of electrons in very thin layers of matter, very high resolution at high luminosity is required; this can be achieved by using a magnetic beta spectrometer with double focusing. Therefore, in addition to the spectrometer described above, we have developed a sector-type magnetic beta spectrometer with double focusing, having flat and parallel pole pieces [4] (Fig. 7). Focusing in the median plane is provided by a homogeneous magnetic field in the gap; focusing in the plane of the magnetic field is achieved by the decrease of the magnetic field toward larger radii at the entrance and exit of the particles from the magnet. This spectrometer is intended mainly for studying the energy losses of electrons in matter in the energy range 0–35 MeV.

Radiation Detectors

In the case of using a sector magnetic beta spectrometer without double focusing, the electron detector at the exit from the spectrometer is a double ionization chamber (Fig. 8). With differential connection, the currents of the individual chambers caused by the gamma background of the accelerator are directed toward one another, which leads to automatic elimination of the background [5]. Provision is made for filling the chambers with a heavy inert gas in order to increase sensitivity. The chamber operates with a standard electrometric amplifier, making it possible to measure currents of \(10^{-13} \div 10^{-6}\) A. The chamber is calibrated by means of a Faraday cylinder.

Fig. 7. Diagram of a beta spectrometer with double focusing.
1—preliminary diaphragm, 2—sample under study, 3—entrance diaphragm, 4—exit diaphragm, 5—radiation detector, 6—radiation detector, 7—entrance chamber, 8—central chamber, 9—exit chamber, 10—working radius — 300 mm

In the case of using a sector magnetic beta spectrometer with double focusing, the radiation detectors are scintillation counters using FEU-12B photomultipliers, one of which is located immediately behind the entrance diaphragm of the spectrometer, while the second is placed at the exit from the spectrometer. The scintillator of the detector located behind the entrance diaphragm has a slot through which the main beam passes; part of the beam traveling in the direction of the pole pieces strikes the edge of the scintillator. The presence of two detectors is necessary in order to eliminate the error that arises in determining

forms of the energy-loss distribution curve due to fluctuations in the intensity of the betatron radiation. If, as the intensity quantity, one uses not the readings of the detector located at the exit of the spectrometer, but the ratio of the readings of the detectors described above, then this error is eliminated [6].

Fig. 8. Double ionization chamber.

In conclusion, it should be said that the trajectory of the electron beam in the various parts of the apparatus is determined by us by photographing the beam at the required location.

REFERENCES

1. T. Westermark, Nucl. Inst. and Meth. 10, 129 (1961).

2. B. A. Kononov. Extraction of electrons from a betatron by an electrostatic method. Izv. TPI, vol. 94, 1958.

3. V. M. Antonov, B. A. Kononov, Yu. N. Beltyaev. Study of the spectrum of electrons extracted from a betatron. Collection of Proceedings of the Fourth Inter-University Conference on Electron Accelerators. Tomsk, 1962.

4. V. M. Antonov, V. V. Evstigneev, B. A. Kononov. Program and abstracts of reports of the 14th annual meeting on nuclear spectroscopy in Tbilisi, p. 107. Nauka Publishing House, 1964.

5. T. Westermark. Nucl. Inst. and Meth. 10, 145 (1961).

6. G. Knop, A. Minten, B. Nellen, Z. Physik 166, 533 (1961).