SOURCE OF IONIZING RADIATION

O. V. SOKOLOV, Yu. Yu. SIDORIN

(Presented by the scientific seminar of the Research Institute of Electron Introscopy)

For the purposes of flaw detection and introscopy of thin-walled articles made of light metals and plastics, X-ray installations are used primarily (for example, RUP-200-20-5). In some cases such installations do not satisfy high-productivity radiation inspection, where parameters such as radiation intensity and exposure time play the principal role. In this respect, pulsed sources of ionizing radiation operating in various modes are of interest; these may include electron accelerators known as “febetrons” [3]. Febetrons are two-electrode tubes operating in the regime of an incompletely developed high-vacuum pulsed breakdown, which leads to the acceleration of electrons with a current of 1–10 kA. A sealed two-electrode electron tube [2] with an energy of 150 keV has been prepared for serial production; its autoemission cathode is capable of producing currents of hundreds of amperes per pulse.

In [1] the mechanism of electrical breakdown of a vacuum gap and the possibility of obtaining accelerated-electron currents of very high strength are described. The appearance of this current is due to the “explosion” of cathode microprotrusions under the action of a strong electric field. The magnitude of the current drawn from the frontal surface by the electric field obeys the “three-halves power” law and depends on the rate of rise of the leading edge of the voltage pulse.

In the laboratory of the Research Institute of Electron Introscopy at Tomsk Polytechnic Institute, we developed and investigated an electron source based on a vacuum discharge. The design of the source discharge tube is a glass chamber with sealed-in electrodes (cathode and anode). The chamber has a vacuum plug with a window for extracting electrons into air (Fig. 1). The plug is necessary to provide possible access to the electrodes when replacing them and adjusting the interelectrode distance. As the generator feeding the tube, a very common circuit of a high-voltage pulse generator was chosen; an analogous circuit is given in [5]. Such a generator made it possible to obtain high-voltage pulses of negative polarity with an amplitude up to 200 kV and a frequency of 50 Hz. To reduce the duration of the voltage front, a spark gap is used, connected in series with the discharge tube. Owing to the delay in breakdown of the spark-gap interval, an overvoltage is produced, and upon its breakdown the pulse front is shortened. Calculations show

[4] that, even at a pressure of 1 atm with a single discharger, the pulse front can be reduced to \(10^{-8}\) s. The sharpening efficiency at the discharger was no worse than 5; in this case the voltage-pulse duration was 400 ns with a leading edge of about 50 ns.

Fig. 1. Circuit for measuring currents and voltage: 1—cathode; 2—anode; 3—collector; 4—plug; 5—Rogowski coil.

Figure 1 shows the circuit for measuring currents and voltage. The current in the anode circuit was measured with a Rogowski coil, and the electron current at the collector was measured from the voltage drop across a resistance \((R_{\mathrm{sh}}=0.16\ \Omega,\ L=0.04\ \mu\mathrm{H})\); the discharge current was determined as the sum of these two currents. Figure 2 shows the dependences of the discharge current and the collector current on the applied voltage at a pressure in the chamber of \(10^{-5}\) torr. The cathode was tantalum foil \(0.1\) mm thick, rolled into a cylinder \(2\) mm in diameter. The maximum current of the accelerated-electron beam at \(U=200\) kV was 240 A in the pulse, which is about 75% of the discharge current (320 A). The current of electrons extracted into air through a window made of polyethylene film \(0.05\) mm thick was \(5\div20\) A in the range of applied voltages \(100\div150\) kV.

Fig. 2. Dependences of the discharge current \(I_{\mathrm{p}}\) and the collector current \(I_{\mathrm{k}}\) on the applied voltage.

At a residual-gas pressure in the tube of \(10^{-1}\div10^{-2}\) torr, most of the current flows through the entire volume of the tube outside the discharge gap. In the pressure range \(10^{-3}\div10^{-6}\) torr, a normal vacuum discharge develops and no noticeable changes in the flow of the currents are ob—

was observed; evidently, in this pressure range the residual gas has little effect on the development of the discharge.

When the tungsten mesh of the anode was replaced by tantalum foil 0.15 mm thick, the accelerated electrons produced X-ray radiation in it. The dose rate, as a function of the residual-gas pressure, was unchanged and depended only weakly on the applied voltage in the range \(10^{-1}\)—\(10^{-2}\) torr, amounting to 100–200 mR/h at a distance of 15 cm from the anode. Reducing the pressure below \(10^{-3}\) torr led to stabilization of the current flow in the accelerating interelectrode gap, and already at 100 kV the dose rate was 3000 mR/h. When the voltage on the tube cathode was increased to 200 kV, an increase in the dose rate to 8000 mR/h was observed. Figure 3 gives the distribution of the dose rate over the beam cross section at a distance of 8 cm from the anode for two cathode configurations. The dependence was recorded at a tube voltage of 100 kV and a pressure in the chamber of \(5 \cdot 10^{-5}\) torr, using a thimble ionization chamber. A “Kaktus”-type roentgenometer served as the measuring instrument. As can be seen from the graph, the cylindrical foil cathode has a focusing property in comparison with the needle cathode.

Fig. 3. Relative distribution of the dose rate over the beam cross section: 1 — cylindrical cathode; 2 — pointed cathode.

When aluminum 10 mm thick was irradiated, the radiographic sensitivity of the source was determined. It was of the order of 1%. When fabric with an applied defect (for example, a thin layer of paint) was irradiated, an image was obtained on which both the defect and the structure of the fabric itself are clearly indicated. In this case, a few X-ray flashes are sufficient for a high-quality image. This indicates that, with the aid of this source, flaw detection of thin-walled products (printed circuit boards, emulsions, films, etc.) can be carried out with high productivity.

In conclusion, one may note the possible areas of application of an electron source based on a vacuum electric discharge:

a) flaw detection of thin-walled products made of light metal and plastics;

b) structural investigation of rapidly moving, dynamically loaded machine parts, as well as observation of flow during casting processes;

c) in experiments on the acceleration of electrons in various media (gases, dielectrics), where a source of electrons with large pulsed currents is required;

d) in electrophysical installations as an injector; for example, it is proposed to be used in a linear induction electron accelerator, since this accelerator is capable of accelerating currents of hundreds and thousands of amperes.

When a sealed-off vacuum tube is used, the source can become portable and have small dimensions and weight. In this case, for purposes of flaw detection, such a source can compete with X-ray apparatuses of similar energy and in many respects surpass them.

REFERENCES

1. I. N. Slivkov. Electrical Insulation and Discharge in Vacuum. Moscow, Atomizdat, 1972.

2. N. G. Pavlovskaya. Abstracts of Reports of the Conference on Accelerators. Tomsk, Tomsk Polytechnic Institute Publishing House, 1972.

3. W. P. Dyke. U.S. Patent, class 250—49.5, No. 33173006, published 9.03.65.

4. G. A. Vorob’ev, G. A. Mesyats. Technique for Forming High-Voltage Pulses. Moscow, Gosatomizdat, 1963.

5. P. V. Belkin. “Instruments and Experimental Techniques,” 1972, No. 2, 194.