Pulsed Radiography

A. A. Vorob'ev

In a number of cases it is necessary to record in space the position of a body moving at high speed in a medium opaque to visible light. For example, it is necessary to photograph moving parts of machines hidden by a casing from direct observation; to record individual stages in the development of the process of an explosion, a chain reaction, or the motion of a projectile inside a cloud of gases opaque to visible light; to establish the stressed state of a crystal lattice under elastic deformation, and so forth. In these cases, when visible light does not make it possible to obtain an image, one may resort to the aid of hard X-radiation.

The average detonation velocity of explosives reaches 5000 m/sec. Elastic shock waves in steel propagate at comparable velocities. Obtaining a distinct, unblurred image on a radiograph, for example of a rapidly flying bullet, is possible if the exposure is of the order of \(10^{-5}\) sec or less. The generation of X-rays during such a short interval of time can be accomplished by applying to an X-ray tube an equally short voltage pulse. Other possible methods of obtaining a brief flash of X-radiation directed at the object—such as pulsed heating of the cathode of an X-ray tube or the opening of some shutter placed between the source of the rays and the object—can provide an exposure, probably, of the order of \(10^{-4}\) sec or more. In this case it is assumed that the voltage between the tube electrodes is applied for a long time, which also has its disadvantages in comparison with pulsed application of the voltage.

In 1937, in the high-voltage laboratory of the Siberian Physico-Technical Institute, the author of the article developed the foundations of the method and circuits of pulsed radiography. The developed technique was successfully applied to the study of a number of phenomena proceeding at high speed and was filed as an application for an inventor’s certificate on March 10, 1938, No. 193/2, with the Department for Inventions of the People’s Commissariat of Defense Industry of the USSR.

The circuit of the installation for pulsed generation of X-rays was proposed as follows: a voltage pulse from a pulse-voltage generator is applied to the X-ray tube synchronously with the phenomenon being recorded at the given moment.

To produce an X-ray photograph during the short time of action of the X-rays, it is necessary to have the greatest possible radiation intensity, sufficient to penetrate the object and record its shadow image on photographic film.

It is known that the intensity of total X-radiation increases in proportion to the square of the voltage between the electrodes of the tube (Ulrey curves) and to the magnitude of the current flowing through the tube (Dauvillier curves).

To reduce the exposure at a given radiation intensity, it is necessary to use photographic films of the greatest possible sensitivity.

The use by us of an ionic X-ray tube with an iron anode, when a large pulsed current of the order of tens of amperes could be passed through the tube, showed that under these conditions a flash on the fluorescent screen is observed only at a distance of several centimeters from the tube window. The voltage on the tube electrodes reached 15–35 kV. This circumstance greatly limits the use of such an apparatus. Seeking to use standard X-ray equipment for taking photographs with a small exposure, we employed diagnostic and therapeutic electronic X-ray tubes with a heated cathode, and tubes for X-ray structural analysis.

In 1937 we had data showing that the electric strength of vacuum devices under pulsed voltage exceeds their electric strength under alternating or direct voltage by a factor of 1.5 or more. This made it possible for us to apply to the X-ray tube a pulsed voltage, for example, twice as high as its static operating voltage. This led to a significant increase in the total radiation of the tube (proportional to \(U^2\)) and to a reduction of the exposure time. Good results were obtained when using high-voltage X-ray tubes with radiation cooling, whose operating voltage was 180 kV and higher.

The advantage of an electronic X-ray tube with a heated cathode is the possibility of regulating, within considerable limits, the electron current through the tube and, consequently, the exposure.

The second important advantage of the electronic tube over the ionic one is the greater hardness of the X-rays that can be obtained with an electronic X-ray tube. It is known that as the hardness of X-rays increases, their penetrating power when transilluminating matter increases, and, consequently, the exposure is shortened, or, at unchanged exposure, the thickness of the layer of matter being transilluminated increases.

Electrons accelerated under the action of the high voltage applied between the tube electrodes bombard the anode. The greater part of the electrons striking the anode, in the process of braking, gradually, through numerous collisions with the atoms of the anode material, expend their kinetic energy. The energy of these electrons goes into raising the temperature of the anode. A very small part of the electrons—about 1%—is rapidly braked after a small number of collisions with the atoms of the anode material, and only then do X-rays arise.

The fraction of the kinetic energy of the electrons carried by the current through the tube and converted into X-radiation depends on the magnitude of the accelerating voltage applied to the tube poles, and increases as this voltage is raised. For example, at a voltage of 100 kV, only 1.11% of the energy carried through the tube by the electron current is converted into X-ray energy; at a voltage of 150 kV, 1.64%; at a voltage of 220 kV, 2.2%; and at a voltage of 2000 kV, already 5%. Calculations also showed that increasing the radiation intensity of the tube would not lead to dangerous overheating of the anode. This was justified in practice. At the same time, with the increase in voltage on the tube electrodes, we also increased the electron current through it by increasing the cathode heating current by 50–80% and more above normal. It is known that the efficiency of a thermionic cathode, i.e., the magnitude of the emission current referred to the unit of power expended on heating, increases rapidly with increasing temperature according to an exponential law. In practice, for a tungsten cathode this quantity lies within the limits of 4 to 10 milliamperes per watt of power expended on heating.

In some modern X-ray tubes for pulsed radiography, the current through the tube reaches 1000 amperes and more.

Fig. 1 shows one of the electrical circuits that served us for the synchronous generation of X-ray pulses. The voltage-doubling circuit outlined by a dashed line is the source of constant high voltage feeding the actual shock circuit, located on the right. The latter consists of capacitors \(C_2\) and \(C_2\), the spherical spark gap \(F\), leakage resistances \(R_2\) and \(R_2\), and the “tail” resistance \(R_3\).

The transformer T, through the kenotrons \(K_1\) and \(K_1\), charges the capacitors \(C_1\) and \(C_1\). Through the charging resistances \(R_1\) and \(R_1\), the left plates of the capacitors \(C_2\) and \(C_2\) connected to them are charged. Like-signed electric charges released on the right plates of the capacitors \(C_2\) and \(C_2\) leak to ground through the resistances \(R_2\) and \(R_2\).

Owing to the grounding of the right plates of the capacitors \(C_2\) and \(C_2\) at point \(b\), the electrodes of the X-ray tube (RT) normally also prove to be grounded. At the moment when a spark jumps in the spark gap \(F\) and the left plates discharge, the bound charges of the right plates are released and charge the line to a high potential, as well as the objects (RT) connected to it. At the same time, the charges in the right-hand part of the circuit leak through the resistances \(R_2\), \(R_2\), and \(R_3\). The superposition of these two processes—charging of the line and leakage of charges to ground and through the resistances \(R_2\), \(R_2\), and \(R_3\)—leads to the formation of a voltage pulse on the electrodes of the X-ray tube (RT). The duration of the voltage pulse is regulated by changing the values of \(C_2\), \(R_2\), and \(R_3\).

With a decrease in the value of \(R_3\), the duration of the voltage pulse and its amplitude will rapidly decrease. In the case where the value of the resistance exceeds the value of the wave impedance of the line, the phenomenon of wave reflection at the end of the line and an increase of voltage on the tube electrodes will occur.

Fig. 1. Electrical circuit for synchronous generation of X-ray pulses using a tube with a hot cathode (after Vorob’ev, 1937).

For synchronizing the moment of application of the voltage pulse with the motion of the body being recorded, one of many possible circuits for forced ignition of the pulse generator was used.

The moving body breaks a conductor (foil, wire) included in the primary circuit of the inductor at point \(d\). The opening extra-current in the secondary circuit of the inductor causes the appearance of a voltage pulse. Under the action of this voltage pulse, breakdown occurs between the auxiliary electrodes \(f\). Ionization of this gap in turn produces a spark in the ignition spark gap \(F\) and operation of the pulse-voltage generator circuit. As a result, a voltage pulse arises on the electrodes of the X-ray tube. In order to protect the inductor from penetration of a high-frequency, large-amplitude overvoltage wave, large inductances \(L\) were included in the circuit.

The synchronization circuit used operated satisfactorily. In the case of radiographing a moving bullet, the broken wire-

the filament was placed at a distance of 5–20 mm in front of the tube axis. The barrel from which the bullet emerged had to be set at a distance of 3–20 mm from the filament. If the filament is pulled taut by the barrel, then, probably, after the wire breaks its pieces slide for some time along the body of the bullet still emerging from the barrel. A prolonged electrical contact and a slow opening of the circuit result. In some cases this leads to disruption of the synchronization and to the absence of an image of the bullet on the photograph. Owing to the increased strength of the insulation under pulses, the entire setup is quite compact.

For radiographing bullets at the moment of their emergence from the barrel, it is more convenient to use other synchronization circuits.

Switching of high-voltage circuits can be carried out, for example, by means of Stekol’nikov’s spark-light relay. These circuits make it possible to produce individual switch-ons at intervals down to \(10^{-7}\)—\(10^{-8}\) sec.

Having at our disposal in 1937 photographic film with a sensitivity of 260 to 700 according to \(XD\), in order to improve the quality of the pictures we used sensitization of the photographic film and fluorescent screens.

Iso-panchromatic film was placed between two fluorescent screens in a cardboard or wooden cassette and was positioned appropriately with respect to the tube and the object being photographed. In our experiments the cassette was usually placed at a distance of 150–200 mm from the tube, and in individual cases even farther away. The X-ray tube was placed in a lead box, which protected the tube from mechanical damage and the personnel from scattered radiation.

By increasing the duration of the voltage pulse, and thereby increasing the exposure, we obtained an image of the bullet blurred along its axis. By determining the magnitude of the path traversed by the moving body during the exposure, from the degree of blurring of the image we find the velocity of this motion. Greater accuracy in determining the velocity of a body can be obtained if two or more of its successive positions are recorded at definite intervals of time. The construction of such a pulsed circuit is possible.

Subsequently, to obtain X-ray pulses of greater power, other authors also used X-ray tubes of two types.

1. Ion tubes with a cold or heated cathode.
2. Electron tubes and kenotrons with increased emission of the heated cathode.

In 1938, Kingdom and Tannis published their investigations on obtaining X-ray radiation pulses during discharge of a capacitor circuit through a special pulsed X-ray tube with a mercury cathode. In studying the biological action of X-rays, these authors dealt with a broad beam of rays.

Steenbeck, also working with a pulsed X-ray tube with a mercury cathode and anode, intended them for the radiography of metals. Using a gas discharge in a capillary, he obtained a large current through the tube, of the order of 1000 amperes. A narrow beam of electrons, well focused along the capillary axis, when bombarding the anode gave an X-ray beam whose focal spot had an area of about \(1 \text{ mm}^2\).

In 1940, Oosterkamp used an X-ray tube with a heated cathode for pulsed radiography. As in our experiments, using a tungsten cathode during its brief overheating, Oosterkamp increased the electron current through the tube from 0.4 to 20 amperes.

In 1942, Zuckerman and Avdeenko, at the Institute of Mechanical Engineering of the Academy of Sciences of the USSR, used a kenotron as a source of X-rays of high intensity,

When a large current passes through a kenotron, the internal voltage drop in it increases and X-rays arise. Their intensity may be very great. For example, in 1936 we observed, with the cathode of a KR-220-type kenotron at elevated temperature, radiation with which it was possible to carry out ordinary fluoroscopy—for example, of an arm, a leg, and so forth—at a considerable distance from the kenotron. The observations were made with an ordinary X-ray screen.

Figure 2 shows the circuit diagram for switching on an X-ray installation for exciting X-ray pulses using a kenotron as the radiation source. Capacitor 1 is charged from transformer 2 through two kenotrons 3. The source of X-rays is kenotron 4, connected in parallel with capacitor 1. When the microsecond switch 5 is closed, capacitor 1 undergoes a pulsed discharge through kenotron 4. The pulse of X-radiation from the anode of kenotron 4 gives an image of the object on screen 6. The voltage drop

Fig. 2. Electrical circuit for generating X-ray pulses using a kenotron as the radiation source (after Zuckerman and Avdeev).

across resistance 7, produced by the pulsed current, is observed with oscilloscope 8. In this way the magnitude of the pulsed current is measured.

In 1943, Al’tshuller, in the X-ray laboratory of the Institute of Mechanical Engineering of the Academy of Sciences of the USSR, proposed an ultrafast method of axial focusing of beams of rays and the application of pulsed radiography methods in structural analysis. Thus pulsed radiography was applied to the study of rapidly occurring structural changes in metals and alloys under mechanical stresses, during phase transformations, and so forth.

To record instantaneous deformations of the crystal lattice, the exposure time must not exceed several hundred-thousandths of a second. To study the transition of austenite to martensite, X-ray radiation pulses of duration \(10^{-4}\) sec. and less are necessary.

Methods have been developed for simultaneous photography of phenomena in visible and X-rays. For this purpose the light of a spark is used, which switches on the X-ray installation and the X-ray radiation pulse. The X-ray and optical images are then photographed separately, in different planes, on X-ray film and photographic film. As a result, for one and the same phase of the process, the spark records, for example during a shot, the outer boundaries of the gas flows, while the radiograph makes it possible to judge the distribution, in the cloud, of the main mass of the substance absorbing the X-rays.

Simultaneous photography of a process or of the motion of a body by means of visible and X-ray radiation, developed in the X-ray laboratory of the Institute of Mechanical Engineering of the Academy of Sciences of the USSR by Zuckerman, provides new possibilities for studying short-duration

processes, for example, the propagation of shock waves in gaseous and liquid media, the motion of a bullet in an opaque cloud of powder gases near the barrel bore, etc.

By switching on the pulsed installation successively many times, it is also possible to carry out radiography of heavy stationary objects that are also photographed by the usual static method. In this case the advantage of the pulsed X-ray installation is its greater cheapness and availability in comparison with the necessary static installation. A pulsed X-ray installation, being a more powerful source of radiation than a static one at the same operating voltage, also makes it possible to reduce significantly the total exposure time. By making use of the known increase in the electrical strength of insulating materials and high-voltage structures under the action of pulsed voltages, as compared with static voltages, one can construct compact high-voltage installations for pulsed radiography.

The method of pulsed radiography with multiple switching-on of the tube can be applied for photographing parts of machines rotating or moving at a constant speed, for example, an engine rotor, a turbine, an engine piston, etc. In this case the action of the pulsed installation must be synchronized with the periodic motion of the object. The installation can be used not only for purposes of flaw detection, but also for other purposes, for example, for adjusting fast-acting automatic devices. In the same way, a pulsed installation, when switched on synchronously, can be used in medicine, biology, and so forth, for example in studying the pulsating heart, respiratory organs, and others.

Since compact pulse-voltage generators and circuits for synchronizing their operation have at present been well developed technically, for the time being the possibilities for applying pulsed radiography are determined by the X-ray tubes. It is necessary to develop a powerful X-ray tube for high operating voltage, for example of the type of accelerator tubes used in nuclear engineering.

As powerful sources of very hard X-rays, betatron installations, synchrotrons, electrostatic generators with a moving belt (statitrons), and cascade generators may be used.

In these cases the accelerator installations must be provided with an additional control and synchronization circuit allowing them to be switched on for a specified interval of time.

LITERATURE

1. Vorob’ev A. A., Onchukov P. N., Bozhkov F. and Maksimovich F. F. Photographing a bullet emerging from the barrel bore by means of X-rays. Report on the work of the Siberian Physico-Technical Institute, High-Voltage Laboratory, 6/VII 1937.

2. Vorob’ev A. A. Device for photographing rapidly moving objects by means of X-rays. Application for an author’s certificate, 10/II-1938, No. 193/2. Department for Invention Affairs of the People’s Commissariat of Defense Industry of the USSR.

3. Vorob’ev A. A. The use of X-rays for recording and determining the velocity of rapidly moving bodies in an opaque medium. Proceedings of the Siberian Physico-Technical Institute, 26, 168–172, 1949.

4. Stekol’nikov I. S. The Electron Oscillograph. Gosenergoizdat, 1949.

5. Tsukerman V. A. and Avdeenko. Obtaining X-ray photographs with a very short exposure time. Journal of Technical Physics, vol. 12, nos. 4–5, 185–194, 1942.

6. Altshuler L. A. Theory of focusing of axial beams and methods of ultrahigh-speed structural radiography. Journal of Experimental and Theoretical Physics, 13, nos. 11–12, 388–398, 1943.

1. Zuckerman, V. A. Radiography of explosion and detonation processes. Reports of the Academy of Sciences of the USSR, vol. 40, no. 7, 305–308, 1943.

2. Strakhovsky, G., and Zuckerman, V. A. Circuits for precise synchronization in high-speed X-ray studies. Bulletin of the Academy of Sciences of the USSR, Division of Technical Sciences, no. 3, 371/385, 1946.

3. Zuckerman, V. A. A combined method for photographing very rapid processes. Reports of the Academy of Sciences of the USSR, vol. 53, no. 4, 323–326, 1946.

4. Uspensky, N. E. The pulsed (ultra-high-speed) method in radiology. Stenogram of a public lecture delivered at the Central Lecture Hall of the All-Union Society for the Dissemination of Political and Scientific Knowledge, 1949.

5. Ivanov, A. A. Electrovacuum Technology. Gosenergoizdat, 1944, pp. 381–382.