A Device for Monitoring the Density and Continuity of Large-Thickness Building Structures Using Betatron Bremsstrahlung

V. A. Vorob’ev, V. A. Vinokurov

(Presented to the scientific seminar of the Research Institute of Electronic Introscopy)

In the practice of construction work, it is often necessary to measure the density of structures or materials. One of the methods that makes it possible to measure the density of a material or structure without impairing its integrity is based on the use of penetrating radiation from various sources.

The X-ray apparatuses and radioactive isotopes currently used for this purpose make it possible to inspect thicknesses not exceeding 40–50 mm of structural concrete [1, 2].

A further increase in the thicknesses accessible for inspection can be achieved by increasing the radiation energy [3]. The use of a betatron as a source of high-energy bremsstrahlung [4] for monitoring the density and continuity of building structures makes it possible to radiograph an article made of structural concrete up to 2 meters thick [5].

For the purpose of carrying out rapid monitoring of the density and continuity of building materials and structures under production conditions, a device was developed that permits continuous monitoring of moving articles.

Because the betatron produces a directed flux of bremsstrahlung with a small divergence angle, the use of compensation circuits ordinarily employed in work with radioactive isotopes [6] is difficult. The instability of betatron radiation intensity in the presence of a stabilization circuit is approximately 1% [4]; therefore, the use of circuits for direct measurement of the radiation intensity beyond the absorber gives a large error.

Measurement of the density of the object under inspection by means of our device is based on measuring the radiation intensity before and after passage through the object under inspection (Fig. 1).

The radiation flux is recorded by means of the scintillation counter of remote unit 2 and, after passing through absorber 3, enters the scintillation counter of remote unit 4.

The signals from units 2 and 4 are processed by an electronic circuit and then compared.

We used an information-processing circuit employing amplitude–time transformation [7].

Let us consider the operation of this circuit using one channel as an example, Fig. 1. Pulses of positive polarity, taken from the last dynode of the FEU-19M [5, 6], are fed to the cathode follower 7, 8, mounted in a remote unit, and then to the amplitude discriminator 9, 10, whose discrimination threshold is selected so as to cut off noise pulses. From the discriminator the pulses enter the differential amplifier 11, 12 [8]. The differential circuit ensures high stability of the gain coefficient of both channels.

From the amplifier the signals are fed to an amplitude–time converter 13, 14, at whose output rectangular pulses are obtained, the duration of which is proportional to the signal amplitude at the converter input. These rectangular pulses are power-amplified by the cathode follower 15, 16 and fed to the integrator.

Fig. 1. Block diagram of the device for monitoring the density and continuity of concrete structures.
1 — radiation source; 2, 4 — radiation detectors in remote units; 3 — object under inspection; 5, 6 — FEU-19M photomultipliers; 7, 8 — cathode followers; 9, 10 — discriminators; 11, 12 — amplifiers; 13, 14 — cathode followers; 15—16 — amplitude–time converters; 17 — balanced amplifier; 18 — stabilized 300-volt power supply; 19 — VSV-0.25.

At the output of the integrator we obtain a pulsating voltage \(u_1\) (from the second, \(u_2\)), which is determined by the amplitude and duration of the charging pulse \(u_1 = f(At)\). Since from converters 13 and 14 we obtain rectangular pulses of the same amplitude but of different duration, \(U_c\) will depend only on the duration of the arriving pulses. Consequently, the quantities \(U_{c1}\) and \(U_{c2}\) will be determined by the amplitudes of the pulses taken from the scintillation counters 2—5 and 4—6.

The voltage pulses \(U_c\) and \(U_{c2}\) are compared by a balanced cascade. The comparison results, proportional to the measured attenuation of radiation in the object under inspection, are recorded on the tape of an EPP-09 self-recording potentiometer and also by means of a pointer instrument calibrated in density units.

The circuit used has the following advantages:

1. Measurement error due to drift of the radiation-beam axis is eliminated.

2. Measurement error due to instability of the radiation intensity is reduced.

3. It has high accuracy and fast response, and is simple and reliable in operation.

Studies of this device showed that, at the maximum bremsstrahlung energy of the betatron of 30 MeV and a radiation intensity of 150–200 roentgens per minute, it is possible to inspect structures

Fig. 2. Schematic electronic circuit of the device for monitoring the density and continuity of concrete structures.

made of structural concrete with a unit weight of 2.3–2.4 t/m\(^3\) and a thickness of up to 2 m [9]. The sensitivity to the detection of discontinuities in a concrete structure is 2–3% of the thickness being radiographed, for an overall structure thickness of more than 1 m.

REFERENCES

1. V. G. Firstov. The Use of Radioactive Isotopes in Construction. Stroiizdat, Moscow, 1964.

2. E. Rohl. Kerntechnik in Banwesen. VEB Verlag für Banflesen, Berlin, 1964.

3. A. A. Vorob'ev, V. A. Vorob'ev, V. I. Gorbunov. Nondestructive methods for testing materials and products. Collection of articles edited by Prof. S. T. Nazarov. ONTIPribor, Moscow, 109, 1964.

4. L. M. Anan'ev, A. A. Vorob'ev, V. I. Gorbunov. Induction Electron Accelerator—Betatron. Gosatomizdat, Moscow, 1961.

5. V. A. Vorob'ev. Candidate dissertation, Tomsk, 1965.

6. A. K. Val'ter, I. N. Plaksin, M. L. Gal'din. Automatic Control of the Density of Iron-Ore Pulp by Gamma Rays. Publishing House of Kharkov University, Kharkov, 1962.

7. A. M. Bonch-Bruevich. The Use of Electron Tubes in Experimental Physics. Gosatomizdat, Moscow, 1950.

8. Bulgakov. Electronic Devices of Automatic Control. Gosenergoizdat, 1953.

9. V. A. Vorob'ev, V. A. Vinokurov, P. N. Musunov. Development, manufacture, and study of a device for contactless measurement of the density of concrete using betatron bremsstrahlung. Registration certificate No. 42673 of February 15, 1964.