DEVELOPMENT AND STUDY OF A BREMSSTRAHLUNG MONITOR FOR A COMPACT BETATRON

A. V. RATNIKOV, M. M. SHTEIN

(Presented by the Research Institute of Nuclear Physics, Electronics, and Automation)

Unlike stationary installations, a portable compact betatron imposes more stringent requirements on the radiation monitor. The dimensions of existing commercially produced roentgenometers differ little from the dimensions of such an accelerator as the PMB-6 [1]. It is obvious that, for compact betatrons, in order to improve mobility and other operational characteristics associated with the specifics of their use, a compact, reliable monitor with a simple electronic circuit is required—one that would not constitute a separate unit of the accelerator installation, but would be placed in the useful volume of the emitter. A monitor installed in the emitter is subjected to its thermal influence and must ensure stability of its characteristics over a wide temperature range.

Fig. 1. Dependence of the gate current of a field-effect transistor on temperature

A monitor consisting of a compact FEU-60 photomultiplier and a NaI(Te) crystal, Ø 10×10, with which the industrial prototype PMB-6 is presently equipped, has a number of substantial shortcomings associated with the nature of the scintillation method.

The phenomenon of “fatigue” of dynodes at high pulsed currents, the sharp dependence of the photomultiplier gain on the supply voltage, and the phenomenon of “saturation”—all this causes ambiguity in the instrument readings. The change with time in the efficiency of the crystal and in the sensitivity of the photomultiplier requires frequent calibration of such a monitor, which complicates operation of the accelerator under nonstationary conditions.

These reasons led to the need to develop a new monitor for compact betatrons.

As is known, one of the best measuring instruments is the ionization chamber, whose current is proportional to the radiation dose rate. The only drawback in this case is the small magnitude of the current, which requires the use of amplifiers, usually made with special electrometer tubes [2]. The amplifier, or its electrometer stage, is as a rule structurally combined with the ionization chamber. This leads to an increase in the dimensions of the external unit and makes it difficult to place it in the radiator of a small-size betatron. In addition, direct-current amplifiers using electrometer tubes have large time drift.

Fig. 2. Schematic design of a monitor based on an ionization chamber and its placement in the radiator of a PMB-6 betatron

The authors investigated the possibility of using field-effect transistors in the monitor amplifier, which have a low leakage current of the control electrode—the gate. Studies of various types of field-effect transistors made it possible to establish that KP301B transistors with an insulated gate have the best characteristics.

The averaged temperature dependence of the gate current at a drain voltage of 10 V and a drain current of 1 mA, measured for 20 KP301B transistors, is shown in Fig. 1. The data obtained show that the transistors

KP301B can be used to amplify currents on the order of \(10^{-12}\) A over a wide temperature range. It should be noted here that the relative spread of gate currents of individual specimens does not exceed \(2 \div 3\) in the temperature interval studied.

The schematic design of the developed monitor and its placement are shown in Fig. 2.

Fig. 3. Electrical circuit of the monitor

To eliminate the influence of fast electrons present in the betatron bremsstrahlung and distorting the monitor readings, a lead shield \(2\) mm thick is attached to the high-voltage electrode.

Fig. 4. Temporal and temperature instability of the monitor readings: \(\delta\) — zero drift at \(P = 18\) R/h; \(a\) — monitor zero drift with the betatron switched off (temporal instability); \(v\) — drift of the monitor readings at \(P = 18\) R/h; \(g\) — change in the temperature of the field-transistor case

Fig. 5. Dependence of the monitor readings on the magnitude of the bremsstrahlung dose rate

The working volume of the ionization chamber is about \(50\ \mathrm{cm^3}\). The monitor is installed in the output window of the radiator at a distance of \(20\) cm from the target. At a bremsstrahlung dose rate of \(18\) R/h·m, the chamber current is \(2 \cdot 10^{-9}\) A.

The monitor’s recording circuit consists of the simplest current amplifier, using a single transistor connected as a source follower, and a 50 μA pointer microammeter installed in the betatron control panel (Fig. 3). The small dimensions of the amplifier make the monitor sufficiently compact, which makes it possible to place it in the betatron emitter without particular difficulty.

In the course of the experimental study of the monitor, its main characteristics were measured. Fig. 4 shows: the time drift of the readings of the recording instrument with the betatron switched off (curve a), the zero drift (b), and the stability of the readings (c) during six hours of continuous operation of the betatron. Simultaneously with the stability study, the temperature of the transistor case (d) was measured with a thermocouple. When curve (c) was recorded, the dose rate of the betatron bremsstrahlung was kept constant.

The data obtained show that time drift is practically absent and, consequently, the stability of the readings with the betatron in operation is associated primarily with the change in temperature at the location where the monitor is installed. After the establishment of the temperature regime, which occurs after 3.5 hours, the monitor readings also stabilize. The maximum deviation is relatively small and does not exceed 5%, which satisfies practical requirements.

It should be noted that the drift of the readings is determined not by a change in the gate leakage current, since even at the highest temperature the leakage current is several orders of magnitude smaller than the current of the ionization chamber, but by the temperature instability of the drain current and the transistor transconductance. A reduction in drift can be achieved by selecting a temperature-stable operating mode of the transistor [3]. For KP301B transistors, the best temperature stability is achieved, on average, at a drain current of approximately 1 mA.

Fig. 5 presents the dependence of the monitor readings on the value of the betatron dose rate, from which it is evident that the monitor provides satisfactory linearity.

Thus, the developed monitor, along with its small dimensions, has comparatively good measuring characteristics and can be recommended not only for compact betatrons, but also for solving other problems associated with the recording of ionizing radiation.

REFERENCES

1. Technical description and operating instructions for the PMB-6 betatron.
2. A. A. Sanin. Electronic Instruments of Nuclear Physics. Moscow, Fizmatgiz, 1961.
3. Field-Effect Transistors. Edited by S. A. Maiorov. “Sovetskoe Radio,” 1971.