FORMATION OF A PRESCRIBED SERIES OF BETATRON RADIATION PULSES

A. P. Grigor'ev, A. F. Shinkevich

(Presented by the scientific seminar of the Scientific Research Institute of Nuclear Physics)

In studying certain parameters of a betatron radiation beam, for example, when measuring the focal spot on a target and in a number of other cases, it is necessary to accumulate radiation doses generated in fractions of a second. Naturally, such a task cannot be solved by manual control, and automatic electronic devices are required, based, for example, on counting a preset number of radiation pulses, after which generation of radiation is stopped. For this purpose, several devices were developed, in particular, a device using trochotrons for betatrons with tube control circuits, as well as a circuit using semiconductor devices, described below.

The operation of the proposed circuit is based on the principle of forming a time interval during which the betatron can generate radiation; in this case the duration of the time interval is set by means of a comparison device based on a diode-regenerative circuit [1].

The use of counting devices in the present case (it is necessary to set from 1 to 100 pulses, i.e., an interval duration \(T_{\text{int}} \leq 2\) sec) would be impractical because of the bulkiness of the circuit.

Figure 1 shows the schematic diagram of the proposed device. It consists of the betatron synchronization circuit proper and a time-interval shaper with a time-selection circuit. The channel of the injection circuit consists of a magnetic-field zero sensor—a peak transformer (PT) with a bias winding for regulating the injection phase, a resistive amplifier on transistor \(T_1\), and an output stage on thyristor \(D_2\), triggered through protective diode \(D_1\); the bias channel consists of a stage with adjustable delay (\(T_3, T_4\)) and output stage \(D_5\); \(D_4\) is a protective diode.

When switch \(P\) is set to position \(1\!-\!1\), the usual operating mode of the circuit takes place, and radiation from the betatron is generated continuously. In position \(2\!-\!2\), between amplifier \(T_1\) and the delay stage of the bias channel, a linear gating circuit (or time-selection circuit) is inserted, made on field-effect transistor \(T_2\), which, from the continuous sequence of pulses, passes to the triggering of the bias channel only a certain counted series of pulses during the specified time interval. Pulse selection only in the bias channel is due to the fact that, for the existing types of injectors, it is not recommended to apply the injection voltage abruptly, in order to avoid breakdown of the injector.

Fig. 1. Circuit diagram of the device.

(Label in the diagram: “resistor bank with switch ‘1’ … ‘10’.”)

Let us briefly consider the operation of the circuit. In the mode of forming a series, pulses from the amplifier \((T_1)\), when switch \(P_1\) is thrown to position 2—2, are fed to the triggering of an asymmetrical trigger \((T_5, T_6)\) and to one of the inputs of the gating circuit \((T_2)\). The trigger \((T_5, T_6)\) is actuated; a positive voltage step appears at its output. With the aid of a differentiating chain a sharp pulse is formed, which is then applied, through the emitter follower \((T_7)\) and a diode, to trigger the strobe shaper—a symmetrical trigger with separate triggering \((T_8, T_9)\). The trigger \((T_5, T_6)\) is needed in order to mark the starting (first) pulse of the series and not to pass subsequent pulses into the circuit. Here it is preferable to use an asymmetrical circuit, since its output is not connected by an RC chain to the input, which makes penetration of subsequent input pulses to the output more difficult.

When the trigger \((T_8, T_9)\) is actuated, a positive voltage step is formed at its output; this is applied to the gates of the transistors of the gating circuit \((T_2)\) and of the key circuit \((T_{10})\), and the current through \(T_2\) and \(T_{10}\) is then in fact completely cut off. The use of field-effect transistors in these stages is due to the need to have a high input resistance.

When transistor \(T_2\) is cut off, pulses begin to pass to the input of the displacement channel until the trigger \((T_8, T_9)\) is returned to its initial state; this occurs after a time interval specified by means of the timing capacitance and the comparison circuit.

Let us consider how this occurs. The voltage step from the collector of \(T_9\) is applied to the gate of the key \((T_{10})\) and cuts it off; charging of the timing capacitance \(C_{22}\) then begins from the stabilized-voltage source \(E_{\text{st}}=-30\ \text{V}\). Up to this moment the current from the source flowed through the open key. The charging of capacitance \(C_{22}\) is carried out, in fact, by a constant current, since, first, only the initial part of the exponential is used, and, second, the correcting chain \(R_{47}\)—\(C_{21}\), connected with the time-setting RC chain through the common resistance \(R_{49}\), exerts its effect [2, 3].

At the moment when the linearly falling voltage \(U_{C0}\) reaches the value of the voltage applied to the anode of diode \(D_{10}\), which is included in the base circuit of the comparison device, it opens. As a result of the flow of current through the base winding of the pulse transformer, a blocking process develops in the blocking generator, and a short positive pulse is formed on the collector of \(T_{11}\), delayed relative to the starting pulse by the time \(T_{\text{int}}\), determined by the constant of the charging circuit. This pulse, through the emitter follower \(T_{12}\), is applied to the second input of the symmetrical trigger and returns it to its initial state.

At this moment transistor \(T_2\) of the gating circuit is opened, and the sequence of pulses from the zero-field sensor no longer passes to the triggering of the displacement circuit. The charge of \(C_{22}\) stops; it discharges through the opened key \(T_{10}\). To obtain the next series of pulses, it is necessary to press the “reset” button \(P_2\) of the asymmetrical trigger in order to bring it into its initial state.

Changing the duration of the strobing pulse, and hence also the number of pulses in a series, is achieved by changing the parameters of the time-setting chain, in particular by changing the values of the charging resistances, which are made in the form of resistance boxes. In the present case the principle of constructing wide-range attenuators is used; two wafer switches make it possible to change the number of pulses in the specified series in steps of 1 and 10 pulses.

Let us note that, when selecting the parameters of the timing circuit, it is necessary to take into account the condition [3]

\[ C_{22}R_{48}\frac{R_{49}}{R_{47}} = C_{21}(R_{49}+R_{47}), \tag{1} \]

where \(R_{49}\) is the common resistor for the RC circuits, connecting them to the source.

The resistor values in the resistance box are chosen from the condition of a linear increase in the charging voltage of the timing capacitor from 0 to \(U_{C_0}\) in the specified range

\[ U_{C_0}=\frac{E_{\mathrm{st}}}{3RC}\cdot t, \tag{2} \]

where

\(E_{\mathrm{st}}\) is the voltage of the voltage source for the charging circuit,
\(U_{C_0}\) is the voltage at which the blocking oscillator must be triggered,
\(t\) is the time interval during which the prescribed series of pulses is formed.

The value of the time interval at a frequency \(f=50\ \mathrm{Hz}\) can be determined from the expression

\[ \sum_{i=2}^{n}\Delta t_i=\Delta t_{i-1}+20\ \mathrm{ms}; \tag{3} \]

the magnitude of the first interval is taken equal to 10 ms. This makes it possible to avoid errors in pulse counting when the pulse repetition frequency is varied, and also to impose less stringent requirements on the resistor tolerance.

When the circuit was tested, the error of the specified counted series was \(<\pm 2\%\); for series from 1 to 50–55 pulses there was practically no error, i.e., the number of pulses set by means of the switches of the resistance box and that measured by means of the pulse counter (PP-15) corresponded exactly to each other; with a further increase in the number of pulses to 100, the error was \(\pm 1\) pulse.

The circuit can also be used in other fields of technology when it is necessary to select a counted number of pulses from a periodic sequence.

REFERENCES

1. B. I. Shvetskii. Electronic Measuring Instruments with Digital Readout. Kiev, “Tekhnika,” 1970.

2. Z. P. Vazhenina et al. Methods and Circuits for Time Delay of Pulse Signals. Moscow, “Sovetskoe radio,” 1971.

3. V. A. Il’in. Telecontrol and Remote Control. Moscow, “Energiya,” 1969.