Digital Exposure Meter for Pulsed Radiation
Abstract Generated abstract
A digital exposure meter is described for controlling the irradiation dose delivered by pulsed accelerator sources, particularly a betatron, where pulse intensity instability makes simple pulse counting or time relay methods insufficient. The proposed circuit measures the amplitude of each bremsstrahlung pulse with a detector, converts the amplitude into a time interval, fills that interval with calibration pulses, and sums the resulting digital counts until a preset value corresponding to the required dose is reached. The system can automatically stop irradiation and coordinate conveyor motion, and the paper derives the relation between preset counter value and dose while noting the unavoidable dose excess caused by completion of the final radiation pulse.
Full Text
Digital Exposure Meter for Pulsed Radiation
A. P. Grigoriev, I. I. Krimker
(Presented by the scientific seminar of the Research Institute of Electronic Introscopy)
At present, charged-particle accelerators are widely used as radiation sources for solving many scientific and technical problems. Since accelerator radiation is pulsed in nature, its recording when irradiating objects in order to give them a specified dose has certain special features, unlike the recording of radiation with intensity constant in time.
In this case it is necessary to solve problems of the following kind: a) automatic switching off of the source after a specified dose has been accumulated when irradiating a stationary object; b) automatic switching on and off of the source for the time during which the irradiated objects, moving continuously on a conveyor, are in the radiation field; c) the same as in b), but with the object stopped within the area of the radiation field in order to accumulate the specified dose.
The last problem, being the most general, is solved by means of a device developed for a betatron, the description of which is given in the present work.
Accumulation of a specified dose with a sufficient degree of accuracy could be carried out by counting the pulses of bremsstrahlung radiation [1], or by using a time relay.
In this case it is assumed that all radiation pulses have the same intensity, that is, they make the same contribution to the dose received by the object. However, since in reality there is instability in the intensity of pulsed radiation, caused by various factors [2], these methods do not solve the problem. The use of special radiation exposure meters is necessary.
Known exposure meters of the integrating type are based on measuring the charge on an input capacitor [3]. However, their substantial drawback is their low accuracy due to the loss of part of the charge during dose accumulation, as well as instability of the triggering threshold, which limits the possibilities of their application.
The proposed circuit is free of these drawbacks (Fig. 1); it makes it possible to record a quantity proportional to the intensity and, consequently, to the dose of each individual radiation pulse, and to sum these data until the specified dose has been accumulated. After this the radiation source, in particular the betatron, is switched off automatically.
The circuit operates as follows. Into the synchronization circuit of the beta-
Fig. 1. Functional diagram of the exposure meter.
An electronic switch 1, normally open for synchronizing pulses and controlled by a strobe pulse formed by the symmetrical trigger 3, is introduced into the circuit of the betatron (for example, into the bias channel). By the “Adjustment” pulse, trigger 3 is transferred into such a stable state in which switch 1 is closed and the betatron is adjusted for radiation. Next, by the “Operation” pulse, which has the opposite polarity, trigger 3 is switched to the second stable state, in which switch 1 is open and the synchronizing pulses do not pass to the output stages of the synchronization circuit. This same stable state of trigger 3 corresponds to the flow of current through the winding of relay 2, which is connected into the collector circuit of one of the transistors of trigger 3. In this case the relay contacts close the power-supply circuit of the conveyor motor that feeds the objects into the area of the radiation field. The signal for the beginning of irradiation is supplied by photodetector 4, which records the appearance of a moving object in the field area [4]. Trigger 3 is switched over, closing switch 1 and opening the contacts of relay 2.
The bremsstrahlung pulses are recorded by detector 8, at the load of which voltage pulses arise whose amplitude is proportional to the intensity of the corresponding radiation pulses.
\[ U_i = k_1 I_i . \tag{1} \]
Next, the amplitude is converted into a time interval (9)
\[ t_i = k_2 U_i , \tag{2} \]
a strobe pulse (10) is formed for switch \(1'\), and the time interval is filled with calibration pulses from generator 6, the number of which \(n_i\) is proportional to the magnitude of this interval
\[ n_i = f t_i , \tag{3} \]
where \(f\) is the generator frequency, and is also proportional to the amplitude of the corresponding voltage pulse. Thus, the pulse amplitude can be measured in the form of a digital code [5].
On decade counter 5, by means of switches \(P_2\)—\(P_n\), the number \(N\) [6] corresponding to the specified dose \(D\) is preset; when it is reached, coincidence circuit 7 is actuated and the entire system returns to its initial state.
In order to determine the correspondence of the number \(N\), which must be set on the counter for accumulating the dose \(D\), it is necessary to know the proportionality coefficients of the circuit converters. Taking into account that the dose \(D_i\) received by the object as a result of the action on it of the \(i\)-th pulse of bremsstrahlung is proportional to the intensity of this pulse,
\[ D_i = k_3 I_i, \tag{4} \]
and also taking into account (1, 2 and 3), we obtain
\[ N = \frac{f k_1 k_2}{k_3} D . \tag{5} \]
It is evident that the accuracy of counting off the specified dose can be increased by raising the generator frequency \(f\), by using more sensitive detectors (\(k_1 \uparrow\)), and by increasing the coefficient \(k_2\) of \(A \to t\) conversion. The limit of this increase may be, for \(f\), the maximum counting frequency of the counters used, and for \(k_2\), the period of repetition of the synchronizing pulses.
It should be noted that, since the specified dose is accumulated from pulsed radiation, in any case the object will receive not the dose \(D\), whose value is set on the counter, but a somewhat larger dose \(D'\), lying within the limits
\[ D \leq D' \leq D_{i \max} + D, \tag{6} \]
where \(D_{i \max}\) is the dose that the object receives from a radiation pulse of maximum intensity.
This is explained by the fact that, if the coincidence circuit is actuated by any pulse from the series arriving at the counter, the bremsstrahlung pulse corresponding to this series will nevertheless not be interrupted in the middle, but will contribute completely to the total dose.
For the case of stable intensity, it is sufficient to use part of the circuit (stages 1—5,7; switch \(P_1\) in position 2); in this case the counter counts the synchronizing pulses, the number of which required to obtain the specified dose is established by preliminary calibration.
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