Device for Protecting a Radiation Detector in Betatron Flaw Detection of Moving Products
Abstract Generated abstract
A protection device is described for betatron radiation flaw detection of moving products, where a detector may be overloaded if an unattenuated beam reaches it while no product is in the irradiation field. The device uses two photosensors spaced by the radiation field width and an electronic attachment to the betatron synchronization circuit to permit bremsstrahlung generation only when the product blocks both sensors. The paper explains the switching sequence, timing conditions, and trigger circuit operation, showing that the arrangement protects the receiver during product gaps, improves radiation conditions, reduces power consumption, and can operate with translational or reciprocating product motion, including variable speeds.
Full Text
DEVICE FOR PROTECTING THE RADIATION RECEIVER IN BETATRON FLAW DETECTION OF MOVING PRODUCTS
A. P. Grigor'ev, I. I. Krimker
(Presented by the scientific seminar of the Research Institute of Electronic Introscopy)
In radiation flaw detection there often arises the need to inspect moving objects, for example on conveyors and rolling mills, where stopping the moving products leads to a significant reduction in productivity, and sometimes is simply impossible for technical reasons.
A similar problem was solved, in particular, when using an X-ray tube as the radiation source [1], where the inspected product was automatically stopped for the duration of the exposure by means of a time relay and mechanical contacts. A method was also proposed for obtaining an intermittent beam of radiation from an isotope source using a pair of photosensors, in which, when products moving without stopping are irradiated, the beam of isotope radiation is interrupted for the time from the moment the product leaves the irradiation field until the moment the next product enters the field [2].
On the basis of this method, a device was developed for protecting the radiation receiver when a betatron is used as the source of $\gamma$-radiation.
The proposed device contains a pair of photosensors with light sources, located from one another at a distance determined by the width of the radiation field of the source, and an electronic circuit made as an attachment to the usual betatron synchronization circuit. It makes it possible to interrupt the betatron radiation beam for the time during which the inspected products are absent from the area of the radiation field, thereby preventing the possibility that a direct beam of radiation, unattenuated by the product, will strike the receiver (especially the detector), protecting it from overloads and from its parameters going beyond the established limits.
In addition, operation with an intermittent radiation beam provides a better radiation environment in the work area and leads to savings in electric power, since the betatron control circuit is switched off for the time when the product is absent from the irradiation field (it is not advisable to switch off the betatron completely); the period of active operation of the injector is increased, etc.
The operating principle of the device is explained by Figs. 1 and 2, which respectively show possible positions of the photosensors and of the inspected product relative to the area of the radiation field, and the basic circuit of the attachment with photosensors connected into the displacement channel of the synchronization circuit. Connecting the attachment into only one cha-
is explained by the fact that the injectors currently in use do not always operate stably when a high voltage is applied to them in stepwise fashion.
Fig. 1. Possible positions of the inspected object relative to the bremsstrahlung-field area
It is seen from Fig. 1 that, when the product moves with velocity \(V\), the radiation beam must not appear at the instant \(t_1\), when the first photodetector along the direction of motion is blocked; it must appear at the instant \(t_2\), when both photodetectors are blocked, and be switched off at the instant \(t_3\), when the first of the sensors is uncovered. The sensors are placed at a distance \(A\) from each other, equal to the width of the irradiation field along the path of the inspected object.
The use of two photodetectors is due to the need to protect the radiation receiver also during the time interval \(t_2 - t_1\), determined by the irradiation-field width \(A\)
\[ t_2 - t_1 = \frac{A}{V}, \tag{1} \]
where
\(V\) is the velocity of motion of the object.
It is possible to construct the device with a single sensor by using, in the accessory circuit, an additional delay stage with \(t_3 = t_1\); however, in this case very stringent requirements must be imposed on the stability of the velocity of motion of the object.
The accessory circuit shown in Fig. 2 contains a pair of series-connected photodetectors FD-3 (FD-1, FSA-1, etc., were also used), a blanking circuit \(T_1\), and a Schmitt trigger, which simultaneously serves as a discriminator of the sensor-signal level and as a shaper of the strobing pulse.
In the initial state both photodetectors are illuminated; their total resistance is very small, and the voltage at point A is insignificant in absolute value. At the instant \(t_1\) photodiode \(D_1\) is shaded, its resistance increases to \(R_{\mathrm{d}1} \gg R_6\), and the voltage at point A increases in absolute value to a value determined by the divider \(R_5R_6\):
\[ U_{A t_1} = - \frac{E_k R_6}{R_5 + R_6}. \tag{2} \]
In this case the condition \(\left| U_{A t_1} \right| < \left| E_{\text{trig. op.}} \right|\) must be satisfied. At the instant \(t_2\) photodiode \(D_2\) is shaded, its resistance increases to \(R_{\mathrm{d}2} \gg R_7\), and the voltage at point A also increases to the value
\[ U_{A t_2} = - \frac{E_k (R_6 + R_7)}{R_5 + R_6 + R_7}. \tag{3} \]
Fig. 2. Circuit of the device for protecting the radiation receiver; a — schematic diagram of the add-on to the betatron synchronization circuit; b — voltage diagrams at the principal points of the circuit
If \(\lvert U_{At2}\rvert > \lvert E_{\text{trig. lock}}\rvert\), the trigger is started, the voltage at its output rises to \(-E_k\), the gating circuit opens, and pulses from the zero-field sensor are passed to the firing circuit of the biasing circuit; generation of hard braking radiation begins.
At the instant \(t_3\), photodiode \(D_1\) is illuminated, and, if the condition \(\lvert U_{At3}\rvert < \lvert E_{\text{trig. lock}}\rvert\) is satisfied, the strobe pulse ends and the passage of firing pulses stops. At the instant \(t_4\), photodiode \(D_2\) is illuminated, and the circuit returns to its initial state.
It is not difficult to see that if \(R_6 = R_7\), then, when the photosensors are illuminated or darkened in the opposite sequence (first \(D_2\), then \(D_1\)), the processes in the circuit do not change. This means that the circuit is equally suitable for use both with translational motion of the inspected products and with reciprocating motion; in this case the speed of motion may vary arbitrarily.
In addition to the described circuit for switching on the photosensors, other circuits were also tested, in particular a balanced stage on voltage followers with a common load, with the photodiodes connected in the “base—ground” arms (through adjustable resistors), as well as variants of the circuit described above; they have no noticeable advantages over one another.
REFERENCES
- A. A. Kiselev. Author’s certificate No. 54105, 1938.
- U.S. Patent No. 3272987, 13/IX 1966.