DEVICE FOR AUTOMATIC COUNTING OF ELECTRICAL PULSES

V. I. GORBUNOV

(Presented by the scientific seminar of the Physics and Technology Faculty)

In the practice of using betatrons for flaw detection of thick-walled metal products, and also in medicine, it is necessary to operate with integral doses, that is, radiation doses over a certain interval of time. When working with radiation sources, portable instruments are usually used to measure the physical dose or the dose rate of gamma radiation; these are called roentgenometers or dosimeters. Each of these instruments consists of an ionization chamber and a device for measuring the ionization current or the quantity of electricity.

In working with the radiation produced by a betatron, we encountered two difficulties: first, the question arose of methods for measuring the intensity of high-energy gamma radiation (25 MeV), since the ionization chambers available to us, included in sets of dosimeters manufactured by domestic industry, can be successfully used only for dosimetry of gamma radiation with hardness of 1–3 MeV [1]; second, in work on the radiographic inspection of metal products, when we have to operate with integral doses, a dosimeter-integrator is necessary.

The use of a dosimeter-integrator largely removes this difficulty; however, the problem will be completely solved only if the installation, automatically and without operator intervention, is switched off after the irradiated object has received the prescribed dose.

At our disposal there was a dosimeter-integrator of the “DIGD” type, which has no interlock circuit for the controlled object, although, as is known [2], dosimeter-integrators of the “RIP” and “MIR” types have recently appeared; these have a relay system that provides interlocking of the controlled object.

Therefore, for the above-mentioned purpose we calculated, designed, and tested a device which, together with an industrially manufactured dosimeter-integrator of the “DIGD” type, provides the required prescribed dose to the irradiated object with sufficient accuracy, after which the betatron is automatically switched off. Thus, the device makes it possible, first, to set the required doses automatically and, second, completely eliminates the need for the operator to monitor the dosimeter, which greatly facilitates the operator’s work and makes operation of the installation simpler.

Principle of Operation and Circuit of the Device

The device is a pulse-counting relay (abbreviated PCR) and is intended for counting pulses of electric current within the range from 1 to 650, with an accuracy of up to 1 pulse. The device operates for counts of any number of pulses within the limits indicated above; after the specified number of pulses has been processed, the relay is automatically reset to zero. The circuit of the device (Fig. 1) contains the following circuits and elements.

Fig. 1

1. The power-supply circuit has a step-down transformer (Tr) 220/70 V with a power rating of \(P = 70\) W and a selenium rectifier (SR) assembled according to a single-phase full-wave circuit. In addition, the power-supply circuit contains a switch (B) and a fuse (PR) rated at 1.5 A.

2. The pulse-receiving circuit consists of a relay (RD) of type Ю1718121 and an intermediate relay (RPo) of type У1718047.

1. The first counting cascade consists of a step-by-step distributor (ShD\(_1\)) for 25 positions, a relay (RP\(_1\)) of the MGU-48 type, and a switch (P\(_1\)) for 25 positions.

2. The second counting cascade likewise consists of a step-by-step distributor (ShD\(_2\)) for 25 positions, a relay (RP\(_2\)) of the MGU-48 type, and a switch (P\(_2\)).

3. The third counting cascade contains a switch (P\(_3\)), a relay (RP\(_3\)) of the MGU-48 type, and also includes 2 contact fields of the step-by-step distributor (ShD\(_1\)).

4. An intermediate cascade, consisting of a relay (RP\(_4\)) of the MGU-48 type.

The operation of the device proceeds as follows. A current pulse, which is supplied from the output of the dosimeter-indicator, passes through the winding of the sensitive relay (RD); the relay operates and thereby closes the normally open contact KRD, located in the circuit of the more powerful intermediate relay (RP\(_0\)). After KRD closes, relay RP\(_0\) operates, closing the normally open contact KRP\(_0\). As soon as KRP\(_0\) closes, current will pass through the coil of the step-by-step distributor (ShD\(_1\)), and its brushes will turn in a jump by the distance between two adjacent contacts of the lamellae.

This process will be repeated until the distributor brushes reach the contact corresponding to the number of pulses set by switch P\(_1\). When the brush of ShD\(_1\) reaches this contact, the circuit ShD\(_1\)—P\(_1\)—coil RP\(_1\) is closed. When the normally open contacts RP\(_1\)—K\(_1\)RP\(_1\) and K\(_2\)RP\(_1\) close, the coil circuit of ShD\(_1\) will close through the normally open contact ShD\(_1\)—KShD\(_1\). In this case the finder will automatically move the brush along the short-circuited contact field of ShD\(_1\) until it reaches zero. In this case the circuit of relay RP\(_1\) is broken, and the step finder is set to its initial position. In addition to contacts K\(_1\)RP\(_1\) and K\(_2\)RP\(_1\), relay RP\(_1\) also has a normally open contact K\(_3\)RP\(_1\), located in the second counting cascade and performing exactly the same function as contact KRP\(_0\) in the first counting cascade. The second counting cascade operates in exactly the same way as the first, with the only difference that the signal for it is the operation of relay RP\(_1\) of the first counting cascade.

At the end of counting off the specified number of pulses, set by switch P\(_2\), relay RP\(_2\) operates; its normally open contact K\(_3\)RP\(_2\) is in the circuit of the intermediate cascade.

As soon as K\(_3\)RP\(_2\) closes, current will flow through the coil of relay RP\(_3\); the relay will operate, closing its normally open contacts K\(_1\)RP\(_3\) and K\(_2\)RP\(_3\). Contact K\(_1\)RP\(_3\) blocks contact K\(_3\)RP\(_2\), which soon opens. Contact K\(_2\)RP\(_3\) is located in the third counting cascade and, by closing, brings this cascade into operation.

The third counting cascade operates in exactly the same way as the first two; moreover, since it is assembled together with the first counting cascade on one and the same step-by-step distributor ShD\(_1\), the signal for operation of this unit is contact KRP\(_0\); at the end of counting off the specified number of pulses set by switch P\(_3\), relay RP\(_4\) operates, thereby switching off the controlled installation (contact K\(_1\)RP\(_4\)).

In addition, with the aid of contact K\(_3\)RP\(_4\), relay RP\(_3\) is de-energized. This completes the operating process of the RSI, and the counting relay is set to its initial position.

Design of the Device

The chassis of the RSI device consists of one horizontal panel, on which the parts of the device are located. The chassis is inserted into the housing and fastened to it with screws. On top of the chassis are located both step-by-step distribu-

...amplifier, four relays of the MGU-4 type, a sensitive relay of the Yu1718121 type, an intermediate relay of the U1718047 type, and a power transformer. At the bottom of the chassis are mounted a selenium rectifier, three switches, and a circuit breaker. In the upper cover of the housing there are seven openings: three of them are intended for the axes of switches \(P_1\), \(P_2\), \(P_3\), one for the head of the toggle switch, and the remaining three serve for observing the switch dials. In the rear side wall of the housing there is a window in which all six terminals are located \((1,2\)—mains, \(3,4\)—sensor, \(5,6\)—command contacts). The switch knobs, terminals, and circuit breaker are provided with the appropriate inscriptions.

Calibration of the DIGD dosimeter and instructions for use together with the RSI instrument

Calibration of the DIGD dosimeter is carried out with the aid of a graphite ionization chamber designed to measure the intensity of radiation with a maximum energy of 25 MeV. Having accurately determined the dose rate of the radiation at the distance we have selected from the target to the object being irradiated, we can determine what dose in roentgens will correspond to one pulse of the dosimeter in the case where the remote unit of the integrator is on the beam side. Knowing the value of each pulse in roentgens and the integral dose specified for irradiation, we determine the required number of pulses. The number of pulses is set by means of the three switches of the RSI.

Let us consider three cases. If the specified number of pulses does not exceed 25, switches \(P_2\) and \(P_3\) are set to position 1, and switch \(P_1\) is used to set the required number from 1 to 25. In the case where the specified number of pulses is a multiple of 25, switch \(P_1\) is set to position 25, switch \(P_2\) is set to the position corresponding to the number obtained by dividing the specified number of pulses by 25, and switch \(P_3\) is set to position 1. Finally, in the case where the specified number of pulses is greater than 25 and is not a multiple of 25, then switches \(P_1\) and \(P_2\) are set exactly as in the second case, while switch \(P_3\) is set to the position corresponding to the number obtained by subtracting from the specified number of pulses the largest multiple of 25 within the specified number, plus one.

The actuating contact of relay \(RP_4\) is normally rated for operation in alternating-current circuits at a voltage not exceeding 380 V and at a nominal current of 2 A. The breaking current of these contacts, depending on the voltage, is 0.3–0.8 A. The RSI operates normally at mains voltages in the range from 80 to 105% of nominal. The service life of the telephone-type step-by-step switches included in the RSI is 300,000 full revolutions of the rotor, provided that cleaning, lubrication, and adjustment of the brushes are carried out after every 50,000 revolutions.

LITERATURE

1. Flowers B. H., Lawson J. D., Fossey E. B. Pros. Phys. Soc., 65B, 286, 1952.
2. Krongauz A. N. Dosimeters for X-rays and \(\gamma\)-rays. Medgiz, 1953.