A New Scheme for Monitoring the Maximum Energy of X-Rays
Abstract Generated abstract
This paper describes a triggering and comparison circuit developed for Tomsk Polytechnic Institute betatrons to control the maximum energy of X-rays with higher precision than earlier published schemes. The design replaces the direct-current amplifier used in a prior circuit with a millivolt-level discriminator, combined with a stabilized cathode follower, compensation diode, differentiating network, trigger stage, and output stage for driving a hydrogen thyratron. The authors estimate the main error sources, including tube instability, pickup, discriminator error, and trigger spread, and conclude that the total error does not exceed approximately ±1.75 keV. They also note that the comparison device may be useful beyond betatron beam-dumping applications because of its stability and fast output-pulse rise time.
Full Text
A NEW SCHEME FOR CONTROLLING THE MAXIMUM ENERGY OF X-RAYS
A. K. BERZIN, R. P. MESHCHERYAKOV, R. G. NEMKOV
(Presented by the scientific seminar of the Faculty of Physics and Technology)
For certain investigations with betatrons manufactured by the Tomsk Polytechnic Institute, a triggering circuit was developed that makes it possible to control, with high accuracy, the maximum energy of X-rays. Previously published circuits do not make it possible to vary the triggering moment over wide limits with an error of less than ±10 keV [1, 2]. In a number of experimental studies the stability of the X-ray energy was kept constant with an accuracy better than ±5 keV over a comparatively short interval of time [3, 4]; however, the parameters of the circuits used were not published.
Using the results of a study of the operation of a discriminator with a millivolt level [5], we succeeded in eliminating the principal source of error in circuit [1]—tested, unlike [2], by a number of authors—the dc amplifier, thereby achieving an accuracy of the comparison device on the order of ±2 keV, while at the same time reducing the delay time of the circuit.
The circuit of the comparison device is shown in Fig. 1, where \(V_2\)—\(V_4\) is the comparison device proper, and \(V_1\) and \(V_{11}\)—\(V_{12}\) are circuits for matching the input and output.
The resistance values are given in Table 1.
Its first stage is a cathode follower \(V_1\), serving as a buffer between the integrator circuit and the discriminator input. As the diode conducting current at the moment when the voltage on the tube anode has reached a fixed value, a double triode 6N15P is used, connected according to a compensation circuit to prevent changes in the characteristic, with its grid connected to the anode. The use of a triode instead of a diode, together with reduced stabilized filament supply, ensures stable operation of the diode and a sharp cutoff. Careful shielding of the diode and of the lead-in wires protects the input of the millivolt discriminator from 50-Hz ac pickup. The operating threshold of the third stage of the circuit—the discriminator with a millivolt level \(V_5\)—\(V_6\)—is about 60 mV. The stage is assembled with 6Zh1P tubes; as the diode operating in the region of negative potential, a 6Kh2P tube is used. The pulse at the discriminator output has an amplitude of 40 V and a rise rate of the leading edge of 10 V per microsecond. A transient \(RC\) circuit, differentiating the signal from the millivolt discriminator, brings the rise time of the leading edge (the first 6 V) to 0.3 μsec. The fourth stage of the circuit is the trigger \(V_7\)—\(V_{10}\) with an operating threshold of \(6 \pm 0.5\) V. The inductance in the anode circuit of the paired 6P9 tubes consists of three series-connected coils in a carbonyl-iron frame with a total induc-
by a factor of 2.1 millihenries. The half-sinusoidal wave at the trigger output has a rate of rise of about 290 V in 0.3 μsec. The output stage \(V_{11}\)—\(V_{12}\) operates into the small input impedance (grid—cathode) of the hydrogen thyratron TGI-1400/16.
Table 1
| Resistors | |||||
|---|---|---|---|---|---|
| \(R_1\) | — | \(R_{24}\) | 350 kΩ | \(V_4\) | 6X2P |
| \(R_2\) | 51 kΩ | \(R_{25}\) | 660 Ω | \(V_5\) | 6Zh1P |
| \(R_3\) | 4 kΩ | \(R_{26}\) | — | \(V_6\) | 6Zh1P |
| \(R_4\) | 1 kΩ | \(R_{27}\) | — | \(V_7\) | 6Zh4 |
| \(R_5\) | 10 kΩ | Capacitances | \(V_8\) | 6P9 | |
| \(R_6\) | 440 kΩ | \(C_1\) | — | \(V_9\) | 6P9 |
| \(R_7\) | 750 kΩ | \(C_2\) | 0.03 μF | \(V_{10}\) | 6X4 |
| \(R_8\) | 750 kΩ | \(C_3\) | 0.03 μF | \(V_{11}\) | \(1/2\) GU-29 |
| \(R_9\) | 100 Ω | \(C_4\) | 300.0 μμF | \(V_{12}\) | \(1/2\) GU-29 |
| \(R_{10}\) | 100 Ω | \(C_5\) | 0.2 μF | \(V_{13}\) | DTs-8 |
| \(R_{11}\) | 22 kΩ | \(C_6\) | 2.0 μF | \(V_{14}\) | DTs-8 |
| \(R_{12}\) | 300 kΩ | \(C_7\) | 40 pF | Supply voltage | |
| \(R_{13}\) | 35 kΩ | \(C_8\) | 6000 pF | \(E_1\) | — |
| \(R_{14}\) | 5.0 MΩ | \(C_9\) | — | \(E_2\) | 420 V* |
| \(R_{15}\) | 75 kΩ | \(C_{10}\) | 0.5 μF | \(E_3\) | 190 V* |
| \(R_{16}\) | 47 kΩ | \(C_{11}\) | 30.0 μF | \(E_4\) | 600 V* |
| \(R_{17}\) | 10 kΩ | \(C_{12}\) | 1000 pF | \(E_5\) | 150 V* |
| \(R_{18}\) | 2 kΩ | \(C_{13}\) | 2.0 μF | \(E_6\) | 127 V* |
| \(R_{19}\) | 500 Ω | Inductances | \([xx]\) | 6.3 V* | |
| \(R_{20}\) | 500 Ω | \(L\) | 2.1 millihenries | ||
| \(R_{21}\) | 5 kΩ | Radio tubes | |||
| \(R_{22}\) | 80 Ω | \(V_1\) | 6N8 | ||
| \(R_{23}\) | 50 Ω | \(V_2\) | 6N15P | ||
| \(V_3\) | DTs-8 |
* Direct current; stabilized.
The error in the first stage of the circuit—the cathode follower—arises mainly from changes in the characteristics of the tube caused by changes in filament heating and by aging of the tube. The use of reduced, stabilized filament supply eliminates rapid changes of these characteristics and sharply reduces the error, making it difficult to determine. The effect of tube aging can be eliminated by regular recalibration of the circuit. The same errors, reduced owing to compensation, are also inherent in the fixing diode. We have taken the instability of operation due to these two effects and to AC-voltage pickup at the input of the circuit to be \(\pm 7\) mV, which corresponds to a spread of \(\pm 1\) keV when an effective AC voltage of 107 volts is applied to the grid.
The error of the discriminator with a millivolt level does not exceed \(\pm 0.5\) keV. The spread in the firing of the trigger and of the hydrogen thyratron TGI-1400/16 amounts in all to \(\pm 0.65\) μsec, which at 1 MeV corresponds to an error of \(\pm 0.25\) keV, decreasing with increasing energy. The total error of the circuit does not exceed \(\pm 1.75\) keV.
In conclusion we note that the field of application of the present comparison device \(V_2\)—\(V_{10}\), with high stability and a relatively high rate of rise of the leading edge of the output pulse, is not limited only to the betatron dumping circuit.
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Fig. 1