Apparatus for Studying Coherent Bremsstrahlung from a Single-Crystal Synchrotron Target

A. F. Genning, V. D. Epaneshnikov, B. N. Kalinin, G. I. Kotlyarevsky,
V. N. Kuzmin, Yu. N. Plotnikov, Yu. I. Sertakov

(Presented by the scientific seminar of the high-energy sector of the Research Institute of Nuclear Physics, Electronics, and Automation at TPI)

Introduction

For experiments in elementary-particle physics, the production of monoenergetic and polarized beams of gamma quanta is of considerable interest. The use of such gamma quanta for studying ion photoproduction makes it possible to obtain additional information on the interactions of elementary particles. One of the experiments prepared and begun at the 1.5 GeV synchrotron of the Tomsk Polytechnic Institute, with the aim of obtaining quasi-monoenergetic and polarized beams of gamma quanta, is the study of electron bremsstrahlung in silicon and diamond crystals. The theory of this effect is well known [1]. At the Research Institute of Nuclear Physics at TPI,

Fig. 1. Experimental layout: 1 — 1.5 GeV electron synchrotron, 2 — goniometer with a crystal target, 3 — collimators, 4 — opening in the concrete wall with lead shielding, 5 — cleaning magnet, 6 — double ionization chamber, 7 — magnetic gamma spectrometer, 8 — scintillation counters, 9 — quantometer.

a procedure and apparatus were developed and prepared for this experiment. The layout of the experimental installation is shown in Fig. 1. The dumping of electrons onto the crystal target is accomplished by switching off the high-frequency accelerating voltage of the accelerator.

The bremsstrahlung beam from the crystalline target is collimated by two lead collimators with apertures of diameter 24 mm and 10 mm, and is cleared of charged particles by an SP-58-type magnet with an air gap of 50 mm and a magnetic-field intensity of 30 kOe. At the entrance to the pair spectrometer the bremsstrahlung beam has a diameter of 16 mm and an angular divergence of 0.8 mrad.

Goniometer

Rotation of the synchrotron crystalline target about two mutually perpendicular axes is accomplished by means of a goniometer. Figure 2 shows the kinematic diagram of the goniometer, designed and manufactured at NIIYaF.

Fig. 2. Kinematic diagram of the goniometer

The goniometer makes it possible to rotate the crystal about the horizontal axis through an angle \(\Theta_{\mathrm{h}} = \pm 2.5^\circ\), and about the vertical axis through \(\Theta_{\mathrm{v}} = \pm 45^\circ\). The resolution is no worse than 10 seconds of arc. In this goniometer design, to ensure high precision in rotating the crystal, micrometer 15 and theodolite limb 9 are used. Crystal 1 is mounted in crystal holder 2 on rod 4. Rod 4 is connected by a hinge to bracket 18, rigidly fastened to tube 5. Tube 5 has a rigid connection with the theodolite limb 9, to which worm gear 10 is attached. The theodolite limb 9 can rotate on a thrust plain bearing.

Rotation of the limb is transmitted through a two-stage worm reduction gear with a gear ratio of 2015. Together with the limb, the rigidly connected tube 5, rod 4, and, finally, the crystal holder with crystal 1 are rotated. Rotation of the crystal about the horizontal axis is carried out by means of the micrometric screw of micrometer 15. The micrometric screw pushes the lower end of rod 4 and thereby rotates it about hinge 3. The rotation angle \(\Theta_{\mathrm{v}}\) is read from the micrometer scale. Reliable contact of the rod with the micrometric screw is ensured by spring 6. Rotation of the micrometric screw is carried out through single-stage worm reduction gear 16 with a gear ratio of 60. The worm wheel of reduction gear 16 is connected with micrometer 15 by means of a guide sliding key. The connection of the goniometer with the vacuum chamber of the synchrotron is made through Sylphon transition piece 7. The necessary operating pressure in the chamber is maintained by vacuum seals 8 and 14.

The entire goniometer is placed on a table (not shown in the figure), which is designed so as to allow the goniometer to be moved vertically in order to set the crystal into the operating position. The goniometer is controlled remotely by means of electric motors 13 and 17 of the SD-54 type and angle readout systems 19, 20. The angles of rotation are indicated by reading the number of revolutions with electromechanical counters brought out to the control panel. The goniometer was calibrated on an OSK-3 optical bench with the aid of an autocollimation tube, which made it possible to check rotations about the horizontal and vertical axes within 1 degree. With the aid of an eyepiece mounted on the base of the goniometer, the rotation angle \(\Theta_{\mathrm{v}}\) can be read with an accuracy of up to 5 sec. For studying the polarization of bremsstrahlung, a goniometer will be used that makes it possible to rotate the crystal in the (110) plane by \(90^\circ\).

Double Ionization Chamber

The rough setting of the crystal to the zero angle is carried out with the aid of a double ionization chamber. Its design consists of two identical ionization chambers placed one behind the other along the beam path. The absorbers in front of them are calculated in such a way that the first chamber is sensitive to the entire bremsstrahlung spectrum of the synchrotron, whereas the second is sensitive only to its hard part. On the bremsstrahlung beam from a tantalum target, the measured ratio of the ionization currents was 0.92.

At angles of incidence of the electrons on the crystalline target close to the normal, an increase is observed in the number of soft quanta in the bremsstrahlung spectrum, while the number of hard quanta remains practically unchanged. Therefore, from the ratio of the chamber readings one can set the crystal to the zero angle. The expected ratio of the ionization currents for a diamond target is 1.01–1.02.

The block diagram for measuring the ratio of the ionization charges of the chamber is presented in Fig. 3a. The pulsed ionization currents, with switch \(K_1\) closed, charge the input polystyrene capacitors \(C_1\) and \(C_2\). After charge has accumulated on them, the capacitors are discharged (switch \(K_1\) open, switch \(K_2\) closed) by means of “current–frequency” integrator-converters.

Each integrator is a blocking oscillator in a waiting mode, built on a 6Zh1B tube operating in the low-grid-current mode (\(10^{-12}\) A). The sensitivities of the integrators are identical and are \(6 \cdot 10^{-10}\) coulomb/pulse.

Capacitors \(C_1\) and \(C_2\) are discharged through resistors with a discharge time of \(0.3\) sec. The charge-accumulation time on the capacitors is selected so that the number of pulses at the output is not less than 100. In this case the error in converting charge into the number of pulses is no more than \(1\%\). The integrators are calibrated by means of a capacitor of known capacitance, \(0.1\,\mu\mathrm{F}\), of the MGPT type. The number of pulses is recorded by counting instruments of the PST-100 type. When

Fig. 3. Measurement of the charge ratio of a double ionization chamber:
a) measurement circuit, b) block diagram of an integrator with a sensitivity of \(2\cdot10^{-13}\) coulomb

one channel has accumulated a number of pulses that is a multiple of 10, counting is stopped in the other channel, and in this way the ratio of the charges collected from the chambers is measured. The time for one measurement of the ratio for a diamond target is on the order of 10–20 min. To reduce the measurement time, an integrator with a sensitivity of \(2\cdot10^{-13}\) coulomb was developed, which makes it possible to measure the charge ratio during each acceleration cycle. The basic operation of such an integrator is based on discharging a capacitor by a device that follows the voltage level on this capacitance. The block diagram is shown in Fig. 3b.

The voltage at the output of the amplifier is monitored by a threshold device. When a certain voltage value is reached at the input, the threshold device is triggered, as a result of which the time-selection circuit passes pulses from the generator to the input of the discharge device. During the action of each generator pulse, the discharge device discharges the capacitor by a definite amount \(q\). By counting the pulses entering the discharge device and knowing the value of one count \((q)\), one can determine the charge arriving from the ionization chamber. The resistance of the input circuit of the amplifier must be large,

on the order of \(10^{13}\)—\(10^{14}\) ohm; therefore, an electrometer tube of the EM-10 type is used as the input tube. To reduce leakage currents, the input circuit is mounted on a Teflon plate, and PSO-type capacitors are used as the storage capacitance. This integrator can also be used to measure ionization currents from monitors, quantometers, etc.

Spectrometer

The spectrum of coherent bremsstrahlung is measured with a three-channel pair magnetic gamma spectrometer having the following characteristics: energy resolution about 1–2%, efficiency of one energy channel \(10^{-6}\) with an aluminum-converter thickness of \(1.1 \cdot 10^{-3}\) radiation length.

A standard electromagnet of type SP-57-A-1, with modernized triangular poles, is used as the analyzing magnet of the spectrometer. To eliminate the influence of the stray magnetic field on the trajectories of electrons and positrons, magnetic shields are provided. The position of the scintillation detectors was determined by calculation and experimentally—by the current-carrying-wire method. For tracing the magnetic field, a special device was designed that made it possible to determine the coordinates of the current-carrying wire with an accuracy of up to 0.05 mm. The tracing made it possible to determine the focal line of the scintillation detectors and the dependence of the energy of the electrons and positrons on the coordinate perpendicular to their trajectory. The error in tracing the magnetic field was \(\pm 0.5\%\). On the basis of the tracing results, lead collimators with slits of 5.1 mm, 5.9 mm, and 6.7 mm were made and installed for three scintillation detectors for electrons and three detectors for positrons. The results of the tracing and the calculation of the magnetic shields agreed to an accuracy of 2%. The field in the air gap of the magnet was measured with an IMI-2 magnetometer. The error in determining the energy of each of the three energy channels of the spectrometer is 1%.

It should be noted that the traced SP-57-A-1 magnet can be used as a source of electrons and positrons of known energy (from 5 to 750 MeV) for calibrating scintillation and Cherenkov counters, as well as for a number of other tasks. The half-width of the spectral line of the selected electrons and positrons is 1%.

The registration system (Fig. 4) consists of the following main units: scintillation counters, shaping and coincidence circuits, counting circuits, and control and monitoring circuits. The scintillation counter consists of a plastic scintillator, a Plexiglas light guide, and an FEU-30 photomultiplier. To shield them from the stray magnetic field, the FEU-30 are enclosed in protective shields consisting of two cylinders: the outer one is steel, with a wall thickness of 10 mm, and the inner one is permalloy, with a wall thickness of 1.5 mm. Signals from the FEU-30, with a duration of 4–5 nsec and an amplitude of 10–15 V, are sent through a 40 m cable to six shapers, and then to three double-coincidence circuits, which register “true + false” coincidences, and, through delay cables (40 nsec), to three double-coincidence circuits registering “false” coincidences. The resolving time of the coincidence circuits is 8 nsec.

Pulses from each of the coincidence circuits are sent to shapers and time selectors, whose purpose is to pass pulses from the coincidence circuits to the counters only during the

during the bremsstrahlung pulse from the synchrotron. From the time-selection circuits the pulses go to 6 counters, 3 of which record “true + false” coincidences and 3 record “false” coincidences. The “true” coincidences for each energy channel can be obtained by subtracting the “false” readings from the “true + false” readings. To switch off the spectrometer when a definite dose of gamma radiation has been accumulated, a special circuit is used which, when a pulse corresponding to the specified dose arrives from the quantometer electronics, switches off the synchronizing pulse coming from the accelerator synchrogenerator.

Fig. 4. Block diagram of the electronics of the magnetic pair gamma spectrometer:
F — shapers, CC — coincidence circuits, DL — delay lines,
TS — time selectors.

In the diagram: counters, channel 1; channel 2; channel 3; control circuit; quantometer.

In addition, there are buttons for stopping the count and resetting the counter readings at the operator’s discretion. For periodic checking of the operation of the spectrometer electronics, a remote GNI-1 generator is used. All electronics circuits are made with semiconductor devices.

Quantometer

To normalize the measurements in determining the radiation spectrum, a quantometer is used. The design of the quantometer is analogous to that used by Wilson [2].

To eliminate the influence of the surrounding medium, the quantometer is sealed and filled with argon. The constant of the gas-filled quantometer is \(K = 4.59 \cdot 10^{18}\ \mathrm{MeV/C}\).

The electronic circuit of the quantometer consists of a remote electrometer unit, the instrument itself, made on the basis of a “Kaktus”-type dosimetric instrument, and a counting circuit.

The operation of the circuit is as follows. The positive charge of the collecting electrode of the quantometer charges capacitor \(C\), which

connected to the grid of an electrometer tube 2E2P. The voltage from the capacitor is amplified by a three-stage dc amplifier. The anode of the last tube of the dc amplifier is galvanically connected to the grid of the first tube of the multivibrator. Capacitor \(C\) is charged until the grid potential of the multivibrator reaches the tube cutoff threshold. The winding of the capacitor-\(C\) discharge relay is included in the anode circuit of the second tube of the multivibrator. When the multivibrator switches over, the relay operates and closes the discharge circuit. Upon complete discharge of \(C\), the multivibrator returns to its initial state and opens the discharge circuit. Each operation of the multivibrator is recorded by a counting device.

The value of one count when operating in range I corresponds to
\(q = 2 \cdot 10^{-9}\ \text{C/count}\), and when operating in range II, \(q = 10^{-8}\ \text{C/count}\). The voltage on the working capacitor \(C\) is also monitored by a pointer instrument, whose full-scale deflection corresponds to one count. The number of effective \(\gamma\)-quanta per one radiation cycle is determined from the formula

\[ N_{\mathrm{eff}\,\gamma}=\frac{KQ}{EN}=\frac{K(qn+B)}{N}, \]

where \(K\) is the quantometer constant, \(q\) is the value of one count (in C/count), \(n\) is the number of counts, \(B\) is the reading of the pointer instrument (in coulombs), \(N\) is the number of acceleration cycles, and \(E\) is the maximum energy of the accelerated electrons.

REFERENCES

1. H. Uberall. Phys. Rev. 103, 1055. 1956.
2. R. Wilson. Nucl. Instrum. 1, 101. 1957.