SETUP FOR THERMOSTATING A SAMPLE

IN A PARAMAGNETIC RESONANCE SPECTROMETER

S. I. Rukoleev

(Presented by the scientific seminar of the Department of Radiation Chemistry, KhTF)

In studying the kinetics of annealing of radiation defects in solids, and in other investigations by the method of electron paramagnetic resonance, it is necessary to vary the temperature of the sample placed in the resonator of the radiospectrometer over a wide temperature interval (from the boiling point of liquid nitrogen to \(+300^\circ\)C). At the same time, the accuracy of maintaining the specified temperature at any point of the indicated interval must be no worse than \(\pm 1^\circ\)C. Often, for this purpose, when recording the temperature dependences of the paramagnetic-resonance signal, various devices are used: a cryostat for operation from \(-4^\circ\)K to \(300^\circ\)K [1] and a furnace of special design, placed in the resonator at temperatures above room temperature [2]. The inapplicability of such devices in the study of temperature-unstable signals over a wide temperature interval is obvious. There are also more universal thermostating devices, similar to those described in works [3, 4], but in these installations the temperature of the sample is regulated and maintained manually, which naturally reduces the accuracy of thermostating and creates considerable inconvenience in operation.

In the installation described here, thermostating of the sample is achieved by placing it in a stream of gas at a specified temperature. The small dimensions of the sample and the high velocity of the gas flow ensure rapid and almost complete equalization of the temperatures of the sample and the thermostating gas. The use of an automatic control system increases the stability of the specified temperature and makes it possible to carry out a smooth change of temperature according to a linear law with high accuracy, which is necessary in conducting certain experiments.

The installation operates as follows.

In a closed metal Dewar vessel (12) (Fig. 1), filled with liquid nitrogen, an excess pressure is created by means of a heater (13); under this pressure the liquid nitrogen is fed through a copper capillary (10) into the heating chamber, formed by a quartz Dewar tube (6), a Teflon plug (9), asbestos thermal insulation (5), and a Teflon seal (4). The use of the Dewar tube (6) is due to the need to reduce heat exchange between the evaporating nitrogen and the surrounding medium, since otherwise it will be difficult to obtain the temperature at the location of the sample.

Fig. 1. Diagram of the setup

close to the boiling point of nitrogen. The heater (8), placed in tube 6, is powered from a LATR (21), controlled by an automatic regulation system.

The evaporated nitrogen gas, heated to the required temperature, is then directed onto the sample 1, located in the resonator (2) of the radiospectrometer, by means of a small-diameter quartz Dewar tube 3, which protects the resonator from temperature changes that affect the parameters of the radiospectrometer during the recording of sample spectra at various temperatures. The gas temperature is monitored by a low-inertia platinum resistance thermometer, similar to that described in [5], which is included in one arm of measuring bridge 14. In the adjacent arm of the bridge an accurate variable resistance with a linear characteristic is connected, serving as the temperature setpoint device. When the resistances of the thermometer and the setpoint device are equal, the bridge is in equilibrium and the entire regulation system is at rest. In the event of bridge imbalance, which occurs either when the temperature changes as a result of destabilizing factors or when the position of the setpoint device is changed because of the need to change the temperature, an imbalance signal appears at the bridge output.

Since the phase of the imbalance signal of the measuring bridge depends on the sign of the resistance difference between the thermometer and the setpoint device, a phase detector is required at the amplifier output, controlling the operation of the actuator mechanism in such a way that the resistance difference between the thermometer and the setpoint device, under the action of displacement of the actuator mechanism, tends to zero. For amplification and phase detec-

For amplification of the imbalance signal of the measuring bridge, an amplifier (16) and a phase detector (17) are used. To increase sensitivity and noise immunity, a narrow-band stage is placed at the output of the amplifier. The introduction into the control signal of a component proportional to the rate of temperature change substantially increases the stability, accuracy, and speed of response of the control system. This is achieved by introducing circuit (18), which extracts the derivative of the bridge-imbalance signal.

The actuating mechanism of the control system is a reversible motor RD-09 (20), whose shaft is mechanically connected to the movable contact of the LATR (21), which supplies the heater (8). In order that pickups on the measuring circuit and on the input circuits of the amplifier at a frequency of 50 Hz—which are very difficult to eliminate—should not affect the operation of the control system, the operating frequency of the automatic-control system was chosen to be 100 Hz. The voltage of this frequency is formed by a special circuit (15) and is then fed to the measuring bridge and the phase detector.

When a smooth linear change of temperature is required, the axis of the temperature setter is mechanically connected to the shaft of a synchronous motor (19) of the DSD-2 type through a reduction gear with variable transmission ratio, which makes it possible to obtain different rates of temperature sweep.

In the case of operation at a temperature above room temperature, the test tube (9) is provided with a channel, closed by a valve (11), through which compressed air from a compressor is supplied to the heating chamber. This creates additional convenience in operation. The schematic electrical circuit of the automatic-control system is shown in Fig. 2.

LITERATURE

1. R. A. Zhitnikov, N. V. Kolesnikov. PTE, No. 5, pp. 236–237 (1965).
2. A. M. Chaikin. PTE, 16, p. 178 (1963).
3. T. M. Connor. British Journal of Appl. Phys. 14, No. 6, p. 396 (1963).
4. R. L. James and W. S. Neikam. Journal of Scientific Instrument 43, 14, p. 272 (1966).
5. A. M. Zaitsev. PTE, No. 1, p. 228 (1964).

Fig. 2. Schematic electrical circuit of the controller system.

Noted Misprints