INSTALLATION FOR STUDYING THERMOLUMINESCENCE

N. L. TERENT'EV, V. T. SHKATOV

(Presented by the seminar of the RAS laboratory, Research Institute of FTT)

The method of thermal bleaching (TB), or thermoluminescence (TL), has at present become widely used, since it makes it possible, by means of comparatively simple equipment, to study the most diverse phenomena connected with the localization of electrons at trapping levels. Usually the TB method is applied to the study of electron-hole processes in solids in parallel with optical methods. But in a number of cases optical methods cannot be used (in the investigation of powders and opaque substances). In such cases the use of the TB method is especially important.

The mechanism of thermoluminescence consists in the liberation of electrons from trapping levels and in their transfer to the conduction band by means of externally supplied thermal energy. Usually heating of the specimen at a constant rate is employed [1], [2].

The error in the parameters determined from TB curves depends on the accuracy with which the linear heating rate is established. In addition, the heating device must make it possible to vary the heating rate from experiment to experiment over wide limits.

In developing an installation for studying TB curves we were forced to abandon the methods of temperature regulation described in the literature, either because of their low accuracy or because of their high cost.

The installation we have constructed consists of two systems:

1) the heating system,
2) the photoregistration system.

A block diagram of the installation is presented in Fig. 1. As the receiver of the light radiation, a FEU-27 photoelectronic multiplier was chosen, having a wide region of spectral sensitivity (3000—4000 Å) and a small dark current \((10^{-8}\ \mathrm{a})\) [3]. The electrical signal from the output of the FEU, through a cathode follower, is fed to a self-recording multipoint potentiometer, which simultaneously records the specimen temperature and the thermal-bleaching curve. The sensitivity of the photoregistration device proved to be \(\sim 10^{-9}\ \mathrm{a}\) and quite sufficient for investigating the thermoluminescence of pure alkali-halide crystals.

Temperature Controller

The temperature controller provides heating according to a linear law in the temperature range from 300 to 550°K and makes it possible to vary the heating rate within the range \(1 \div 20^\circ\mathrm{C}/\mathrm{min}\). In principle, it can be used for linear heating starting from the temperature of liquid gas.

Fig. 1. Block diagram of the thermoluminescence installation

The heating rate, if necessary, can be changed during the heating process. In addition, the circuit makes it possible, at the desired moment, to switch from linear heating to stabilization of the temperature at a specified level, and then to subsequent continuation of heating at the same rate. The stabilization time and the number of stops are not limited by the circuit capabilities. The accuracy with which the temperature follows the specified law is no worse than \(\pm 1.5\%\).

The controller is an electronic automatic-control circuit with program control and contains the following principal elements (Fig. 1).

1. Program device is based on an electronic voltage integrator [4]. It supplies to the comparison circuit a linearly increasing voltage with a rate depending on the magnitude of the integrated voltage, which can easily be changed.

It should be noted that the need to obtain long (1–2 h) heating durations requires the use of a high-quality capacitor \(C_1\) in the integrator. In the installation described, a program with a nonlinearity of 1.5% was obtained using an oil-paper capacitor \(C_1 = 120\ \mu\mathrm{F}\) and \(U_{\mathrm{work}} = 3\ \mathrm{kV}\), although the voltage in the circuit does not exceed 200 V.

1. Comparison circuit produces a mismatch signal between the program voltage and the voltage of two thermocouples connected in series. One of the thermocouples is in thermal contact with the furnace heater, the other with its mass. Electrically, however, they are insulated.

Such a connection ensures a good response of the circuit to the furnace temperature and gives high regulation quality.

1. Mismatch-signal amplifier, having high sensitivity (\(10 \div 20\ \mu\mathrm{V}\)), causes the phase-sensitive control device to operate reliably even from such a low level. In this installation a standard amplifier from an automatic potentiometer is used.

1. The phase-sensitive device is assembled on a 6P6S tube, in whose anode circuit there is an MKU-48 regulating relay.

Special attention was paid to the construction of the furnace. The heater must fit as tightly as possible against the brass or copper base of the furnace, which in turn is designed with a view to minimizing temperature gradients in the crystal.

Operation of the Regulator

In the initial position, switch \(P_1\) is in position 1. At the output of the programming device there is no voltage, and the furnace is disconnected from the power supply (since it is switched on only when the program voltage exceeds the thermocouple voltage by a certain amount).

The capacitor \(C\) in the integrator is charged to \(U_{\text{init.}}\). In position 2, the integrated voltage is applied to the integrator, and capacitor \(C\) begins slowly to recharge, causing an increase in the program voltage \((U_{\text{prog.}})\).

Under the condition \(U_{\text{prog.}} > U\) of the furnace thermocouple, the furnace is switched on; when \(U_{\text{prog.}} < U\) of the thermocouple, it is switched off. As a result, the furnace temperature is forced to follow a linear law.

The transition to temperature stabilization is carried out by shifting the upper limit of integration to a specified level. This is achieved by a simple change in the anode voltage of the integrator by means of \(R_{12}\). When \(U_a\) reaches the set value, the integrator tube is cut off at the anode, and the sensitive indicator of this state (\(T_1\) and the polarized relay RP-4) switches the integrator input from the integrated voltage to the holding voltage. If, after some time, \(U_a\) of the integrator is increased slightly again, the anode current will reappear and relay RP-4 will perform the reverse switching. Heating will continue at the same rate as before the stop.

The apparatus operated reliably for 11 months.

REFERENCES

1. M. L. Katz. Luminescence and Electron-Hole Processes in Photochemically Colored Crystals of Alkali-Halide Compounds. Saratov University Press, 1960.

2. Ch. B. Lushchik. Investigation of Capture Centers in Alkali-Halide Crystals. Tartu, 1955.

3. N. A. Soboleva, A. G. Berkovskii, N. O. Chechik, R. E. Eliseev. Photoelectronic Devices. Nauka, Moscow, 1965.

4. J. Markus. Application of Electronic Automation. Foreign Literature Publishing House, 1962.