LOW-TEMPERATURE MICROCALORIMETER FOR DETERMINING STORED ENERGY IN IRRADIATED CRYSTALS

V. V. GERASIMOV, K. A. EMEL'YANOV, I. A. NASKIDASHVILI,
L. S. TOPCHYAN

The passage of nuclear particles through a solid causes the appearance of various types of defects in the crystal lattice. The accumulation of defects leads to an increase in the energy of the crystal. In this case the free energy of a crystal containing \(n=\Sigma n\) \((i-\min)\) defects increases, and its increment will be equal to \(\Delta F=\Sigma n_i E_i - T\Delta S'\), where \(\Sigma n_i E_i\) is the increase in internal energy, or the stored energy of the crystal; \(E_i\) is the formation energy of an \(i\)-type defect; \(T\) is the absolute temperature; \(\Delta S'\) is the increase in entropy [1]. The excess (stored) energy caused by the presence of a nonequilibrium number of defects at a given irradiation temperature can be released in the form of heat during annealing of the crystal, which is associated with the disappearance of defects or their rearrangement. Experimental study of the energy spectrum of stored energy may serve to clarify the existence of intermediate states of defects and their complexes [2].

Measurement of the energy released is associated with a number of experimental difficulties, since the quantity of stored energy being determined is very small. The highest measurement sensitivity is achieved by differential-thermal annealing in a calorimeter of the investigated and control specimens [3]. Using experience gained with calorimetric systems developed in recent years [4—7], we succeeded in developing a low-temperature differential microcalorimetric installation of increased sensitivity with continuous recording of the thermal effect.

The general appearance of the microcalorimeter is shown in Fig. 1. The paired ampoules (5), into which the specimens (1, 2) are placed, are surrounded by radiation shields (6) to eliminate heat losses by radiation. This system is placed in a copper block (8, 9), which equalizes the internal thermal field. The calorimeter jacket (7) is immersed in a Dewar with liquid nitrogen, and its cavity is cooled by gaseous helium (15). The irradiated specimens placed in containers are delivered in special Dewars. The containers are uncorked in the transfer Dewar, and the irradiated specimen is introduced along the guide tube (11). After this operation the guide tube is pulled up, the calorimeter cavity is evacuated (16), and during the experiment a pressure of approximately \(10^{-5}\) mm Hg is maintained.

Figure 2 shows the block diagram of the microcalorimeter. The investigated and control specimens (1) are heated by heaters connected

together with standard resistors and resistance boxes into the circuit of a type U119 dc voltage stabilizer. With the aid of the resistance boxes the system is balanced so that the initial heating rates of both ampoules are the same. Further adjustment, for small thermal effects, is accomplished by the thermal conductivity of the thermobatteries, and there is no need for artificial compensation by thermal power [4].

The signal sensor for the thermal imbalance is a manganin-constantan thermobattery (2). The thermal-imbalance signal taken from the thermobattery is amplified by an F116/1 photoamplifier and recorded by an EPP-09 electronic potentiometer. By means of a sensitivity switch, and also by changing the resistance on the MSR-63, it is possible to divide the scale of the EPP-09 recorder into different recording ranges for measurements of the thermal effect [8].

The sensors for the imbalance signal between the ampoules and the subsequent radiation shields (3, 4) are differential copper-constantan thermocouples (5). The tracking system for heating the first radiation shield is assembled on the basis of F116/2, PSR1-09 instruments and a dc amplifier using a 6S18S tube. The signal of the thermal imbalance between the ampoules (1) and the first shield (3) is amplified and fed to the PSR1-09, which, by means of contact \(K_1\), shunts the grid resistance of tube \(L_2 6S18S\), thereby increasing the current in the heater of the first radiation shield, connected in the cathode circuit of the tube.

Fig. 1. 1 — specimen under investigation, 2 — control specimen, 3 — container for the irradiated specimen, 4 — thermobattery, 5 — double ampoule with heaters, 6 — radiation shields with heaters, 7 — vacuum jacket, 8 — copper beaker, 9 — copper lid, 10 — wire feedthrough, 11 — tube for dumping specimens, 12 — insulating plug, 13 — transfer Dewar, 14 — cooling Dewar, 15 — valve for admitting gaseous helium, 16 — vacuum valve

The tracking system for heating the second radiation shield operates in an analogous manner. However, because of the increase in heat losses with rising temperature to the outer jacket of the calorimeter, it is necessary to increase the supplied thermal power. An approach to adiabatic conditions in this case is achieved as follows: the signal of the thermal imbalance between the first and second shields, taken from the ...

from a differential thermocouple, is fed to the input of the F116/2 instrument, amplified, and supplied to the PSR1-09, which regulates the heat balance by shunting the resistances connected into the grid circuit of the L1, 6S18S tube. Contact \(K_2\) serves to supply additional thermal power, regulated by contact \(K_2\), that is insufficient to maintain the heat balance. In this case contact \(K_3\) closes and switches on the discharge circuit of capacitor \(C\) through the winding of the step finder ShI \(=25\), which stepwise changes the resistance of the grid circuit of the tube. This leads to an increase in the anode current, and the system, having received additional

Fig. 2. 1—ampoules of the specimen under study and the control specimen; 2—thermobattery; 3—first radiation shield; 4—second radiation shield; 5—differential thermocouples

thermal power, returns the slider of the PSR1-09 into the regulation zone by means of contact \(K_2\) [9].

To calculate the observed thermal effects, the following principle of calibrating the thermograms is used: with complete heat balance of the system, the current in one of the ampoule heaters is artificially increased. The change in the heating current is determined by measuring the potential across a standard resistance. Knowing the initially flowing current \(I_0\) and the current \(I_1\) after the change in the resistance of the heater circuit, one can calculate the released thermal power \(\Delta Q\) that was caused by the change in current in the heater, \(\Delta Q=(I_1^2-I_0^2)R\), where \(R\) is the resistance of the heater. The ratio of the calibration mark to the mean fluctuation of the record determines the sensitivity of the instrument. Figure 3 shows a thermogram of the melting of Wood’s alloy weighing 0.002 g (sharp peak at \(72^\circ\)C) and of pure indium weighing 0.003 g (sharp peak at \(156^\circ\)C). After each peak, artificial marks are given, obtained by the release of Joule heat in one of the ampoules. The sensitivity for measuring the thermal effect, calculated from these two different types of calibration marks, was \(6\cdot10^{-6}\) W.

Using the setup described, the stored energy and the formation energy were determined in LiF irradiated with \(\gamma\)-rays (dose 1.5 Mrad) at \(77^\circ\)K. Figure 4 presents two thermograms (\(a\) and \(b\)) of irradiated LiF crystals exposed to identical doses but annealed at different rates (\(a\)—\(V_1=2^\circ/\text{min}\), \(b\)—\(V_2=1^\circ/\text{min}\)).

The released stored energy in the temperature range 113–153°K was \(\Delta Q = 0.13\) cal/g, and the activation energy of the defect formed, determined from the formula

Fig. 3. Thermograms of calibration marks

\[ \ln \left( \frac{V_2}{V_1}\frac{T^2}{T_2^2} \right) = \frac{E}{K}\left( \frac{1}{T_1} - \frac{1}{T_2} \right), \]

where \(T_1\) and \(T_2\) are the temperatures corresponding to the same concentration \(n\) at two different heating rates [1], is equal to 0.065 eV.

Fig. 4. Thermograms of irradiated crystals

The described calorimetric system proved itself well in operation with a specified sample-heating rate (5–50° per hour) over a wide temperature range (80–600°K), with high sensitivity in measuring the thermal effect (\(6 \cdot 10^{-7}\) W), and completely eliminated operator interference in the operation of the tracking systems.

REFERENCES

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