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MEASUREMENTS OF STORED ENERGY RELEASE
OF X-IRRADIATED ALKALI HALIDES
P. Schrey, R. Balzer
To cite this version:
P. Schrey, R. Balzer. MEASUREMENTS OF STORED ENERGY RELEASE OF
JOURNAL DE PHYSIQUE Colloque C7, supplkment au no 12, Tome 37, Dkcembre 1976, page C7-489
MEASUREMENTS
OF
STORED ENERGY RELEASE
OF X-IRRADIATED ALKALI HALIDES
P. SCHREY and R. BALZER
Institut I fiir Festkorperphysik der Techn. Hochschule Darmstadt, F R G
ResumB. - Des cristaux de KBr ont et6 irradies a 10 K par des rayons X de 100 kV. Le dkgage- ment d'energie par recombinaison des defauts au cours d'une croissance lineaire de la temphature a et6 determine. A I'aide de mesures d'absorption optique les maxima ont pu Etre attribues B la recombinaison de configurations variees de deux paires de Frenkel differentes, d'une part une lacune et un ion interstitiel, et d'autre part un centre F et un atome interstitiel. Les knergies stockks par paire de Frenkel ont et6 evaluks a E ~ + I = (4,O 0,8) eV et EF+H = (2,7 -C 0,6) eV.
Abstract. - KBr crystals were irradiated at about 10 K with 100 kV X-rays. During a linear
rise of temperature the energy release due to the recombination of defects was determined. With the aid of optical absorption measurements the peaks could be attributed to the annealing of various configurations of two different Frenkel pairs, namely vacancy and interstitial ion, and F-center and interstitial atom. The stored energies per Frenkel pair are determined to be E,+I = (4.0 f 0.8) eV
and EF+H = (2.7
-+
0.6) eV respectively.The formation of lattice defects in a crystal leads to a n increase of internal energy. This extra energy is stored in a crystal and can be set free again when the defects recombine. For heavy irradiation this energy release can reach amounts sufficient to heat up the crystal, and even destroy it.
Irradiation at elevated temperatures as well as n- and heavy particle irradiation leads to complex defect structures, and no information about individual defect contributions can be obtained.
The aim of this work was to study the contributions ,c,,,[
due to the different point defects, which act as primary 5 9 p p h t r e P I O I C
defects for all complex defect structures. heat /IOW calorimeter Alkali halide crystals were chosen for the investiga-
tions for several reasons :
1) Low temperature X-irradiation primarly leads to the formation of point defects.
2) The concentrations of the different defects can easily be obtained by optical absorption measure- ments.
3) Much detail information is already available about defects in these crystals.
The stored energy release was measured in a specially developed heat flow calorimeter. Since the energy release due to point defects is not expected to be very large, the calorimeter had to be constructed as sensitive as possible. For KBr at 10 K a sensitivity
of l X 10d5 cal/K for a typical sample mass of 5 X 10-3 m01 was achieved. Figure 1 shows a cross
section of the calorimeter. It consists of a copper cylinder with holdings for a thermometer and a
FIG. 1. - Heat flow calorimeter.
heater. The sample is hooked up inside the calorimeter on a thermopile. With the thermopile the temperature difference between sample and calorimeter is measured. During an energy release measurement the tempe- rature of the calorimeter is raised with a constant rate, and the calorimeter temperature, and the tem- perature difference between calorimeter and sample are recorded.
Figure 2 shows the results of a typical measurement. The temperature difference between calorimeter and sample is plotted against the calorimeter temperature. The temperature of the calorimeter was raised at a constant rate of 0.36 K/min.
Curve 1 was obtained from an unirradiated crystal, and the curve 2 was measured on the same
C7-490 P. SCHREY AND R. BALZER
2 . 1:before irradiation KBr 3h irradia led 2 : o f t e r irradiation 01 11K
G-+
0.8- 0.6-
0.4 - 0.2-
0 , 10 20 30 CO 50 TK I K I-
FIG. 2. -Temperature difference between calorimeter and
sample versus calorimeter temperature for an unirradiated (1) and an irradiated sample (2).
diation times from 20 min. to 30 h. Several peaks can be separated, which change considerably in intensity with irradiation time. The 1. peak at 14 K
first grows with irradiation time, shows saturation after about 3 h, then decreases, and after 30 h irra- diation time this peak has almost vanished. It is also found that the broad peaks obtained for longer irradiation times are composed out of several indivi- dual peaks. This becomes evident, if one compares the spectrum for 20 min irradiation time with the energy release spectrum after 3 or 6 h irradiation. From other experiments [l, 21 it is known that low
temperature X-irradiation in alkali-halides predo- minantly leads to 2 different Frenkel pairs, namely the a
+
I-centers, and the F+
H-centers. Figure 4 shows the energy release spectrum after 6 h X-irra- diation compared with annealing spectra for the interstitial ion and the F-center. Since always corres-ponding Frenkel pairs recombine, it is sufficient to
KBr ,rmd,oled 01 IIK measure the concentrations of only one partner of
the Frenkel pair, here the I-center for the ionized Fren- 0 5 kel pair, and the F-center for the neutral Frenkel pair. It becomes evident from the comparison of the curves that the low temperature peaks are predomi- nantly caused by a
+
I-center recombination, and the high temperature peaks are mainly due to the recombination of the neutral Frenkel pairs. Since these data were obtained from different experiments with different annealing rates, detail interpretations cannot be made from this.To get detail informations about the energy release per defect pair, measurements of the energy release and the optical absorption change have to be made simultaneously on the same crystal.
'P
-
'P-
In addition, to separate the contributions of theFIG. 3, - Energy release spectra for different irradiation different FreIIkel pairs, it is necessary to do this
times. experiment in several steps.
crystal, but after a 3 h X-irradiation at 11 K. The - difference between the 2 curves can be interpreted as due to an energy release connected with the annealing
l', K B r
I I
I \
-
1 6 h l -I
'
of radiation defects. The change of internal energyof the sample is described by the relation 2 -
9 dTp dE 2 .- c 3 mCp(Tp) =
it
4- K(T)*A.
Q..
D."
dE/dt is the released energy per time, A the temperature
,
difference between calorimeter and sample, m the mass of the sample, C , the specific heat of the sample,and K ( T ) describes the heat flow, where thermal conduction along the thermopile dominates.
K ( T ) is obtained by a measurement on the unirra- o t
10 20 30 4 0 50
diated crystal with dE/dt = 0. A subsequent measure- T ~ ~ K I
-
ment on the same crystal, irradiated in situ, gives
dEldt (T), the released energy rate. FIG. 4. - Comparison of an energy release spectrum with
MEASUREMENTS OF STORED ENERGY RELEASE OF X-IRRADIATED ALKALI HALIDES C7-491
The energy release and the change in optical absorption were measured for the intervals 10-16 K, 16-32 K, and 32-50 K.
To keep experimental errors small, the intervals were chosen from minimum to minimum in the energy release spectrum.
The results of such an experiment are plotted in figure 5. After irradiation the optical absorption was measured at 10 K (curve 0). Then the energy release was measured while warming up the crystal to 16 K. The optical absorption was measured at 10 K again
K B , 4 h irradiofed 01 11 K during m r m I : I 0 - 16 K
" p 2 : 1 0 - 3 2 K
FIG. 5.
-
Energy release and optical absorptions from simul- taneous measurements.(curve 1). In a similar manner the optical absorption curves 2 and 3 after energy release measurements up to 32 K and 50 K respectively have been obtained. Thus it is possible to separate the contributions of a
+
I, and F+
H Frenkel pairs.A stored energy release of
E,,,
= (4+
0.8) eV for the a+
I Frenkel pair, andE,+, = (2.7
+
0.6) eV for the F+
H Frenkel pair has been obtained.There are no other experimental values to be compared with, but theoretical values for the forma- tion energy of Frenkel pairs exist. Schulze and Hardy [3] obtained 4.17 eV, and Diller [4] 3.11 eV for the a
+
I Frenkel pair in KBr, using different parameters for the lattice potential. The agreement is good, but may be fortuitous, because the theoretical values depend strongly on the choice of the potential para- meters.Fuchs and Taylor [5] have observed a strong thermo- luminescence, which can be connected with the recombination of F
+
H-centers. We assume therefore that part of the stored energy of F+
H-centers is released by emission of photons. Our value therefore gives a lower limit of the stored energy due to the formation of a neutral Frenkel pair.References
[l] ITOH, N., ROYCE, B. S. H. and SMOLUCHOWSKI, R., Phys- [3] SCHULZE, P. D. ~ ~ ~ H A R D Y , J. R., Phys. Rev. B 6 (1972) 1580.
Rev. 137 (1965) A 1010. [4] DILLER, K. M., thesis Oxford (1975).
[2] BALZER, R., Z. Phys. 234 (1970) 242. [5] F u c ~ s , W. and TAYLOR, A., Phys. Rev. B 2 (1970) 3393.
DISCUSSION
H. PEISL.
-
What is the ratio between the energy K. ROSSLER. - IS there no interference of energy deposited in the crystal during irradiation and the release from probably damaged structural materials energy released as stored energy ? of the cryostate, the sample holder for example ?R. BALZER. - From the temperature change of the R. BALZER.
