On the Temperature Stability of NaI(T1) Scintillators
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some critical one, there occurs a decomposition of the solid solution. This defines the optimal content of the dopant in crystals for standard scintillation detectors. On the other hand, with the temperature rise, the equilibrium concentration of the dissolved ions increases. Thus, there appears a possibility of a partial compensation of the luminescence efficiency decrease because of the increase of the luminescent center concentration. Let us consider a standard phase diagram in the temperature range of interest (293-673 K). The curve defining the region of solid solution existence may be constructed on the basis of the experimental data 3 in the low temperature range and on the basis of the Becker equation4 in the high temperature range. With a sufficient degree of accuracy, this curve may be approximated by a second-order function to describe the dependence (T/Tm,)(c), where Tm is the maximum temperature of the solid solution decomposition. In order to obtain the dependence on temperature for the amount of T1+ ions available in the solid solution, we shall transform the dependence (ThTm)(c). By trinial transformations and substitution of the experimental data one can obtain: CT!
1_ T -,
--1 2
where C is the molar share of Tl + ions in the solid solution that depends on temperature. 313 Mat. Res. Soc. Symp. Proc. Vol. 348. 01994 Materials Research Society
(1)
Thus, we obtain the dependence of the content of the activator centers in the form of the partial function from total concentration Co of T I and temperature: C
-
I
CTI (T),Co >
CT1
(T)
(2)
C0 < CTI (T)
C0,
Correspondingly, the expression for the intensity of the luminescence with allowance for temperature quenching takes the form: I=const
C u
1+Ae
(3)
kT
The consequences of this formula are obvious: at low concentrations of T1+, the impurity is
present in solid solution so that with increasing temperature it does not increase and temperature quenching produces the usual effect. With a higher total concentration of the active ions, light output increases up to the condition CO =CTI(T) after which the level of the emission is defined only by the temperature dependence of the luminescence quenching. At high concentrations of TI+, the increase of the light output may look as follows. Figure 1 (curve 1) shows the behavior of the temperature quenching of the luminescence of a weakly doped NaI(TI) crystal in accordance with Eq. (3) where the energy of the luminescence quenching activation U=0.75 eV, A=2x 107 is the constant value. 2 The curves 2 and 3 correspond to the cases of the increase of the Ti+ ion concentration in the solid solution.
I,
a.u. 10
0
50
100
150
250T, C
Figure 1: The temperature dependence of light output NaI(T1) calculated using Eq. (3). 1-0.1 mol% T1, 2-0.4 mol% T1, 3-0.7 mol% T1.
314
-
9 8 7
1
10 1 -2 3 6 __
15 5
Figure 2: General schematic of the measuring system. 1-sample 2,3-lightguides, quartz and organic glass respectively 4-photomultiplier 5-light diode 6-fiber lightguide 7-brass holder 8-Ni-Cr all
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