Short Notes K289

phys. stat. sol. (a) 130, 289 (1992)

Subject classification: 71.55; 78.60; S10

Chemical Department of the E.-M.-Arndt-University Greifswald I)

Equipment to the Method of Fractional Glow Technique

BY W. LANGE, G. HERZOG, U. SCHMIDT, E. BRUNNER, W. KLINGNER, and F. MULLER

Introduction Thermoluminescence is a method to detect traps caused by disturbances of the crystal lattice, foreign phases or foreign activation. Generally the traps effect a decrease of the efficiency of phosphors.

By the conventional thermoluminescence glow curve the luminescence intensity I and the temperature T of the phosphor are simultaneously recorded at a constant heating rate (Fig. 1).

By means of this it is possible to determine the kinetic parameters of the activation energy E, and the frequency factor s.

A whole lot of different expressions essentially exists for the determination of E, and s in the literature 111.

Difficulties for the calculation of E, and s appear if the premise, the existence of discret, monoenergetic traps is not realized.

Compared to the conventional glow technique the method of the fractional glow technique (FGT) at oscillating heating permits the determination of trap depths with greater accuracy than is obtainable from a normal glow curve when the different groups of traps are not well separated in energy or are conti- nuously distributed 12, 31.

The FGT method The FGT is similar to the initial rise method given by Garlick and Gibson 141, but in the fractional glow technique alternating heating and

Fig. 1. Thermoluminescence glow curves of Ba,,,Eu,,,Mg,Al,,O,, for selected constant heating

. . .- 0 ' I I I rates w = (1) 1.388, (2 ) 1.282, (3)0.737, (4)0.560 Ks-'

100 150 200 250 TIW-

') Soldtmannstr. 16, 0-2200 Greifswald, FRG.

K290

d i g i t a l - vo l tmeter

physica status solidi (a) 130

T ( K )

recorder

Fig. 2. Block diagram of the equipment (FGT), aiou analogic input-output unit, ps phos- phor screen, ph heating power

cooling of the sample causes the whole light sum to be emitted in many small fractions. Fig. 2 shows the block diagram of the recording device which was constructed for the fractional glow investigations. The equipment on the computer-controlled base allows simultaneously the measurement of the luminescence intensity I , the temperature T, and the partial light sum L as a function of the voltage V , the working up to the neccessary parameters A(l/T), A In I , and L for each heating cycle y1 by the computer and finally the representation of the energetic trap distribution H ( E , ) as a function of the activation energy E,.

The determination of the energetic trap distribution is made as follows / 5 / : An independent way of determining the depth of the traps can be derived by studying

the temperature dependence of the phosphorescence with constant trap population. Assuming monoenergetic traps the trap depth E , can be determined from the slope of the low-temperature part of the glow curve with In I plotted against 1/T,

Fig. 3. Representation of the alternating heat- ing and cooling cycles for the determination of A In I , A (l/T), and L by the computer

Short Notes K29 1

Fig. 4. Energetic trap distribution of Ba0.9Eu0.1Mg2A116027

The share of the energetic trap distribution H(E, ) for each heating cycle n is given by the simplified expression

The total number of traps which must then be coordinated to each E , is obtained by summing up all contributions,

Here k is the Boltzmann constant and AG is the width over the average energy interval ~~

IEA,n-l - EA,n l .

Results In Fig. 3 the behaviour of the oscillating parameters temperature T and inten- sity I is illustrated for each cycle n. The method consists of about 70 temperature cycles with an increasing amplitude of about 3 to 4 K and decreasing amplitude of 2 to 3 K. During the experiment the amplitude of cycles can be controlled and changed in dependence on the value of the partial luminescence intensity.

Fig. 4 shows the energetic trap distribution of the Bao,9Eu~,lMg,Al,,0,, phosphor excited by 254 nm UV-rays at liquid N, temperature.

The trap spectrum of this phosphor does not exhibit a single line, but consists of a broad distribution between 0.1 and 0.9 eV with more or less significant peaks at 0.2,0.3, and 0.5 eV.

Moreover, the fractional glow technique gives better resolving power and does not require knowledge of the frequency factor s and retrapping probabilities.

References

/1/ P. KIVITS and H. J. L. HAGEBEUK, J. Lum. 15, 1 (1977). /2/ H. GOBRECHT and D. HOFMANN, J. Phys. Chem. Solids 2, 509 (1966)., /3/ H. L. OCZKOWSKI, J. Lum. 17, 113 (1978). /4/ G. F. J. GARLICK and A. F. GIBSON, Proc. Phys. SOC. @, 574 (1948). /5/ W. LANGE, G. HERZOG, and H. L. OCZKOWSKI, Z. phys. Chem. (Leipzig) 2J,

653 (1990).

(Received February 17, 1992)