Production of LiF:Ti thermoluminescence dosimeter material

ht. J. Appl. Radiat. ht. Vol. 36, No. 7. pp. 566568. % Pergamon Press Ltd 1985. Printed in Great Britain. 0020-708X/85 S3.00 i 0.00

Production

sources and an industrial type Picker-Anrex x-ray generator were used. Thermoluminescence measurements were made on I j mg powdered samples by heating them linearly at a rate of4’Cis with a Pitman mode! 654 TL reader at nitrogen atmosphere (400’ cm:‘,min). Comparable results were also obtained with a Harshaw 2000 series TL analyzer.

1985

of LiF:Ti Thermoluminescence Dosimeter Material

Results and Discussions

A. AYPAR” and H. DEMIRTA$ Ankara Nuclear Research and Training Center, Besevler, Ankara, Turkey (Received

I3 December

1984)

Titanium (Ti) doped LiF:Ti thermoluminescent crystals were prepared and their dosimetric properties were studied. They showed different thermoluminescence sensitivies with increasing amounts of Ti dopings. reaching maximum for 400 ppm in LiF. In the glow curves of a!! samples, two peaks at 140°C and 210°C were observed. The stability of the traps connected with these peaks were studied for a period of one month. Dose-response and energy dependence properties of the stable 210°C peak were investigated between dose ranges of 200 mR and 200 R and 50 mR and 100 R for “‘Cs y rays and x-rays respectively. Dose-response curves were found to be linear in these ranges. The energy dependence was !.70’, between 33 and 662 keV when the thermoluminescent responses were normalized to “‘Cs 7 rays. It is suggested that LiF:Ti (400ppm) may be used as a personnel dosimeter.

Introduction Ionizing radiation dosimeters, which rely on the thermoluminescence (TL) properties of materials, have helped in the solutions of the many dosimetric problems due to their long time storage capacities, independence of dose with radiation intensities, ease with which measurements are done and light weights”’ When a TL dosimeter is exposed to ionizing radiation (e.g. u.v., x, p ray etc.) material colour centers are created. The number of centers, i.e. the radiation doses, can be measured by one of the TL reading techniques. So far as many as ten TL materials have been developed and put into use.‘r.3)The most commonly used TL material for personnel dosimetry is lithium fluoride (LiF) due to its tissue equivalent quality. ‘Ji However LiF is still the most mysterious thermoluminescent phosphor although great efforts have been made to examine and explain its whimsical thermoluminescence behaviour. In this study, preparation and TL dosimetric characteristics of titanium-doped LiF material are reported.

If an irradiated thermoluminescent material with crysa! structure is heated under control, in the g!ow curve obtained are observed several peaks which, accordmg to band theory, correspond to the different traps in the forbidden band gap of the crystal. The dosimetric properties which were examined were as follows: simplicity of glow curve. stability of traps connected with this glow curve, dose-response, and energy dependence. 1. Glorc cnrce strnc~~lre Referring to Fig. 1. when a LiF:Ti phosphor was irradiated by 5 R from a “‘Cs .’ radiation source. the glow curve obtained shows two peaks’at l4O’C and Z!O’C and this was observed for a!! samples. The change in thermoluminescence yields with increasing ratios of Ti were inv-estigated separately for these peaks, (Fig. 2). It is seen that the highest efficiencies are obtained for 200 and 400 ppm Ti additions for the 14O’C and 21O’C peaks respectiv-ely. 2. Stabilities

of traps

It is important to know the stabilities of the traps connected with the peaks. since these reflect the storage capacities of the traps. To determine the stabilities, thermoluminescence measurements were performed for a period of 30 days on the samples irradiated with “‘Cs :: rays of I50 R. With reference to Fig. 3, the traps corresponding to the 210’C peaks were considered to be stable since the readings for them remained constant during the measurement period,

Peak 1 Peak 2

A

Experimental Mixtures of purified LiF (BDH Chemicals, Poole, England) and stoichometrically prepared solutions of titanium (Ti) in HF at different ratios were heated in a furnace at 9OO’C in a platinium crucible for 2 h and left there for cooling. The colourless and transparent crystals so obtained were then ground in a mortar by hand to an 80-100 mesh crystalline powder. Samples labelled A, B, C, D, E, F, G with Ti contents of 0, IO, 200, 400, 1000, 1500 and 2000 ppm respectively were prepared. They were then given similar heat treatment for I h at 400°C and 24 h at 100°C. To irradiate the samples. “‘Cs

I 50

I

I

I

I

I

I

loo

150

200

250

300

350

Temperature

* Present address: University of Ankara, Faculty of Science, Department of Physics, Beqevler, Ankara, Turkey. 566

(‘C)

Fig. I. Thermoluminescence glow curve of LiF:Ti, irradiated with “‘Cs 7 source, 5 R, heating rate 4’Cjs.

Technical Note

567

Peak 2

Peak 1

I

I *

I

IO‘

10'

IO3

Titanium

additions

in Lif ( ppm

)

Fig. 2. Improvement of thermoluminescence readings with the additions of titanium in LiF for the 1st and 2nd peaks of B, C, D, E, F, G, samples.

and sample D with 400 ppm Ti content was chosen to be the most efficient, since the readings for it were the-highest. 3. Dose-response Having determined that the LiF with 400 ppm Ti was stable and the most sensitive phosphor, the response curve (TL vs R) was measured to find out the useful range. As can be seen from Fig. 4, TL readings increase with increasing radiation dose for 7 rays of 200 mR-200 R and x-rays of 5@jmR-lOOR to be linear in the range I-100 R. Slight differences in the tangents of the two dose-response curves suggest that the LiF:Ti phosphor has different qualities for “‘Cs 7 rays and x-rays.

lo*

1

I

I

I

02

03

04

Illlll 06

CL6 1

4. Energy dependence

Absorption of ionising radiation is a fairly well-known process. (I) We have limited our measurements, for the time being, to the investigation of x- and y-ray absorptions, since these measurements are the most common in dosimetric practice. Therefore the responses of LiF:Ti (400 ppm) to xand 7 rays were investigated over a range of energies from 30 to 662 keV. An isotope emitting y rays of known energy and filtered x-rays from a constant potential x-ray machine were used (Table 1). X-ray output was measured with an appropriate Victoreen R-meter chamber (Victoreen Instr., Co., Cleveland, Ohio, U.S.A.) Referring to Fig. 5, the energy dependence curve changes with a quality of 1.7%

I

I

I

Illlll

2

3

4

5678910

Storage

time ( days

I 20

I 3040

I

I Illll 60mlw

1

Fig. 3. Decay of stored energy in LiF:Ti for the 1st and 2nd peaks of differently titantium doped (B, C, D, E, F, G) samples, exposure 150 rad of “‘Cs y radiation, stored in the dark. A.RI. 36,7-D

Technical Note

568

TLD -la,.----.~zrn~,~~ 011 ‘-.30l-f&l,34R

A_ __., ld'

lo-’

100

lo2

10

Exposures

(R

lo’

1

Fig. 4. Dose-response curve of LiF:Ti (400ppm Ti, 2nd peak) exposed to “‘Cs 7 and x-radiation.

keV or isotope

Filter

70 70 100 120 190 190 “‘CS

Al Al Al CU CU CU

x-ray x-ray x-ray x-ray x-ray x-ray 7 rav

k

(mm)

keV

0.5 4.0 5.0 1.07_ 2.09 7.49

33 41 49 70 105 140 662

1.0-

H !! 1 z

I 15c-c

I 2m-c

Temperature

I 27vc3xK

I

I

350-c

(*Cl

Fig. 6. Thermoluminescence glow curve of TLD-100 and LiF (Ti: 400 ppm, Mg: 2000 ppm) (sample Dl I), irradiated with “‘Cs 7 source, 34 R, heating rate 4”C/s.

According to the results so far obtained we may conclude that the phosphor prepared with the additions of 400ppm Ti to LiF has very good dosimetric properties. Considering its tissue equivalence, low dose sensitivity, radiation storage capacity and energy independence, it may be proposed as a personnel dosimeter because of its 210°C peak, after having cancelled out the first unstable one with an initial annealing, or putting limits before and after the 210°C peak while counting the dosimeter with a TL reader such as Toledo-654 suitable for these purposes. Later studies have been directed towards the preparation of titanium (Ti) and magnesium (Mg) doped LiF(Ti, Mg) dosimeter crystals with a profile similar to those of TLD-100 produced by Harshaw Ltd. In Fig. 6, the profiles of TLD-100 and our LiF (Ti:4OOppm, Mg:2OOOppm) are compared. They show similarity except for the peak around ZOOC, which is caused by the addition of titanium. Further studies need to be concentrated on more dopings of both titanium and magnesium to LiF matrix.

References

5 0.1

=i

I lcG-c

where the TL readings were normalized to unity for 662 keV. This shows that the phosphor prepared is energy independent.

Table I. Sources for energy dependence studies SOUP%

I

SO’C

10

Effective

I

I

102

103

energy CkeV I

Fig. 5. The energy dependence of LiF:Ti (400 ppm Ti, 2nd peak) which was normalized to i3’Cs (662 keV).

Attix F. H. and Tochilin F. Radiation Dosimetry (Academic Press, New York, 1969). Burke G. P. Compilation of available studies on TLD stability, 5th Int. Congr. on Luminescence Dosimetry, Sao Paulo, Brazil. p. 84 (1977). Aypar A. Int. J. Appl. Radiat. Isot. 29, 369 (1978). Mansfield C. N. and Suntharalingam N. The usefulness of thermoluminescence dosimetry in clinical radiation therapy. Biomedical Dosimetry, Proc. of a Symposium, IAEA, Vienna, lo-14 March. p. 335 (1975).