Some studies of thermoluminescent MgF2 phosphors

International Journal o/Applied Radiation aml Isotopes Vol. 32, pp. 147 Io 151

0020-708X 81 030147-05$02 00 0

© Pergamon Press Ltd 198l. Printed in Great Brilain

Some Studies of Thermoluminescent MgFE Ph osphors J. S. N A G P A L ,

V. K. K A T H U R I A

Division of Radiological Protection, Bhabha Atomic Research Centre, Trombay, Bombay 400 085, India and V. N. B A P A T Health Physics Division, Bhabha Atomic Research Centre, Trombay, Bombay 400 085, India (Received 5 August 1980)

Optical grade MgF 2, fused in nitrogen atmosphere with individual dopants Mn, Dy, Tm and Tb (0.1°;i by wt), has been studied. After ),-ray irradiations phosphors exhibit 10 TL glow peaks between RT and 400°C around 80, 115, 137, 165, 190, 225, 265, 290, 315 and 360°C. Their response to u.v. and 7-rays, their PTTL, TL emission spectra, photoluminescence spectra and absorption spectra are reported. Rare earth ions in a triply ionized state replace Mg 2 + in MgF 2 and take part in TL emission, whereas Mn is in a doubly ionized state in the doped phosphor, High temperature treatment in air enhances the high temperature peaks in MgF2 :Th, while the contribution from lower temperature peaks becomes negligible. MgF 2 phosphors show y-induced sensitization. For a post-annealing for 1 h at 30&C and a test exposure of I0 R at RT, the sensitization factor is 2.2 for MgF2 :Mn (pre-exposed to 103 R) and 25 for MgF2 :Tb (pre-exposed to 5 × 104 R).

1. Introduction OF ALL the alkaline earth fluorides, MgF z remains the least studied from point of view of radiation effects. Radiation-induced color centres in M g F z were studied by BLUNT and COHENt~ and by FACEY and SIaLEV,12~ whereas the thermoluminescence of co-precipitated M g F z : M n has been examined for radiation dosirnetry by PAUN et al. ~3"+~ and BRAUNLICH et alJ ~ We present here certain studies conducted on Mn-, Dy-, Tm- and Tb-doped thermoluminescent MgF2. Rare earth activation has been found to be very effective in alkaline earth salts like Ca, St, Cd and Ba fluorides and sulfates. 16'~'8) We have, therefore, for what we believe is the first time, investigated the effect of rare earth doping of MgF2.

2. Experimental Details Samples of doped MgF2 were prepared by dry mixing of MgF2 (optical grade) with individual dopant oxides (99.9~o purity) and then by fusing the mixture in a nitrogen atmosphere for 1 h at 1200°C. The fused mass was cooled to room temperature, crushed and seived to the size 75-210pm. A TL reader having a programmed linear heating rate of 25°C min-1 was used for analysing the glow curve structure. For normal studies and other dosimetric purposes, a reader having a fast heating rate (-~10°Cs -1) was used. This reader has an E M I 9635 QB photomultiplier as the detector, and it records the

glow curve and the integrated TL output simultaneously. Emission spectra were recorded using a 0.25 m Jerell-Ash monochromator in conjunction with an EMI 9558 QB photomultiplier and maintaining the TL samples at temperatures ~ - 2 0 C lower than the glow peak to be analysed. A scanning rate of 100 nm m i n - ~ was used, employing a motor drive for the grating. The grating has 1180 grooves mm l and was blazed at 300 nm and 600 nm separately. Photoluminescence spectra were measured using an Aminco~Bowman spectrophotofluorimeter having a solid state accessory. The sample, in the powder form, was packed in a silica cell and mounted vertically. Absorbance of the samples was measured by the reflectance method described earlier/9~ For 7-ray irradiations, a 6°Co 7-teletherapy machine and a y-cell were employed. Exposure rates were measured using a pre-calibrated Victoreen condenser R-meter. The )'-cell was calibrated using the ferrou~ferric chemical dosimeter. X-ray irradiations were performed using a Siemens stabilipan X-ray machine (maximum operating voltage 250kV, current 20mA). Different X-ray spectra were obtained by various filtrations of the X-ray beam. X-ray spectra were identified by the experimentally measured HVL in terms of thickness of AI and Cu. Based on the HVL, the effective energy of an X-ray spectrum was defined as the energy of the monochromatic beam having the same HVL as the given spectrum. 147

J.S. Nagpal el al,

148

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Heat treatment of the samples was done in a muffle furnace having a normal atmosphere. Temperatures were controlled within +5~C of the specified values. The powder was contained in pure alumina boats during the heat treatment.

3. Results and Discussion 3.1. TL glow curves Thermoluminescence glow curves of various MgF2 samples, irradiated with gamma rays (I04 R) at RT (25°C) are shown in Fig. 1. All the samples, irrespective of the dopant added (Mn, Dy, Tm or Tb) show ten glow peaks between RT and 400°C (at 25~Cmin-~) around 80, 115, 137, 165, 190, 225, 265, 290, 315 and 360°C (shown by arrows). Most of them are clearly identifiable. Some, however, had to be isolated by a partial heating method. It is well known that a TL glow curve is characteristic of the various traps existing in a phosphor which are thermally deactivated as the sample is heated. Similarity of the glow peak temperatures obtained for the samples demonstrates that the dopants have not altered the basic nature of the traps already existing in the host matrix. Thus the activators give only different luminescence efficiencies.

Effect of pre-irradiation heat treatment on the TL of various MgF2 samples was studied. Samples were heated in air for 30 min at various temperatures in the range 400 800C. For MgF2 :Mn samples, a steady decrease in the TL response to photons was observed for 600°C and higher temperatures. Ultraviolet response deteriorated even for temperatures of 500°C and beyond. Slight colouration of the sample was also observed. This was probably due to oxidation of the constituents. There were no major changes in glow curve structure. In RE-doped MgF 2 samples, heat treatment from 400°C to 800°C enhances the high temperature peak of 225"C and drastically reduces the lower temperature peaks. There is no quantitative change in the TL output for Dy- and Tin-doped samples but an enhancement by a factor of 3 to 4 is observed for Tbdoped MgF2 for 7-irradiations and a marginal increase in intrinsic u.v. response is observed. 3.5. Gamma-induced sensitization Gamma-induced sensitization of Mn- and Tbdoped MgF 2 was studied after annealing the highly 10 6 5 2 i0 5

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3.2. Gamma response TL response of MgF2 phosphors in supralinear right from 10 R upwards (Fig. 2). Even the individual peaks grow in a supralinear fashion with exposure. Of the samples prepared, MgF 2 :Mn (0.1 wt~o) exhibits the highest TL sensitivity, but the best achieved with the fused samples is only 75?/0 of that of LiF (TLD-100). TL output increases supralinearly up to 103 R, beyond which the response maintains sublio nearity up to 106R. However, saturation is not reached even at 106 R. At lower gamma exposures, contributions from the 8 0 C and the 115r'C peaks are significant. As the level

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samples (Fig. 4). Essentially, lower temperature peaks are only observed on exposure to 254 nm u.v. The intrinsic u.v. response is supralinear with respect to radiant energy for all the samples. It is highly dependent on the u.v. flux. The higher the u.v. flux, higher is the TL output for the same total radiant energy. A log-log plot of the integral TL response of MgF 2 :Tb against the radiant flux of 254 nm u.v. (for 30 s exposure at each flux) gives a straight line having a slope of 1.48. It means that the induction of TL by 254 nm u.v. is a multi-stage process similar to that for diamond. (12~ Ultraviolet response of the phosphors on an integrated TL basis is comparable to that of CaSO4:Dy, CaSO4 :Tm and AI~Os(Si, Ti) phosphors.

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Such a degree of sensitization in MgF 2 :Tb, as compared to M g F 2:Mn, may be due to the higher amount of residual thermoluminescence (RTL) in Tbdoped samples. Glow peaks at temperatures higher than 300°C in Tb-doped samples of MgF 2, as compared to other dopants, are far more prominent.

i-Iigh-dosed (103-106 R) MgF2 samples which are partially annealed, on exposure to 254 nm and 365 nm photons, exhibit PTTL. In all the doped samples, the peaks repopulated in P T T L are the same as those observed on ),-irradiation of virgin phosphors. It means the high temperature peaks responsible for PTTL and the repopulated peaks are due to similar charge carriers. PTTL intensities, in the case of MgF2 :Tb posttreated at temperatures of 480cC and higher (for 6min) and for M g F 2 : M n similarly treated at temperatures higher than 4 0 0 C , are negligible. It can be inferred that there are no high temperature peaks beyond 400°C in M g F z : M n and beyond 480°C in MgF 2 :Tb. PTTL of the phosphors is a function only of the total radiant energy and does not depend on the radiant flux. This aspect is very useful for low-level u.v. dosimetry employing the technique of PTTL.

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Fzo. 3. Photon energy dependence of MgF 2 :Mn (0.1% wt).

irradiated (103-106 R) samples for 1 h at 300°C and giving a test exposure of 10R of 6°Co ?-rays at RT. This response (S) was compared with that of the virgin phosphor (So). For the Tb-doped sample, the sensitization factor ( S / S o ) i s ~13 at 103R, and increases gradually to 25 for 5 x 104 R and decreases at still higher exposures. In the case of Mn-doped specimens it is ~-2.2 for 103R and decreases at higher exposures.

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TL emission spectra of the samples, maintained isothermally at the temperatures indicated, are shown in Fig. 5. The Mn emission band is present to a greater or lesser extent in all the samples. This is mainly because the starting material has Mn as an inseparable impurity. The characteristic Mn 2 + emission occurs at 575 nm with a F W H M of 50 nm. The characteristic spectrum of Dy 3. can be seen superimposed over the Mn 2 ÷ spectrum for MgF2:Dy. On the other hand, in the Tb-doped sample, the emission is of Tb 3÷ and the contribution from Mn 2+ is very low. For MgF 2 :Tm, the spectrum of Tm 3+ is superimposed over that of Mn 2+. Thus it is evident that Dy 3+, Tm 3+ and Tb 3+ take part in TL emission together with Mn 2+ in individually doped samples. Hence it can be inferred that rare earth ions in the trivalent state are replacing Mg 2+ in MgF 2. 3.9. Photoluminescence spectra The before Fig. 6. doped

photoluminescence spectra of the samples and after ),-irradiation at RT are given in Non-irradiated starting samples as well as Mnsamples do not show any fluorescence under

J.S. Nagpal et al.

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2 6 0 n m excitation. It is already known that Mn 2÷ does not fluoresce on its own unless the p h o s p h o r has some other absorption bands through which it receives excitation via energy transfer/~°~ On irradiation, undoped and Mn-doped samples show two fluorescence emission bands, one at 420 nm and the other at 580nm. The latter matches with the M n : ' emission as observed in TL emission of M g F , :Mn. The enhanced output of the 420 nm band on addition of the Mn impurity also confirms that it is due to

Mn 2÷. An additional excitation band at 3 7 0 n m is also observed on irradiation for all the samples. This corresponds to the intense absorption band detected in irradiated samples and has been assigned to M centres. (11~ The fluorescence b a n d at 420 nm may be assigned to an optical centre produced on 7-irradiation. The excitation m a x i m u m for the 420 nm band is at 260 n m while that for M n emission {580 nm) contains maxima at 2 6 0 n m and 420 nm. This goes to show that the M n 2 + fluorescence is due to excitation energy transfer from the centre responsible for 420 nm fluorescence emission or in other words, from one having 260 n m absorption band. Tb-, Tm-, and Dy-doped MgF2 show fluorescence even m the non-irradiated state, which is characteristic of Tb 3+. Tm 3. and Dy 3+ ions respectively and compares well with the characteristic TL emission spectra. O n ?,-irradiation, the characteristic fluorescence of RE 3+ vanishes and the fluorescence observed is similar to that of the undoped samples. The absence of fluorescence typical of RE 3+ on 7-irradiation means that RE 3 + ions are absent. The trivalent ions are converted, most probably, to divalent ones on irradiation. • It is interesting to note that 420 nm band, so predominantly observed in fluorescence, is absent in the TL emission. This is because in the TL emission process, the recombination energy excites the host centres to higher excited states from which transfer of excitation to T m 3+, Dy 3. , Tb 3+ or M n 2+ is much more efficient than relaxation to a lower state of the host centre, which could result in 420 n m emission. 3.10. Absorption spectra In addition to the intense absorption band at 260 nm. already detected by Kou)mJs eta[. and attributed to the F centre, ¢ ~ irradiated MgF2 exhibits absorption spectra as shown in Fig. 7. There is a sharp absorption band at 380 nm with minor satellite

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151

Some studies of thermoluminescent MgF 2 phosphors

MgF 2 phosphors. RE ions do get seated in the matrix in the triply ionized state and the TL emission spectra are characteristic of the same. The energy dependence of the phosphor is low as compai'ed to other phosphors, such as CaF2 and CaSO4, when in use. It may be worthwhile improving the sensitivity of the phosphor, particularly MgF2 :Tb, by making use of the co-precipitation method for preparation. As the y-induced sensitization factor (25) for MgF2 :Tb is perhaps the highest so far reported for any phosphor, it can be put to maximum use for low level dosimetry after improving its initial sensitivity.

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Acknowledgements--The authors thank Dr K. G. Vohra for constant encouragement. Many useful discussions with P. Gangadharan are also acknowledged.

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1. BLUNT R. F. and COHEN M. I. Phys Rev. 153, 1031 (1967). 2. FACEY O. E. and SIBLEY W. A. Phys. Rev. 186, 926 (1969). 3. PAUN J. and JIPA S. Radiochem. Radioanal. Lett. 24, 263 (1976). 4. PAUN J., JIPA S. and ILIE S. Radiochem. Radioanal. Lett.

40, 169 (1979). 5. BRAUNLICH P., HANLE W. and SCHARMANN A. Z.

bands around 40(N420nm. However, even after extended ),-ray exposures in the range 10s-5 x 107 R, it has not been possible to identify any absorption bands which can be assigned to R E : * ions.

4. Conclusions Even though the fusion method does not yield a sensitive TL phosphor based on the MgF2 matrix, RE doping and the subsequent heat treatment shows that higher temperature peaks become more prominent in

Naturforsch. 16(a)~ 869 (1961). 6. DIXON R, L. and EKSTRANDK. E. J. Lumin. 8, 383 (1974). 7. SUNTA C. M. Ph.D. thesis, Agra University, India, (1971). 8. NAMal K. S. V., BAPAT V. N. and GANGULYA. K. J. Phys. C 7, 4403 (1974). 9. NAGPALJ. S. Int. J. Appl, Rad, Isot. 31,333 (1980). 10. MEDLIN W. L. In TL of Geological Materials (Edited by McDoUGALL D. J.) pp. 201. (Academic Press, London, 1968). 1l. KoLoPUS J. L., LEwls J. T., UNRUH W. P. and NELSONS L. G. J Phys. C 4, 3007 (1971). 12. CHENR. and HALPERINA. Proc. Int. Conf on Luminescence, Budapest, p. 1414 (1966).