Photoluminescence and absorption spectra of various common TL phosphors—Interpretation of TL mechanisms

Photoluminescence and absorption spectra of various common TL phosphors—Interpretation of TL mechanisms

Inrernutionul Journal 01 Applid Rudiurwn uncl 1sorope.s. Vol. 0 Pergamon Press Ltd 1980. Printed in Great Britain 31. pp. 333 lo 337 Photoluminescen...

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Inrernutionul Journal 01 Applid Rudiurwn uncl 1sorope.s. Vol. 0 Pergamon Press Ltd 1980. Printed in Great Britain

31. pp. 333 lo 337

Photoluminescence and Absorption Spectra of Various Common TL Phosphors-Interpretation of TL Mechanisms J. S. NAGPAL Division of Radiological Protection, Bhabha Atomic Research Centre, Trombay, Bombay 400085, India (Received 6 November 1979) Photoluminescence and absorption spectra of TL phosphors TLD-100, CaF*:Dy, CaSO,:Dy (0.05%wt), CaS04:Tm (0.05%wt) and Mg,SiO,:Tb have been measured. The absorption spectra are typical of RE 2+ in rare earth doped irradiated phosphors. On heating RE2+ -+ RE3+ and emission from the excited states of RE3+ is observed.

Intruductiun

Experimental details

A BASIC understanding of the thermoluminescence mechanism involves the identification of the trapping centers and recombination centers for the observed light emission. Normally, electrons and holes trapped at defects or impurities form color centers and introduce optical absorption bands into the transparent crystal. The defect energy levels lie in the forbidden gap of the insulating crystals. Therefore, occupation and detrapping of the charge carriers should be correlated with changes in the optical absorption bands. A study of these changes, coupled with photoluminescence spectra, can give additional information about the thermoluminescence process. Color center absorp tion has been extensively studied in the alkali halides (including LiF), but little attention has been paid to the materials not readily- available in single crystal form of size big enough for spectrophotometric measurements. Thermoluminescent materials such as TLD-100 (LiF: Mg, Ti), TLD-200 (CaF2 : Dy), CaSO.,:Dy, CaS04:Tm, LitB407 : Mn, Mg,SiO*:Tb and several others are readily available commercially and are being used for radiation dosimetry in various fields. This paper presents the results of radiation-induced optical absorption and photoluminescence spectra of some of these phosphors and their interpretation towards the TL mechanism.

Details of the various thermoluminescent phosphors used in the present study are given in Table 1. A 6oCo y-cell @recalibrated with a Fricke Chemical dosimeter) was used for all irradiations. All the irradiations and spectral measurements were made at room temperature (25-30°C). An Aminco-Bowman spectrophotofluorimeter in conjunction with a solid sample accessory was employed for the studies. Light from 100 W xenon lamp was dispersed by the exciting monochromator (grating type) onto the sample. The sample consisted of powder packed in a 1 mm silica cell. For reproducible readings, a powder sample weighing SO-1OOmg was found to be sufficient. The sample was held in a vertical position as shown in Fig. 1. Diffusely scattered light from the sample was reflected by a front polished plane mirror onto another monochromator. This monochromator disperses light onto the photomultiplier (RCA 11 21) and the PM tube current is fed to an X-Y recorder after amplification. Apparent optical density (AOD) of the irradiated phosphor samples was defined as.

TABLE

Phosphor TLD-100 (LiF) TLD-200 (CaF,) CaSO, caSo* MgaSiO,

*.1.1.31/6--A

(AODh = logI

+ D

where Iu, II, are the diffusely scattered light intensities for unirradiated and irradiated specimens, respectively,

1. Thermoluminescent phosphors Supplier

Particle size

Harshaw Chemical Co., U.S.A. Harshaw Chemical Co., U.S.A. DRP, BARC, Trombay, India DRP, BARC, Trombay, India Dai Nippon Toryo Co. Ltd, Japan

75-211 pm 75-211 q -

Activator Mg, Ti? DY 0.05% wt Dy 0.05% wt Tm 0.001 gm atom/m01 Tb

333

J. S. Nagpal

334

FROM EXCllATlON MON3CliXWKX?

(”

t

TO EMISSION MONOCHROMATOR

FIG.

1.

Schematic of the experimental set-up.

at wavelength 1. Light intensity readings with MgO powder were used as a standard for normalization of the observations made at different intervals. 1OOpm wide slits were used for excitation as well as emission monochromators. For each excitation wavelength (at intervals of 10 nm in the range 300600 nm) the light scattered from the sample (setting the same wavelength for emission monochromator) was determined. Since AOD is defined as a ratio, it is independent of the lamp intensity and the monochromator response. Photoluminescence spectra were recorded in a way which is well established. For this purpose, a 1 mm slit was used for the emission monochromator. A wider slit was necessary because of the very low fluorescence intensities.

Results and discussion The spectra for TLD-100 are shown in Fig. 2. There is an absorption band around 450nm (M-band). Smaller bands around 360 nm (R-band) and 550 nm

(N2-band) are also present. These results exactly reproduce the observations of VAUGHAN and MILLER(‘) and CLAFFY et aI.(‘) In fact the absorption spectra of the single crystals of LiF:Mg, LiF:Ti and TLD-100 have been extensively studied!3*4) The present set of observations taken with the powder sample is shown here only to indicate the validity of the method adopted. Photoluminescence spectra of the unirradiated phosphor have an excitation peak at 220nm with a broad emission peaking around 420nm. The elongated nature of the emission curve on the higher wavelength side indicates that it is an envelope of various bands. The 220 nm excitation band coincides with the absorption band (now identified as the Z3-band”)) produced in the material on y-irradiation. On y-irradiation, the photoluminescence spectra undergo a drastic change with excitation maxima around 425 and 470nm and emission peaking at 520 nm with humps around 550-570 nm and 635 nm. TL emission spectra are reported to have bands at 300, 420, 520 and 650 nm3(@and these coincide with

TLD-100

,/ 300

,

I 400

Wvelength

500

600

m

( nm I

FIG. 2. TLD-100. (a) Absorption spectrum: y-irradiated 2.32 x 103CKg-‘; (b) Excitation spectrum: unirradiated; (c) Emission spectrum: unirradiated; (d) Emission spectrum: y-irradiated; (e) Excitation spectrum: y-irradiated.

335

Photoluminescence and absorption spectra of TL phosphors

I

TLD-200

1

wavelength(m)

FIG. 3. TLD200. (a) Absorption spectrum: y-irradiated 2.32 x lo3 C Kg-‘; Emission spectrum.

(b) Excitation spectrum; (c)

spectra (Fig. 3b) has peaks coinciding with the absorption bands observed on y-irradiation. X-ray excited optical luminescence (XEOL) studies indicated that the Dy ion is in the Dy”+ state in the unirradiated phosphor!“) CaSO, :Dy (0.05% wt) exhibits y-ray induced absorption bands as shown in Fig. 4a. Within the experimental limits, the bands are characteristic of DY2+. Undoped CaSO*, for the same y-irradiation, shows an almost uniform coloration over the wavelength range 300--6@3nm, and the apparent optical density is lower by a factor of three. Isothermal annealing studies at 120, 150, 250, 300 and 400°C have failed to establish the correspondence of any of the TL glow peaks observed around 85, 120,220,275, 375,500 and 550°C with any of the optical absorption bands observed in the irradiated phosphor. Photo-

the emission bands of the photoluminescence spectra of the irradiated phosphors. TLD-200 (commercial quality of CaFx:Dy) exhibits, in powder form, absorption spectra on y-irradiation at room temperature, as shown in Fig. 3a. Absorption bands are observed at 310, 360, 390, 410, 435, 410, 500 and 550nm. FONG”)observed bands around 314, 421, 458, 488, 544, 577, 715 and 91Onm for CaF? : Dy single crystals whereas Krss(*) reported bands at 303, 337,383,406,446,475,531 and 566 nm for similar material. These bands are characteristic of Dy’+. The TL emission spectrum (sharp bands at 485 and 580nm) coincides with the photoluminescence spectrum of the powder (Fig 3~). The two emission bands at 485 and 580nm can be attributed to transitions from 4re,I to 6H,,,l and 6H,,,1 respectively.“’ The excitation part of the photoluminescence

CaSO,: Dy

/

L_

350

-_

450 Wavelength

550

hm)

FIG. 4. CaSO,: Dy (0.05% wt). (a) Absorption spectrum: y-irradiated 2.32 x lo3 C Kg-‘; (b) Excitation spectrum; (c) Emission spectrum.

336

J. S. Nagpal loo

&SO, Tm om

1

I

xx)

400

500

600

(nm)

Wovelength

FIG. 5. CaSO,:Tm (0.05% wt). (a) Absorption spectrum: y-irradiated 2.32 x 10’ C Kg-‘; (b) Excitation spectrum; (c) Emission spectrum.

and the TL emission spectra are characteristic of the Dy3+ ion. CaSOo:Tm (O.OS%wt), on y-ray irradiation, exhibits absorption bands as shown in Fig. Sa. The bands are characteristic of Tm’+, and are similar to those observed in case of CaF2:Tm on y-irradiation. KI&“’ reported bands around 208,230, 310, 335, 410 and 450nm with a small hump around 570nm for CaF2:TmZ+. The photoluminescence spectra of the unirradiated phosphor is characteristic of Tm3+ and this intensity diminishes on y-irradiation, as some ions get converted to Tm2+. On thermal stimulation of the phosphor, the ions get converted back to Tm3+ and the intensity of the photoluminescence is restored. The photoluminescence spectra of Mg,SiO,:Tb, luminescence

shown in Fig. 6, is typical of Tb3+ ions, and on ir-

radiation the luminescence is diminished. The powder is highly phosphorescent and this characteristic greatly interfered with the present method of measurement of absorption spectra, and hence it is not possible to state emphatically that on y-irradiation Tb is in the divalent state. However, the reduction in luminescence intensity leads us to infer that Tb3+ ions are reduced on irradiation and the original concentration of Tb3+ ions is restored on heating the TL phosphor. Conclusions In TL phosphors CaF,:Dy, CaSO.,:Dy and CaSO.,:Tm, rare earth activator ions are in trivalent

MsSi04 = Tb

t 20< 1

300

400 Wwelength

FIG.6. Mg,Si04:Tb.

5fx

600

hm)

(a) Excitation spectrum; (b) Emission spectrum.

Photoluminescence and absorption spectra of TL phosphors

state, and on y-irradiation, absorption spectra corresponding to RE’+ are produced. On heating the phosphors RE’+ -+ RE’+ and emission spectra typical of RE3+ are produced. Acknowledgements-The author is grateful to Dr K. G. Vohra for constant encouragement. Many useful discussions with U. R. Kini and P. Gangadharan are also acknowledged.

References 1. VAUGHANW. J. and MILLERL. 0. HIth Phys. 18, 578 (1970).

331

2. CLAFFYE. W., GORBICSS. G. and Arnx F. H. 3rd Int. Con& Luminescence Dosimetry, Rise, Denmark. p. 756 (1971). 3. NAKAJIMAT. .I. Phys. C4, 1060 (1971). 4. ZIMMERMANN D. W. and JONESD. E. Appl. Phys. Lett. 10, 82 (1967). 5. JAINV. K. and KATHURIAS. P. Physica stat. sol. (a) SO, 329 (1978).

P. D. and TOWNSEND S. 6. CR&N&N G. C., TOWNSEND E. J. Phys. D7, 2397 (1974). 7. FONG F. K. 4th Con& Rare Earth Research. p. 373 (1964). 8. Krss Z. J. Phys. Rev. 137A, 1749 (1965). 9. DIEKEG. H. and CROSSWHITE H. M. Appl. Opt. 2, 675 (1963).

10. NAGPALJ. S. and SARANATHAN T. Unpublished data. 11. Kiss Z. J. Phys. Rev. 127, 718 (1962).