bos (2006) - optically and thermally stimulated luminescence characteristics of mgo tb 3+

7/28/2019 Bos (2006) - Optically and Thermally Stimulated Luminescence Characteristics of MgO Tb 3+

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OPTICALLY AND THERMALLY STIMULATED LUMINESCENCECHARACTERISTICS OF MgO:Tb3

A. J. J. Bos1,, M. Prokic2 and J. C. Brouwer1

1Delft University of Technology, IRI, Mekelweg 15, 2629 JB Delft, The Netherlands2Institute of Nuclear Sciences, Vinca, P.O. Box 22, 11000 Belgrade, Yugoslavia

In this paper main optically stimulated luminescence (OSL) and thermoluminescence (TL) characteristics are presented of anewly synthesised material MgO doped with terbium (Tb) developed at the Institute of Nuclear Science, Vinca. A thermallystimulated emission spectrum showed the characteristic lines of Tb3

in a wide range of wavelengths. The TL sensitivity of the

main TL glow peak at 315C is 1.7 times higher than the TL of Al2O3:C. The highest OSL sensitivity was obtained undergreen lamp (500570 nm) stimulation. The fast component in the OSL decay curve is 2.4 times faster than Al 2O3:C. The OSLsignal is linear with dose up to 10 Gy. The lower limit of detection was found to be 100 mGy. These first results show that thenewly synthesised material has some promising properties for the application in radiation dosimetry.

INTRODUCTION

Although the application of optically stimulatedluminescence (OSL) was suggested long time ago(1)

the use of OSL in radiation dosimetry has not beenextensively reported, mainly because of the lack ofgood luminescent materials which were both highlysensitive to radiation and showed high optical stimu-lation efficiency. At present the dosimetric applica-tion of OSL is dominated by Al2O3:C (Zeff 11.3),which can be considered as a standard material inthis field(2). In order to enlarge to number of suitableOSL materials for dosimetric applications we stud-ied magnesium oxide (MgO) activated with terbium

(Tb). MgO is known for a long time as thermolu-minescent material(3). Current interest in this mater-ial stems primarily from its potential as UVdosemeter(4) and its use in neutron dosimetry(5). Inthose applications transition metal ions (notably Fe,Cr, Mn, Al, Ti, Ni, V) as dopant play a key role inthe luminescence mechanism. MgO doped with rareearth elements like Tb is relatively few investigated (6)

and its properties for the application in radiationdosimetry is unknown. In this paper the synthesisprocedure of a new MgO:Tb3 (Zeff 10.8) phos-phor has been described and its thermoluminescence(TL) and OSL characteristics are determined. It isinvestigated which light source (wavelength) stimu-

lates the luminescence the most efficient. Some maindosimetric characteristics (sensitivity, dose response)are measured. The first results show that the newlysynthesised material has some promising propertiesfor the application in radiation dosimetry.

MATERIALS AND METHODS

The polycrystalline MgO:Tb phosphor was synthesisedat the Institute of Nuclear Sciences, Vinca, Belgrade,by starting decomposing a Mg(NO3)26H2O com-pound at 600C. Because of its high melting temper-ature (>2000C) MgO needs the addition of a fluxmaterial with a lower melting point to realise a liquidphase which allows the introduction of impurity ionsinto the MgO lattice. For this purpose an optimalquantity of Na2CO3 was used, a compound that actsboth as fusing compound and as ingredient whichenhances the TL characteristics. The optimal activ-ator (Tb) concentrations were found to be 0.2 wt%

of Tb, in form of Tb(NO3)36H2O. The mainfeatures of the initially prepared materials havebeen dramatically improved by adding a chemicalcompound, which was incorporated with the phos-phor to form a stable compound. After drying for16 h at 120C it was sintered in air at 1650C for 3 hand cooled down very slowly. Finally, the materialwas crushed, sieved, washed withconcentrated hydro-chloric acid, washed once more with hot distilledwater and finally dried. After sieving the polycrys-talline powder (with grain size


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density of 480 mW cm2, (2) a halogen lamp ofwhich the output is filtered resulting in a green lightwith wavelengths between 500 and 570 nm and apower density of %10 mW cm2, (3) a 10 m WNdYVO4 solid-state laser emitting at 532 nm witha power density of%5 104 mW cm2 and (4) a ringof blue light emitting diodes (Blue LED) emitting at470 nm and a total power density of 16 mW cm2.Each stimulation source requires a carefully chosenoptical filter or combination of filters in front of thePM tube in order to cut-off the scattered light of thestimulation source and thereby transmitting as muchas possible stimulated light from the sample. The TLemission spectrum was measured using an emissionspectrometer described elsewhere(7).

Samples were irradiated at room temperatureusing a build-in 90Sr/90Y beta source. Two sourceswere available with dose rates in air of 0.72 mGy s1

and 1 mGy s1 of which only one is present in thereader.

RESULTS AND DISCUSSION

The TL emission spectrum of a MgO:Tb3 sample isshown in Figure 1. It shows many lines, all of whichcan be attributed to transitions of Tb3. Most trans-itions are due to transitions 5D4!

7FJ (J 0, ... 6) butthere is also a considerable contribution due to emis-sion from the higher level emission 5D3!

7FJ. Sincethe J values involved in the transitions are high, thecrystal field splits the levels into many sublevels,which gives the spectrum its complicated appear-ance. Around 600 nm the Tb3 lines are superim-

posed on a weak broad emission band, which seemsto have another origin (F-centre?).MgO:Tb3 shows a bright TL signal. Glow curves

measured after different doses are shown in Figure 2.Between the irradiations no special annealingprocedure was applied. For each glow curve the TLafter a reader anneal (zero dose) was subtracted asbackground. Three glow peaks can be distinguished.The main glow peak (no. 3) has its peak maximum at300C. At higher doses (not shown in the figure) ahigh temperature peak at 400C can be noticed. TheTL sensitivity of Peak 3 in terms of counts mg1

mGy1 is 1.7 times that of the TL sensitivity ofAl2O3:C (chips, 5 mm diameter, 1 mm thick read

out at 5C s1).The OSL properties of the material were investig-

ated using various stimulation sources. Blue LEDOSL was observed but the luminescent signal wasvery weak. This can be explained by the fact that theU340 filter used in combination with this stimula-tion source only transmit a very small part of thestimulated spectrum (see Figure 1, the same emissionspectrum under TL and OSL is assumed). Theefficiency for green and infrared stimulation hasbeen investigated by bleaching the irradiated sample

(D 10 mGy) for different exposure times beforea TL read-out. The results are shown in Figure 3. Itis seen that the infra red (IR) laser does only effectthe low temperature glow peaks 1 and 2 but not the(stable) 300C TL glow peak even after an exposure

time of 1600 s. Green lamp light, however, bleachesthis glow peak. The OSL experiments were, there-fore, continued using the green lamp and the greenlaser.

The OSL decay curves under green lamp(500570 nm) stimulation after different beta dosesare shown in Figure 4. These decay curves weremeasured with two optical filters (5 mm SchottBG4 and 2 mm Schott BG37) in front of the PMtube. Although there is some leakage of the scatteredstimulation light (see the signal at zero dose) this

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Figure 1. TL emission spectrum of MgO:Tb3 integratedbetween 100 and 500C after a dose of 475 Gy. All peakscan be attributed to transitions between energy levels ofTb3 as shown in the decay scheme on the right-hand side.

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Figure 2. TL glow curves of MgO:Tb3 after differentdoses read out at 5C s1 and measured with a 2 mmSchott BG 39 filter in front of the PM tube. For eachglow curve the TL after a reader anneal (zero dose) was

subtracted as background. The sample mass was 14 mg.

OSL AND TL OF MgO:Tb3

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filter combination gave the highest signal-to-noise(S/N) ratio for this source among the different filterstested. The OSL signal is considerable lower thanthe TL signal but is still bright. The OSL signaldecays in 27 s by a factor 2 but after that time thedecay elapses very slowly. The integrated OSL signalincreases linearly with dose in the 1 mGy10 Gydose range.

In Figure 5 the OSL decay curves stimulated withthe green laser are shown. The stimulation was per-formed using the single grain attachment to the Risreader(8). The sample powder was placed in the holesof a Ris aluminium disc which contains 100 holes

in a 1010 grid, each hole being 300 mm deep and300 mm in diameter. The laser is focussed into abeam with diameter of$120 mm and is assumed tostimulate the sample in one hole. The signals shownin Figure 5 are all originating from the same holeposition. The sample mass in that hole is estimatedat 50 mg. The decay curves were measured with a2 mm Schott UG1 optical filter in front of the PMtube. The OSL signal decays by a factor 2 in 37 mscompared to 27 s under green lamp stimulation. Theratio of these decay rates reflects the ratio of the

power densities of the two stimulation sources.Another difference is that under green laser stimula-tion the slow component in the decay is muchless pronounced. This needs further research. Theintegrated OSL signal (i.e. during the first 0.5 s)minus the background (signal of a not irradiatedsample) has been plotted against the dose inFigure 6. It is seen that the signal is linear withdose over almost four decades.

To put the OSL signal in perspective a comparisonwith Al2O3:C has been carried out. This comparison

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Figure 3. TL glow curves of MgO:Tb3 after bleachingwith (a) IR (830 nm) laser and (b) green light (500570nm) for different lengths of time. For each glow curve the

background (second read-out) was subtracted.

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Figure 4. CW-OSL decay curves of MgO:Tb3 powderunder green lamp light (500570 nm) stimulation afterdifferent beta doses of 90Sr/90Y source. Optical filters infront of the PM tube are 5 mm Schott BG 4 and 2 mm

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Figure 5. CW-OSL decay curves of MgO:Tb3 under greenlaser light (532 nm) stimulation after the following betadoses of a 90Sr/90Y source: (a) 0 mGy, (b) 64 mGy, (c) 256mGy, (d) 512 mGy, (e) 1024 mGy, (f) 2048 mGy, (g) 4096mGy and (h) 8192 mGy. Optical filter in front of the PMtube is 2 mm Schott UG1. The laser is switched on at t

0.5 s.

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was hampered since only chips of the standardmaterial were present. Under green lamp stimulationthe OSL intensity of Al2O3:C is higher but a directquantitative comparison could not be made. Undergreen lamp stimulation a lower detection limit(based on three times the standard deviation of thebackground) of the (transparent) Al2O3:C chips wasfound to be 1 mGy while the same limit forMgO:Tb3 (white powder) was found 100 mGy.However, for a correct comparison of the OSL signal

it is necessary to use Al2O3:C in powder form.Experiments performed by Botter-Jensen(9) onMgO:Tb3 have shown that using a powerful greenlaser (532 nm) as a stimulation source, MgO:Tb3

and Al2O3:C have similar OSL sensitivities. Thedecay by a factor 2 is for the green lamp stimulationfor MgO:Tb3, a factor 2.4 times faster.

Fading of TL has been measured after severalperiods of time up to 36 h after irradiation. Thelow temperature peaks 1 and 2 faded 28% after36 h but the main TL peak only 7%. The greenlamp OSL signal faded surprisingly faster (43%after 36 h) but half of it during the first hours afterirradiation. Apparently there are traps which do play

a role in the OSL but are not in involved in theTL production. The fading can be decreased by apre-heat procedure but this needs further study.

CONCLUSIONS

It has been made possible to synthesise a new MgOphosphor with Tb3 as luminescent centre. The TL

stems from 4f4f transitions of Tb3 with emissionlines over a wide range of wavelengths. The poly-crystalline powder shows a bright luminescent signalboth under thermal and optical stimulation. The TLsignal is 1.7 times higher than Al2O3:C with the mainTL glow peak at 300C (b 5C s1). MgO:Tb3

shows luminescence under IR and green light stimu-lation but IR does not bleach the main TL peak. Thebest results so far (in terms of S/N ratio) are obtainedwith green light stimulation. Under green lampstimulation the fast component in the OSL-decayof MgO is a factor 2.4 times faster than of Al2O3:Cbut the OSL intensity of Al2O3:C (chips) is higher.Further experiments are planned using green laserstimulation and Al2O3:C in powder form. The OSLsignal is linear with dose up to at least 10 Gy. Thefading is not negligible but pre-heat procedures maybe improving this property.

In conclusion: first measurements on MgO:Tb3

material show very promising TL and OSL proper-

ties for dosimetric applications in personal and med-ical dosimetry. Since it is developed very recentlythere is a potential to improve the present properties.

REFERENCES

1. Antonov-Romanovskii, V. V., Keirum-Marcus, I. F.,Poroshina, M. S. and Trapeznikova, Z. A. In: Con-ference of the Academy of Sciences of the USSR onthe Peaceful Uses of Atomic Energy, Moscow, 1955,USAEC Report AEC-tr-2435 (Pt. 1), p. 239 (1956).

2. Btter-Jensen, L., McKeever, S. W. S. and Wintle, A. G.Optically Stimulated Luminescence Dosimetry(Amsterdam: Elsevier) (2003). ISBN 0 444 506845.

3. McKeever, S. W. S., Moscovitch, M. and Townsend, P.D. Thermoluminescence Dosimetry Materials: Propertiesand Uses (Kent: NTP) (1995). ISBN 1 870965 19 1.

4. Kortov, V. S., Milman, I. I., Monakhov, A. V. andSlesarev, A. I. Combined TSL-ERS MgO detectors

for Ionising and UV Radiations Radiat. Prot. Dosim.47(1/4), 273276 (1993).

5. Dolgov, S., Kaber, T., Lushchik, A., Maaroos, A.Nironova-Ulmane, N. and Nakonechyi, S. Thermo-luminescence Centres created selectivily in MgO Crystalsby fast neutrons. Radiat. Prot. Dosim. 100(1/4), 127130(2002).

6. Rao, R. P. and Duley, W. W. Preparation and lumines-cence of MgO:Tb phosphors. J. Mat. Sc. 27, 58835886(1992).

7. Bos, A. J. J., Winkelman, N. J. M., Le Masson, A. V.,Sidorenko, C. W. E. and van Eijk, A. TL/OSL emissionspectrometer extension of the Ris reader. Radiat. Prot.Dosim. 101(14), 111114 (2002).

8. Btter-Jensen, L., Bulur, E., Duller, G. A. T. andMurray, A. S. Advances in luminescence instrumentsystems. Radiat. Meas. 32, 523528 (2000).

9. Btter-Jensen, L. and Thomson, K. TL and OSLproperties of MgO:Tb, Private communication (2004).

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IntegratedOSLsignal(arb.

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Figure 6. Doseresponse curve of the net integrated OSLsignal under green laser stimulation for the materialexposed to different beta doses of a 90Sr/90Y source. Thedotted line is the line of linearity. The error bar (1 SD) is

only distinguishable from the black dot for lower doses.

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bos (2006) - optically and thermally stimulated luminescence characteristics of mgo tb 3+

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