www.elsevier.com/locate/nimb

Nuclear Instruments and Methods in Physics Research B 251 (2006) 133–142

NIMBBeam Interactions

with Materials & Atoms

Correlation between TL and OSL properties of CaF2:N

George S. Polymeris a,b,*, George Kitis b, Nestor C. Tsirliganis a

a Cultural and Educational Technology Institute (CETI), R.C. ‘‘Athena’’, Archaeometry Laboratory, Tsimiski 58, GR-67100 Xanthi, Greeceb Aristotle University of Thessaloniki, Nuclear Physics Laboratory, 54124 Thessaloniki, Greece

Received 14 March 2006; received in revised form 11 May 2006Available online 10 July 2006

Abstract

Natural CaF2 is very well known thermoluminescent (TL) material, since it has been extensively used as a dosimeter. Its basic advan-tage is the exhibited high TL sensitivity. In the present work, the optically stimulated luminescence (OSL) sensitivity of this material wasstudied by exposing it to environmental radiation for time intervals of few hours up to a few days, and was found to be very high. Byanalyzing the TL glow curves and the OSL decay curves into their individual glow-peaks and components respectively, a relation betweenspecific glow-peaks and OSL components was established. An intense thermal transfer effect occurring during optical stimulation at hightemperature was observed and an explanation is offered according to existing models.� 2006 Elsevier B.V. All rights reserved.

PACS: 78.60.Kn; 78.45.+h; 78.70.�g; 87.58.Sp; 91.60.Mk

Keywords: Calcium fluoride; TL/OSL dosimetry; Lowest detectable dose limit; Thermal transfer

1. Introduction

Calcium fluoride, CaF2, is one of the earliest knownthermoluminescence (TL) materials and was among thefirst used for TL dosimetry. It is available and used as adosimeter in both natural (fluorite) and synthetic forms.Fluorite, also known as fluorspar commercially, occurs asa natural mineral in the form of crystalline blocks, in cer-tain tin veins found in many places around the world,namely Nigeria [1], Brazil [2], etc. Sunta [3] and Mishev[4] among others have studied the different varieties ofCaF2 and their applications to TL dosimetry.

A significant advantage of natural calcium fluoride(CaF2:N), besides its high TL intensity, is its low cost. Anumber of synthetic CaF2 TLD phosphors have beendeveloped based on doping the naturally occurring mineralby Mn or earth impurity elements, such as Dy, Nd, Eu and

0168-583X/$ - see front matter � 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.nimb.2006.05.023

* Corresponding author. Address: Cultural and Educational TechnologyInstitute (CETI), R.C. ‘‘Athena’’, Archaeometry Laboratory, Tsimiski 58,GR-67100 Xanthi, Greece. Tel.: +30 2541 078787; fax: +30 2541 063656.

E-mail address: polymers@auth.gr (G.S. Polymeris).

Tm. The popularity of synthetic CaF2 as a dosimetricmaterial stems from its enhanced sensitivity and its simpleglow curve structure, depending upon the dopant [5]. Thelack of tissue equivalence [5,6] precludes its use as a per-sonal dosimeter. However, the higher sensitivity means thatit is an excellent environmental dosimetric material, sinceexposure times can be reduced substantially [5].

There has been an extensive study into the TL propertiesof both natural and synthetic fluorite [1,7–9]. For anextended review, the readers could refer to [5].

OSL studies on CaF2 based materials since 1982, haveshown that the CaF2:Mn detectors are very efficient fordosimetry in mixed neutron gamma fields [10]. However,relatively few studies exist on the OSL properties of CaF2

dosimeters, despite the fact that it was reported to be extre-mely sensitive to light, independently of the dopant used[5,11]. Bernhardt and Herforth [12] suggested the use ofCaF2:Mn as an OSL dosimeter as early as 1974. Specifi-cally, they monitored the optically stimulated phosphores-cence, also known as DOSL [13]. By stimulating gammairradiated CaF2:Mn discs with broad band light from atungsten lamp, they showed that the DOSL integrated

Fig. 1. Dose–response curve for the OSL signal of CaF2:N, for extremelylow doses. Experimental points represent the mean values of twomeasurements carried out for each exposure time. Errors indicate 1r.The background OSL levels measured after 95–100 s blue light exposurewere subtracted from the initial luminescence intensity (0–1 s) of the shine

134 G.S. Polymeris et al. / Nucl. Instr. and Meth. in Phys. Res. B 251 (2006) 133–142

signal was linear with the absorbed dose over five orders ofmagnitude, starting from a low dose of 10�2 Gy. Allen andMcKeever [14] studied the excitation mechanism ofconventional continuous wave-OSL (CW-OSL) fromirradiated CaF2:Mn discs. They concluded that low dosemeasurements with OSL from this material will be difficult,due to a non-radiation-induced background [13,14].

Recently, Polymeris et al. [15] have proposed that natu-rally occurring sensitive TL and OSL materials, under spe-cial conditions of very low environmental radioactivity,could be potentially used as time-integrating luminescencedetectors for setting bounds on dark matter particle char-acteristics. Calcium fluoride based materials like CaF2

doped by Eu act as scintillators, which are frequently usedin investigations for rare events such as dark matter [16],and neutrinoless double b decay [17].

The aim of the present work is to investigate the OSLsensitivity and the lower dose detection limit (LDDL) ofthe CaF2:N, and to find the relation, if any, between itsTL individual glow-peaks and its OSL components.

down curves measured. Solid line indicates the linear fit for doses largerthan 10�5 Gy. The OSL LDDL of CaF2:N could be considered to be ofthe order of 10�6 Gy, similar to that reported for TL.

2. Sampling and apparatus

The study was performed on natural fluorspar. The ori-ginal sample was initially crushed, and grains with dimen-sions between 40 and 80 lm were selected and placed onstainless steel disks of 1 cm2 area. Each sample contained10 mg of material.

The measurements were performed using a RISO TL/OSL reader (model TL/OSL-DA-15) equipped with ahigh-power blue LED light source, an infrared solid statelaser and a 0.085 Gy/s 90Sr/Y b-ray source [18]. The readeris fitted with an EMI 9635QA PM Tube.

All TL measurements were performed using a combina-tion of a Pilkington HA-3 heat absorbing and a Corning7-59 blue filter, using a heating rate of 1 �C/s in order toavoid thermal gradient, and a maximum temperature of400 �C. OSL measurements were performed using a HoyaU-340 filter; power level was software controlled and setat 90% for blue LEDs (�40 mW/cm2).

3. Experimental results

3.1. OSL lowest detectable dose limit (LDDL)

The lowest dose detected using CaF2:N as a TL dosim-eter is 10�6 Gy [5,19,20]. However, a similar study regard-ing the OSL LDDL has not been reported. The LDDL wasobtained in the present work by exposing 10 mg samplesof CaF2:N to the background environmental radiationfor time intervals ranging from 1 to 15 days. After theend of each exposure, samples were measured usingCW-OSL for 100 s. The environmental radiation level atthe laboratory (Xanthi, North-East Greece) was measuredusing a proportional counter monitor (type FH 40 G-L)and it was found to be 225 nGy/h.

Fig. 1 presents the OSL dose–response curve of CaF2:N,in a low dose region. Two measurements were carried outfor each exposure time, in order to use their mean value.Errors indicate 1r. The background OSL levels measuredafter 95–100 s blue light exposure were subtracted fromthe initial luminescence intensity (0–1 s) of the obtainedshine down curves. The growth curve obtained has anintercept on the y-axis, namely it does not pass throughthe origin. This feature could be attributed either to theself-dose occurring in the phosphor, or to the phenomenonknown as recuperation in the growth curves measuredusing single aliquot procedures. However, the growth curveevidently becomes linear for doses larger than 10�5 Gy.

The LDDL can be defined as the TL signal which ishigher than three standard deviations of the zero dose TLreading [6,21]. Following the same definition for the caseof OSL, the LDDL of CaF2:N is found to be roughly ofthe order 10�6 Gy, which is similar to that of TL [5,19,20].

3.2. The choice of the stimulation duration

The optimum stimulation time for the OSL measure-ments was found by studying the OSL signal as a functionof stimulation time, between 5 and 500 s, on eight differentsamples. After each OSL measurement, the residual TLwas measured for each sample by performing a readoutup to 400 �C with 1 �C/s. Fig. 2 shows the integratedOSL signal as a function of illumination time, as well asthe integrated residual TL, which belongs mainly to themain peak. It is obvious that the OSL signal increases withthe illumination time, approaching a saturation level after500 s of illumination. The corresponding residual TL signalexhibits a decreasing pattern as the stimulating time

100 300 500

Illumination time (sec)

8.0E5

1.6E6

No

rmal

ized

OS

L-R

-TL

R-TL

OSL

150 200 250 300 350 400

Temperature (°C)

1.0E3

1.2E4

TL

(a.

u.)

1

7

Fig. 2. Upper figure: The integrated CW-OSL signal measured at roomtemperature and the residual TL (R-TL) signals obtained after CW-OSLby readout up to 400 �C, using a heating rate of 1 �C/s, as a function ofillumination time. Lower figure: Residual TL glow curves obtained afterincreasing stimulation times, ranging from 5 s (curve 1) to 500 s (curve 7).

G.S. Polymeris et al. / Nucl. Instr. and Meth. in Phys. Res. B 251 (2006) 133–142 135

increases, reaching a stable residual level after 500 s. Notethe symmetric behavior of the two curves.

3.3. Method of analysis

All TL glow curves and OSL decay curves obtained wereanalyzed into their individual components by following acomputerized curve deconvolution procedure. In the caseof TL glow curves the deconvolution was performed usingthe following general kinetics order expression [22]:

IðT Þ ¼ Im � bb

b�1 � expEkT

T � T m

T m

� �

� ðb� 1Þð1� DÞ T 2

T 2m

� expEkT

T � T m

T m

� �þ ðZmÞ

� �� bb�1

;

ð1Þ

where D = 2kT/E, Dm = 2kTm/E and Zm = 1 + (b � 1)Dm.In the case of CW-OSL the decay curves where trans-

formed into pseudo-linear modulated OSL curves(pseudo-LM-OSL), according to the transformation pro-posed by Bulur [23] and further transformed by Polymeriset al. [24]:

IðuÞ ¼ Im

um

� u � b� 1

2 � b �u2

u2m

þ bþ 1

2 � b

� � b1�b

; ð2Þ

where Im and um are the values of OSL intensity and timerespectively for the maximum of the pseudo-LM-OSLpeak, u is the time transformed and b stands for the param-eter of the kinetic order. The background was measuredexperimentally using non-irradiated samples at the stimu-lating temperatures of room temperature (RT), 90, and180 �C and it is shown in the left-hand side of Fig. 3.The pseudo-LM transformation of the background signalis shown in the right-hand side of Fig. 3 and was fitted toan equation of the form:

bg ¼ zD

P

� �� ta; ð3Þ

where zD is the zero dose value of the CW-OSL signal afterblue stimulation and P is the total stimulation time. Thevalue of the parameter a was found to be very close to unityfor the case of RT background, 1.14 for the case of 90 �Cand 1.32 for 180 �C. Values greater than unity could be re-lated to thermal transfer effects as is discussed below.

All curve fittings were performed using the MINUITcomputer program [25], while the goodness of fit was testedusing the figure of merit (FOM) of Balian and Eddy [26]given by:

FOM ¼X

i

jY Exper � Y FitjA

; ð4Þ

where YExper is the experimental glow curve, YFit is the fit-ted glow curve and A is the area of the fitted glow curve.The obtained FOM values were in all cases less than 2%.

An example of the deconvolution of a CaF2:N TL glowcurve is shown in Fig. 4. The glow curve was measured upto 400 �C, using a heating rate of 1 �C/s. The main glow-peak appears at high temperature, near 275 �C, with prom-inent peaks also at around 90 �C and 183 �C. The mainglow-peak and the one at 90 �C, have one satellite peakeach. Computerized glow curve analysis, was used in orderto analyze the glow curve into its five individualcomponents.

An example of the deconvolution of the pseudo-LM-OSL curve is given in Fig. 5. The original OSL decay curveappears in the inset of the same figure. All OSL curves wereanalyzed into four individual components, assuming gen-eral order kinetics. The values of the parameters um andb for each one, as they resulted from the curve fitting pro-cedure, are given in Table 1. In accordance with the threecomponents of quartz reported by Bailey [27], OSL compo-nents of CaF2:N may be termed ultra-fast, fast, mediumand slow respectively. It should be mentioned that best

100 300 500

u (sec)

50

150

250

OS

L

RT

90

180

100 300 500 700

u (sec)

0

100

200

300

I(u

)

RT

90

180

Fig. 3. Left-hand side: Background CW-OSL shapes, as measured at room temperature, 90 and 180 �C. Right-hand side: Pseudo-LM-OSL curvesobtained after the transformation of the respective left-hand side figures.

200 300 400 500 600 700

Temperature [K]

0

400

800

1200

TL

(a.

u.)

Fig. 4. TL glow curve of CaF2:N, measured up to 400 �C using a heatingrate of 1 �C/s, analyzed into its five individual glow-peaks, using generalorder kinetics.

Fig. 5. The pseudo-LM-OSL curve analyzed into its four individualcomponents. Experimental points due to their high number and scatter areseen as a shadow. Solid lines represent the fit and individual components.The values of the trapping parameters are listed in Table 1. Inset:Continuous wave blue OSL curve of CaF2:N.

Table 1OSL characteristics of CaF2:N

Peak number um (s) b r (10�18 cm2)

1 13.55767 1.56 24.991492 80.65983 2 0.603123 195.1535 2 0.103044 649.29629 1.75 0.01015

136 G.S. Polymeris et al. / Nucl. Instr. and Meth. in Phys. Res. B 251 (2006) 133–142

fitting results were obtained for both fast and medium com-ponents using second-order kinetics.

Both curves of Fig. 5 indicate that 500 s of illuminationare adequate to empty almost all traps responsible for theOSL signal. Whereas ultrafast component has decayed tozero, fast and medium components have decayed substan-tially. Only the traps of the slow component still containcharge after 500 s of illumination. However, the slow com-ponent needs almost another 500 s in order to decay tozero, whereas all three others have substantially or evencompletely emptied.

3.4. Sensitization

Monitoring sensitivity changes of the TL and OSL sig-nal as a function of successive TL and OSL readout cycles

Fig. 6. Sensitivity of the TL signal as a function of successive cycles ofirradiation–TL readout, normalized over the sensitivity of the first cycle.Glow-peak at 90 �C with its satellite (squares), glow-peak at 180 �C(circles) and glow-peak at 275 �C with its satellite (triangles).

G.S. Polymeris et al. / Nucl. Instr. and Meth. in Phys. Res. B 251 (2006) 133–142 137

is a very useful test, especially when single aliquot measure-ments are carried out.

The TL sensitivity of all glow-peaks of CaF2:N wasstudied for nine successive irradiation–TL readout cycles.It was found that the sensitivity of all peaks remains stablewithin less than 1.5%. This result indicates the remarkablestability of the TL signal from CaF2:N. Fig. 6 presents thesensitivity of the TL signal as a function of successive cyclesof irradiation–TL read out, normalized over the firstmeasurement.

In the case of the OSL, seven successive irradiation–OSL measurement cycles were carried out. The normalizedOSL sensitivity for each component is shown in Fig. 7. Thefast and the ultra-fast components seem to be extremely

1

Illumination Cycle

0.5

1.0

1.5

2.0

2.5

3.0

3.5

No

rmal

ized

OS

L

C2

C1

C4

C3

2 3 4 5 6 7

Fig. 7. Sensitivity of the OSL signal as a function of successive cycles ofirradiation–OSL measurement normalized over the sensitivity of the firstcycle. Component C1 (squares), component C2 (circles), component C3

(diamonds) and component C4 (triangles).

stable, whereas the medium component seems to be rela-tively stable. The slow component appears to be highlysensitized over the seven successive cycles. However, theincrease in its signal with cycle may also be attributed toits incomplete bleaching after each cycle, since the slowcomponent has not decayed after 500 s of stimulation(Fig. 5). Thus, incomplete bleaching gives rise to an accu-mulating signal which is never completely removed.

3.5. Quantitative correlation between TL and OSL signal

The correlation between the individual TL glow-peaksand individual OSL components was investigated by anexperimental procedure, termed thermal cleaning proce-dure, which consists of two sub-sequences. The first oneincludes the following steps:

Step 1: Irradiation–blue OSL at RT–residual TL.Step 2: Irradiation–TL up to 140 �C–blue OSL at RT–

residual TL.Step 3: Irradiation–TL up to 210 �C–blue OSL at RT–

residual TL.Step 4: Irradiation–TL up to 400 �C–blue OSL at RT–

residual TL.

The second sub-procedure includes the same steps,except that the OSL measurements were performed at90 �C in step 2, at 180 �C in step 3 and at 275 �C at step 4.

The reasoning of the above experimental procedure isthe following: In step 1 the OSL signal is correlated withall TL glow-peaks. In step 2 the OSL is correlated withglow-peaks 2 and 3, in step 3 it correlated with glow-peak3 and in step 4 the OSL is not correlated with any TL glow-peaks. Glow-peaks which could exist above 500 �C are nottaken into account.

The pseudo-LM-OSL curves were analyzed using fourcomponents. The characteristic parameters of each compo-nent are given in Table 1. Fig. 5 shows the deconvolutionof pseudo-LM-OSL at room temperature when all TLglow-peaks are present, whereas Fig. 8 shows the deconvo-lution of a pseudo-LM-OSL curve measured at roomtemperature with glow-peaks 2 and 3 present, after theglow-peak 1 was erased in step 2 by readout up to140 �C. The pseudo-LM-OSL curve shape obtained in step3 with only the glow-peak 3 present, is exactly similar tothat of step 2 (Fig. 8). Fig. 9 shows the deconvolution ofa pseudo-LM-OSL curve measured at 180 �C with glow-peaks 2 and 3 present, whereas glow-peak 1 was erasedin step 2 by the readout up to 140 �C. In the CW-OSLshape of Fig. 9 the high value of the background is appar-ent (see Fig. 3). However, as it will be shown later, this is areal background, which is due to intense thermal transfereffects. In the case of step 4, where all the TL glow-peaksare erased by the TL readout up to 400 �C, there is noOSL signal. Detailed results are shown in Table 3 for thefirst group i.e. OSL at RT and in Table 4 for the secondgroup i.e. OSL at elevated temperatures. The third (C3)

0 200 400 600u (sec)

100

300

500

700

I(u

)

Fig. 8. Pseudo-LM-OSL curve of CaF2:N measured at room temperaturewith the TL glow-peak at 90 �C previously erased by readout up to 140 �C(see text) analyzed into its individual components. Experimental pointsdue to their high number and scatter are seen as a shadow. Solid linesrepresent the fit and individual components.

0 200 400 600

u (sec)

100

300

500

700

I(u

)

Fig. 9. Pseudo-LM-OSL curve of CaF2:N measured at 180 �C with the TLglow-peaks at 90 and 183 �C previously erased by readout up to 210 �C,analyzed into its individual components (see text). Experimental pointsdue to their high number and scatter are seen as a shadow. Solid linesrepresent the fit and individual components.

Table 2Integral of individual TL glow-peaks of CaF2:N as obtained from theGCD analysis

Peak at (�C)

90(1a) 110(1b) 183(2) 280(3a) 320(3b)

Group 1 365,700 65,810 118,600 1,305,000 201,110Group 2 335,966 62,333 109,950 1,153,000 249,200

138 G.S. Polymeris et al. / Nucl. Instr. and Meth. in Phys. Res. B 251 (2006) 133–142

and fourth components (C4) were treated as a sum. Thereason is that the fourth component has the lower intensityand is poorly resolved as it is between the higher intensitycomponent C3 and the background.

Table 3Results of cleaning procedure with OSL measurements at room temperature

Step Glow-peaks contributing C1 C2

1 All 324,100 1,172,0002 180, 275 �C – 1,100,0003 275 �C – 961,0004 None – –

Integrals of individual component of OSL curves as obtained from the decon

The TL glow curves obtained from steps 2 to 4 of bothgroups were analyzed by GCD and the individual integralof each glow-peak was evaluated. The results are given inTable 2, for the two samples used for the two groups ofmeasurements respectively.

A direct observation from the LM-OSL shapes is thatthe first component of the OSL curves is related to theglow-peak at 90 �C, since this component is recorded onlywhen glow-peak 1 is present during the OSL measurement.This observation is confirmed by the results shown inTables 2 and 3 where one can see how the integral of theglow-peak 1a matches the integral of the first OSL compo-nent (C1) for both groups of measurements. Furthermore,the kinetics order found is the same for both entities. Onthe other hand a very strong correlation seems to existbetween glow-peak 3a at 280 �C and the second OSL com-ponent (C2), for two reasons: (i) their integrals are verysimilar and (ii) the deconvolution analysis showed that theyare both of second-order kinetics (see Table 1).

The glow-peaks 1b at 90 �C and the glow-peak 2 at183 �C are of relatively low intensity, so that their contribu-tion to the OSL signal cannot be accurately acknowledged.It seems, however, that at least the glow-peak 1b at 90 �C,has no correlation with the OSL signal, since the integral ofthe OSL component C1, which appears only when the sys-tem of glow-peaks 1a and 1b is present, matches the inte-gral of the glow-peak 1a only.

The relation of the glow-peak 3b at 320 �C to the OSLsignal is not clear. Comparing the integral of this glow-peak, as it was found from the deconvolution analysis(Table 2), with the integral of the OSL component C3 (plusC4), the former is found to be much lower. However, theintegral of the same glow-peak at 320 �C, is also lower thanthe integral of the residual TL (R-TL) glow-peak, whichappears at exactly the same peak maximum temperaturei.e. 320 �C. It is evident to conclude that the trap responsi-ble for the glow-peak 3b at 320 �C is extremely difficult tobleach. Therefore it is meaningless to correlate it with anyOSL component. Note that in the case of the second group

C3 + C4 OSL total TL total R-TL

787,200 2,283,300 2,056,220 705,060701,100 1,801,100 1,624,710 623,950530,800 1,491,900 1,506,110 564,110– – – 218,270

volution analysis. Comparison of OSL with TL.

Table 4Results of cleaning procedure with OSL measurements at elevated temperatures

Step Tmeasur (�C) Glow-peaks contributing C1 C2 C3 + C4 OSL total TLi R-TL

1 RT All 283,900 1,167,000 635,510 1,986,410 1,910,449 516,9802 90 180, 275 �C – 1,092,566 690,033 1,782,599 1,512,150 557,4603 180 275 �C – 1,125,571 494,377 1,619,948 1,402,200 975,8204 275 None – – – – – 1,238,400

Integrals of individual component of OSL curves as obtained from the deconvolution analysis. Comparison of OSL with TL.

G.S. Polymeris et al. / Nucl. Instr. and Meth. in Phys. Res. B 251 (2006) 133–142 139

of OSL measurements at elevated temperature the R-TL isenhanced (Table 4). For the OSL components C3 and C4,although there is not a direct indication about their relationto TL glow-peaks, their integral must be taken into accountin order to match the total OSL signal to the total TLsignal.

The behavior of R-TL at elevated temperature, given inTable 4, could be attributed to some kind of either photo-transfer or thermal transfer effects, or even both. Thishypothesis was further investigated by performing theexperiments described in the next sub-section.

Since some of the above OSL measurements are per-formed at elevated temperatures it is necessary to examinewhether CaF2:N suffers from thermal quenching of itsluminescence. The simplest way to examine that is to studythe TL as a function of the heating rate. If the TL peakintegral decreases as a function of the peak maximum tem-perature (or as a function of the heating rate), then thermalquenching is present, whereas if it is stable then thermalquenching is absent. The results of this experiment areshown in Fig. 10, where one can see the glow curves forheating rates 1, 2, 4, 8 and 20 �C/s. These results are incomplete agreement to what it is expected from the theory.

100 200 300 400

Temperature (°C)

0E0

1E5

2E5

3E5

TL

(a.

u.)

1

5

Fig. 10. Thermal quenching test in CaF2:N. As the heating rate increases(1, 2, 4, 8 and 20 �C/s, glowcurves 1 to 5 respectively), the glow-peaks areshifted towards higher temperatures and the full width at half maximumincreases with a simultaneous decrease of the peak maximum intensity inorder for the glow-peak integral to be stable.

Namely, as the heating rate increases, the glow-peaks areshifted towards higher temperatures and the full width athalf maximum increases with a simultaneous decrease ofthe peak maximum intensity in order for the glow-peakintegral to be stable. The stability of integrals of all glow-peaks as a function of the heating rates ensures the absenceof thermal quenching in this phosphor.

3.6. Transfer effects

Since, the R-TL glow-peak appears at the position of theglow-peak 3a at 320 �C (in fact they coincide) the transfereffects were investigated on samples, which were previouslyreadout up to 400 �C in order to erase completely the320 �C glow-peak.

The investigation consists of two steps. In the first stepthe OSL measurements were performed for a fixed timeinterval, 500 s, at temperatures ranging from RT up to300 �C. In the second step the OSL measurements wereperformed at a fixed temperature, 275 �C, for times rangingfrom 5 up to 600 s.

The shapes of the CW-OSL measured at temperaturesfrom 200 up to 300 �C are shown in Fig. 11. The respectiveshapes for lower measuring temperatures are shown inFig. 3 in the discussion for the background. Besides theunusual shape of the CW-OSL, the most interesting featureis the strong increase of the CW-OSL intensity, as well as,of the R-TL, as a function of temperature. These results are

0 100 200 300 400 500

Time (sec)

300

800

1300

1800

CW

-OS

L

300

275

250

225

200

150

T : °C

Fig. 11. CW-OSL curves received at various elevated temperatures for astimulation time of 500 s. The solid lines through the experimental points,represent the fit according to Eq. (5).

0 50 100 150 200 250 300

Temperature (°C)

0.0E0

4.0E5

8.0E5

1.2E6

R-T

L

1.0E6

3.0E6

5.0E6

7.0E6

OS

L

Fig. 12. The integrated CW-OSL signal for 500 s, received at variouselevated stimulation temperatures and subsequent R-TL glow-peak signalas a function of the stimulation temperature. The measurements wereperformed on samples, which after a test dose of 0.425 Gy were readout upto 400 �C.

20 25 30 35 40

1/ k T

9.0

12.0

15.0

Lo

g(O

SL

)

E=0.368 eV

Fig. 14. The plot of log(OSL) of Fig. 11 versus 1/kT.

140 G.S. Polymeris et al. / Nucl. Instr. and Meth. in Phys. Res. B 251 (2006) 133–142

shown in Fig. 12. It is interesting to note that for the sam-ple whose TL was erased up to 400 �C and subsequentlymeasured by CW-OSL at 225 �C, the R-TL at the positionof the glow-peak 3b (320 �C), has an intensity equal to theintensity of the glow-peak 3a. This is shown in Fig. 13.The CW-OSL emitted light is even higher. For examplethe CW-OSL at 300 �C gave an integrated intensity, whichis 4.5 times the TL integral of all glow-peaks given inTables 3 and 4. The question regarding the origin of thisintense luminescence signal is intriguing.

The increase of the CW-OSL signal as a function of tem-perature, shown in Fig. 12, seems to follow an exponentiallaw, which corresponds to thermally activated processes. It

0 100 200 300 400Temperature (°C)

1000

7000

13000

19000

TL

(a.

u.)

a

b

c

d

e

Fig. 13. Residual TL glow curves of CaF2:N after OSL at RT (curve b),after OSL at 90 �C (curve c), after OSL at 180 �C (curve d) and after OSLat 275 �C (curve e). Curve (a) indicates the TL glow curve, measuredwithout any preceding optical treatment.

was found that the plot of log(OSL) versus 1/kT , shown inFig. 14, is strictly linear with a slope corresponding to anactivation of 0.368 ± 0.006 eV (R = 0.999). On the otherhead the R-TL signal, as can be seen in Fig. 11, increasesup to the temperature of 225 �C and then decreases.

In the second step of this investigation, the CW-OSLmeasurements were performed at a temperature of 275 �Cfor various stimulation times. The integrals of the CW-OSL, as well as of the R-TL are plotted in Fig. 14 as a func-tion of the illumination time. The behavior of the R-TL asa function of time shown in Fig. 15 is exactly similar tothe behavior of CW-OSL as a function of time, whichwas shown in Fig. 11. Since, the sample on which thesemeasurements were performed has no glow-peaks due tothe TL readout up to 400 �C, it is clear that the appearanceof the R-TL signal (Fig. 15) is due to a thermally activated

0 200 400 600

1.0E5

3.0E5

5.0E5

R-T

L

5.0E5

2.0E6

3.5E6O

SL

Time (sec)

Fig. 15. Integrated CW-OSL signal measured at 275 �C and the subse-quently obtained integrated R-TL signal as a function of illuminationtime. The measurements were performed on samples, which after a testdose of 0.425 Gy were readout up to 400 �C. The solid line through theOSL experimental points is an eye guide, whereas the solid line throughthe R-TL experimental point is the fit with Eq. (5).

G.S. Polymeris et al. / Nucl. Instr. and Meth. in Phys. Res. B 251 (2006) 133–142 141

(assisted) photo-transferred TL effect. On the other handthe CW-OSL signal in Fig. 11, is a combination of ther-mally activated (assisted) photo-transfer and opticallystimulated effects. A working hypothesis here is that therate of thermal transfer is high enough to provide chargedcarriers for both CW-OSL (Fig. 11) and R-TL (Fig. 15).The trap which provides the charged carriers is apparentlya very deep one.

The photo-transferred TL (PTTL) is a well knowneffect [13,28]. The simplest model that describes thephoto-transfer effect assumes a shallow trap into whichthe charge is transferred, one deep trap from where thecharge is excited and one recombination center. Chenand McKeever [28], using simplifying assumptions on thedifferential equation governing the PTTL effect, arrivedto the following analytical expression for the PTTL inten-sity as a function of time:

SðtÞ ¼ C � expð�t=sÞ � N 1 � ½1� expð�B � tÞ�N2

n20� expð�f � tÞ

; ð5Þ

where N1 is the number of shallow traps, N2 the number ofdeep traps, n20 the concentration of deep traps at t = 0, s aconstant related to the recombination probability, B a con-stant related to the probability of trapping electrons to theshallow trap, f the rate at which electrons from the deeptraps are transferred to the conduction band and C is aconstant.

Eq. (5) can be used to fit experimental data using certainsimplifications. Setting N1 = 1 in order to have only theconstant C by which the rest equation is multiplied andassuming N2/n20 = D > 1, Eq. (5) provides a very good fitto the experimental data concerning the behavior ofR-TL versus time. The result of this fitting procedure isshown by the solid line in Fig. 15.

Eq. (5) was also applied to the experimental behavior ofthe CW-OSL as a function the OSL measuring temperatureshown in Fig. 11. The fitting results are represented by thesolid lines through the experimental points. As can be seenfrom Fig. 11, Eq. (5) fits very well these experimental data.

The fit of experimental data to Eq. (5) supports the sug-gestion that the observed effects are due to charge transfermechanisms. However, further experimental work isneeded in order to investigate the origin of these charges.The present evidence is that the origin should be a verydeep trap, above 500 �C, and highly populated.

3.7. Photo-ionization cross-section

The deconvolution of the experimental pseudo-LM-OSL curves gives the possibility to have an estimation ofthe photo-ionization cross-section of each component ofthe OSL decay curve. The evaluation is performed asfollowing:

The time at which the maximum of the pseudo LM-OSLpeak takes place is closely related to the photo-ionizationcross-section, r, according to the equation [29]

um ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

2PrI0ðbþ 1Þ

s; ð6Þ

where P is the total illumination time, I0 is the stimulationintensity (W/cm2) and r is the cross-section. Solving to-wards r one obtains

r ¼ 2Pu2

mI0ðbþ 1Þ : ð7Þ

In the case of CW-OSL curves, which later are transformedinto pseudo-LM-OSL curves, the stimulation intensity isconstant during the measurement. Therefore, for the eval-uation of r, one has to convert the intensity of the stimula-tion I0 from Watts into photon number, i.e. to obtain thevalue of the coefficient cf for the expression

r ¼ 2Pcf � I0 � u2

mðbþ 1Þ : ð8Þ

For the stimulation light intensity expressed in W/cm2 oneobtains

cf ¼Joule

s cm2¼ 6:24146� 1018 eV

s cm2: ð9Þ

For a given stimulation photon energy E/

cf ¼6:24146� 1018

Eu

eV

eV s cm2

� �: ð10Þ

Eventually

r ¼ 2P � Eu

6:24146 � 1018 � u2mI0 � ðbþ 1Þ

: ð11Þ

The latter is an expression, which relates the photo-ioniza-tion cross section, r, with experimental parameters such asthe total stimulation time, P, and the value of the stimula-tion intensity used, I0, (expressed in W/cm2), as well as withoperational characteristics such as the photon energy of thestimulation light, E/ (expressed in eV) along with fittingparameters. The resulting values are listed in Table 1.

4. Conclusions

The OSL signal of CaF2:N consists of four individualcomponents. Two of them are qualitatively and quantita-tively connected to two of its TL glow-peaks. The resultsof the present work suggest that there is a correlationbetween specific previously studied TL properties, such assensitivity changes and lowest detectable dose limit, andthe respective OSL properties. Strong photo- and thermaltransfer effects were observed while measuring OSL atelevated temperatures. Finally, the second component ofthe OSL signal measured, either at room temperatureor at some higher temperature, due to its very low dosedetectable limit, could be very useful for measuringextremely low environmental doses of known and unknownorigin (when, for example, CaF2:N is considered a timeintegrated luminescence dark matter detector).

142 G.S. Polymeris et al. / Nucl. Instr. and Meth. in Phys. Res. B 251 (2006) 133–142

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