investigation of order of kinetics for optically stimulated ... cross-section of trapped electrons...

International Journal of Engineering Technology, Management and Applied Sciences

www.ijetmas.com March 2017, Volume 5 Issue 3, ISSN 2349-4476

399 S. Dorendrajit Singh, Ritesh Hemam, L. Robindro Singh, Munish Kumar

Investigation of Order of Kinetics for Optically Stimulated

Luminescence (OSL) of Li2B4O7:Ag Nanoparticles

1S. Dorendrajit Singh,

2Ritesh Hemam,

2L. Robindro Singh, and

3Munish Kumar

1Department of Physics, Manipur University, Canchipur, Manipur

2Department of Nanotechnology, NEHU, Shillong, Meghalaya

3Radiological Physics and Advisory Division, Bhabha Atomic Research Centre,

Mumbai, Maharashtra, India

CW-OSL and LM-OSL curves of LTB:Ag nanoparticles were studied using RISO TL/OSL reader system. It was been

found that CW-OSL curve cannot be fitted with sum of three first order exponential decay curves. It was also found

thatthe shape factor μgfor experimental LM-OSL curve was ≈ 0.77, which represents non first order kinetics. The effect of

optical bleaching on LMOSL curves was studied and it was found that peak position “tm” is affected by optical

bleaching and shifts towards higher side of time. The studies show that the CW-OSL and LM-OSL glow curves does not

follow first order kinetics.

Keywords: LTB:Ag nanoparticles, CW-OSL, LM-OSL, optical bleaching, decay constants.

1. Introduction

In the estimation of various kinetic parameters involving in optically stimulated luminescence,

continuous wave (CW) and linearly modulated (LM) processes are commonly used methods. In CW-OSL,

the recorded luminescence appears like a decaying (exponential) curve whereas in LM-OSL, the recorded

luminescence appears peak shaped visually like that of the thermoluminescence (TL) glow curve. The CW-

OSL and LM-OSL represents the same physical information describing the same phenomenon under

different stimulation profiles [1-4]. In this paper CW-OSL and LM-OSL processes for Li2B4O7(LTB)

nanophosphor doped with Ag are studied. LTB is selected for the study because it is one of the most widely

studied excellent tissue equivalent dosimeter [5-10]. The effect of dopant concentration on CW-OSL decay

curves is studied and also the TL and OSL correlation of the traps of the phosphor is studied.

2.Theoretical basis of OSL

General order kinetics equation of Optically Stimulated Luminescence (OSL) process can be written

as:

𝐼 = −𝑑𝑛

𝑑𝑡= 𝑓

𝑛𝑏

𝑁𝑏−1 (1)

Where,b is the order of kinetics, N is total density of available traps (m-3

), n is the density of filled

traps (m-3

) and f is the optical excitation rate. The optical excitation rate 𝑓 = 𝜎𝑝𝐼/ℎ𝜈 where 𝜎𝑝 is the photo

ionisation cross-section of trapped electrons for stimulating radiation, I is the intensity of stimulating light and

ℎ𝜈 is the energy of the stimulating light.It can further be written as𝑓 = 𝜎𝑝(𝜆)𝜑(𝜆) where,𝜑 𝜆 = 𝐼/ℎ𝜈.

Thevariation of f is dependent on stimulating light flux; 𝜑(𝜆) as for a given wavelength λ, 𝜎𝑝 is constant.

When b = 1 in equation (1) will represent the first order kinetics and b = 2 will give the second order kinetics.

2.1. First order kinetics

A CWOSL curve follows first order kinetics if it is fitted by single decaying exponential and the

decay constant/pattern is always independent of radiation dose and is unaffected by optical bleaching. The

plot of ln(ICW) versus time is always a straight line where ICW is CW-OSL intensity. The peak position tm is

always independent of radiation dose as well as optical bleaching for LM-OSL curves. ForLM-OSL curves

following first order kinetics, the shape factor μg lies in the range 0.55–0.58 (4). Further for LM-OSL curves

obeying first order kinetics, the numerical values of ω/tm, δ/tm and τ/tm are around 1.602, 0.92 and 0.68,

respectively [4].




2.2. Non-first order kinetics

The decay pattern for CW-OSL curve is not a perfect decaying exponential. The decay constant is

dose dependent in this case. The decay constant for CW-OSL curves is also influenced by optical bleaching.

The peak position tm for LM-OSL curvesis dose dependent and shifts toward higher side in time with the

decrease of radiation dose. Also under optical bleaching, the peak position tm for LM-OSL curves shift

towards higher side in time. The value of the shape factor μg for second order kinetics lies in the range 0.65–

0.68 for LM-OSL curves whereas the shape/geometrical factor μg values in the range 0.59–0.65 correspond

to order of kinetics between one and two. Further the numerical values of ω/tm, δ/tm and τ/tm in the range

from 1.60–2.10, 0.92–1.40 and 0.68–0.70, respectively represent order of kinetics between one and two (4).

Non-first order CW-OSL curves may be fitted as sum of two, three or more first order exponential fits,

however that may not be the actual situation always. Higher values of μg (>0.68), may represent LM-OSL

curve resulting from superposition of more than one LM-OSL curves obeying first or non-first order kinetics

or their mixture. Further the prevalence of mixed order kinetics is not ruled out. Details regarding the

analysis of LM-OSL curves using peak shape methods based upon general and mixed order kinetics can be

had from the papers of Kitis et al. [11,12].

3. Materials and Methods

In the adjoining paper (13), we have discussed the synthesis method and characterizations of the

LTB:Ag nanoparticles(). The formation and average size of the nanoparticles were confirmed using XRD and

TEM analysis. The synthesized LTB:Ag was found to be of average size 25nm. The TL and OSL

measurements were carried out using RISO TL/OSL system, TL/OSL-DA-15, in which a cluster of 42 blue

light emitting diodes (λ = 470±30nm) were used for stimulation. The standard PMT in RISO TL/OSL reader

is a bialkali EMI9235QA PMT, which has minimum detection efficiency between 200 and 400 nm. Irradiation

of the sample was carried out using a 90

Sr/90

Y source house in the system.

4. Results and Discussion

4.1. Effect of dopant concentration

To study the effect of dopant concentration on the CWOSL glow curves the LTB:Ag nanoparticles doped with

different concentrations (0.025, 0.05, 0.1, 0.5, 1, 3 and 5 at.wt%) of Ag were irradiated with test dose of 200

mGy of β-radiation. The CW-OSL measurements were carried out using blue LED stimulation (60s,

λ ≈ 470 nm) and 72 mW/cm2 powers. The CW-OSL decay curves of LTB nanoparticles doped different

concentrations of Ag are shown in figure1. Inset of the figure1 shows the plot of CW-OSL counts against the

dopant concentration in atomic weight percent. As we can see from the figure, the LTB doped with Ag shows

concentration quenching phenomenon. The intensity of the CW-OSL increases from 0.025% Ag doped LTB

till 1% Ag doped LTB and decreases with higher concentration of Ag after 1%. Thus LTB:Ag(1%) is found to

be the most sensitive among the synthesized LTB nanoparticles doped with different concentrations of Ag. So,

LTB:Ag(1%) is selected and used for further studies to determine the kinetic parameters of the OSL

processes. The CW-OSL decay curves do not obey first order kinetics as it cannot be fitted by single

exponential decay curve. The CW-OSL decay curves can be fitted with sum of three first orders exponential

decay curves with three different photoionization cross-sections [13].

4.2. LM-OSL Glow Curves

The LTB:Ag(1%) nanoparticle is irradiated with 2 Gy of β-dose and measure LM-OSL using blue

LEDs stimulation with power variation between 0% and 100%.The recorded LMOSL glow curve is shown in

figure 2. The shape factor μg = δ/ω = (t2-tm)/(t2-t1) was calculated for the LM-OSL glow curve and was found

to be ≈ 0.77 which indicates that the LM-OSL glow curve does not follow first order kinetics. Since μg is

found to be greater than 0.68 it can be deduced that it may represent LM-OSL curve resulting from the

superposition of more than one LM-OSL curves having close photoionization cross sections. This observation

is in agreement with the CW-OSL decay curve fitted as the sum of three first orders exponential decay curves.

Though the LM-OSL glow curves is higher values ofshape factor, LM-OSL curve having multiple peaks

could not be observed experimentally.




4.3. Effect of optical Bleaching

To study effects of optical bleaching on the OSL glow curves, the LTB samples were irradiated with

dose of 2 Gy of β-radiation. The irradiated samples were bleached using blue LEDs operating at power of

72mW/cm2 for different bleaching time and compare with the OSL glow curves of unbleached samples. The

samples were bleach for 1, 5, 10, 25, 50 and 100 seconds respectively. Table 1 gives the different decay

constants (t1, t2, and t3) of the three individual first order CW-OSL curves for different bleaching time. The

decay constants of the CW-OSL curves varies with the bleaching time which is evident of the non-first order

kinetics of CW-OSL curves of LTB:Ag. And Figure 3 shows the LM-OSL glow curves of different bleaching

time. As we can see from the figure the tm of LM-OSL glow curves shifts towards the higher time scale. The

tm increases with the increases in bleaching time which suggest that it follows non first order kinetics.

Table 1: Decay constants of CW-OSL glows for different bleaching time

Bleach time(s) t1 t2 t3

0 1.65743 5.53582 30.05027

1 1.63702 4.82309 28.19868

5 1.93376 6.78974 32.42843

10 1.91777 7.67649 40.91012

25 1.58029 11.25869 75.49445

50 1.4191 3.32107 59.12729

100 1.75288 5.10598 111.80685

4.4. TL – OSL correlation

Figure 1: CW-OSL glow curves of LTB for different concentrations of Ag. Inset is the plot of CW-

OSL counts vs dopant concentration.

Figure 2: LM-OSL glow curve of LTB:Ag(1%).




Figure 3: LM-OSL glow curves of LTB:Ag(1%) for different bleaching time.

The LTB:Ag(1%) sample was irradiated with 2 Gy dose of β-radiation. The TL glow curves of this irradiated

sample were recorded at the heating rate 1oC/s at room temperature. The recorded TL glow curve has three

distinct peaks at around 125, 200 and 320 oC respectively. Again the sample was irradiated with same dose as

above in same condition and CW-OSL with 72mW/cm2 power blue LEDs stimulation for 60 second

stimulation time was recorded. After the CW-OSL measurement TL glow curve of the sample was recorded

Figure 4: TL glow curves recorded before and after CW-OSL of 60s stimulation time.

again. The two TL glow curves were compared to understand the participation of the TL traps in the OSL

process. The results shows that all the three TL traps of the LTB:Ag nanoparticles participated in the OSL

process. It was observed that 92.3% of the first TL peaks is participated in the OSL and 75.44% of the second

TL peaks participated in the OSL process whereas only 25.3% of the third peak is contributed towards the

OSL of the LTB:Ag(1%) nanoparticles. This also agrees with the results that the CW-OSL decays curves can

be fitted with the sum of three first orders exponential decay curves. This confirms that the OSL glow curves

of LTB:Ag(1%) follows non first order kinetics.

4.5. Effect of Dose

To study the effects of dose on the CW-OSL glow curves of the LTB:Ag(1%) nanoparticles, samples

were irradiated with different doses of β-irradiation varying from 0.1Gy to 40 Gy. The CW-OSL glow curves

of the LTB nanoparticles were recorded for 60 seconds stimulation with blue LEDs operating at same powers

as mention earlier. All the CW-OSL decay curves were fitted with the sum of three first orders exponential

decay curves. The decay constants(t1,t2 and t3) of the fitted CW-OSL glow curves are given in table 2. As

from the table we can see that the decay constants were affected by the variation of doses though there is not

much change in the total characteristics of the CW-OSL curves.




5. Conclusion

The OSL of LTB:Ag follows non first order kinetics. Concentration quenching of dopants is also

observed in LTB:Ag nanoparticles and LTB:Ag was found to be the most sensitive. The CW-OSL curves of

LTB:Ag is not a perfect exponential decay and can be fitted as sum of three first order exponential decay

curves using conventional fitting methods. The peak shape factor of LM-OSL glow curves was found to be ≈

0.77. The LM-OSL glow curves of LTB:Ag is the superposition more than one LM-OSL curves. The CW-

OSL curves were affected by optical bleaching as well as doses. The peak (tm) of LM-OSL glow curves shifts

towards higher temperature with the increase in bleaching time. The TL glow curves have three peaks which

all of the three TL traps contributed in the OSL process.

Table 2: Decay constants of fitted CW-OSL curves for different doses.

Dose(Gy) t1 t2 t3

0.1 0.92882 4.52016 19.67808

0.2 0.95363 3.69025 35.79286

0.5 0.64747 2.65799 28.44455

1 0.98066 3.03193 24.08264

2 1.06677 2.94639 23.51157

5 1.29998 3.47826 25.60473

10 1.04573 2.96755 23.97211

20 1.08212 3.05588 24.2978

40 1.2979 3.43367 24.66747

Acknowledgement

The authors would like to thank BRNS Mumbai for the financial assistance under BRNS project no.

2012/36/59-BRNS/1301.

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investigation of order of kinetics for optically stimulated ... cross-section of trapped electrons...

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