A new high sensitivity Na₂LiPO₄:Eu OSL phosphor

Abstract

A new high sensitivity Na₂LiPO₄:Eu optically stimulated luminescence dosimeter (OSLD) material was prepared by a simple solid-state diffusion method. The formation of the material was confirmed by comparing the experimental data with that available in the literature (JCPDF # 80-2110). The dosimetric properties of the phosphor material using a continuous wave-optically stimulated luminescence (CW-OSL) technique were studied. The material was studied for different concentrations of the impurity and also for different heating treatments. The material doped with 1.0 mol% and annealed at 873 K was found to be the most sensitive. The phosphor was found to have all the good dosimetric characteristics, such as tissue equivalence (low-Z, Z_eff ∼ 10.8), high sensitivity (∼3 times less than the commercially available Al₂O₃:C, Landauer Inc., USA and BeO, Thermalox® 995, Materion Inc., USA), low fading (∼6.2% in 40 days), wide range of dose response (0.1–1.0 kGy), excellent reusability, easy optical bleaching (annealing) for its reuse, etc., which makes the material useful for dosimetry of high-energy radiations using OSL. The advantages of our OSLD phosphor are the easily available and inexpensive ingredients and a very simple method of preparation that makes it cost effective as compared to the commercially available OSLD phosphors. Also, it is easy to handle, unlike the Thermalox® 995 (BeO) dosimeters which are very toxic, and require a special method of preparation and handling.

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Introduction

Optically stimulated luminescence (OSL) has established itself as an improved and more reliable technique for dosimetry of high-energy radiations. The technique is becoming more and more popular because of several advantages over thermoluminescence (TL) dosimetry, such as the completely optical nature of the instrumentation, possibility of online (in situ) measurements using optical fiber, low power consuming LED sources for stimulation, facility of reestimation of radiation doses in case of any doubts and, over and above all, no structural changes¹ due to heating that lose its reusability, unlike in some TLD phosphor materials.^2–5 However, there is not much choice due to the paucity of commercially available OSL phosphors and the high fading in others. The OSL phosphors available are also costly due to their difficult methods of synthesis. For example, for producing Al₂O₃:C,^6,7 one needs to have a very high temperature vacuum furnace as the doping of carbon is not possible in an atmospheric environment; for producing another OSL phosphor, BeO,⁸ special arrangements are needed for its production and handling as it is very toxic to human beings.

The general requirement of a good OSL phosphor is that the emission should be between 350 and 425 nm (where most common detectors are most sensitive) and the defects (traps) should have a high photo-ionization cross-section in the blue-green region (450–550 nm) or IR region (650–800 nm). The sensitivity of an OSLD phosphor depends on the kind of defects, stimulation sources and availability of suitable filters and on the detector (generally a wide-band PMT). The tissue equivalent (low-Z) phosphors are much preferred due to their flat energy response and their suitability to be used in a mixed radiation field. Attempts are, therefore, going on to study new (preferably low-Z) materials for their applications as OSL phosphors, such as, LiMgPO₄:Tb,B,⁹ Na₂SiF₆:Cu,P,¹⁰ Al₂O₃:B,¹¹ Y₃Al₅O₁₂:C,¹² Cu-doped quartz (SiO₂:Cu),¹³ LiAlO₂,¹⁴ MgO:Tb,¹⁵ NaMgF₃:Eu,¹⁶ etc. However, all these phosphors are at the developmental level and the issues related to their sensitivity and fading need to be addressed before they are finally accepted as OSLD phosphors. More details and a comparative study can be seen in some recent review papers.^17–19

In the present paper we report optically stimulated luminescence (OSL) and thermoluminescence (TL) studies of a new high sensitivity low-Z Na₂LiPO₄:Eu phosphor for its application for OSL dosimetry. Na₂LiPO₄:Eu is really a multifunctional advanced ceramic as it has already been proved to be a red phosphor for solid-state lighting and display applications²⁰ and a high-sensitivity TLD phosphor for dosimetry applications.²¹ It was intuitive to study this material for its application as an OSL phosphor and it proved to be satisfying. The phosphor Na₂LiPO₄:Eu is found to have several ‘good’ characteristics, such as low-Z material, very easy method of preparation, nontoxic in nature, highly sensitive, linear OSL dose response over a wide range, low fading, excellent reusability, emission well separated (peaking at around 420 nm) from that of the excitation source (470 nm blue LEDs). Phenomenological studies have also been done to understand the process of OSL and a model for trapping and emptying and recombining the traps with luminescence (hole) centers has been proposed.

Result and discussions

XRD analysis

XRD patterns of the NaLi₂PO₄:Eu³⁺ phosphor are as shown in Fig. 1. The peak positions in the diffraction pattern of the synthesized material were indexed by comparing them with the standard data available in the literature (JCPDF # 80-2110)²¹ and found to be in agreement, confirming the formation of the material. All the XRD peaks are indexed (hkl) for different lattice planes. The material is found to be in the orthorhombic crystal system, with the space group Pmnb (62).²² The stick pattern of the standard data (JCPDF # 80-2110) along with the experimental one has also been given for a ready reference. No separate peaks corresponding to any impurity phase were observed at low concentrations (0.05–0.1 mol%), showing that the impurity has a good solubility in the matrix in this range. However, when the impurity concentration was increased beyond this range (<0.2 mol%) new peaks corresponding to impurity clusters of monoclinic phase of Eu₂O₃ (JCPDF # 43-1009) were observed. This may be attributed to precipitation of the impurity ions and forming clusters. Consequently, it has also been observed that the cell parameters and the cell volumes of the samples gradually decreased with the impurity concentration due to a decrease in the crystallinity of the materials with the addition of the Eu₂O₃ phase.²³ This is very important from the application point of view as the precipitation of impurity clusters and/or the stress/strain developed due to changes in the cell volume could change the luminescence characteristics of the phosphor material.²⁴ The details of the study can be found in our earlier paper.²⁵

Fig. 1 XRD pattern of NaLi₂PO₄:Eu based on our experimental data. The stick pattern of the XRD pattern plotted using the data available in the literature (JCPDF file # 80-2110)²¹ is also given for comparison.

OSL measurements

Effect of excitation source. Two different excitation sources (∼470 nm, 80 mW cm⁻², blue LED cluster and ∼530 nm, 40 mW cm⁻², green LED cluster) were used for optical stimulation to see which one is more suitable. The results are as shown in Fig. 2. It can be seen that optical stimulation by the blue LED yielded more (∼3 times) OSL signal (intensity) as compared to that of the green LED for the samples irradiated at a nominal dose of 0.1 Gy. This shows that stimulation by the blue light is more suitable than by the green light due to its photo-ionization cross-section (as mentioned in Table 1). However, it is to be mentioned here that the power of the green LED cluster at the sample is just half that of the blue LED cluster power at the sample. Therefore, it is difficult to conclude that the difference in OSL intensities on stimulation by green or blue LEDs is really due to difference in energies (wavelengths) of the light from these sources or due to their powers (intensities) as their intensities at the sample position are not the same. However, further dosimetric studies were done using the blue LEDs only.

Fig. 2 Effect of the stimulating source on the CW-OSL decay curves of NaLi₂PO₄:Eu, (a) decay curve stimulated by blue LED light (470 nm) and (b) decay curve stimulated by green LED light (530 nm). The material was irradiated by 10.0 Gy of γ rays from a ¹³⁷Cs source. The samples were annealed at 873 K for 1 h before irradiation.

Table 1 Data of photo-ionization cross-sections and decay constants for CW-OSL decay curves stimulations by green and blue LEDs and their deconvoluted components

OSL component	OSL response stimulated by green LED (570 nm)			OSL response stimulated by blue LED (470 nm)
	Coefficients	Decay constant (s)	Photo-ionization cross-section σ (cm^2)	Coefficients	Decay constant (s)	Photo-ionization cross-section σ (cm^2)
Fast	75 656.8 (A1)	1.13 (τ1)	0.82 × 10^−17	191 734.5 (A1)	3.45 (τ1)	0.15 × 10^−17
Medium	107 279.3 (A2)	0.10 (τ2)	0.91 × 10^−16	—	—	—
Slow	34 103.1 (A3)	11.69 (τ3)	0.8 × 10^−18	440 293.7 (A2)	0.50 (τ2)	0.11 × 10^−16

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Effect of impurity concentration on the CW-OSL. Fig. 3 shows the effect of impurity concentration on the CW-OSL decay curves. From the figure it can be seen that the CW-OSL intensity is maximum for the impurity concentration (Eu ions) of 0.1 mol%. Doping the material with more Eu concentration shows quenching. It may be mentioned here that when studied as a TLD phosphor, the same material showed maximum sensitivity at 0.5 mol% of the impurity concentration.²¹

Fig. 3 Effect of Eu-impurity concentration on the CW-OSL decay curves: (a) pure sample, (b) 0.1 mol%, (c) 0.5 mol%, (d) 1.0 mol% and (e) 2.0 mol%. The variation of the CW-OSL intensity with impurity concentration is also shown in the inset. The samples were annealed at 873 K for 1 h and irradiated by 1.0 Gy of γ rays from a ¹³⁷Cs source.

Effect of annealing temperature. The effect of annealing temperature was also studied. The results on CW-OSL measurements are as shown in Fig. 4. It can be observed from the figure that the material shows a maximum CW-OSL sensitivity for the samples annealed at 873 K. The diffusion of environmental oxygen at higher temperatures^26–29 might be responsible for the quenching on annealing at higher temperatures.

Fig. 4 Effect of annealing temperatures on the CW-OSL decay curves: (a) as-prepared sample, (b) annealed at 473 K, (c) annealed at 673 K, (d) annealed at 873 K and (e) annealed at 1073 K. The variation of the CW-OSL intensity with annealing temperatures is also shown in the inset. The samples used for these studies are doped with 0.1 mol% Eu concentration (which shows the maximum CW-OSL sensitivity) and were irradiated by 1.0 Gy of γ rays from ¹³⁷Cs source.

Dose response

Na₂LiPO₄:Eu samples annealed at 873 K for 1 h were irradiated at low doses (in the range of 0.003–0.14 Gy) of β rays from the ⁹⁰Sr–⁹⁰Yr source and the CW-OSL decay curves were recorded. The results are as shown in Fig. 5(a) and (b). For high doses, however, the samples were irradiated with different doses of γ rays from a ¹³⁷Cs source. No difference in the shape of the decay curve was observed except for the increase in intensity with the dose. The response curves for stimulation by both the blue and green LED lights, i.e., 470 and 530 nm, respectively, are as shown in Fig. 6. From the figure it can be seen that both response curves are very linear in the dose range (i.e., 0.05–10 Gy). However, beyond this range (10 Gy–100 Gy) they show supralinearity before saturation. No further studies, therefore, were done thereafter. These results show that there is little effect between the different types of radiations (i.e. electrons and photons) and their energies due to the low effective atomic number (Z_eff ≈ 10.8). The dose response is found to be within the same range as in the case of the commercially available Al₂O₃:C (Landauer, USA) OSL phosphor. Several batches of the material for the optimized impurity concentration to see the batch to batch variation and were given proper annealing treatments. No appreciable variation was observed in their decay curves and dose response.

Fig. 5 (a): CW-OSL decay curves for Na₂LiPO₄:Eu (0.1 mol%) samples irradiated in situ with β rays from a ⁹⁰Sr–⁹⁰Yr source for different doses: (a) 0.003 Gy, (b) 0.03 Gy, (c) 0.05 Gy, (d) 0.08 Gy, (e) 0.1 Gy and (f) 0.14 Gy. (b): CW-OSL decay curves for Na₂LiPO₄:Eu (0.1 mol%) samples irradiated with γ rays from a ¹³⁷Cs source for different doses: (a) 1.0 Gy, (b) 5.0 Gy, (c) 10.0 Gy, (d) 50 Gy, (e) 100.0 Gy and (f) 500.0 Gy. The samples were annealed at 873 K for 1 h before irradiation.

Fig. 6 CW-OSL dose response curves of Na₂LiPO₄:Eu phosphor material: (a) stimulated by green LED light (530 nm) and (b) stimulated by blue LED light (470 nm). Theoretical linear fittings of the curves (solid lines) are also shown in the figure. The dotted curves are guides for eyes.

Correlation of CW-OSL with TL peaks and supralinearity of the dose response

The CW-OSL dose response of Na₂LiPO₄:Eu phosphor is shown in Fig. 6. It can be seen in the figure that the dose response is supralinear in the dose range 10.0–100.0 Gy. To understand the phenomenon in more detail, some more experiments were performed. It was found that in CW-OSL the intensity is reduced by almost three orders of magnitude after the first readout (Fig. 7). However, it can be seen (inset of Fig. 7) that the CW-OSL intensity does not completely diminish even after repeated readouts or by optical annealing. This indicates some optically sensitive traps (though negligibly small in numbers) still remain inside the material and get depleted on subsequent readouts. The material was optically annealed for half an hour but still some (though very weak) OSL signal was observed. In order to investigate which of the TL peaks are contributing to the CW-OSL in our material, the irradiated sample was subjected to thermal bleaching at different temperatures. For this, the NaLi₂PO₄:Eu sample was irradiated with beta radiation of 0.1 Gy each time and heated with a heating rate of 5 K s⁻¹ on metal strip (in the Riso TL/OSL Reader) up to 373, 423, 463, 505, 560, 603, 663 and 698 K, held for 30 s, cooled to room temperature rapidly by switching off the heater and then the CW-OSL was recorded.³⁰ These results were plotted and are shown in Fig. 8. The results show that all the peaks are OSL sensitive. To confirm these results further, the phosphor material was irradiated by β rays, different TL peaks were cleaned by a thermal cleaning method and the intensity of the CW-OSL decay curves was plotted with the temperature up to which the TL glow curves were recorded before taking the CW-OSL. The results are as shown in Fig. 9. It can be seen that OSL intensity was observed even after cleaning the first two peaks and even the third peak partially. It seems that some of the deeper traps were transferred to the vacancies created by thermal cleaning on subsequent illumination by the blue light during CW-OSL readout. This also indicates that all the TL peaks are contributing in OSL one way or another. Finally, TL glow curves were recorded after each subsequent CW-OSL readouts a number of times (Fig. 10). It was surprising that there was no decrease in the TL intensity as compared to the subsequent OSL readouts. This might be occurring due to a lower probability of transitions of the deeper traps to the conduction band and the subsequent recombination. From the above discussion it seems that even after optical annealing or thermal cleaning a small fraction of the deep traps still remain inside the material, which could be responsible for the supralinearity of the dose response. The amount of these reminiscent traps may depend upon the total number of traps created during irradiation (dose). This amount would be negligibly small for low doses and will not be that prominent, but at higher doses the effect is prominent until saturation occurs.

Fig. 7 Plot of the CW-OSL intensity with number of readout cycles. It can be seen in the figure that more than 99.8% intensity is lost during the first readout itself. An enlarged view of the intensity after the successive readouts is also shown in the inset.

Fig. 8 Effect of thermal cleaning of the TL glow curve on the CW-OSL curves of Na₂LiPO₄:Eu phosphor. The phosphor was stimulated by blue LED light (470 nm). The samples were annealed at 873 K for 1 h and irradiated by 1.0 Gy of γ rays from a ¹³⁷Cs source. Glow curves taken after the thermal cleaning are also shown in the inset.

Fig. 9 The plot of CW-OSL intensity of Na₂LiPO₄:Eu phosphor with the temperatures up to which the TL glow curves were taken before the CW-OSL. A typical glow curve before taking the CW-OSL is also shown in the figure. The samples were annealed at 873 K for 1 h and irradiated by 1.0 Gy of γ rays from ¹³⁷Cs source.

Fig. 10 TL glow curves recorded after successive CW-OSL decay curve readouts. The samples were annealed at 873 K for 1 h and irradiated by 1.0 Gy of γ rays from ¹³⁷Cs source.

Comparison of the OSL sensitivity of Na₂LiPO₄:Eu with those of Al₂O₃:C and BeO dosimeters

The CW-OSL intensity of the newly developed homemade Na₂LiPO₄:Eu OSLD phosphor was compared with that of commercially available Al₂O₃:C OSL dosimeters (Landauer, USA) and that of the BeO hot pressed chips (Thermalox® 995, Materion, USA). The results are as shown in Fig. 11. It can be seen from the figure that the intensity of our phosphor was found to be approximately 3 times less than that of the commercially available phosphors in the linear dose response range (i.e. 0.05–10.0 Gy). However, the advantage of our OSLD phosphor is the very easily available and inexpensive ingredients and a very simple method of preparation that make it cost effective as compared to the commercially available ones. Also, there is ease of handling, unlike with the Thermalox® 995 (BeO) dosimeters which are very much toxic.

Fig. 11 Comparison of the OSL sensitivity of Na₂LiPO₄:Eu phosphor material with that of the commercially available Al₂O₃:C (Luxel™, Landauer Inc., USA) and BeO (Thermalox® 995, Materion Inc., USA) phosphors.

Fading

To study fading, the samples were irradiated by 1.0 Gy of γ rays from a ¹³⁷Cs source. The samples were stored at room temperature (∼300 K) in the dark and CW-OSL was taken at different intervals of time. The results are as shown in Fig. 11 (inset). A few CW-OSL decay curves recorded initially and after 3 h and 20 h are also given. Other decay curves are not given for clarity of the figure. It can be seen from Fig. 12 that ∼6.2% fading of the OSL signal was observed in 40 days. This is very much comparable with that of the commercially available phosphors, i.e., Al₂O₃:C (Luxel™, Landauer Inc., USA) and BeO (Thermalox® 995, Materion Inc., USA). This has also been accepted for medical and personnel dosimetry.

Fig. 12 Typical CW-OSL decay curves just after irradiation and after storing in the dark at room temperature: (a) at 0 h, (b) after 3 h and (c) after 20 h. The decay curves recorded after 20 h are not shown in the figure for better clarity. The fading curve is also shown in the inset. Around 6.2% fading was observed in 40 days.

Optical annealing (bleaching) and reusability

Several batches of the newly developed NaLi₂PO₄:Eu OSLD phosphor were prepared and used for OSL studies. The phosphors were also irradiated to various test doses; OSL was taken and optically annealed using an annealing system consisting of 8 high output T5 fluorescent lamps (Annealing System, Model no.: GS-00004, Gammasonics, Australia). The system can remove a dose of a few mGy in a few minutes.³¹ The samples were optically annealed for different times after the OSL readouts until the entire OSL signal was erased. After the first CW-OSL, the remaining signal is of the order of only a few mGy. The moderate time for annealing/bleaching was found to be around half an hour. The OSL was again taken to see that no OSL signal was left inside the material. Several samples were irradiated at different test doses and the procedure was repeated several times. A few results are shown in Fig. 13. The material shows excellent reusability.

Fig. 13 Normalized intensity curve for repeated reusability of the Na₂LiPO₄:Eu phosphor material. The material was irradiated by 1.0 Gy of γ rays from a ¹³⁷Cs source and the CW-OSL was taken. The material was optically annealed for half an hour; and the OSL was taken before irradiation to see that no OSL signal was left after the annealing. The material was irradiated again for the same dose as for taking the CW-OSL. The process is repeated several times. The dotted line shows the least squares fitting.

Theoretical fitting of the CW-OSL decay curve and photo-ionization cross-sections

Fig. 14 and 15 show theoretical curve fittings of typical CW-OSL decay curves for the OSL stimulated by the blue (470 nm) and the green (530 nm) light. The figure of merit (FOM) for both the curve fittings was approx. 2%. It can be observed from the figure that the blue light stimulated decay curve consists of two components, a fast component and a slow component, while that stimulated by the green light consists of three components, fast, medium and slow. The sum of these components is also shown as theoretically fitted curves (curves with symbols). The curve stimulated by blue light can, be represented by the following equation (eqn (1)).where A₁ = 391 734.5, τ₁ = 3.45, A₂ = 4 440 293.7, τ₂ = 0.50 and the other one (stimulated by green light) by another equation as given below (eqn (2)),where A₁ = 75 656.8, τ₁ = 1.14, A₂ = 107 279.3, τ₂ = 0.10, A₃ = 34 103.1, τ₃ = 11.7.

Fig. 14 Theoretical fitting of a typical CW-OSL decay curve stimulated by blue LED light (470 nm). The theoretically fitted curve is shown in symbols and the experimental as a solid curve. The components are: (a) fast component (dash dot curve) and (b) slow component (dot dot curve).

Fig. 15 Theoretical fitting of a typical CW-OSL decay curve stimulated by green LED light (530 nm). The theoretically fitted curve is shown in symbols and the experimental as a solid curve. The components are: (a) fast component (dash dot dot curve), (b) medium component (dash dot) and (c) slow component (short dash dot curve).

From the above discussion, it is clear that the decay curve stimulated by green light is more complicated than that stimulated by blue light. Therefore, stimulation by blue light is preferred. It may also be inferred that different kinds of traps might have been involved in the two different stimulation processes.

The photo-ionization cross-section was determined by using the following formula for photoionization cross-section, σ = 1/(ϕ × τ), where τ is the decay constant (in seconds) and ϕ is the incident photon flux (photons incident per cm² per second). The corresponding values of decay constants and photo-ionization cross-sections are as given in Table 1. It can be seen from this table that the photoionization cross-sections depend not only on the stimulating wavelength (energy) but also on the total incident photon flux at the sample. In the present case, large differences in the photo-ionization cross-sections may also be attributed to different incident photon fluxes due to the different powers of the stimulating sources.

Experimental

Material and the method

Eu³⁺-doped orthophosphate NaLi₂PO₄ phosphor was synthesized by a simple solid-state diffusion method.²⁰ This is described here briefly. The starting materials LiOH, NaH₂PO₄ (CDH, 99.5%) and the impurity salt EuCl₃ (Alfa Aesar, 99.9%) were used to synthesize the material. All the chemicals were used as received without any further purification as they were of high purity. The samples were prepared by considering the following chemical reaction:

The impurity (in appropriate amounts, i.e., 0.05–3.0 mol%) was firstly dissolved in LiOH water solution and the solvent was evaporated slowly in a vacuum oven. The Eu-doped LiOH (thus obtained) and NaH₂(PO)₄ (in 2 : 1 molar ratio) were mixed thoroughly, using an agate mortar and pestle, in the presence of ethanol for better mixing. A temperature-controlled programmable furnace having a temperature stability better than ±1 K was used for synthesis of the material. The mixture was heated initially at 673 K for 12 h in air and then cooled slowly to room temperature, crushed again to a fine powder and reheated at 1073 K for the same period. The ingots thus obtained were finally crushed and sieved to get particle sizes in the range of 100–200 μm, given proper heat treatment and were used for further characterizations.

Characterization

The powder X-ray diffraction (XRD) patterns were recorded on a high-resolution D8 Discover Bruker X-ray diffractometer using a monochromatic Cu-K_α1 line obtained through a pair of Göbel mirrors. The scan rate was kept at 1.0 s per step with a step size of 0.02 s. The XRD recording was done at room temperature. The CW-OSL decay curves were recorded on a Risø TL/OSL Reader Model TL/OSL-DA-20TL (Risø, DTU, Radiation Research Division, Denmark) available at AIIMS, New Delhi, India. The decay curves were recorded for 300 s using blue (470 nm, FWHM ≈ 20 nm) and green (530 nm, FWHM ≈ 20 nm) LED lights (variable from 50 to 100 mW cm⁻²) at the sample position for stimulation and a wide band photomultiplier tube (EMI 9235QB15 PMT, ET Enterprises Ltd, UK) for the detection of light. A UV transmitting broad-band glass filter (Hoya U-340, 7.5 mm total thickness, transmission between 270 nm and 380 nm, Hoya Corporation, Japan) was used (across the PMT) to prevent a stimulation signal from reaching it and a long pass green filter GG-420 (3.0 mm thickness, Schott, Germany) was used (across the LED) to cut off the stimulation wavelengths below 420 nm. For low doses the samples were exposed in situ to the ⁹⁰Sr–⁹⁰Yr β rays source (dose rate 0.16 Gy min⁻¹) attached to the main reader unit (∼3.0 mGy–140 mGy). However, for higher doses a ¹³⁷Cs γ rays source (dose rate 0.6 Gy min⁻¹) was used (1.0 Gy–1 kGy). Every time ∼10 mg preirradiated sample was used for the CW-OSL decay curve recordings. For comparative and other studies, some TL measurements were also carried out using the same TL/OSL set up. Recordings of the TL glow curves and CW-OSL decay curves after partial thermal cleaning were also done to study whether all the traps take part in the OSL. All the samples were protected from sun/room light during irradiation as well as during storage and measurements.

Conclusions

The newly developed NaLi₂PO₄:Eu phosphor material shows good OSL characteristics useful for the dosimetry of high-energy radiations. The newly developed phosphor material was compared with the commercially available Al₂O₃:C (Luxel™, Landauer, USA) and BeO (Thermalox® 995, Materion, USA) phosphors. Though the phosphor is a little less sensitive (∼3 times less), the very easy method of synthesis makes it very cost effective. The fading in the material was found to be around 6.2% in 40 days and is comparable with that of the commercially available phosphors and thus is acceptable for dosimetry. The dose response was found to be linear in the dose range of 0.05–0.0 Gy but shows supralinearity at higher doses (from 10.0 to 100 Gy) before it gets saturated. The supralinearity and the contribution of different kinds of traps were also studied in detail and explained.

Acknowledgements

We are thankful to the Inter-University Accelerator Center, New Delhi, for partial financial assistance under the research project (UFR # 45318) and also University of Delhi for the research grant under the R & D scheme (RC/2014/6820). We are also thankful to the Director, BRAIRCH, AIIMS, New Delhi for permitting us to use the facilities at the hospital. We are also thankful to Mrs Debbie Handley from Materion, USA, for generously donating a few BeO (Thermalox® 995) dosimeter chips.

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