Thermoluminescent response of nanocrystalline Ba 0.97 Ca 0.03 SO 4 : Eu for proton beam

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1 Indian Journal of Pure & Applied Physics Vol. 48, July 2010, pp Thermoluminescent response of nanocrystalline Ba 0.97 Ca 0.03 SO 4 : Eu for proton beam Shaila Bahl a, S P Lochab b *, V E Aleynikov c, A G Molokanov c, A A Rupasov d, A Pandey e & Pratik Kumar a a Medical Physics Unit, All India Institute of Medical Science, New Delhi b Inter-University Accelerator Centre, Aruna Asaf Ali Marg, New Delhi c Joint Institute for Nuclear Research, Dubna , Russia d P N Lebedev Physical Institute, Russian Academy of Sciences, Leninsky Prosp. 53, Moscow , Russia e Department of Physics, Sri Venkateswara College, University of Delhi, New Delhi * lochab@iuac.ernet.in Received 22 April 2010; accepted 12 May 2010 Nanocrystalline Ba 0.97 Ca 0.03 SO 4 doped with Eu, was prepared by the chemical co-precipitation method. The formation of the compounds was confirmed by XRD. The particle size was calculated by the broadening of the XRD peaks using Scherrer s formula. The particle size was found to be around 45 nm. Samples in the form of pellets were irradiated by 150 MeV proton beam. Thermoluminescence (TL) glow curves of the irradiated samples were recorded and studied. It has been found that there is a prominent TL glow peak at 498 K with a small shoulder at around 465 K. The TL response is linear in the range Gy and then saturates for higher doses. The wider linear TL response of nanocrystalline Ba 0.97 Ca 0.03 SO 4 : Eu and low fading makes it a good candidate as a dosimeter to be used for detecting the doses of protons which are commonly being used in proton therapy for the treatment of cancer. Keywords: Thermoluminescence, XRD, Proton beam, Nanocrystalline 1 Introduction Thermoluminescence (TL) is the thermally stimulated emission of light from a semiconductor or insulator, following the previous absorption of energy from ionizing radiation such as X-rays, γ-rays, β-rays, α-particles, energetic ions, etc. It is an extensive technique used for dosimetry of ionizing radiations as the intensity of light emitted by the phosphor on stimulation (heating) reflects the irradiation flux (dose). Thermoluminescent materials exhibit differences in dose response between sparsely ionizing radiation (high energy photons like X-rays and γ-rays) and densely ionizing radiation like heavy charged particles (HCP). This is due to different spatial dose distribution 1. There is an increasing demand for dosimetry of the beams of charged particles because they are increasingly used for diagnostic and therapeutic purposes 2,3. Beams of negative pions, protons and the nuclei of helium and heavier ions such as carbon and argon, known as heavy charged particles, are of interest in radiation biology and radiation therapy because of their various physical properties 4,5. The treatment with such highly energetic ion beams has several advantages in comparison with the conventional irradiation with photons. The physical advantage is a significantly more favourable dose deposition profile. With conventional irradiation, the dose deposition decreases in proportion to the penetration depth, whereas with ion beams it gradually increases and then declines rapidly beyond a sharply defined maximum known as the Bragg Peak near the end of range of ion beam. Therefore, the penetration depths of those ions can be projected to have a maximum absorption at the tumors positions. Moreover, the position of the Bragg peak is also energy dependent. Proton, helium and heavier ion beams have been used for biomedical purposes since the 1950s; pions have been used since the 1970s. Clinical trials using these beams are now in progress in several countries. With these modalities, theoretical considerations suggest the possibility of increasing the local tumor control without increasing normal tissue complications. However, the dose of these energetic ions needs to be measured with great precision and accuracy, especially, while dealing with human beings. This has triggered investigations to use

2 BAHL et al.: THERMOLUMINESCENT RESPONSE OF NANOCRYSTALLINE Ba 0.97 Ca 0.03 SO 4 : Eu 501 thermoluminescent dosimeters (TLD) for dose verification. The mixed sulphate, Ba 0.97 Ca 0.03 SO4 doped with rare earth impurity, Eu has been reported 6 to have a higher sensitivity even than the commercially available standard TLD phosphor, CaSO4:Dy (~2.3 times of CaSO4:Dy) and its TL sensitivity is very stable over many successive irradiation-readout cycles. Moreover, its nanocrystalline have also revealed very imperative characteristics such as high sensitivity, saturation at very high doses and low fading 7,8. This has encouraged us to study further the TL characteristics of nanocrystalline Ba 0.97 Ca 0.03 SO4: Eu with proton beam (a beam which is commonly used in radiotherapy). We have irradiated Ba 0.97 Ca 0.03 SO 4 :Eu sample by 150 MeV proton beam with a dose range of Gy to find its application to be used as a dosimeter in proton therapy. 2 Experimental Details Nanocrystalline sample of Ba 0.97 Ca 0.03 SO4:Eu was prepared by the same method adopted earlier by Lochab et al 7. In this method, analytical reagent (AR) grade barium chloride and calcium chloride of analar grade were taken in a stoichiometric ratio (0.97 Ba and 0.03 Ca) and dissolved in triply distilled water. Europium chloride of AR grade (0.2 mol %) was then added to the solution. To control the size of particles to be produced on precipitation, ethanol was added to the solution. Further, ammonium sulphate was added drop wise to the solution until the precipitation was complete. The mixture was centrifuged each time so that the precipitate settled at the bottom of the tube and a clear liquid was seen at the top. The liquid on the top was then decanted and the precipitate was washed repeatedly in distilled water and dried at 90 C for 4 h. The nanocrystalline powder thus obtained was further annealed at 700 C for 1 hr under an argon atmosphere in a quartz boat and quenched by taking the boat out of the furnace and placing it on a metal block for better sensitivity. Pellets of the prepared nano powder with 0.6 mm thickness and 1 cm diameter were prepared taking 100 mg of the sample and 1 mg of teflon powder, mixing together, putting in a die and applying 0.1 ton/cm 2 pressures each time by a hydraulic press. The pellets were again annealed at 973 K for 1 hr in argon atmosphere and quenched rapidly to anneal out the deformations, if any, due to applied stress. Formation of the compound was confirmed by taking the X-ray diffraction (XRD) pattern that matched with the standard data available in literature. The broadening of the XRD lines was used to determine the particle size and the nanocrystalline nature of the material. The samples in the form of pellets were irradiated at room temperature by proton clinical beams with energy of 150 MeV for different doses ranging from 0.1 to 325 Gy, using a phasotron at the Laboratory of Nuclear Problems of the Joint Institute for Nuclear Research (LNP JINR), Dubna, Russia. (The full details of this set-up are given by Molokanov et al 9. The detectors were positioned orthogonal to the beam direction. Three pellets were exposed to the same dose every time. For taking TL the irradiated surface of the pellet was kept facing upwards towards the detector (PMT) of the TLD reader. TL glow curves were recorded using a Harshaw TLD reader (Model 3500). The heating rate was 5Ks 1. The sample was kept on a small heating plate which also acts as a sample holder. 3 Results and Discussion 3.1. Particle size When crystallites are less than approximately 100 nm in size appreciable broadening in the X-ray diffraction lines occurs. The observed line broadening can be used to estimate the average grain size (Fig. 1). The full width at half maximum (FWHM) of the diffraction peaks can be used to calculate the average crystallite size using the Debye Scherrer formula: 0.9 D = λ β cos θ Fig. 1 X-Ray diffraction pattern of Ba 0.97 Ca 0.03 (SO 4 ) 3 :Eu prepared by Co-precipitation method

3 502 INDIAN J PURE & APPL PHYS, VOL 48, JULY 2010 where D is the average grain size of the crystallites; λ the incident wavelength; θ the Bragg angle; β is the diffracted FWHM (in radians) caused by the crystallites. The XRD pattern is well fitted with the orthorhombic structure having: lattice parameters: a= , b= , c= lattice volume =103.75Ǻ and χ 2 = The grain size was calculated to be 45 nm. 3.2 TL glow curves Figure 2 shows the typical TL glow curves of Ba 0.97 Ca 0.03 SO 4 :Eu nanocrystalline phosphor with varying doses with a linear heating rate of 5 K/s. The TL glow curve exhibits two glow peaks with the main peak at 498 and a small shoulder at 465. The appearance of two peaks in the glow curve indicates that there are possibly two kinds of trapping sites due to proton irradiated effect, one which is shallower leading to the peak at lower temperature and the other which is deeper leading to the peak at higher temperature. No appreciable shift in T m has been observed with the variation in doses. The TL intensity increases with the increase in dose becomes maximum for 305 Gy and decrease with further increase in dose (Fig. 2). This is the point at which TL saturation occurs. 3.3 TL response The TL response curve of nanocrystalline Ba 0.97 Ca 0.03 SO 4 :Eu samples exposed to different doses of proton beam is shown in Fig. 3.The peak heights were used for measuring the TL intensities. On exposing the nanophosphor to doses ranging from 0.1 to 325 Gy, it was found that it exhibits a linear response over a range of Gy after which there was a small change in the intensity. So the region after 150 Gy may be speculated as a region of saturation. This range is a good result for the nano material to be used as a dosimeter for dose estimation of proton beams, commonly used for the treatment of cancer in radiotherapy. The linearity, supra-linearity and sub-linearity of the response curve of Ba 0.97 Ca 0.03 SO 4 :Eu nanoparticles (Fig. 2) may be explained by the track interaction model 10,11 (TIM), a model that is used to explain behaviour of TL materials when irradiated to heavy ions having energies in the order of MeV. This model suggests that the number of generated traps as a result of irradiation depends on both the crosssection of the tracks and the length of tracks in the matrix. In nanomaterials, the length of such a track may be a few tens of nanometers, so the number of trap centres or luminescence centres will be less for lower doses than their micro-crystalline form. According to this model, a densely ionized region created mainly through inelastic collisions surrounds proton track in the TL material. At low doses of proton, the distance between the nearest neighbour tracks is large and therefore, the recombination leading to TL occurs entirely within the track. The electrons escaping from the track are captured by the non-radiative centers in the inter-track region and do not yield TL. Thus, the TL signal is simply proportional to the number of proton tracks. As the Fig. 2 TL Glow curves for Ba 0.97 Ca 0.03 (SO 4 ) 3 :Eu readout after different exposures of proton beam Fig. 3 TL response of nanophosphor Ba 0.97 Ca 0.03 (SO 4 ) 3 :Eu

4 BAHL et al.: THERMOLUMINESCENT RESPONSE OF NANOCRYSTALLINE Ba 0.97 Ca 0.03 SO 4 : Eu 503 dose increases, the average distance between the nearest neighbour tracks decreases and electrons escaping the parent track can reach the luminescent center in neighbouring tracks, thus, resulting in increased production of light leading to supralinearity. With further increase in doses, the tracks overlap each other as a result of which the effective ionization is reduced which may not give extra TL, resulting in sub-linearity and finally, saturation. But in case of nanomaterials, there still exists some particles that would have been missed while being targeted by high-energy radiation, due to their very tiny size. Thus, on increasing the dose, these nanoparticles which had earlier been left out from the radiation interaction, now generate trapping and luminescence centres (TC/LC). So in comparision to its microcrystalline form reported in literature, we do not get saturation in nanomaterials even at higher doses. However, further increasing the dose results in saturation or even a decrease in TL intensity, due to the same reason of overlapping of tracks. This can be further explained in terms of high surface to volume ratio which results in a higher surface barrier energy for the nanoparticles. Thus, on increasing the dose, the energy density crosses the barrier and a large number of defects are produced in the nanoparticles which ultimately keep on increasing with the dose till saturation is achieved. A further increase in dose, after 150 Gy, a decrease in TL intensity has been noticed. Charlesby and Partridge 15 have reported such an effect where they describe a decrease in the TL intensity with an increase in the radiation damage in the material. Lawless et al. 16 have mentioned many such reports by various researchers and suggested a model for non-monotonic dose dependence of TL, to explain the effect without considering the radiation damage in the material, and concluded that radiation damage is not necessarily the governing process and the effect may simply be the result of interplay between radiative and non-radiative centres, and sometimes between different kinds of trapping centres as well, contradicting earlier claims. This interplay may take place during the excitation stage or during the read-out (heating) stage. Chen et al. 17 have also reported a model explaining the non-monotonic dose dependence of thermoluminescence which requires further investigations for better understanding of this effect. From the application point of view, the easy method of preparation, good sensitivity, simple glow curve structure and TL response in the doses of interest, are some of the good characteristics of the presented Ba 0.97 Ca 0.03 SO 4 :Eu nanophosphor. Therefore, it might be used as a dosimeter for the estimation of proton beam doses used in proton therapy. 4 Conclusions A thermoluminescence study of proton beam irradiated Ba 0.97 Ca 0.03 SO 4 doped with Eu nanostructures has been presented. We noted that the nanoparticles exhibited grain size of around 45 nm. Samples in the form of pellets had been exposed to 150 MeV proton beam for varying doses. The TL glow curve exhibited two glow peaks with the main peak at 498 and a small shoulder at 465. TL intensity increased with the increase in dose, became maximum for 305 Gy, and finally decreased with further increase in dose. The nanophosphor exhibited a linear TL response in a range from 0.1 to 150 Gy. Further increase in the dose first showed a supra-linear response, and then sub-linear, finally resulting into saturation. Its linear response in this range makes it a suitable candidate for its application in estimating the doses of proton beam in proton therapy. Acknowledgement The authors are thankful to Dr Amit Roy, The Director, Inter-University Accelerator Centre (IUAC), New Delhi, for providing the experimental facilities. References 1 Geiß O.P, Krämer M & Kraft G, Nucl Instr & Meth B, 142 (1998) Barth W, Dahl L, Glatz J, Groening L, Richter S & Yaramishev S, Proceedings of the European Workshop on Beam Diagnostics and Instrumentation for Particle Accelerators,Mainz, Germany, 2003, p Strehl P, Proceedings of the 4th European Workshop on Beam Diagnostics and Instrumentation for Particle Accelerators, Chester, UK, 1999, p Wilson R R, Radiological Use of Fast Protons Radiology 47 (1946) Raju M R, Heavy particle radiotherapy (Academic Press, New York), Lochab S P, Sahare P D, Chauhan R S, Salah Numan & Pandey A, J Phys D: Appl Phys 39 (2006) Lochab S P, Sahare P D, Chauhan R S, Salah Numan, Ranjan Ranju & Pandey A, J Phys D: Appl Phys, 40 (2007) Lochab S P, Kanjilal D, Salah Numan, Habib Sami S, Lochab Jyoti, Ranjan Ranju, Aleynikov V E, Rupasov A A & Pandey A Journ of Appl Phys, 104 (2008) Molokanov A G, Begusova M, Spurny F & Vlcek B, Rad Prot Dosim 99, 1-4(2002) 433.

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