A new highly sensitive phosphor for carbon ion dosimetry

Abstract

Dy³⁺-doped CaMg₃(SO₄)₄ (CMS) phosphor was prepared by the acid distillation method and examined in detail with a thermoluminescence (TL) study whereby the phosphor was irradiated with γ-rays and a carbon ion beam. A good dosimetric glow curve was observed that is stable against both types of radiation. The CMS doped with 0.2 mol% of Dy³⁺ showed 3.5 times more sensitivity than commercially available CaSO₄:Dy³⁺ TLD phosphor when irradiated with a carbon ion beam. The observed glow curve variation and resultant variation in the values of trapping parameters with a change in ion beam energy suggest more complex interactions of ion beam within the phosphor at higher energies.

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1. Introduction

The demand for the dosimetry of charged particle beams has taken a decisive lead because of its utility in cancer diagnosis and therapy.^1–3 Ion beam therapy has been found to be a crucial tool in clinical practice for curing tumors in comparison to conventional radiation beams.^4,5 Using ion beams in cancer therapy is based on the phenomenon of dose deposition at Bragg's peak. In comparison to conventional radiation therapy such as γ-rays and high energy photon beams, heavy charge ion therapy is superior. The heavy charge particles (HCP) such as C⁵⁺ ions are heavier than the constituent particles in conventional radiation, which ensures penetration into the final treatment locations deep inside the body with minimum scattering and stiffer particle trajectories, low straggling effects, and sharper field edges.⁴ Ion beam therapy appears to be a more favorable option with several advantages when compared to conventional photon therapy. Ion beams have a well-defined range and small angular scattering compared with conventional photon or electron beam radiotherapy. Conventional photon beams have a limited depth of penetration proportional to the energy of the accelerator.

Beams of protons and carbon ions have a much more favorable dose-depth distribution than photons and are the new frontiers of cancer radiation therapy.⁵ Heavy ion beams deliver a larger mean energy per unit length of their trajectory in the body than proton and photon beams.⁶ Bone and soft-tissue tumors are usually surgically removed because they are generally radio-resistant, and the effect of photon radiation is not sufficient for long-term control. However, when surgery is difficult to perform, radiotherapy is often employed as a sole treatment. In this sense, carbon ion radiotherapy exhibits more significant biological effects than other types of therapy.⁷ These factors are crucial for tumors located close to critical organs such as the eye or ear. Tumors that are to be treated with higher doses require special care so that healthy tissues are not influenced by the incident ion beam. This opens up a promising potential for their highly effective use in the treatment of intractable cancers. For this, the dose delivered to the tumor requires an accurate calibration of radiotherapy sources and their proper dosimetry. These ideas encourage us to find a suitable TLD phosphor for carbon ion dosimetry.

For the high energies of heavy charged particles in the entrance region, low ionization density commonly produce repairable damages. However, with increased energy, energy loss towards the Bragg's peak is more significant, which produce irreparable damages and results in a higher relative biological efficiency (RBE). The biological effectiveness refers to the difference in rate at which cells are killed with the same radiation dose. We have selected the carbon ion for study because heavier and lighter ions possess their own drawbacks. The limitation of using heavier ions such as neon or argon is that they cause irreparable damages in the entrance channel (surface), and thus significantly damage the healthy tissues in front of the tumor.⁸ For very light ions such as protons, the scattering effect is large, and therefore, no damage potentiation (the increase in strength of nerve impulses along pathways that have been used previously) can be observed in the target volume.⁸ Hence, carbon ion beams represent a most favorable option in heavy ion therapy and for enhancing the biological efficiency in tumor therapy.⁹

Sulphate-based thermoluminescence (TL) materials are known for their high sensitive TL response. It has been found that mixed sulfates form a class of TL phosphors with good TL characteristics when doped with the appropriate activators.^10–13 This family includes several materials such as CaSO₄:Dy³⁺, K₂Ca₂(SO₄)₃:Eu, LiNaSO₄:Eu, Na₂₁Mg(SO₄)₁₀Cl₃:Dy³⁺, and K₃Na(SO₄)₂:Eu, which are studied due to their excellent thermoluminescent properties such as high TL sensitivity, high TL efficiency, linear dose response over a wide range of doses, and reproducibility. But all these phosphors suffer from one or the other problem. More work is going on either to improve TL properties of these existing materials or to develop new high sensitive TL phosphors with ideal TL characteristic. Therefore, there is a great demand for highly efficient dosimetry materials for radiation dose assessment and to meet technological challenges. Such efforts are also crucial in a number of other areas such as human exploration in outer space.¹⁴ Several methods and treatments have been adopted (1) to improve TL sensitivity, (2) to know the responsible defects in these materials, and (3) to understand the phenomenon of TL in more detail.

To the best of our knowledge, the literature describing the effect of carbon ion beams on phosphor materials especially with reference to dosimetry is very limited. This study is an attempt to obtain useful data regarding the TL response of carbon beam-irradiated highly sensitive phosphor, which would be helpful in the dosimetry of carbon ion beams.

2. Experimental

2.1. Synthesis method

The phosphor studied in the present work was synthesized using the method described by Yamashita et al.¹⁵ For the preparation of the Dy³⁺-doped CaMg₃(SO₄)₄ (CMS) phosphor, all of the starting materials used were of analytical grade. CMS doped with different concentrations of Dy³⁺ was prepared by dissolving CaSO₄, MgSO₄, and a stoichiometric amount Dy₂O₃ 15 ml of hot sulfuric acid (the excess acid used). The mixture was allowed to heat at approximately 300 °C for 20 h, and during heating, the highly active acid vapors were condensed using a water-cooled condenser assembly so as to prevent any spontaneous reactivity. After cooling the mixture to room temperature, excess acid in the sample was repeatedly washed out with distilled water, and a water-insoluble compound was obtained in the form of small crystals. After washing the sample 4 to 5 times with distilled water, the remaining sample was dried in an oven at 80 °C. No further heat treatment was given to the samples.

2.2. Experimental details

5 mg of CMS phosphor was exposed to γ-rays from ⁶⁰Co and ¹³⁷Cs sources for various doses to see the glow curve structure variation with dose and the linearity of the phosphor. The samples in the form of pellets were irradiated at room temperature by a C⁵⁺ ion beam at energies of 50 MeV and 75 MeV for different ion fluences in the range of 15 × 10¹⁰ to 30 × 10¹² ions per cm², using a 16 MV tandem Van de Graaff type electrostatic accelerator (15 UD pelletron) at the Inter-University Accelerator Center, New Delhi, India. The full details of this setup are given by Kanjilal et al.¹⁶ For irradiation, the sample pellets were mounted on a copper target ladder, as shown in Fig. 1(a) and (b). The copper ladder prevents heating of the sample during swift heavy ion (SHI) irradiation. For irradiation, the ladder was kept inside the evacuated irradiation chamber, as shown in Fig. 1(c). The ion beams were magnetically scanned on a 10 mm × 10 mm area of sample surfaces for uniform irradiation. The beam spot size used was 2.5 mm². The pressure of the vacuum chamber during ion beam irradiation was 5 × 10⁻⁴ mbar.

Fig. 1 (a) and (b) Copper ladder with pellets of CMS sample mounted on it; (c) inner view of the C⁵⁺ ion irradiation chamber.

The diffraction pattern of the CMS phosphor was examined using synchrotron XRD (SXRD) measurements at the ADXRD beamline (BL-12) of the Indus-2 synchrotron source at the Raja Ramanna Center for Advanced Technology (RRCAT), Indore, India.^17,18 An image plate area detector (Mar 345 Dtb) was used to record the diffraction pattern. Lanthanum hexaborate (LaB₆) was used as a standard material for calibration of the beam energy and the sample to detector distance. The wave-length used was 0.77774 Å. The TL glow curves were recorded using a Harshaw TLD reader (Model 3500) fitted with a 931B photomultiplier tube (PMT). The heating rate used was 5 °C s⁻¹. The photoluminescence (PL) emission spectra of the samples were recorded using a RF-5301 PC Shimadzu spectrofluorophotometer. Emission and excitation spectra were recorded using a spectral slit width of 1.5 nm.

3. Results and discussion

3.1. X-ray diffraction

The X-ray diffraction pattern of the CMS phosphor was recorded using the beam energy of 15.94 keV with a sample-to-image plate distance of 151.2 mm. These refined values were obtained using the FIT2D program, and the image plate data files were also integrated using FIT2D, incorporating polarization correction.¹⁹ Fig. 2 shows more detail regarding the phase of this phosphor by presenting the high resolution synchrotron X-ray diffraction data collected at room temperature. The angle dispersive synchrotron XRD patterns are nearly pure phase and in good agreement with the standard ICDD file no. 19-0241. The different Dy³⁺ concentration-doped XRD patterns of the CMS phosphor are shown in Fig. S1.†,²⁰ All four XRD patterns are almost identical, which suggests that the incorporation of Dy³⁺ into the CMS lattice does not influence the crystal structure. Fig. S2† shows the modification of the XRD pattern of the CMS phosphor after C⁵⁺ ion beam irradiation. A video illustrating the ion beam irradiation on sample pellets is also given in the ESI (file M1†). No new peaks were observed after ion beam irradiation, indicating no change in phase of the phosphor. Due to ion beam irradiation, the relative intensities of some dominant peaks change, and some minor peaks are diminished. This alteration is small, and indicates that there was a small reduction in the crystallinity of the phosphor after ion beam irradiation. Therefore, we can say that this phosphor is stable against C⁵⁺ ion beam irradiation.

Fig. 2 (a) High-resolution synchrotron X-ray diffraction patterns of the CMS phosphor; (b) the raw image plate X-ray diffraction data.

3.2. SEM study

The SEM images of the as-synthesized samples are shown in Fig. 3. The large particles were formed when the acid distillation method was used, and some grains have a disk shape while others are irregularly shaped with fine surfaces. The figure shows the microstructures consisting of large particles in the 5–15 μm range, which would enable the phosphor to be useful in dosimetry application.²¹

Fig. 3 SEM images of the CMS phosphor prepared by the acid distillation method.

3.3. TL study

3.3.1. Effect of irradiation. Thermoluminescence is commonly used for dose estimation of high-energy ionizing radiation absorbed by materials. In this work, the TL study of this phosphor was carried out to propose its use in monitoring not only conventional radiation beams but also heavy ion beams such as carbon. The TL glow curves were recorded with the help of the Harshaw TLD reader. For the TL readout, the fixed mass (5 mg) of the irradiated sample was placed on a heating planchet, which was then allowed to heat at the rate of 5 °C s⁻¹. The light released from the sample was detected with a photomultiplier tube, which was placed perpendicular to the sample. Fig. 4(a) shows typical TL glow curves of the CMS phosphor, irradiated with γ-rays from a ⁶⁰Co source at a 15 Gy dose. The glow curve consists of a good dosimetric peak at 260 °C with a small low-temperature peak at 150 °C. Among the four different concentrations of Dy³⁺, the maximum TL sensitivity was observed for 0.2 mol% concentration.

Fig. 4 TL glow curves of the CMS phosphor irradiated with (a) γ-rays from a ⁶⁰Co source at a 15 Gy dose, (b) γ-rays from a ¹³⁷Cs source at a 2300 mRad dose, (c) C⁵⁺ ion beam at 75 MeV with a 216 kGy dose, and (d) comparison between TL glow curves of the CMS phosphor irradiated with a C⁵⁺ ion beam at 50 and 75 MeV energies, and γ-rays from a ⁶⁰Co source at a 15 Gy dose and a ¹³⁷Cs source at a 2300 mRad dose.

To compare the TL sensitivity, we also measured the TL of the standard TLD (CaSO₄:Dy³⁺) phosphor. It was observed that the sensitivity of the CMS phosphor is approximately 50% compared to the sensitivity of CaSO₄:Dy³⁺. Fig. 4(b) shows the TL glow curve of the CMS phosphor for γ-ray irradiation from the ¹³⁷Cs source at 2300 mRad. The glow curve shape of the ¹³⁷Cs-irradiated samples was found to be similar to ⁶⁰Co-irradiated samples, but there is a shift in the glow peak position towards lower temperatures when irradiated with the ¹³⁷Cs source. The observed shift was approximately 3 °C for peak 1 and 28 °C for peak 2. This shift may be due to the alteration in the position of trapping levels inside the forbidden band gap. As can be seen from Fig. 4(c), the glow curve structures of CMS exposed to carbon ions (50 and 75 MeV) are similar to that of γ-irradiated samples. The glow curves of CMS exposed to two different energies of carbon ion beams are qualitatively similar, but quantitatively, there is a difference in the temperature and relative intensities of the glow peaks, which leads to the modifications in the trapping parameters. Various authors have reported significant variations in the glow curve structures and the positions when samples were irradiated with different sources.^22–24 The CMS phosphor was found to possess a glow curve that is stable with variation in irradiation species, as the carbon ion- and γ-ray-irradiated samples show 11 and 8% fading, respectively, over 16 days of storage (see Section 3.6). In comparison to our phosphors, the standard TLD phosphor CaSO₄:Dy³⁺ showed significant change in the shape of the glow curve when exposed to the carbon ion beam.

As can be seen in Fig. 4, the glow curve structure of CMS exposed to carbon ions at two different energies (i.e., 50 and 75 MeV) is similar to that of the ¹³⁷Cs and ⁶⁰Co γ-irradiated sample. There is a small difference between the TL glow peak temperature for the ⁶⁰Co and ¹³⁷Cs γ-irradiated sample. In the case of γ-irradiated samples using the ⁶⁰Co source, the second peak at 260 °C is more prominent than the lower temperature peak at 146 °C when compared with TL glow curves of γ-irradiated samples using the ¹³⁷Cs source. The lower temperature peak shows significant growth compared to the high temperature peak. This clearly indicates that the number of traps responsible for these peaks is not in the same proportion in the two cases. For the 75 MeV C⁵⁺ ion-irradiated samples, additional trapping levels form, which yield two more glow peaks compared to γ-ray-irradiated samples. The occurrence of additional glow peaks at 185 °C and 232 °C suggests the formation of intermediate trapping levels in-between the trapping levels responsible for the occurrence of glow peaks at 143 °C and 242 °C. Moreover, the C⁵⁺ ion-irradiated samples (50 MeV) show significant change in the position as well as intensity of the glow curve compared to glow curves of C⁵⁺ ion-irradiated samples at 75 MeV. The C⁵⁺ ion irradiation at 50 MeV not only alters the positions of trapping levels, causing a shift in the glow peak temperatures, but it also creates one additional trapping level that results in an additional glow peak. Upon irradiating the samples with C⁵⁺ ions, there may be the formation of new trapping and luminescent centers that are responsible for this anomalous behavior. Similar effects in the TL response of the CaSO₄:Dy³⁺ phosphor upon C⁶⁺ ion beam irradiation have been reported by some authors.²⁵

It is expected that upon irradiating samples with a highly energetic ion beam such as that of 50 and 75 MeV C⁵⁺ ions, there will be changes in the TL glow curve structure because the TL trapping and recombination mechanisms are very sensitive to any perturbation. The variation is more when atomic displacements occur due to non-elastic collisions, and ionization due to secondary particles takes place.²⁶ The obtained results show that there is not much variation in the structure of the glow curve, but there are minor changes in glow peak temperature, number of glow peaks, and TL intensity, which are responsible for the variation in trapping parameters. These variations in trapping parameters are due to the disorganization of the initial localized energy levels in the mixed sulfate host as a result of the high energy ion irradiation.

3.3.2. Dose response. The TL response curves of the CMS phosphor irradiated by γ-rays from ⁶⁰Co and ¹³⁷Cs sources and a carbon ion beam are shown in Fig. 5 (a)–(c), respectively. The TL response curve of the materials irradiated by γ-rays within a dose range 10 Gy–10 kGy shows a linear response up to 1000 Gy; above 1000 Gy, slight saturation is observed. The phosphor irradiated with γ-rays from the ¹³⁷Cs source also shows a linear response from 100 mRad to 5 Rad. Unlike γ-rays, the TL response of the CMS phosphor towards the carbon ion beam shows early saturation when irradiated within a dose range of 22 kGy to 4 MGy. The saturation in the TL response of the CMS phosphor can be explained by the track interaction model (TIM).^27,28 The intensity of the TL signal increases in proportion to the number of ion tracks. At higher fluences (due to a large flux), the distance between nearest neighboring tracks decreases and the probability of electrons escaping from the host track ion and reaching neighboring tracks increases. This results in increased luminescence recombination and ultimately increases the TL intensity. As the fluence increases, further saturation effects occur where the distances between nearby tracks decrease and the tracks begin to interact and overlap. These overlapping regions do not give additional TL because they do not form additional trapping charge carriers due to the full occupancy of the available trap and luminescence centers. As the energetic ions are implanted in the matrix, they may be creating new kinds of defects, which make the process more complicated.

Fig. 5 The TL response curves of the CMS phosphor irradiated by γ-rays from (a) ⁶⁰Co, (b) ¹³⁷Cs, and (c) C⁵⁺ ion beams.

The influence of 50 and 75 MeV carbon ion beams is definitely more than that of γ-rays, and therefore, the corresponding cross section of C⁵⁺ ion tracks inside the CMS phosphor is higher. Low linear energy transfer (LET) radiation such as γ-rays and electrons confers higher luminescence efficiency as compared to high LET radiation consisting of heavily charged particles. However, high LET radiation may induce additional defects in the host material as compared to low LET radiation, and with increase in the radiation exposure, the density of defects inside the host increases, which leads to an increase in peak intensity.

The dose delivered by carbon beam was calculated using eqn (1):

where dE/dX is the mean energy loss, ρ is the density of the target material, and ϕ is the ion fluence. The energy loss (linear energy transfer) was calculated using a Monte Carlo simulation based on TRIM code given by Ziegler et al.²⁹ The maximum penetration depth inside the material was calculated to be 89 μm, which is less than the thickness of the pellet i.e., 0.075 cm. The energy loss values for 50 MeV and 75 MeV are 2330 and 1760 (MeV cm² g⁻¹), respectively. These values are an indication of the amount of ionization caused by ions inside the target material, and the values suggest that the ionization caused by the 50 MeV ion beam is higher than that produced by the 75 MeV beam. This is also reflected in the TL, where a higher sensitivity was observed for the 50 MeV irradiated sample. Moreover, the penetration depth for 50 MeV and 75 MeV energies was calculated to be 48 and 89 μm, respectively. The present phosphor shows a decrease in TL sensitivity after a 23 kGy dose of the C⁵⁺ ion beam. This remarkable result is extremely important for CMS to be used as a dosimeter in such high doses of ion beam irradiation.

3.4. PL study

The photoluminescence (PL) excitation and emission spectra for different mol% concentrations of Dy³⁺-doped CMS pristine phosphor are shown in Fig. 6(a). The excitation spectrum of pristine CMS phosphor shows a number of excitation peaks with a prominent peak at approximately 349 nm. The emission spectrum that was recorded at this excitation wavelength shows characteristic emission peaks of Dy³⁺ at approximately 484 nm and 574 nm, which were assigned to the transitions ⁴F_9/2 → ⁶H_15/2 and ⁴F_9/2 → ⁶H_13/2, respectively.³⁰ Fig. 6(b) shows the PL excitation and emission spectrum of 1 mol% Dy³⁺-doped CMS phosphor irradiated with 50 and 75 MeV carbon ion along with that of pristine phosphor. From the excitation and emission spectra, it was observed that phosphor exposed with a carbon ion beam exhibited a decrease in PL emission intensity, and with a further increase in the energy of the carbon ion beam, the emission intensity continued to decrease. There are several possible reasons for this decrease: (1) destruction of luminescence centers that are responsible for emission, (2) residual absorption of both the excitation light as well as emission light due to the irradiation, and (3) damage to the microstructure of the phosphor due to ion-induced defects.³¹

Fig. 6 (a) PL of the CMS phosphor for different concentrations of Dy³⁺; (b) PL of carbon ion-irradiated CMS (Dy³⁺ = 0.2 mol%) phosphor at 50 and 75 MeV and pristine sample.

At high ion fluences, structural changes occur inside the sample that may reduce its PL output. This effect is directly influenced by the energy loss of the ions and thus by the size and the damage concentration. The observed saturation in the TL glow curves and Dy³⁺ PL emission of the samples exposed to the 75 MeV C⁵⁺ ion beam might be due to the increased penetration depth of the ion beam. However, for the 50 MeV ion beam, the penetration is less and the backscattering could be more, resulting in a smaller number of implanted ions; therefore, it still exhibits a higher TL and PL efficiency.

3.5. Glow curve analysis

To further verify the energy levels and other kinetic parameters of the glow peaks in all three cases, the glow curve deconvolution was performed using glow curve deconvolution (GCD) functions as previously reported by Kitis et al.³²

For general order,

where I (T) is the TL intensity at temperature T (K), I_m is the maximum peak intensity, E is the activation energy (eV), and k is the Boltzmann constant.

These parameters were applied to the experimentally obtained glow curves to isolate each peak. Firstly, the order of kinetics and activation energy of one of the peaks was found using Chen's set of empirical formulae.³³

The trap depth is calculated using the following equation, which is independent of order of kinetics:

E = c_γ(kT_m²/γ) − b_γ(2kT_m) (3)

where γ is τ, δ or ω are the constants c_γ and b_γ for the three equations (τ, δ, or ω), k is the Boltzmann constant, and T_m is the maximum peak temperature.

The frequency factor (s) is:

where Δ_m = 2kT_m/E, b is the order of kinetics, k is the Boltzmann constant, and β is the linear heating rate (5° C s⁻¹).

To determine the order of kinetics (b), the form factor μ_g (μ_g = (T₂ − T_m)/(T₂ − T₁)), which involves T₁ and T₂ (temperatures corresponding to half of the intensities on either side of the maximum), was calculated. This form factor μ_g is independent of the activation energy (E) and strongly depends on the order of kinetics (b). Finally, the peak was theoretically generated using these parameters and separated from the main experimental glow curve. The benefit of using the GCD method is that most of the parameters used for generating a theoretical curve are easily derived from the experimentally recorded glow curve. The thermal activation energy (E) was again calculated using the same set of equations. This procedure was repeated for each TL peak until a good fit between the experimental and theoretical glow curve was obtained.

The deconvolution of experimentally obtained glow peaks was carried out using the glow curve deconvolution program. The deconvolution of experimental glow curves was carried out in order to reveal the number of individual glow peaks present in a complex glow curve, as shown in Fig. 7. The isolated glow peaks were then examined to obtain data regarding trapping parameters. The generation of new absorption peaks with the increasing energy of the carbon ion beam also illustrates the changing position of the trapping levels (see Table 1).

Fig. 7 Comparison between the experimental (—), theoretically fitted (•), and deconvoluted (—) TL glow curves of the CMS phosphor exposed to (a) 15 Gy of γ-rays from ⁶⁰Co, (b) 2300 mR of γ-rays from ¹³⁷Cs, (c) 216 kGy from the 75 MeV C⁵⁺ ion beam, and (d) 108 kGy from the 50 MeV C⁵⁺ ion beam.

Table 1 Trapping parameters of the CMS phosphor for different types of irradiation calculated by Chen's method

Type of irradiation	Peak number	Tm (°C)	μg	E (eV)	S (s^−1)
^60Co	1	146	0.44	0.60	4.11 × 10^6
	2	258	0.48	0.61	9.07 × 10^4
^137Cs	1	145	0.5	0.83	2.6 × 10^9
	2	230	0.5	0.87	9.3 × 10^7
C^5+ 75 MeV	1	143	0.5	0.93	5.5 × 10^10
	2	185	0.5	0.67	4.44 × 10^6
	3	232	0.48	0.43	1.64 × 10^3
	4	242	0.5	1.21	2.3 × 10^11
C^5+ 50 MeV	1	167	0.5	1.09	9.14 × 10^11
	2	234	0.48	0.36	2.9 × 10^2
	3	280	0.49	1.25	6.51 × 10^10

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3.6. Fading study

For the fading study, the γ-ray- and carbon ion-irradiated samples were stored for a few days without taking any precautions to shield them from light or moisture. The glow curves were then recorded for a period of approximately 16 days, as shown in Fig. 8, which illustrates the fading plot of the γ-ray- and carbon ion-irradiated phosphor. It was observed that the higher temperature glow peak is quite stable over the storage time. The carbon ion- and γ-ray-irradiated samples exhibited 11 and 8% fading, respectively, over 16 days of storage.

Fig. 8 The fading data of the CMS sample.

3.7. Absorption spectra

Fig. 9 shows the absorption spectra of the CMS phosphor for 0.2 mol% Dy³⁺ doping. The calculated band gap energy of the pristine sample is 5.34 eV. It was observed that the absorption of the phosphor increases with increasing energy of the ion beam, which is due to the generation of a large number of defects. The absorption was found to be shifted towards a higher wavelength with increases in beam energy. The difference between the band gap energy and the energy of the absorption peak represents the activation energy of trapped charge carriers. The sample irradiated by the 50 MeV ion beam demonstrated absorption at 219, 258, and 317 nm corresponding to 5.66, 4.80, and 3.911 eV energies, respectively; the 75 MeV ion beam resulted in absorption at 204, 243, 254, 313, and 459 nm corresponding to 6.07, 5.10, 4.88, 3.96, and 2.70 eV energies, respectively. The activation energies calculated from the absorption spectra are nearly same as those calculated by Chen's peak shape method.

Fig. 9 The absorption spectra of the CMS sample.

4. Conclusion

In conclusion, we provide insight for synthesizing a CMS phosphor by the acid distillation method with remarkable TL properties. The phosphor was found to be 3.5 times more sensitive than a standard CaSO₄:Dy³⁺ phosphor with a stable dosimetric peak for C⁵⁺ ion irradiation. The TL response was nearly identical for different types of irradiation. The irradiation resulted in a slight variation in glow peak position, and ion beam irradiation caused additional glow peaks, which influences the trapping parameters. The obtained results can be easily correlated with the physical phenomenon, suggesting that this phosphor can be used for carbon ion beam dosimetry along with γ-ray dosimetry over a wide range of exposures.

Acknowledgements

Authors are grateful to Inter University Accelerator Center (IUAC) New Delhi, India for providing financial assistance to carry out this work under research project UFR-56301. The authors would like to thank the Director of the Inter-University Accelerator Centre (IUAC), New Delhi, for providing beam time. The authors are also grateful to the Director of the Center for Advanced Technology, Indore, for providing access to the ADXRD measurement facility and Dr A. K. Sinha and Dr M. N. Singh for their support during the ADXRD measurement.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra08742a

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