CHAPTER 4 MAGNESIUM BORATE (MgB 4 O 7 )

Transcription

1 CHAPTER 4 MAGNESIUM BORATE (MgB 4 ) This chapter deals with the synthesis of nanocrystals of MgB 4 : Dy using combustion method. TL characteristics of the synthesized material have been studied in details. 4.1 INTRODUCTION Magnesium borate (MgB 4 :Dy) phosphor is nearly tissue equivalent material with an effective atomic number equal to 8.4 compared to 7.4 for water and soft biological tissues so it is commonly used for medical dosimetry of ionizing radiations such as gamma and X rays using the TL technique. In the present work, the synthesis of nanocrystals of MgB 4 : Dy using combustion method have been presented. The formation of material in the nano size has been confirmed by using XRD and TEM techniques. TGA has been performed to study the thermal stability of synthesized material. The prepared MgB 4 : Dy phosphor have been irradiated to gamma rays and their TL properties have been examined that includes annealing conditions, TL glow curves, TL response, fading and reproducibility. Further the TL properties of MgB 4 :Dy samples, irradiated with proton (3 MeV and 150 MeV) beam and electron (4 MeV, 9 MeV) beam have been studied. These results may be helpful in the development of tissue equivalent TL nanocrystalline detectors best suited for the wide range of radiation exposures. 4.2 EXPERIMENTAL Synthesis MgB 4 :Dy samples have been synthesized by using combustion method (Mimani and Patil, 2001; Lochab et al, 2007) where commonly available materials like urea and ammonium nitrate work as fuel and oxidizer respectively. Magnesium nitrate (Mg(NO 3 ) 2 ), boric acid, ammonium nitrate, urea act as the starting materials and Dysprosium chloride acts as an activator. The starting mixture, with a molar ratio of Mg(NO 3 ) 2 :H 3 BO 3 :NH 4 NO 3 : NH 2 CONH 2 = 1.0: 3.2: 10.2: 10.2 and appropriate amounts of DyCl 3 :6H 2 O (for Dy concentration ranging from 500 ppm to 3000 ppm), was put in a large quartz crucible and introduced in a muffle furnace preheated to 550 C. The mixture undergoes combustion to produce the 84

2 corresponding borate. The phosphor thus obtained was then crushed to get the powder form. Finally the nanocrystalline powder was annealed at 400 C for 60 minutes in crucible in the presence of air and was quenched by taking the crucible out of the furnace and placing it on a metal block. The synthesized phosphor was then used for studying its TL properties Characterization The formation of the compound has been confirmed by analysis of XRD pattern taken at room temperature by using Cu-target (Cu-K α1 line, = Å). The slow scan was performed in the 2θ range from 10 o 60 o with a scan step of 0.01 o. The results obtained were matched with the standard data available (JCPDS card No ). The particle size and shape of the concerned phosphor was analysed by using TEM. The thermal stability of the concerned phosphor was obtained by TGA. Samples were analysed in the air atmosphere with a heating rate of 10 C/min upto 1200 C taking 25 mg of sample. To study TL properties, the annealed samples were then irradiated with γ-rays using calibrated 137 Cs source for low dose ranging 4x10-4 Gy to 1x10 0 Gy and calibrated 60 Co source for high dose ranging 5x10 1 Gy to 1x10 5 Gy at room temperature. The TL properties of samples irradiated with electron beam of energies 4 MeV, 9 MeV for dose ranging1x10-2 to 1x10 1 Gy each, proton beam of energy 150 MeV for dose ranging 1x10 0 to 1x10 2 Gy and proton beam of energy 3 MeV for fluence ranging 1x10 11 to 1x10 15 ions/cm 2 were also studied. TL glow curves have been recorded using TLD reader. The heating rate of 5 K/sec and the amount of 5 mg of the sample has been taken for each TL recording. 4.3 RESULTS AND DISCUSSIONS X-Ray diffraction (XRD) Formation of the compound was confirmed and its particle size was determined by studying the XRD pattern. Figure 4.1 shows the X-ray diffraction pattern of synthesized MgB 4 :Dy with hkl values. The average grain size can be estimated from the observed line broadening. Assuming the particles are stress free, the size can be estimated from single diffraction peak using Debye scherrer s formula (given in equation 3.1). The average grain size of the concerned phosphor was calculated to be approximately 22 nm. When the data is fitted with the powder X-ray data analysis system, it is found that the compound exhibits an orthogonal structure having lattice parameters, a = Å, b = Å, c = Å. 85

3 4.3.2 Transmission electron microscopy (TEM) The transmission electron microscopy (TEM) has been used to determine the shape and size of the particles of the concerned phosphor. The TEM photograph of MgB 4 : Dy is shown in Figure 4.2. It reveals that the particles are of tubular shape with their diameter ranging between 18 to 30 nm approximately. However, the difference in the estimation of the particle size by XRD and TEM may be due to the two different methods used for estimation Thermogravimetric analysis (TGA) The thermal behaviour of sample has been studied using thermogravimetric analysis (TGA) (shown in figure 4.3). TGA curve reveals that the weight of MgB 4 :Dy samples at 35 C is 100 %. When the sample is heated to 95 C, it shows a weight loss of 2.4 %, this weight loss is most probably due to the trapped moisture in the samples. As the temperature is increased further to 450 C, the total 45 % of weight loss is observed. MgB 4 :Dy shows a total weight loss of 47 % at 1150 C, close to its melting point, which may arise from the decomposition of bonds. Since the weight loss is 47 % before the phosphor reaches its melting point so, the synthesized MgB 4 :Dy is found to be thermally unstable compound at high temperatures TL properties of gamma irradiated MgB 4 :Dy The TL properties of synthesized MgB 4 : Dy, irradiated with gamma rays using 137 Cs (for low doses) and 60 Co (for high doses) sources have been studied. TL properties examined in this study include annealing conditions, glow curves, TL response curves, reproducibility and fading Annealing conditions To study the pre-irradiation annealing condition, samples of synthesized MgB 4 :Dy nanophosphor in a powder form have been subjected to different annealing temperatures of 200 C, 300 C, 400 C and 500 C for a time period of 5, 10, 15, 30 and 60 minutes each. The annealed samples have been irradiated by a test dose of 1x10 3 Gy emitted from 60 Co gamma source and TL glow curves have been observed. It has been found that the optimum pre-irradiation annealing condition is 400 C for 60 minutes. 86

4 Effect of heating rates on TL glow curves The influence of different heating rates on TL glow curves of synthesized nanocrystalline MgB 4 : Dy phosphor has been investigated. Figure 4.5 shows the TL glow curves of MgB 4 :Dy exposed to 1x10 3 Gy of gamma rays at heating rates of 1 K/s, 5 K/s, 10 K/s, 15 K/s. It is revealed from figure that with increase in heating rates the maximum temperature peak shifts to the higher temperature side and the heating rate of 5 K/s shows the maximum peak intensity. Further with increase of heating rate to 10 K/s, the intensity decreases and again starts increasing for 15 K/s which indicates that MgB 4 :Dy have some complex distribution of traps in the lattice that is in accordance with the previous results (Sharma et al., 2010) Effect of dopant concentration on TL response The effect of dopant concentration on the TL response (peak intensity) has been investigated and the optimum concentration of Dy in MgB 4 has been determined. The dopant concentration has been varied from 500 ppm to 3000 ppm and its effect on TL response has been shown in Figure 4.4. For MgB 4 : Dy samples irradiated with 1x10 3 Gy, 1000 ppm of Dy yields maximum TL response and with further increase in dopant concentration, the response decreases. This is in good agreement with the phenomenon of concentration quenching (Dekker, 1957). The incorporation of more dopant ions, Dy in MgB 4 may leads to lattice perturbation. An activator which acts as a luminescent centre is surrounded by the non luminescent host centres. Therefore the released charge carriers cannot recombine directly with the luminescent centres. Most probably the energy is transferred non-radiatively through the host lattice to the activator, which on recombination gives characteristic emission (Kumar et al., 2009). With increase in dopant concentration, distance between the activator ions decreases as a result of which energy levels of activator ions perturb each other in a way to quench each other s emission causing decrease in the TL intensity. These studies reveals that concentration of 1000 ppm of Dy in MgB 4 :Dy is optimum for maximum TL output Effect of dose on TL glow curves The effect of gamma doses ranging 5x10-3 to 5x10-2 Gy, 1x10-1 to 1x10 0 Gy and 5x10 2 to 5x10 3 Gy on TL glow curves of synthesized MgB 4 :Dy nanophosphor has been shown in Figure 4.6 (a), (b) and (c) respectively. It is found that for a dose of 5x10-3 Gy, MgB 4 :Dy shows only one peak at 408 K. With 87

5 further increase in dose upto 5x10-2 Gy, the intensity of peak keeps on increasing. As the dose increased to 1x10-1 Gy (shown in Figure 4.6(b)), MgB 4 :Dy shows two peaks, the main peak is shown at 428 K and small peak at 565 K. With increase in dose to 1x10 0 Gy, the intensity of both peaks increases and the main peak is shifted to 436 K. With further increase in dose to 5x10 2 Gy, the main peak is shown at 416 K and small peak at 525K and the intensities of both the peaks increases (shown in figure 4.6 (c)). When the dose reaches 5x10 3 Gy, main peak is shifted to 413 K, the small peak to 521 K and intensity of both the peaks keeps on increasing. This may be attributed to the fact that on irradiation to high energy radiation, the population of trapping/ luminescent centres (TC/LC) got changed, which reflected on the occurrence of different intensities for the TL glow peaks TL response The TL response of the synthesized MgB 4 :Dy at low doses has been shown in Figure 4.7(a). It has been observed that nanophosphor shows a remarkable linear pattern at lower doses in the range 4x10-4 to 4.5x10-2 Gy. When dose is increased to 1x10-1 Gy, the material shows sublinear behaviour. This shows that the synthesized MgB 4 :Dy nanophosphor can be used as dosimeter for low doses in the range 4x10-4 to 4.5x10-2 Gy. Further the TL response of MgB 4 :Dy in the range 1x10-1 to 5x10 4 Gy (Figure 4.7 (b)) reveals that the synthesized nanophosphor MgB 4 :Dy exhibits a sublinear behaviour below 1x10 0 Gy dose and becomes linear in the range 1x10 0 Gy to 5x10 3 Gy. With further increase of dose, the response is initially supralinear (from 5x10 3 Gy to 1x10 4 Gy), then sublinear and finally results into saturation over 1.5x10 4 Gy. The linear behaviour over a wide range of dose can be explained on the basis of track interaction model (as explained in section ) Fading To determine the fading characteristics of synthesized nanophosphors, the γ exposed samples (at a dose of 1x10 3 Gy) have been stored for one month at room temperature in dark conditions. The results of fading of synthesized MgB 4 :Dy have been presented in Figure 4.8. In has been revealed that, the fading recorded on the third day is 4.6%, on 7 th day it is 6.4% and on 15 th day it is 7.6% and the total fading recorded in one month is 8%. 88

6 Reproducibility In order to assess the reproducibility for the dose measurements of synthesized MgB 4 :Dy material, a large set of repeated readouts have been carried out at a dose level of 1x10 3 Gy each. The results show that the material can measure gamma doses with ± 5% variation for 10 sequential measurements Glow curve deconvulation The trapping parameters associated with the various TL bands, are evaluated after deconvolution of composite glow curves. In the present work, the trapping parameters associated with the glow peaks have been calculated using glow curve deconvolution (GCD) glow fit and peak shape method. (a) Glow fit method The recorded composite glow curves for gamma dose of 5x10 3 Gy have first isolated by a computer program, GlowFit (Puchalska and Bilski, 2006) and are shown in Figure 4.9. It has been found that the composite glow curve composed of six peaks. The parameter describing the quality of fitting, called Figure of Merit (FOM) has been found to be 2.11 %. These all isolated six peaks show first-order kinetics, i.e. the probability of electron re-trapping during thermoluminescence process is negligible. The values of trapping parameters of all the six isolated peaks for the glow curve have been calculated by GCD glow fit method and are shown in Table 4.1. It has been observed that the energy levels (activation energy) of various traps (corresponding to various peaks) are very much different. Therefore it is clear that there are some deep and shallow traps. The competition among these traps might be giving various releasing probabilities, which might have resulted in different frequency factors (Salah et al., 2004). (b) Peak shape method The values of trapping parameters of all the six isolated peaks for the glow curve have been calculated by peak shape method and are shown in Table 4.2. It can be observed that all the peaks have μ g close to 0.42, so all the peaks show first order kinetics. The values of trap depth calculated by peak shape method (shown in table 4.2) shows a good agreement with the values of trap depth reported in Table 4.1 using GCD glow fit method. 89

7 (221) Synthesized MgB 4 :Dy Intensity (a.u.) (300) (020) (302) (321) (031) (332) 20 (500) (511) (311) (602) (133) (441) (613) (351) (015) (Degree) Figure 4.1: XRD spectra of synthesized nanocrystalline MgB 4 : Dy compound. Figure 4.2: TEM image of synthesized nanocrystalline MgB 4 :Dy compound. 90

8 synthesized MgB 4 :Dy Weight(%) Temperature ( C) Figure 4.3: TGA thermogram of synthesized nanocrystalline MgB 4 :Dy compound. TL Intensity (a.u.) 3.0x10 8 b 2.8x10 8 d 2.6x x10 8 c 2.2x x x x x x10 8 a 1.0x x x x x x10 7 a 1K/s b 5K/s c 10K/s d 15K/s Temperature (K) Figure 4.4: Influence of different heating rates on TL glow curves of MgB 4 :Dy exposed to gamma rays. 91

9 3.0x10 8 Synthesized MgB 4 :Dy 2.5x10 8 TL Response (a.u.) 2.0x x x x Concentration (ppm) Figure 4.5: Effect of dopant concentration on TL response of MgB 4 : Dy samples exposed to gamma rays 92

10 TL Intensity (a.u.) 7x10 4 6x10 4 5x10 4 4x10 4 3x10 4 2x10 4 d c MgB 4 : Dy irradiated to gamma dose of a 5x10-3 Gy b 1x10-2 Gy c 3x10-2 Gy d 5x10-2 Gy 1x10 4 b a 0 (a) Temperature (K) TL Intensity (a.u.) 1.0x10 5 c 8.0x10 4 b 6.0x x10 4 MgB 4 : Dy irradiated to gamma dose of 1x10-1 Gy 5x10-1 Gy 1x10 0 Gy 2.0x10 4 a 0.0 (b) Temperature (K) 1.6x10 9 c 1.4x x10 9 MgB 4 : Dy irradiated to gamma dose of a 5x10 2 Gy b 1x10 3 Gy c 5x10 3 Gy TL Intensity (a.u.) 1.0x x x x10 8 b a 2.0x (c) Figure 4.6: Temperature (K) Effect of gamma doses ranging (a) 5x10-3 to 5x10-2 Gy (b) 1x10-1 to 1x10 0 Gy and (c) 5x10 2 to 5x10 3 Gy on TL glow curves of synthesized MgB 4 :Dy nanophosphor. 93

11 10 5 Gamma irradiated MgB 4 :Dy TL Response (a.u.) Dose (Gy) (a) 10 9 Gamma irradiated MgB 4 :Dy 10 8 TL Response (a.u.) Dose (Gy) (b) Figure 4.7: TL response of MgB 4 :Dy nanophosphor for (a) low γ-doses (b) high γ-doses. 94

12 synthesized MgB 4 : Dy Peak Intensity (relative units %) Storage time (days) Figure 4.8: Fading of synthesized nanocrystalline MgB 4 :Dy nanophosphor irradiated to gamma rays TL Intensity (a.u.) 1.6x x x x x x x10 8 a c b d a Experimental curve b glow fit curve c Peak 1 d Peak 2 e Peak 3 f Peak 4 g Peak 5 h Peak 6 2.0x10 8 e f g h Temperature (K) Figure 4.9: Deconvulation of TL glow curve of MgB 4 :Dy exposed to 5x10 3 Gy of γ-dose. 95

13 Table 4.1: Values of trap depth (E) and frequency factor (s) for isolated peaks calculated by GCD glow fit method S. No PEAK T m (K) E (ev) s (s -1 ) 1. Peak x Peak x Peak x Peak x Peak x Peak x10 15 Table 4.2: Values of trap depth (E) and order of kinetics for isolated peaks calculated by Peak shape method S. No PEAK T m (K) E (ev) μ g 1. Peak Peak Peak Peak Peak Peak

14 4.3.5 TL properties of MgB 4 : Dy irradiated with Proton (150 MeV) beam The TL properties of synthesized MgB 4 : Dy irradiated with proton beam of energy 3 MeV and 150 MeV have been studied. TL properties examined in this study include TL glow curves, TL response curves and fading.these results may be helpful in the development of tissue equivalent TL nanocrystalline detectors best suited for wide range of proton beam exposures. TL glow curves of synthesized nanocrystalline MgB 4 :Dy, exposed to 150 MeV proton beam of different doses in the range 1x10 0-1x10 2 Gy are shown in Figure It reveals that for a dose of 1x10 0 Gy, the nanophosphor MgB 4 :Dy shows two peaks, prominent peak at 412 K and small peak at 509 K. The appearance of two peaks at higher doses in the glow curve indicates that there are possibly two kinds of trapping sites, one which is shallower leading to the peak at lower temperature and the other which is deeper leading to the peak at higher temperature (Lochab et al., 2007). With increase in dose to 1x10 1 Gy, the peak intensity of both the peaks increases. It is also found that with increase in dose to 3x10 1 Gy, the TL intensity keeps on increasing and the main peak is shifted to 420 K and small peak to 518 K. On further increasing the dose to 6x10 1 Gy, the TL intensity still keeps on increasing, the prominent peak is observed at 412 K and a small peak is observed at 513 K. Finally when the dose is increased to 1x10 2 Gy, the intensities of both the peaks keeps on increasing. TL response of MgB 4 :Dy exposed to proton beam (150 MeV) of doses in the range 5x10 0 Gy to 1x10 2 Gy are shown in Figure It has been found that MgB 4 :Dy nano phosphor irradiated with 150 MeV proton beam exhibits a linear response in the range 5x10 0 Gy to 1x10 2 Gy. This shows that synthesized nanophosphor MgB 4 :Dy can be used as a TL dosimeter within a range 5x10 0 to 1x10 2 Gy of 150 MeV proton beam. The results of fading of synthesized nanocrystalline MgB 4 : Dy irradiated to 150 MeV proton beam of dose 6x10 1 Gy are presented in Figure It reveals that the fading on third day is 4.8%, on 7 th day it is 6.6% and on 15 th day it is 7.6%, whereas in one month the total fading recorded is 8.2%. The maximum fading recorded in our system is seen in first week after exposure of the samples. 97

15 4.3.6 TL properties of MgB 4 : Dy irradiated with Proton (3 MeV) beam The TL characterstics of nanocrystalline MgB 4 : Dy samples exposed to 3 MeV proton beam have been studied. The TL properties examined in this study include glow curves, TL response and fading. These results may be helpful in the development of tissue equivalent TL nanocrystalline detectors best suited for wide range of proton beam exposures. TL glow curves of synthesized nanocrystalline MgB 4 :Dy, exposed to 3 MeV proton beam of different fluences in the wide range (1x x10 15 ions/cm 2 ) are shown in Figure It reveals that for a fluence of 1x10 11 ions/cm 2, the nanophosphor MgB 4 :Dy shows two peaks, prominent peak at 422 K and small peak at 526 K. With increase in fluence to 1x10 12 ions/cm 2, the peak at high temperature becomes the prominent peak and that of lower temperature becomes the small peak. When the fluence was increased from 1x10 12 to 1x10 13 ions/cm 2, the TL intensities of both the peaks keeps on increasing, small peak is shifted to 420 K and prominent peak is shifted to 521 K. With further increase in fluence to 1x10 14 ions/cm 2, the peak intensities of both the peaks decreases drastically. Finally at a fluence of 1x10 15 ions/cm 2, the peak intensities of both the peaks further decreases and both the peaks (at 426 K and 524 K) are found to have almost same intensities. This may be attributed to the fact that on irradiation to high fluence of radiation, the population of trapping/ luminescent centres (TC/LC) got changed, which reflected on the occurrence of different intensities for the TL glow peaks. TL response of MgB 4 :Dy exposed to proton beam of energy 3 MeV for higher fluences in the range of 1x10 11 to 1x10 15 ions/cm 2 is shown in Figure It has been found that MgB 4 : Dy nano phosphor irradiated to 3 MeV proton beam exhibits a linear response in the range 1x10 11 ions/cm 2 to 1x10 13 ions/cm 2. With further increase in fluence to 1x10 15 ions/cm 2, sublinear behaviour has been noticed. This shows that synthesized nanophosphor can be used as a TL dosimeter within a range of fluence 1x10 11 to 1x10 13 ions/cm 2. In order to determine the fading characteristics of MgB 4 : Dy, several samples were irradiated to 3 MeV proton beam of fluence 1x10 13 ions/cm 2 and were stored in dark conditions at room temperature. The results of fading of synthesized nanocrystalline MgB 4 : Dy have been presented in Figure It reveals that in case of MgB 4 :Dy samples, the fading on third day is 3.2%, on 7 th day it is 5.2% 98

16 and on 15 th day it is 6.8%, whereas in one month the total fading recorded is 7%. The maximum fading recorded in our system is seen in first week after exposure of the samples TL properties of MgB 4 : Dy irradiated with electron (4 MeV) beam The TL properties of synthesized MgB 4 : Dy irradiated with electron beam of energy 4 MeV and 9 MeV have been studied. TL properties examined in this study includes TL glow curves, TL response and fading.these results may be helpful in the development of tissue equivalent TL nanocrystalline detectors best suited for electron beam exposures. TL glow curves of synthesized nanocrystalline MgB 4 :Dy exposed to 4 MeV electron beam of different doses in the range 1x x10 1 Gy have been shown in Figure It reveals that for a dose of 1x10-1 Gy, the nanophosphor MgB 4 :Dy shows two peaks, the prominent peak at 579 K and small peak at 432 K.With increase in dose to 2x10 0 Gy, the prominent peak at higher temperature is shifted to 551 K, small peak to 430 K and TL intensities for both the peaks increases. Further with increase in dose to 1x10 1 Gy, the prominent peak is shifted to 529 K and small peak to 426 K and their TL intensity keeps on increasing. TL response of MgB 4 :Dy, exposed to electron beam (4 MeV) of doses in the range of 1x10-1 to 1x10 1 Gy are shown in Figure It has been found that MgB 4 :Dy nano phosphor irradiated to 4 MeV electron beam exhibits a linear response in the range 1x10-1 Gy to 5x10 0 Gy. With further increase in dose to 5x10 0 Gy, the nanophosphor shows a supralinear behaviour. This shows that synthesized nanophosphor MgB 4 : Dy can be used as a TL dosimeter within a range of 1x10-1 to 5x10 0 Gy of electron beam (4 MeV). The results of fading of synthesized nanocrystalline MgB 4 : Dy irradiated to 4 MeV electron beam of dose 5x10 0 Gy are presented in Figure It reveals that the fading on third day is 3.4%, on 7 th day it is 5.2% and on 15 th day it is 7%, whereas in one month the total fading recorded is 7.3%. The maximum fading recorded in our system is seen in first week after exposure of the samples. 99

17 4.3.8 TL properties of MgB 4 : Dy irradiated with electron (9 MeV) beam The TL characterstics of nanocrystalline MgB 4 :Dy samples exposed to 9 MeV electron beam have been studied. The TL properties of synthesized MgB 4 : Dy examined in this study include glow curves, TL response and fading. TL glow curves of synthesized nanocrystalline MgB 4 : Dy exposed to 9 MeV electron beam of different doses in the range 1x x10 1 Gy are shown in Figure It reveals that for a dose of 1x10-1 Gy, the nanophosphor MgB 4 : Dy shows two peaks, prominent peak at 578 K and small peak at 445 K. With increase in dose to 2x10 0 Gy, the prominent peak at higher temperature is shifted to 560 K, small peak to 433 K and TL intensities for both the peaks increases. Further with increase in dose to 1x10 1 Gy, the prominent peak is shifted to 538 K and small peak to 430 K and their TL intensity keeps on increasing. TL response of MgB 4 :Dy exposed to electron beam (9 MeV) of doses in the range 1x10-2 to 1x10 1 Gy are shown in Figure It has been found that MgB 4 : Dy nano phosphor irradiated to 9 MeV electron beam exhibits a linear response in the range 1x10-2 Gy to 2x10 0 Gy. With further increase in dose from 5x10 0 Gy to 1x10 1 Gy, the nanophosphor shows a supralinear behaviour. This shows that synthesized nanophosphor MgB 4 : Dy can be used as a TL dosimeter within a range of 1x10-2 to 2x10 0 Gy of 9 MeV electron beam. The results of fading of synthesized nanocrystalline MgB 4 : Dy, irradiated to 9 MeV electron beam of dose 5x10 0 Gy are presented in Figure It reveals that the fading on third day is 4 %, on 7 th day it is 5 % and on 15 th day it is 6 %, whereas in one month the total fading recorded is 7%. The maximum fading recorded in our system is seen in first week after exposure of the samples. 100

18 TL Intensity (a.u.) 3.4x x10 6 f 3.0x x x x x x x x x x x x x x x e c d b a MgB 4 : Dy irradiated to proton (150 MeV) of dose a 1x10 0 Gy b 5x10 0 Gy c 1x10 1 Gy d 3x10 1 Gy e 6x10 1 Gy f 1x10 2 Gy Temperature (K) Figure 4.10: TL glow curves of MgB 4 :Dy irradiated to proton (150 MeV) beam MgB 4 : Dy irradiated to proton (150 MeV) beam TL Response (a.u.) Dose (Gy) Figure 4.11: TL response of MgB 4 :Dy irradiated to proton (150 MeV) beam 101

19 Fading of MgB 4 : Dy irradiated to proton (150 MeV) beam Peak Intensity (relative units %) Storage time (Days) Figure 4.12: Fading of synthesized nanocrystalline MgB 4 :Dy irradiated to proton (150 MeV) beam TL Intensity (a.u.) 8x10 7 7x10 7 6x10 7 5x10 7 4x10 7 3x10 7 2x10 7 1x10 7 c d b a e MgB 4 : Dy irradiated to proton (3 MeV) of fluence a 1x10 11 ion/cm 2 b 1x10 12 ion/cm 2 c 1x10 13 ion/cm 2 d 1x10 14 ion/cm 2 e 1x10 15 ion/cm Temperature (K) Figure 4.13: TL glow curves of MgB 4 :Dy irradiated to proton (3 MeV) beam. 102

20 10 6 MgB 4 :Dy irradiated to proton (3 MeV) beam TL Response (a.u.) fluence (ion/cm 2 ) Figure 4.14: TL response of MgB 4 : Dy irradiated to proton (3 MeV) beam Fading of MgB 4 : Dy irradiated to proton (3 MeV) beam Peak Intensity (relative units %) Storage time (Days) Figure 4.15: Fading of synthesized nanocrystalline MgB 4 :Dy irradiated to proton (3 MeV) beam 103

21 TL Intensity (a.u.) 4.0x x x x x x10 5 e MgB 4 : Dy exposed to electron (4 MeV) of dose a 1x10-1 Gy b 5x10-1 Gy c 2x10 0 Gy d 5x10 0 Gy e 1x10 1 Gy 1.0x10 5 d 5.0x c b a Temperature (K) Figure 4.16: TL glow curves of MgB 4 :Dy irradiated to electron (4 MeV) beam MgB 4 : Dy irradiated to electron (4 MeV) beam TL Response (a.u.) Dose (Gy) Figure 4.17: TL response of MgB 4 :Dy irradiated to electron (4 MeV) beam 104

22 200 Fading of MgB 4 : Dy irradiated to electron (4 MeV) beam 180 Peak Intensity (relative units %) Storage time (Days) Figure 4.18: Fading of synthesized nanocrystalline MgB 4 :Dy irradiated to electron (4 MeV) beam. 2.5x10 5 TL Intensity (a.u.) 2.0x x x10 5 MgB 4 : Dy exposed to electron (9 MeV) of dose a 1x10-1 Gy b 5x10-1 Gy c 2x10 0 Gy d 5x10 0 Gy e 1x10 1 Gy d e 5.0x c b a Temperature (K) Figure 4.19: TL glow curves of MgB 4 :Dy irradiated to electron (9 MeV) beam. 105

23 10 3 MgB 4 : Dy irradiated to electron (9 MeV) beam TL Intensity (a.u.) Dose (Gy) Figure 4.20: TL response of MgB 4 : Dy irradiated to electron (9 MeV) beam Fading of MgB 4 : Dy irradiated to electron (9 MeV) beam Peak Intensity (relative units %) Storage time (Days) Figure 4.21: Fading of synthesized nanocrystalline MgB 4 : Dy irradiated to electron (9 MeV) beam. 106

24 4.4 CONCLUSIONS Nanocrystalline MgB 4 doped with Dy has been prepared by combustion method. It is concluded from the results that the TL response of MgB 4 :Dy doped with 1000 ppm Dy is found to be maximum when irradiated to gamma dose of 1x10 3 Gy. MgB 4 :Dy exhibits a linear response in the gamma dose range 4x10-4 Gy to 4.5x10-2 Gy and 1x10 0 Gy to 5x10 3 Gy. Finally the trapping parameters associated with the glow peaks calculated using glow curve deconvolution (GCD) glow fit and peak shape method were found to show good agreement. The fading was found to be around 8 %. It can be concluded that the easy and inexpensive method of preparation, good sensitivity, simple glow curve structure, linear response over a wide range of exposure, low fading and excellent reproducibility are some of the good characteristics of MgB 4 :Dy nanophosphor, making it useful for its application in radiation dosimetry of high dose measurements of gamma radiations. Nanocrystalline MgB 4 : Dy shows a linear response in the range 1x10 11 ions/cm 2 to 1x10 13 ions/cm 2 for 3 MeV proton beam. Since the energy is very low, so this material can be useful for their applications in radiation dosimetry for the treatment of very superficial skin cancer using proton (3 MeV) beam. Moreover MgB 4 :Dy shows a linear response in the range 5x10 0 Gy to 1x10 2 Gy for 150 MeV proton beam. Since the energy is high, so MgB 4 : Dy can be used as a radiation dosimeter for the treatment of deep tumors using proton (150 MeV) beam. Nanocrystalline MgB 4 :Dy shows a linear TL response in the range 1x10-1 to 5x10 0 Gy for 4 MeV and 1x10-2 to 2x10 0 Gy for 9 MeV electron beam. So this material can be used as a radiation dosimeter for the treatment of skin cancer at different depths using electron beam. 107