Thermoluminescence study of aluminium oxide doped germanium prepared by combustion synthesis method

The present paper reports the optimum concentration of germanium (Ge) dopant in aluminium oxide (Al2O3) samples prepared by combustion synthesis (CS) method for thermoluminescence (TL) studies. The samples were prepared at various Ge concentration i.e. 1 to 5% mol. The phase formation of un-doped and Ge-doped Al2O3 samples was determined using X-ray Diffraction (XRD). The sharp peaks present in the XRD pattern confirms the crystallinity of the samples. The samples were then exposed to 50 Gy Cobalt-60 sources (Gamma cell 220). TL glow curves were measured and recorded using a Harshaw Model 3500 TLD reader. Comparison of TL peaks were observed to obtain the best composition of Ge dopants. A simple glow curves TL peak at around 175°C for all composition samples was observed. It was also found that the composition of aluminium oxide doped with 3.0% of germanium exhibits the highest thermoluminescence (TL) intensity which is 349747.04 (a.u).


Introduction
The application of thermoluminescence (TL) dosimetry in radiation protection has grown steadily due to the worldwide progress of the development of solid thermoluminescent dosimeter (TLD).Various kinds of thermoluminescence (TL) dosimeters have been used in radiation dosimetry.One of the materials that can be considered for TLD was aluminium oxide.This is due to aluminium oxide (α-Al2O3) shows an effective number close to that of bone (Zeff = 11.28), it becomes a better choice to be used in medical and environmental dosimetry (Bos, 2001).The TL properties of Al2O3 samples of different dopants materials which can enhance TL properties have been studied by several researches for nearly 50 years [1,2] The materials were generally synthesized by sol-gel method [3], ion beam implantation [4], solvent evaporation [5] and combustion synthesis (CS) [6,7,8].Among this method, combustion synthesis (CS) is an effective and low-cost method for production of various industrially useful materials [9,10].The solution preparation only requires low processing temperatures and short reaction times (̰ ~seconds) making it a quick and easy process [11,12,13].This process is used directly in the production of high purity and homogeneous ceramic oxide powders.The combustion process is due to an exothermic, redox reaction between nitrate ions and fuel.The large volume of gases released during the reaction promotes a rapid disintegration of the precursor gel, yielding the desired nanocrystalline materials [14].
The presence of impurities in a material is important to the thermoluminescence process.Recently, to have a better performance in the dosimetric purpose, Al2O3 is always doped with impurities that induce many different types of trapping centers at which charged particles produce ionizing radiation [15].Since then, there are a lot of efforts have been directed towards the improvements on its sensitivity via various dopants such as Si, Ti, [16], Mg and Y [17], Cr and Ni [18].Study by Ravichandran., (2014) [19] of undoped and (2-10mol%) Al doped ZrO2 nano powder were synthesized using solution combustion method having glycine as a fuel which is pre-heated at approximately 500C.However, the study only concerned on the optical properties of the samples and not on the thermoluminescence properties.Barros et al, (2008), reported the highest sensitivity were found for Al2O3 samples doped with 5% Tb, 5% Eu, and 5% Si.The samples were exposed to 1.5Gy Co-60 gamma rays.It stated that the TL response were 5000 times higher than the un-doped Al2O3.They also reported that a well defined glow curve peaks was observed at approximately 200°C which is well suited for radiation dosimetry.
However, thermolumiescence properties of Ge doped Al2O3 prepared by CS method have not been reported so far.Recently, interest of Ge as doped for dosimetric purpose has increased based from previous study Ge doped optical fibre as a thermoluminescence dosimeter in radiotherapy is a promising material because it gives good response and full fill the criteria of thermoluminescence properties [20].However, the preparation of optical fibre is not simple as CS technique [21].Hence, the aim of this present study is to prepare undoped and germanium doped aluminium oxide (Al2O3: Ge) nanocryatalline powder synthesis by CS method at various concentrations (1-5mol%).The optimum Ge concentration is chosen for TL study.

Materials and methods
Al2O3 doped with Germanium was synthesized using the combustion synthesis method.The samples were prepared using aluminium nitrate (Al (NO3)3.9H2O)as an oxidizer and urea (CH4N2O, 98%) as a fuel, by Sigma Aldrich and Germanium dopant were added with concentration between 1 to 5 mol %.Urea is suitable fuels because they are amino acid that can act as complexings agent of the metal ion in the solution and also serves as fuel to the synthesis nanocrystalline metal oxides.The starting materials taken by mixing 15 g of aluminium nitrate, Al(NO3)3, 6 g of urea, CO(NH2)2 and different weight dopant concentration from 1% to 5%.Then, 20 mL distilled water was added to the samples and stirred for 15 minutes using magnetic stirrer in order to obtain a homogeneous solution for completely dissolved.Then the solution was poured into alumina crucible and transferred to muffle-furnace pre-heated at 500°C.The sample ignited spontaneously with few seconds and produced white foamy product.The sample were then taken out from the mufflefurnace and allowed to cool down before grinding using mortar.X-Ray diffractometer (XRD) was used to identify the crystalline phase of the samples.Before the samples were EPJ Web of Conferences 156, 00011 (2017) exposed to gamma radiation pre-annealing process were carried out.This procedure was done to remove any effects due to any unwanted previous irradiation without damaging the material.The powders were annealed at 400°C for 1h followed by cooling process at 80°C for 16 h [22].The prepared samples were then irradiated at 50 Gy sources of Cobalt-60 gamma Cell Model 220 Excel at Universiti Kebangsaan Malaysia, Bangi, Malaysia.The TL response from the glow curve was measured using a TLD reader Harshaw Model 3500 with optical detection of light with wavelength between 400 and 430 nm.

X-Ray Analysis (XRD)
Figure 1 represents the XRD pattern of the as synthesized Al2O3 and Al2O3 doped with germanium at 3% mol concentration.XRD was done to confirm the phase of the sample either amorphous or crystalline.The diffraction pattern depicted intense peaks thereby, gives a strong indication that the prepared sample has high degree of crystallinity.The peaks found match with the International Centre for Diffraction Data (ICDD) which are Al2O3 (01-070-5679) and Al2O3: Ge (01-088-2013).Pure Al2O3 sample was observed and no other impurity phase was detected.It is to be noted that small amounts of dopants do not produce significant effect on the basic crystal structure of Al2O3.The addition of dopant will not change the unit cell parameters must not change [23].

TL intensity
Figure 2 represents the TL intensity of Al2O3: Ge with different concentration (1 to 5mol%).In the same Figure, the standard deviations are also plotted against the doped concentration.The TL intensity of the sample was found to vary with increasing Ge concentration.The TL intensity increase from 1 mol% Ge to maximum intensity at 3mol% Ge concentration 349747.04(a.u).Then the intensity decreases with further increase of Ge concentration.This may be due beyond 3 mol% concentration, quenching is bound to occur hence the optimum concentration for Ge only suite for this concentration.Highest TL intensity at 3 mol% Ge is attributed to the recombination of the excited electrons from the valences band with defects produced via irradiation [24].As a conclusion, it is found that the optimum concentration value of Ge dopants is at 3 mol% with very low standard error.
Fig1.XRD pattern of (a) Al2O3 doped 3 mol % Ge samples and (b) un-doped Al2O3  4, it is observed that the intensity of TL peaks increases with the increase in Ge composition where the highest peak found is for 3 mol% Ge concentration.The result is similar with the work reported by Barros et al. (2008) for Al2O3 prepared by CS method as mention before.The results also agree with the work done for Al2O3 doped with Tb 3+ and Tm 3+ where the Tl glow curve peak was observed at 220°C with a linear response in the dose range of 0.1-1 Gy prepared by same method [25].Study by J.V.Soares, 2014, [26] showed an intense peak at ~190°C and two other with low intensity at 290°C and 350°C.Hence, based from this of the presence of germanium dopants in Al2O3 sample enhanced the TL response and this will full fill the major tools to decide on the TL dosimetry potential of the material.The glow curves are particularly important since they are the main indicators of whether a material can be used for TL dosimeter purposes or not.According from Pekpak, 2010, [27] to obtain a good glow curve, a single peak was normally found at approximately 200°C temperature.Peak at low temperature (approximately of 100°C) at below will normally fade quickly and could not yield any information about radiation content.

Conclusion
From the results above it is possible to conclude that CS method is very suitable for the preparation of Al2O3 doped Ge for dosimetric applications.The technique is fast, low cost and produces well defined materials that can be used for dosimetric applications.The concentration analysis shows that the Ge concentration that optimizes the TL response is at 3.0 mol% which has highest TL intensity which is 349747.04(a.u).The crystallinity of the samples is proven by the XRD sharp peaks.Based from the TL intensity and TL glow curve figure, it can be concluded that the best Ge concentration as Al2O3 dopants is 3 mol% where it provides high TL intensity response with low standard error compare other concentration.This value can be used to further study the TL properties.

Fig2.Figure 3
Fig2.TL intensity and standard error of Al2O3: Ge with different concentration Fig3.TL glow curve of Al2O3: Ge at different mole concentration exposed to 50 Gy Cobalt-60