Synthesis of MgB4O7:Dy3+and Thermoluminescent Characteristics at Low Doses of Beta Radiation

The synthesis and thermoluminescent characteristics of dysprosium-doped MgB4O7 are analyzed. The phosphor at different concentrations (0, 0.1, 0.5, 1, 2 and 4 mol%) of the dopant was prepared by the solution-assisted method. The magnesium borate compound was confirmed by X-ray diffraction. The annealing and dopant concentrations effects on the crystalline matrix were investigated. The highest thermoluminescent sensitivity was found with 450°C of annealing temperature and at high Dy3+ concentration too. The un-doped MgB4O7 phosphor shows a broad glow curve which peaked at 199°C and about 306 °C. Introducing Dy3+ dopant in the matrix that behavior was strongly changed. The wide glow curve shows three glow peaks; two small shoulders at 124 and 195 °C, and a highest peak between 323 and 336 °C temperature range. A large linear dose-response (5 – 2000 mGy) beta dose was obtained. The complex glow curves were deconvolved and the kinetics parameters were determined considering the general order kinetics model.


Introduction
Alkali earth ions such as Li, Zn, Sr, Ca, and Mg based on tetraborate compound have been considered as important luminescent materials because of their excellent thermal and chemical stability, simple synthesis and cheap reagent material [1][2][3][4][5][6][7][8]. In particular, the MgB 4 O 7 phosphor is attractive due to its low effective atomic number (Z eff = 8.4) close to that of soft biological tissue (Z eff = 7.42) and less value than that of the compact bone (Z eff = 13.59), which implies a small photon energy dependence [9][10]. However, the undoped magnesium borate has a low thermoluminescent (TL) sensitivity. Due to this disadvantage, MgB 4 O 7 has been doped with different trivalent ions mainly of rare earths such as Gd, Ce, Tm, Nd, Dy, Eu, Ho [11][12][13][14][15][16], since it is known that the addition of small amounts of activators with another charge number of valence [9,[17][18] may increase the concentration of imperfections that are important factors improving the TL property of the host matrix. Special attention has been given to the TL study of Dy 3+ doped MgB 4 O 7 [5,18]. From the first studies of this phosphor material, carried out by Hitomi et al. [7,19], considerable effort has been devoted to improves its TL properties. In different works [5,10,15,[20][21], it has been reported that MgB 4 O 7 : Dy 3+ in addition being a material closely equivalent to the soft biological tissue presents a high sensitivity, a single thermoluminescent peak at about 200 °C, with linearity in the dose-response curve over a wide range of doses, good reproducibility and low fading of the TL signal. Currently, the works related to MgB 4 O 7 :Dy 3+ focused on evaluating the optimal conditions of a highly sensitive materials [22], also have been leaning to the determination of the influence of their structural characteristics [23]. In this sense, knowing that the TL characteristics of the magnesium borate material are determined by its preparation method [5,17,[23][24], in this work the TL characteristics of MgB 4 O 7 :Dy 3+ were obtained by the, scarcely known, solution-assisted method.  3 (99.9%), from Sigma-Aldrich brand. The reagents, in suitable stoichiometric amounts, were dissolved and stirred in the deionized water. Subsequently, the solvent was evaporated and the resulting solid was placed in an alumina crucible which was sintered at 800°C for 2 h. The molten mass was then cooled to room temperature, crushed and sieved. Finally, a series of dysprosium doped MgB 4 O 7 samples batches was considered for all measurements.

Experimental Procedure
Formation of the crystalline compound in powder form was determined by analysis the X-ray diffraction.
The diffractogram was carried out in a Bruker Phaser D2 diffractometer and considering the range from 10° to 55° in the Bragg angle (2θ), using an angular rate counting of 0.02°/s. This apparatus was equipped with a Ni filter and Cu-anode X-ray tube operating with 30 kV and 10 mA. The samples were attached on a Bruker AXS Si-Einkristalle sample holder. Thermoluminescence of the phosphor samples were measured on double aliquots and by using a Harshaw model 3500 TLD reader. Each sample was weighted with 8.0 ± 0.1 mg. The thermoluminescent glow curves were obtained at constant heating rate of 2°C/s, from room temperature up to 450 °C, and under nitrogen atmosphere to avoid the spurious termoluminescent signals. The samples were irradiated with β radiation from a 90 Sr/ 90 Y source which provided 0.1099 mGy/s dose rate. Figure 1 shows the X-ray diffraction (XRD) patterns of our host material matrix and from MgB 4 O 7 (PDF-00-031-0787) standard. The adjustment between the experimental signals and the diffraction lines from MgB 4 O 7 standard makes evident that the crystalline phase of MgB 4 O 7 was obtained This phase corresponding to an orthorhombic crystalline system with lattice parameters of a 0 = 8.596 Å, b 0 = 13.729 Å, c 0 = 7.956 Å and the space group Pbca. The diffractograms of the compounds doped with Dy 3 + revealed the crystalline matrix formation and the absence of signals associated to second phase of dysprosium. of un-doped MgB 4 O 7 presents two peaks at 199 and 306 °C which can be related to the intrinsic TL emission from the lattice of this compound. However, the thermoluminescence from dysprosium doped MgB 4 O 7 with different impurity concentration presents a complex glow curves consisting by a broad glow peak with maximum temperature (T m ) located between 323 and 336 °C, and two other peaks of lower intensity about 124 and 195 °C widely overlapped to each other. These changes imply that, when the Dy 3+ is incorporated into the matrix, it originates or improves the activation of other types of traps that participate in the thermoluminescent phenomenon. On the other hand, for 2.0 and 4.0 mol% of dysprosium concentration (as can be seen in Figure 3), the TL peak of higher intensity (323 -336 °C) was strongly increased.  The TL intensity was growth as the concentration increases making evident the role of the Dy 3+ as good activator ion in the magnesium borate optimizing the TL property. However, the fast intensity growth occurs when the impurity concentration increases from 1.0 to 2.0 mol%.

Dopant Concentration Effects on TL
It is important to mention that at higher concentration of dysprosium (4.0 mol%) a TL intensity reduction was not observed, contrary to finding by other authors in MgB 4 O 7 : Dy nanophosphor [23]. While at higher concentration (4 mol%) of dysprosium in our MgB 4 O 7 samples without annealing temperature, the TL intensity was very slowly increased respect to 2.0 mol% of Dy 3+ . In that case, may be the dysprosium impurity aggregation occurs in the lattice. That observation also is according to other authors for dysprosium doped borate nanophosphor [23], due to the different ionic radii of the magnesium borate ions (B 2+ , Mg 2+ and Dy 3+ ), so the ion impurity due of its larger size cannot replace either of them in the phosphor matrix. In consequence the dysprosium ion at higher concentration, 4.0 mol% in that case without annealing treatment, it can be incorporated as interstitial ions causing their aggregation in the structure matrix. Due to the higher TL intensity of MgB 4 O 7 :Dy 3+ (4.0 mol%), it was considered for the TL response, i.e., it is an integral under the glow curves. Figure 4 shows the linear TL dose-response in the range from 5 to 2000 mGy. A superlinear stage also was observed, this behavior may be ascribed to the low TL sensitivity of this magnesium borate at low beta doses. However, several types of traps in competition may be are included in the recombination process during the TL signal emission producing the superlinear and linear stages, in these cases a deep analysis should be carry out with more details [26][27]. The superlinear stage with a lower growth rate than that of the linear region occurred.

Analysis of the Complex Glow Curves by Deconvolution
The structure of the complex glow curves from MgB 4 O 7 :Dy(4.0 mol%) was analyzed by the computerized glow curve deconvolution program (CGCD) considering the general order kinetics (GOK) model [24]. The samples were exposed with 1000 mGy beta dose filling the traps levels in the band gap and very clear glow curves using 2 °C/s heating rate were obtained. The samples were readout immediately after irradiation. The glow curves obtained by fading at the end of one day (24 h) were considered in order to analyze their structure. Figure 5. (a) shows the experimental glow curve deconvolved by four glow peaks. The activation energy (E) values (0.59 -1.16 eV) increased as the maximum temperature (T m ) increases (195-462°C) ( Table 1). While the first peak (195 °C) was fast decay their TL intensity and shifted to 188 °C in the glow curve after one day ( Figure 5.b), and slightly diminish for the second and fourth peaks at 270 and 466 °C, respectively, was observed ( Table 1). The main glow peak at 333 °C practically remains at the same temperature (334 °C) after one day of fading. Finally, T m shows variation about 1 to 7 °C that may be ascribed to the retrapping charges in the distribution levels. This fading effect on the structure of the glow curve seems reflects in the kinetic order (b) values which were close to the second order. It was observed that the main glow peak, experimental and deconvolved, was located at about 333 °C and in consequences it can be considered as a good prospect for dosimetric peak if the synthesis of this magnesium borate is carried out by solution-assisted method. The TL phenomenon in this case, were well described by the general order kinetics (GOK) model, because the Figure of Merit (FOM) for the glow curves was less than 1.2 %.

Conclusions
Dysprosium doped MgB 4 O 7 and un-doped crystalline phosphor were synthetized using the solution-assisted method. The formation of the crystalline matrix was determined by means of X-ray diffraction. The highest thermoluminescent sensitivity was obtained with annealing at 450°C. The TL intensity of magnesium tetraborate was improved with 4 mol% of Dy 3+ dopant. The introduction of Dy 3+ with varying concentrations in the matrix strong activates the TL phenomenon and changed the structure of the glow curves of the MgB 4 O 7 . It seems that the experimental TL glow curves was forming by three overlapped peaks at about 124, 195 and about 336 °C. The complex glow curves were acceptable deconvolved by four peaks, with FOM less than 1.2%, indicating that the general order kinetics model was appropriate to well described the thermoluminescence of this phosphor. MgB 4 O 7 :Dy 3+ compound presents an interesting and useful linear dose-response for measuring low β-radiation doses, which can be useful in radiation therapy doses.