Dosimetric properties of potassium magnesium borate glass doped with copper

The present article deals with potassium magnesium borate glass activated with copper that was prepared by a melting quenching technique to study the feasibility of this glass as a radiation dosimeter. The prepared glasses were characterized through X-ray diffraction and FTIR spectroscopy and some of the glasses optical and physical parameters have been interpreted. The thermoluminescence properties of the prepared samples were studied, and the results revealed that the potassium magnesium borate doped with 0.07 wt% with copper has the highest thermoluminescent response. The glow curve of potassium magnesium borate doped with 0.07 wt% of copper has seven overlapping peaks with maximum temperatures of 383.09, 400.71, 413.26, 426.59, 445.98, 476.57 and 534.04 K, and the TL-response is linear from 0.11 to 165 Gy. From the present study, the potassium magnesium borate doped with 0.07 mol% from copper may be applicable for measuring the radiation dose.


Introduction
Nuclear and radiation technology has become one of the most used technologies in many fields, especially in medicine and radiation therapy, where precise dose measurement is needed. Currently, there are several types of radiation detectors available and the thermoluminescent dosimeter is the most important of them [1].
There is a wide range of TL materials in use, and the glassy system is among them. It is particularly appealing for radiation processing applications [2][3][4][5].
In recent years, the study of radiation-induced defect centers in glasses has been beneficial in evaluating the quality of glasses for radiation dosimetry applications.
One of the essential glass-forming materials is boron oxide. Because of its high transparency, low cost, low melting point, high sensitivity, great thermal stability as compared to other TL materials, as it can be present with two groups, tetragonal BO 4 and trigonal BO 3 in the glass sample, the network improves the ability to modify its coordination state from boroxol rings to triangles and tetrahedral BO 4 coordination's appearing short-range structure of triangles BO 3 and tetrahedral BO 4 groups [6][7][8][9].
Copper is one of the transition metal oxides (TMO) that can exhibit in different oxidation states Cu 2? , Cu ? , and Cu 0 , which can coexist. The other states of copper ions in the glass matrix led to the intense greenish and bluish color of the glass samples.
Copper is one of the best activators used to enhance the luminescence properties of borate dosimeters as copper can form luminescence centers, which are essential for increasing phosphor strength [18] also, copper has non-hygroscopic nature which increases the chemical stability of borate dosimeters [19].
Upon irradiation, numerous typical defects of borate could appear including boron E-center, nonbridging oxygen NBO's hole centers,which may be clarified as hole center HC, hole trapped on a single NBO, and HC2 hole trapped on two NBO hole center. [20] The main aim of the present study is to investigate the physical properties of potassium magnesium borate glass doped with copper, such as density, polaron radius, molar volume, average molecular weight, ion concentration, and TL properties, to use it as a high-dose radiation dosimeter.

Detailed measuring techniques and calculations for some physical parameter
The Shimadzu X-ray diffractometer was used to assess the condition of the produced glasses within a 2h angle range of 5°to 60°at 40 kV and 30 mA. Cu Ka radiation and a scan rate of 6°per minute were used to collect data, as well as an X-ray tube. A Bruker's VERTEX 70 FTIR spectrometer was utilized to investigate the spectra of the produced samples at room temperature in the range 4000-400 cm -1 , and the absorption spectra of the glasses were recorded. The density (D), molar volume (V m ), oxygen packing density (OPD), the ionic concentrations of the glass samples (N), Internuclear distance (r i ) and polaron of each glass sample were calculated [21,22]. The density was calculated according to the formula: q = w a /w a -w b 9 q o , where w a is the weight of the glass sample measured in air, w b is the weight when the glass is immersed in liquid, and q o is the density of azylene. The molar volume (V m ) was calculated according to the formula V m = M/q, where M is the total molecular weight of the multi-component sample. The optical UV-visible absorbance spectra were measured within the UV-visible range (200-1100 nm) using a recording double-beam spectrophotometer (Type JASCO Corp, v-570, Rel-100 Japan) by finely polished glass system.
The Lexsyg Smart TL/OSL luminescent available at the Nuclear Radiation Measurements lab in Department of Physics, Faculty of Science, Ain Shams University was used to measure the glow curve and other TLD-data for the study samples. Lex Studio 2.0 operating software, which runs on a personal computer linked to the reader, controls the reader. The reader is also linked to the nitrogen supply for cooling reasons.
The study samples were irradiated with betaparticles using a beta source of 90 S/ 90 Y, at a dose rate of 110 mGy/s built into the TLD-reader. The result glow curves after being measured were deconvoluted using El-Kinawy TL-software [23].

X-ray diffraction (XRD)
As shown in Fig. 1, XRD was carried out on the selected prepared glass sample. It has revealed that peaks of diffraction did not appear. In contrast, only broad bands appeared at around 2h = 25°for select samples (0, 1.2 mol% CuO), approving the noncrystalline nature of the produced glass.

FTIR spectroscopy
The FTIR analysis of the analyzed samples was optimized to obtain information on the structural units. In this study, it was observed that the vibrational modes of the borate network are mainly active in three infrared spectral regions from 1200 to 1500 cm -1 (B-O stretching of trigonal BO 3 units), from 800 to 1200 cm -1 (B-O stretching of tetrahedral BO 4 units), and from 600 to 800 cm -1 (bending vibrations of various borate segments) [7,24,25]. As shown in Fig. 2. The undoped glass reveals several absorption bands extending from the beginning of the measurements at 400 up to 2000 cm -1 . The main prominent and characteristic absorption bands usually observed in these three spectral regions are observed at 450, 530, 610, 830, 1070, 1110, 1240, 1380, and 1460 cm -1 . The spectral bands in the region of 420-470 cm -1 are assigned to specific vibrations of Na-O bonds [26]. The band around 530 cm -1 in the region of 500-550 cm -1 is assigned to the borate deformation modes [27]. The band located at 700 cm -1 is due to B-O-B bending in trigonal BO 3 units [28]. The absorption in the region 650-700 cm -1 is attributed to the bending vibrations of B-O-B in BO 3 [26]. The spectral band in the region of 780-1100 cm -1 is due to the B-O symmetric stretching of tetrahedral BO 4 units and vibrations of diborates bridging to pentaborate groups [29]. The spectral band in the region of 1200-1300 cm -1 is due to the B-O bond stretching vibrations and B-O bridging between B 3 O 6 and BO 3 triangles [30]. The band at 1100 cm -1 may be due to the stretching vibration of B-O bonds in BO 4 units from tri, tetra, and pentaborate groups [28]. The spectral band in the region 1200-1300 is attributed to anti-symmetrical stretching vibrations with three NBOs of B-O-B groups [31]. Figure 2 also reveals the FT-IR of samples containing progressive addition of CuO. The careful inspection of all the spectral curves before and after doping marked a slight change in the intensity of bands. Some bands of BO 4 groups shifted to a higher wavenumber, while the BO 3 groups were shifted to a lower wavenumber which is in agreement with other research [7,32,33]. At the same time, the absorption band becomes broader and accompanied by a shift to a longer wavelength. This may be due to the introduction of CuO transforming some of the BO 3 triangles to BO 4 tetrahedra or vice versa. So, authors used deconvolution method to estimate the fraction of N4 of BO 4 units in the borate network using the Peak Fit program (version 4.12). As shown in Fig. 3a and b and Table 1, for 0 and 1.2 mol% CuO glass sample, they stem from asymmetric stretching vibrational modes of orthoborate, pyroborate, and triangle BO 3 group, respectively. The deconvoluted data of this band also helped to calculate the relative areas of the absorption peaks related to BO 4 and BO 3 groups (N4 and N3, respectively) and the fraction of the four coordinated boron atoms (N4) from the infrared spectra. Besides, the relative area of the deconvoluted bands indicates the variation in N4 in the structure [28,34]. The tetrahedrally coordinated boron atoms fraction N4, as well as N3, can be calculated using the following formulas: where N3 and N4 represent the area of peaks corresponding to BO 3 and BO 4 groups. Figure 4 shows the relation between copper oxide concentration and N4 content, indicating that N4 in a glass is reduced as the copper oxide level rises. On the contrary, N3 increases with increasing the copper oxide content. It can be shown that increasing the copper oxide level significantly influences increasing NBOs. The negative charge on NBOs makes it easier for electrons to be excited at longer wavelengths [7,35,36]. The above arguments suggest that the N4 ratio is the most prominent factor in increasing density and decreasing molar volume.

Density and other parameters
The addition of copper oxide to the glass network may cause some changes in the structure. These changes include rearrangement of the atoms, coordination numbers of the constituents, and variations in the geometrical configuration of the glass that are directly affected by the density. The investigated glass density was changed with the increment in the content of CuO at the expense of the B 2 O 3 content, as shown in Fig. 5. The increase in density might be due to the substitution of lower atomic mass (B 2 O 3 = 69.62) with higher atomic mass (CuO = 79.547). It is possible that most of the Cu ions enter the glass samples network as a modifier, which consolidates their structure and raises their density [7,28].
In the current work, the molar volume and the density display opposite behavior, which is the normal trend [28,37].
The following relationship can expect the ion concentration of Cu as given in Table 2: According to the current study, Cu ion concentration values increase with increasing the Cu content. This increase is associated with reductions in the molar volume.
The relationship (4) will be used to calculate the interionic distance (r i ) between two Cu ions It was observed that as the concentration of CuO increased, the values of r i reduced, and this reduction was associated with a decrease in the molar volume. Figure 6 shows the UV-Visible optical absorption spectra of the investigated polished glassy samples at room temperature against the 300-1100 nm wavelength. The obtained results of undoped copper are neutral, while the samples doped with copper oxide have a bluish-green color with cumulative copper oxide, then the color changes to dark green. The green color is an indication that the copper has a Cu 2? divalent oxidation state. The glass samples have a broad visible band around 780 nm [38], which may be recognized as a d-d transition due to Cu 2? (d 9 ) ions in the octahedral field, owing to the 2 B 1g ? 2 B 2g octahedral transition of Cu 2 ? ions [28]. The peak position is shifted toward higher wavelengths due to the ligand field around the Cu 2? ion. An electronic hole in the degenerated orbital disturbs the Cu 2? ions, causing the tetragonal distortion. The E g optical bandgap values with the concentration of Cu 2? are listed in Table 2.  [42].

Optical absorption spectroscopy
The molar refraction R m is styled by the following Lorentz-Lorenz equation [43]: R m is the molar refraction, E g is the energy bandgap, and V m is the molar volume.
The findings showed that adding CuO reduced the optical basicity, optical bandgap, and electronegativity while increasing the refractive index. The metallization criterion has been estimated as M ¼ 1 À R m =Vm. Table 2 revealed that the metallization value decreased by adding Cu 2? [21,36].

Thermoluminescent properties
The TL-intensity and glow curve of potassium magnesium borate doped with 0, 0.02, 0.07, 0.1, 0.2, 0.5, 0.7, 1, and 1.2 mol% concentrations of copper after being exposed to b-dose 55 Gy have been investigated, and the results are displayed in Fig. 8a and b.
This result showed that the doping of potassium magnesium borate with copper significantly increased the TL-intensity compared to undoped potassium magnesium borate. The resulted increase in the TL-intensity is dependent on the mol % of copper.
On the other side, the potassium magnesium borate doped with different mol% of copper has the  The presence of one broad peak in the glow curve of potassium magnesium borate doped with copper means that this peak may be composed of more than one glow peak overlapping with each other. Each peak is specific to one trap in the crystal lattice. The positions of these traps in the crystal lattice of prepared glass are very close to each other. So, when the prepared glass was heated during reading, these peaks overlapped and appeared as one peak.
The highest TL-intensity was found in potassium magnesium borate doped with 0.07 mol% of copper. As a result, this specific concentration will be mainly discussed in the following section.

Kinetic parameters
Calculating the kinetic parameters of the electron traps inside the material is necessary to understand its behavior in recording the electrical charges and its  ability to maintain this charge. However, to do this, the number of peaks that make up the curve should first be determined; if the glow curve has overlapping peaks, they must be separated first.

T m -T stop method
The T m -T stop technique [44] was used to determine the predicted number of the glow peaks in the glow curve of potassium magnesium borate doped with 0.07 mol% of copper. For this purpose, we performed the following: • The samples were irradiated at a dose of 132 Gy of beta particles. • The glow curve was recorded immediately after irradiation from room temperature to a temperature called T stop (stopping temperature) at a linear heating rate of 5 K/s. • The samples were cooled to room temperature and reheated to record the whole glow curve up to 723 K with the same linear heating rate. • The temperature corresponding to the first maximum intensity in obtained glow curve (T m ) was determined. • The process of heating to a T stop and cooling to room temperature, and then reheating to record the whole glow curve was repeated for different values of T stop ranging from 328 to 648 K, with an increment of 5 K. • The corresponding T stop and T m were determined at each heating cycle, and a graph between various values of T m against T stop was obtained. The results are shown in Fig. 9.
From Fig. 9, the relation between T m and T stop appeared like a staircase structure where the relation began straight line; then a plateau occurred (flat region), after that, the relation retrained as a straight line, then the plateau region appeared again. The number and location of the plateau regions represent the number and locations of the glow peaks that make up the glow curve. So, the glow curve of potassium magnesium borate doped with copper 0.07 mol% has seven expected peaks at 383.09, 400.71, 413.26, 426.59, 445.98, 476.57, and 534.04 K.
Based on the expected number of glow peaks, the Glow Curve Deconvolution (GCD) was performed using El-Kinawy TL software [23], which determined the activation energy, frequency factor, and kinetic order for each peak after deconvolution. The quality of GCD fit between theoretical and experimental results has been tested with the Figure of Merit (FOM) [45,46], as seen in Table 3 and Fig. 10. As shown in Table 3, the FOM value states the goodness of fit, where the values FOM less than 2.5% refer to a good fit [46].

Linearity and dose response
To establish the possibility of any material being used as a measure of radiation dose, it is needed to know the sort of connection between the radiation portion and the reaction of this material to radiation portion. It is favored that this relationship be linear.
The glass system doped with copper 0.07 mol% was exposed to various doses of b-particles from 0.11 to 165 Gy. After that, the TL-response of the irradiated glass system doped with copper 0.07 mol% samples was recorded, and the obtained glow curves corresponding to each dose were deconvoluted by El-Kinawy TL-software [23].
The shape and position of the peaks for the glow curve of the glass system doped with copper 0.07 mol% did not change with the variation in the radiation dose as seen in Fig. 11, except the area under the glow curve and the corresponding TL-intensity increased with increasing the b-particle doses. This may be due to defects caused by b-particles plus the different types of defects already in the glass before irradiation, which causes an increase in the luminescence intensity [47].
Furthermore, increasing b-dose also induces additional luminescence centers. this may be due to the formation of some defects such as, non-bridging boron oxygen hole centers (NBOHC) / boron electron centers (BEC), and boron oxygen hole centers (BOHC).
The total area under the glow curve of the glass system doped with copper 0.07 mol% was plotted as a function of b-doses. The obtained relation was fitted as a linear fit with the expression (Y = 5.13 ? 0.92x), and the regression square fitting value equalled 0.998. These findings revealed that the glass system doped with copper 0.07 mol% has a linear response from 0.11 up to 165 Gy from b-particle as seen in Fig. 12.
The area under each peak after deconvolution of the glow curve was also calculated as a function of beta dose. The results showed that the seven peaks had a linear relationship with the b doses in the range of 0.88 up to 165 Gy and the results are presented in Fig. 13.

Reproducibility
It is necessary to verify the studied sample repeatability to be used as dosimeters to determine their accuracy. The ability to keep the TL response following the repeated use of the same dosimeter at the same applied dosage is referred to as ''reproducibility''. If the coefficient difference between readings after five to ten cycles is less than 5%, the dosimeter is regarded as having a high repeatability. [48,49]. Thus, the TL-response of a sample from the glass system doped with copper 0.07 mol% was studied after six cycles of irradiation at 0.22 Gy from b-particles and the results showed that the coefficient difference is 2.21% as shown in Fig. 14. This result showed that the glass system doped with copper 0.07 mol% has a good reproducibility.

Sensitivity
One of the essential features that distinguish a dosimeter is its sensitivity. The sensitivity of the dosimeter could be assessed using Eq. (5) [49].
Hence, the sensitivity of the prepared glass system doped with copper 0.07 mol% was calculated using Eq. (6) for dose range from 0.11 up to 165 Gy, and the  results are shown in Fig. 15. It could be observed from this result that the sensitivity was decreased as the applied dose increased, where the average sensitivity overall studied dose range was 3309.49 ± 20%.
In addition, the relative sensitivity of the dosimeter could be assessed by comparing the intensity of the studied samples after being exposed to a certain dose with that obtained from a reference dosimeter (LiF, TLD-100) after being exposed to the same dose [49,50]. Thus, samples from the glass system doped with copper 0.07 mol% and TLD-100 were irradiated at 11 Gy, then the TL-intensity was recorded. The results revealed that TLD-100 has a relative sensitivity of 128.5 times as compared to that of a prepared glass system doped with 0.07 mol% copper (Fig. 15).

Fading
It is critical to understand the degree of electrical charge stabilization within the traps in the dosimeter over time [44]. So, a glass system doped with copper 0.07 mol% sample was irradiated at 11 Gy from b-particle and stored in the dark at room temperature for an interval time. The results revealed that the residual TL-intensity was 10% after 72 h from the first reading and it remained constant over the remaining studied period of 696 h, as seen in Fig. 16.
The glow peaks at 383.09, 400.71, and 413.26 K disappeared after 72 h from irradiation which explains the quick fading in the first 72 h, hence these peaks are considered lower temperature peaks, however the glow peaks at 426.59, 445.98, 476.57 and 534.04 K remain constant overall the study period. Fig. 10 The glow curve of the glass system doped with copper 0.07 mol% after deconvolution Fig. 11 The effect of different doses of b-particles on the glow curve of the glass system doped with copper 0.07 mol% Fig. 12 The dose-response for the glass system doped with copper 0.07 mol% Thus, it is recommended that when using the prepared glass pre-annealing should be performed before reading to temperature 415 K.

Conclusions
Based on the results, it can be concluded that: • The potassium magnesium borate doped with copper was prepared by a melting quenching technique to study the thermoluminescent behavior after b-particle exposure. According to the results, FTIR and N4 values established the occurrence of BO 3 and BO 4 groups in the glass network and the addition of CuO until the maximum concentration is reached (1.2 mol%) has no significant effect on the primary distinctive borate groups in the study, but slight changes in the intensities or places of some of the IR bands are identified. • The density of the glass system showed an increment in the content of CuO increases and molar volume decreases. • IR spectra reflect characteristic absorption bands due to specific borate network Fig. 13 The dose-response for five peaks of the glass system doped with copper 0.07 mol% after deconvolution Fig. 14 The reproducibility of the radiation response of the glass system doped with copper 0.07 mol% for six successive cycles Fig. 15 The sensitivity of the glass system doped with copper 0.07 mol% in the dose range from 0.11 up to 165 Gy Fig. 16 The residual TL-the intensity of the glass system doped with copper 0.07 mol% after different storage times from irradiation •

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