3.1 Optical UV- visible measurements
3.1.1 Optical UV- visible spectra before and after gamma irradiation
Changes in the glass lattice because of doping an element or irradiation can be determined by optical UV spectroscopy, where it gives information about radiation-induced defect centers .
The existence of TM ions even in very small concentrations reasons an electron transfer effect, including the movement of an electron from a coordinating oxygen atom orbital to the metal ion orbital. Consequently an obvious UV band is determined on the UV spectrum of TM ions containing glasses . Fig.1 illustrates the UV- visible optical spectra of Cu and Zn borate glasses. It is observed from the figure that there are small UV peaks appeared in both glasses in the UV region 200–270 nm, correlated to the presence of the triplet Fe3+ impurities accompanied to the chemical materials used in the glass preparation process [16, 17, 23]. For Cu- glass there are two observed peaks are detected; (a) sharp peak in the UV region at 320 nm (b) a broad visible-near IR band at ~ 735 nm. CuO can exist in two valence states; the monovalent Cu1+ (3d10) and the divalent state Cu2+ (3d9). Therefore, it participates in the glass network either in the form of CuO4 or CuO6, respectively . Cu1+ has five occupied d-orbitals therefore; it does not produce separate coloring, however, the cupric Cu2+ gives definite coloring centers in the visible region because of the creation of strong Cu2+ tetragonal distortion in octahedral units . Therefore, Cu-containing glass displays a bluish green color demonstrating the presence of Cu2+ divalent state referring to the broad visible-near IR peak at ~ 735 nm. But, the identification of the monovalent Cu+ ions seems to be restricted to the observed UV sharp peak at 320 nm . For Zn- glass, there are two detected distinct sharp peaks in the UV region at ~ 290 and 300 nm without the appearance of the peaks in the visible region. This behavior refers to the presence of Zn ions of their divalent state as Zn2+ (3d10) and explaining in turn the white colorless of Zn- glass.
Fig.2 (a & b) illustrates the effect of progressive γ-irradiation on the optical behavior of the studied Cu and Zn glasses; respectively. From the figures it is observed that there is a gradual decrease in the absorbance intensity for the both glasses with increasing the absorbed doses. This performance can be discussed according to the direct effect of gamma radiation on the studied glasses which directly affects the host glass matrix by changing the number of bridging oxygens BO and non-bridging oxygens NBO via creating or breaking bonds with the neighboring oxygens and atoms. This effect of such ionizing radiation takes place through some photochemical reactions either photo-oxidation or photo-reduction processes of TM ions in the lower valence states. For example the photo-oxidation process of cuprous Cu1+, the lower valence state of Cu into cupric Cu2+, the higher oxidation of Cu [23, 29,30].
Cu1+ + hυ ↔ Cu2+ +e- + h+ (1)
This recommendation was approved by , who assumed that metal ions (such as Ce3+, U4+) as well as d10 s2 ions have the capability to absorb radiation via some electronic transitions including energetic changes when an electron transfer takes place from an atomic orbital to another .
According to the observed spectra in Fig.2, the gradual decrease in the UV absorbance intensity may be related to the formation of more bridging oxygens with the progressive irradiation of glasses. Because of the ability of transition metal ions to contract with the intrinsic defects in the glassy network by capturing or trapping the radiolytic electrons or holes and then the formation of intrinsic color centers recovery rates, they are so called as “potential traps” . Trapping or capturing processes depends essentially on the doped TM ion; its kind and concentration, in addition to the main composition of the host glass matrix . Since the main composition of the studied glasses (60-x B2O3+ 25 SrO+ 15 Na2O) and the concentration of TM (x=15 mol.) are constant, then the optical behavior depends only on the type of the transition metal ion Cu or Zn as well as the dose of radiation.
3.1.2 Optical response curve
The optical density readout was done at two wavelengths 320 and 300 for Cu and Zn glass samples, respectively. Fig. 3 shows the response curves of Cu and Zn borate glass samples in terms of change in absorption coefficient, ΔA. mm-1 at 320 and 300 nm, versus the absorbed dose, in line with the relation (ΔA= Ao-Ai), wherein Ao and Ai are the values of the absorption coefficient for the un-irradiated and irradiated glasses; respectively. From the response curve, the effective dose range of the two glass systems is between 0.4 and 15 kGy depending upon the glass structure and the sort of the introduced transition metal ion. Response curves show additionally that both Cu and Zn glass samples are sensitive to gamma-rays in the dose range between 0.4 and 15 kGy and then their possible use as radiation dosimeters.
3.1.3 Post irradiation stability of Cu/ Zn glass samples
Both Cu and Zn borate glass system irradiated with 3 kGy, have been stored at room temperature (25oC ±2) in the dark and under laboratory fluorescent light. The absorbance of the two glass samples was measured at 320 and 300 nm exclusive time intervals through the post-irradiation storage duration of two months. Change in absorbance at the two wavelengths at one of a kind conditions of storage as a characteristic of storage time is relative to that when storage as indicated in Fig.4. From this figure, it’s far found that the two irradiated glasses exhibit good stability beneath the storage conditions except for the primary 7 days.
3.1.4 Optical energy band gap Eopt
Optical energy band gap (Eopt) can be identified by by plotting (αhν)n against the incident photon energy (hν) as given in Eq (2) ;
Where α (ν) is the absorption coefficient, αo is a constant, n is a constant depends on the mechanism of electron transition; direct or indirect, allowed or forbidden . In the amorphous structure of the glass system; n can be taken as 0.5 for the indirect allowed transitions.
The optical energy band gap is linked directly to the cross-linking between individual atoms in the glass structure. Therefore, it is controlled essentially by the glass composition and any structural modifications take place because of specific factors like the addition of modifier ions and/or irradiation processes. Eopt can be adjusted by generating positively or negatively charged defect centers “color center” causing the formation of energy levels between the original bands . Fig.5 (a &b) show the linear plots of Eopt values for the studied Cu and Zn borate glasses before and after the progressive irradiation in the dose range 0.4 - 15 kGy, where there is a gradual increase in the Eopt values with increasing the dose of radiation. This behavior can be correlated directly to the effect of radiation in making some structural changes inside the glass matrix by changing the oxygen bond strength of the glassy network . Mott and Davis  have assumed that the alteration in Eopt of the glass is related to the conversion between bridging oxygens and non-bridging oxygens. According to the response curve shown in Fig.6, the gradual increase in Eopt with irradiation doses may be due to the combination between positive holes and negative electrons and then the conversion of some NBOs into BO, creating new bonds with lower defect centers and more closed structure. This process introduces a large energy gap between the ground and excited levels of atoms giving in turn larger Eopt. Some authors [35, 36] have reported that rising in the bridging oxygens number gives more covalent bonds and reinforces the bond strengths in the glass structure causing a large splitting between the valence and conduction bands or higher Eopt values. The presence of 3d transition metal ions in the glass structure like Cu2+ or Zn2+ ions provides this process because of their variable configuration  and their tendency to change the outer electronic configurations. The progressive increase of Eopt with irradiation refers also to the sensitivity of Eopt values of the studied glasses to the dose of radiation.
3. 2 Electron paramagnetic resonance measurements
3.2.1 EPR spectra of free radical development of the glass samples
EPR is a very interesting and perfect measuring instrument for testing dosimetric systems [1-3]. To obtain a reproducible and useful applied dosimeter, it is necessary to obtain main essential requirements such as; the stability of free radicals released by the effect of gamma rays, regular or reproducible changes in the increase of free radicals according to each absorbed dose, as well as the instrumental analysis used in the EPR technique [11,12]. Therefore, the intensity of EPR spectra has been recorded at different time intervals. Fig. 7 demonstrates the measured signal intensities of irradiated and un-irradiated Zn-glass samples where there is a good peak height or signals with a single EPR line. Also, it is observed that the EPR signals begin to develop upon irradiation and its amplitude increases gradually with increasing the absorbed doses of gamma-rays without any change in its shape. So, it can be recorded that Cu and Zn glass samples are sensitive to γ-rays from 0.5 to 12 kGy and from 0.5 to 10 kGy respectively with similar behavior.
Moreover, the EPR signal intensities have been plotted against the square root of the microwave power (range 0-5 mW) as shown in Fig. 8. The power 2.012 mW, corresponding to P1/2 = 1.4184 mW has been selected for carrying out the measurements, as it is almost at the upper end of the linear range of the dependence. Furthermore, the intensity of the signals increases in proportion to the square root of microwave power (P1/2) up to high microwave power. Consequently, an appropriate set of microwave power level is important for the EPR measurement and dosimeter sensitivity.
3.2.2 EPR Dose-response function
Fig. 9 depicts the dose-response functions of EPR signal intensities for the two types of glass samples in the dose range of 0.5 to 12 and from 0.5 to 10 kGy for Cu/ Zn samples; respectively. Certainly, the signal intensities increase linearly with the increase of absorbed dose (correlation coefficient, r2= 0.9967049836 in case of Zn, and r2= 0. 9996760285 in case of Cu glass samples), better linearity and higher r2 value reflect the validity of the two glass samples to be precise dosimeters for dose process control and the good reproducibility of EPR signal measurements. Based on these consequences those glass samples may be carried out food irradiation processing, medical sterilization, and healthcare sterilization. So that it can be used and applied as radiation dosimeters within the range between 0.5 and 12 kGy relies on the conduct of the form of the introduced transition metal ions.
3.2.3 Stability of glass sample's response
(a) Pre-irradiation Stability
One of the most important dosimetric characteristics of Fig. 10 a and b is the pre-irradiation stability over 63 days of glass samples (Cu/Zn), which was studied by measuring the EPR signals of different types of glass samples during storage periods. The samples are conditioned in the dark and under laboratory fluorescent lights at 33% relative humidity and at room temperature (25 ±20 C). From the results, under different storage conditions, no detectable EPR signal was generated during the storage period.
(b) Short term post-irradiation stability
By continuously reporting the EPR spectrum every 10 minutes for 100 minutes, the stability of the radiation-induced paramagnetic center in a 3 kGy glass sample over time is estimated. As shown in Figure 11 (a), the relative response attenuation of the two glass samples during this period was very good.
(c) Long term post-irradiation stability.
The post-irradiation stability of of Cu/ Zn glass samples stored under light and dark irradiation at 33% relative humidity (RH) and at room temperature (25 ± 2 0C) was investigated. Three small pellet samples were irradiated with a dose of 3 kGy. Measure the signal height of the EPR signal of the studied glass samples immediately after irradiation. The samples were then stocked for approximately 8 weeks, and EPR measurements were taken at various times before this storage period. The post-irradiation stability curve of the glasses is illustrated in Fig. 11 (b). The Long-term stability after irradiation is also excellent, In the first five days of storage after irradiation, the EPR signal only dropped by about 1%. The amount of radical decay comes from the structure of the glass network. The spin–spin interaction belongs to the concentration of spins in the glass samples, which increasing with increasing the spin density in the sample [37, 12].
3.3 Electrical conductivity measurements
Specific electrical conductivity (σ) of the glass samples can be calculated according to the relation σ = (L/A) (1/r), where (σ) is the electrical conductivity of the glass sample, (A) the cross sectional area in cm2, (L) thickness in cm and (r) the measured resistance in ohm. Fig.12 shows the relation between electrical conductivity of Cu and Zn glasses against the dose of gamma radiation at room temperature. Where, Cu-glass has a relatively higher electrical conductivity (σ) than Zn-glass and an obvious gradual decrease in σ for both glasses with the progressive irradiation is noticed.
Electrical conductivity in glasses depends mainly on the movement of alkali modifier ions that present in the interstitial positions inside the glass network. The most common conducting species in alkali oxide glasses are Na+ and Li+ ions since these cations are quite mobile to escape from their positions to the nearby vacancies or interstices in the glass matrix, they are so called as “current carrier ions” . Hence, electrical conductivity of glass is directly proportional to valence and the size of the charge carriers, their concentrations and the ability to move or dispersion under the effect of an external electric field. When the glass contains both alkali ions and transition metal ions, the glass would have a mixed electronic–ionic, pure ionic or pure electronic conduction mechanism  according to the internal structure of the glass, concentration of the alkali ions and the electron hopping between the valence states in the TM ion. In the studied glasses, the alkali Sr2+ and Na+ ions act as network, modifying ions creating NBO and enhance the process of charge transport. On the other side, TM ions with their variable configuration can lose or accept electrons to provide the electrical conducting motion by the hopping mechanism of small polarons . The hopping mechanism depends on the type and concentration of TM ion. Cu-glass reveals slightly higher EC than Zn –glass, where Cu2+ ions participate as CuO4 or CuO6 in the glassy network [26, 27] so, the conduction process is enhanced by the electronic hopping between Cu1+ and Cu2+ states giving the chance of an easier mobility for the charge carriers. The observed decrease in EC values with irradiation is correlated directly to Eopt values, where increasing Eopt values with radiation dose refers to more BO and more covalent bonds giving closer or more compacted glassy structure. Consequently, a diminish in the mobility of light charge carriers takes place, giving then a relative decrease in electrical conductivity with the progressive irradiation. This behavior shows a good and suitable electric sensitivity of the glasses to gamma radiation doses. Fig.13 shows the long electric stability of the prepared glasses at the room temperature for 40 days. Where the electrical conductivity values of the two glasses give an excellent stability with time of storage.