SPECTROSCOPIC PROPERTIES OF THE GAMMA IRRADIATED ErF3-DOPED BaF2 CRYSTALS

The spectroscopic properties of the gamma irradiate BaF2:Er 3+ crystals were studied. BaF2:0.2 mol% ErF3 crystal was grown using the vertical Bridgman meth od. The Judd-Ofelt intensity parameters, Ωt (t=2,4,6) for f-f transitions of Er 3+ ions were determined from optical absorption spectra recorded before and after gamma irradiation at various doses. The influence of gamma irradiation dose on the Judd-Ofe lt parameters and on the radiative transition probability, on the branching ration and on the radiative lifetime was also investigated. The predicted radiative lifetimes wer e compared with those measured and with those reported by other authors. Apart from the gre en and red emissions, arising under excitation at 378 nm, a new UV emission band around 321 nm was observed under excitation at 290 nm. This emission was not reported before. U sing the Füchtbauer-Ladenburg method, the emission cross section and optical gain paramet er of the red and green emission band


Introduction
In the last time, trivalent rare-earth (RE) ions doped fluoride MeF 2 (Me=Ca, Sr, Ba) crystals have been extensively investigated. Due to their optical properties, such as low phonon energy, broadband transmission over a wide spectral range from ultraviolet to farinfrared (200-14.000 nm), large bandgap energies, low spectral dispersion and relatively low refractive index, the MeF 2 crystals have been studied for various applications (optical windows, lenses, optical communications, laser host materials, scintilators, etc.) (Hahn, 2014). In BaF 2 crystals containing various RE ions, only trigonal spectra were detected by (Wells et al., 2002;Ranon and Yaniv,1964;Petit et al., 2007;Nicoara et al., 2016). They attributed these spectra to sites where F − ions occupy interstitial body centers of next-nearest cubes along the [111] direction, the so-called NNN sites.
The choice of host materials is of great importance in the designing of RE doped luminescent materials for various practical applications. Fluorides are considered good hosts for upconversion luminescence of RE 3+ ions. The emission spectrum of Tb 3+ -doped BaF 2 consists of two sets of lines, so-called the blue and the green emission (Witkowski and Wojtowicz, 2011). Orlovskii et al., (Orlovskii et al. 2010) studied the double doped BaF 2 :Tm 3+ + Ho 3+ crystals as room temperature laser in IR (3.9 µm) domain. The idea was to improve the pumping scheme of the initial laser level with sensitization of Ho 3+ fluorescence by Tm 3+ in BaF 2 . They used low concentration RE doped BaF 2 (0.35÷1.3 at% HoF 3 and 0.3÷2.1 at% TmF 3 ). The Er 3+ ions in fluoride crystals have spectroscopic properties that are favourable for cascade excitation (Kaminskii, 1996(Kaminskii, , 2007Joubert et al., 1993). Er 3+ ions doped BaF 2 crystals were investigated especially for upconversion luminescence (Johnson and Guggenheim, 1975;Patel et al., 1998;Wojtowicz and Wojtowicz., 2009,;Zhang et al., 2015;Bitam et al., 2018). Patel et. al. (Patel et al., 1998) demonstrated that, under excitation at 805 nm, BaF 2 :Er 3+ crystal is more efficient in generating red, green, and UV emissions than the CaF 2 :Er 3+ crystal. Wojtowicz (Wojtowicz, 2009) studied the emissions under excitation in VUV spectral region. He identified a number of emission bands in VIS, UV and VUV. Zhang et al. (Zhang et al., 2015) detected very weak green and red emissions in Er 3+ : BaF 2 crystals in comparison of the Er 3+ doped BaCl 2 crystal when excited by 808 nm. Under 976 nm excitation, due to 2 H1 1/2, 4 S 11/2 → 4 I 15/2 transition, the green emission becomes dominant. The green luminescence, under excitation at 378 nm ( 4 I 15/2 → 4 G 11/2 transition) was investigated by Bitam et. al. (Bitam et al., 2018) in BaF 2 :2 mol% ErF 3 . A very weak red emission (200 times weaker than the green emission), corresponding to the 4 F 9/2 → 4 I 15/2 transition, was reported. It has been proved that the use of radiation in UV range is an efficient tool for medical purposes such as the treatment of tissue, improving the phototherapy methods of some skin diseases, such as psoriasis, vitiligo or lymphoma (Vanó-Galván, 2012). Finding a laser material with emission in near UV range, for the construction of a solid, miniature laser, easy to manipulate by the patient itself is important.
Some radiation damages appear when the fluorides crystals are exposed to various radiations (x-rays, γ-rays, neutrons, etc). The radiation damage consists in appearance of new absorption bands caused by the color centers formation. These absorption bands reduce the transmission of the light through the crystal and change the optical properties of the crystals.
Efforts have been made to find and understand the causes of the radiation damage. The presence of impurities and defects in crystal is the major reason for the radiation damage (Fedorov et al., 2016, Cooke andBennett, 2003;Ren et al., 1994;Yin, 1994). Sabharwal et al. (2002), Shaoxia et al. (1994), Lingyan et al. (1994) and others also showed that the radiation damage is induced by oxygen and hydroxyl impurities in BaF 2 crystals. Ren et. al. (Ren et al., 1994), have investigated the effect of γ-rays on the optical properties of BaF 2 crystals grown under different vacuum level various conditions. They showed that new optical absorption bands occur in the UV-VIS spectral domain after γ-rays irradiation due to presence of the oxygen ions. The radiation damage produced by a trace amount of Pb 2+ ions in BaF 2 crystals was studied by Nicoara et al., (Nicoara et al., 2020). Rare-earth (RE) ions, such as Sm, Eu, Dy and Yb, which change its valence under irradiation, can eliminate the absorption bands of the color centers in the visible domain (Egranov and Sizova, 2013;Lingyan et al., 1994). There are few papers that analyze the influence of gamma radiation on the emission properties of the RE doped BaF 2 crystals, especially with Er 3+ ions. We will mention only some of them. How the different impurities affect the radiation resistance of  calculate the emission transition probabilities, branching-ratios and radiative lifetimes (Judd, 1962;Ofelt, 1962). The influence of gamma-rays doses on the J-O parameters and on the radiative lifetime is also investigated. To our knowledge, no other report of the influence of γ-rays on the emission spectrum of Er 3+ -doped BaF 2 can be found in the literature.

Experimental
The ErF 3 -doped BaF 2 crystal with a concentration of 0.2 mol% ErF 3 added to the melt was grown using the vertical Bridgman technique. The crystal was obtained in a spectral pure graphite crucible in vacuum (~0.1 Pa). The temperature distribution along the graphite heater and the corresponding used power during the heating and the cooling stages are shown in Fig.   1a,b. In order to obtain the temperature distribution, a shaped graphite heater was used (Nicoara and Nicoara, 1988). The temperature gradient, for a nominal power of 4.3 kW, is 35°C/cm (Fig. 1c). The pulling rate was ~4 mm/h. The obtained crystal is transparent, 6 cm in length and 1 cm in diameter. No visible inclusions were observed. The studied sample was cleaved from the middle of the crystal (Fig. 1d).

Optical absorption spectra
The UV-VIS-IR absorption spectrum of the sample (2.83 mm thick) is shown in Fig. 2. The absorption bands used for the least-square fitting procedure are highlighted. These peaks correspond to the transitions from the ground state, 4 I 15/2 , to the excited states of Er 3+ ions, specified in the figure. The absorption spectrum before and after gamma irradiation ranging from 190 to 1000 nm consists of 12 absorption bands In order to preserve the electrical neutrality of the system, the trivalent erbium ion is compensated by a fluorine ion. The charge compensation is effected by interstitial fluorine ions in various positions relative to Er 3+ ions, giving rise to various sites with cubic (O h ), tetragonal (C 4v ) and trigonal (C 3v ) site symmetries, the so-called isolated centers and various clusters (Petit et al., 2007). For low ErF 3 concentration (<0.1%), the so-called isolated centers predominate in the crystal. As the dopant concentration increases, besides of isolated centers additional complex defects, the clusters, are present in crystals. All these lead to a multisite structure of the absorption bands.
Using laser selective excitation, Wells et al. (Wells et al., 2002) showed that the dominant isolated centre in lightly doped BaF 2 :Er 3+ crystals has C 3v symmetry, against the case of CaF 2 :Er 3+ , where the dominant centre is C 4v . The Gaussian multi-peaks decomposition reveals two major peaks in the case of the highlighted bands: at 372 nm and 378 nm, at 511nm and 521 nm, respectively at 640 nm and 649 nm. Taking into account the results of Tallant and Wright (Tallant and Wright, 1975) we attributed the 372 nm, 511 nm and 640 nm peaks to C 3v (NNN) site and the 378 nm, 521 nm and 649 nm peaks to clusters (aggregates).
The radiation damage is related to the appearance of the new absorption bands due to color center formation. Ren et al., (Ren et al., 1994) investigated the effects of various RE ions on the radiation damage of BaF 2 . The elements which changes its valence from +3 to +4 under irradiation are harmful, in contrast to the stable RE ions that have no effect on the radiation resistance of BaF 2 .
No change in our OA spectrum was observed after irradiation, even at doses up to 270 kGy related to fact that no new absorption bands appeared after irradiation. Changes were observed related to the intensity of the every absorption bands. After irradiation, the intensity of the absorption bands decrease. Analyzing the absorption bands after irradiation, it was found that the intensity of the component corresponding to the clusters decrease. This means that after irradiation some of the clusters were "dissolved". Therefore, the areas corresponding to the OA bands are changed. The absorption line strengths for various transitions before and after γ-irradiation is calculate using the area of the band. This explain why differ the various JO parameters before and after irradiation.

Judd-Ofelt analysis
The behaviour of luminescence spectrum of ErF 3 doped BaF 2 crystal can be explained using the Judd-Ofelt model (Judd, 1962;Ofelt, 1962). The J-O intensity parameters (Ω 2 , Ω 4 , Ω 6 ) are used to evaluate the transition probabilities, the branching ratios and the radiative lifetime. These parameters are obtained using the best fit between the experimental absorption oscillator strengths and the theoretical ones. The J-O intensity parameters were calculated using a set of four absorption bands, highlighted in Fig. 2. These bands correspond to the transitions from 4 I 15/2 ground state to 4 I 9/2 (802 nm), 4 F 9/2 (653 nm), 2 H(2) 11/2 (519 nm) and 4 G 11/2 (377 nm). In order to study the influence of gamma rays irradiation on the J-O spectroscopic parameters, the sample was irradiated at 70, 130 and 270 kGy The experimental line strength, , has been evaluated from the absorption spectrum.
And then was used to obtain the J-O parameters by solving simultaneously a set of four equations. In order to resolve this system of equations the Levenberg-Marquardt algorithm was used. The theoretical line strength = S + is calculated using the obtained J-O parameters and the values of the reduced matrix elements for the chosen Er 3+ bands from those tabulated in (Kaminskii, 1996). The measured and calculated absorption line strengths for transitions 4 I 15/2 →[ 4 I 9/2 , 4 F 9/2 , 2 H(2) 11/2 , 4 G 11/2 ], before and after gamma irradiation are shown in Table 1 Table 1 for each transition. The obtained J-O parameters and the spectroscopic quality factor, χ=Ω 4 /Ω 6 , are given in Table 2. A comparison of our J-O parameters to those obtained by other is also provided.  (Bitam et al., 2018) BaF 2 : 2 mol% Er 3 (Patel et al., 1998)  The influence of gamma irradiation dose on the J-O parameters is shown in Table 3; after irradiation the values of the J-O parameters decrease a little. The error of J-O parameters was estimated using the method described by (Görller-Walrand, 1998). These errors are not usually reported in the literature and therefore it is difficult to compare them. The main reason of the lack of this information in the literature could be related to the large error in the estimation of J-O parameters. In our case the errors varies between 2% and 90%. These large errors are not only due to inadequacies of the Judd-Ofelt theory, but also due to the difficulty of obtaining accurate absorption line strengths, especially in the case of overlapping bands. We calculated the spontaneous emission probabilities, A JJ' , of the transitions to their corresponding lower-lying manifold states. The radiative lifetime τ rad for an excited state J is calculated by τ = 1/ ∑ " , where the sum is taken over all final lower-lying states J'. The fluorescence branching ratios, β JJ' , can be calculated from the radiative decay rates and the radiative lifetimes using the relationship # = τ " , where the sum runs over all final states J ' . The value of the radiative emission probabilities, the radiative lifetimes and the branching ratios before and after gamma rays irradiation at the maximum dose are given in Table 4. The dependence of the calculated radiative lifetime on the gamma rays dose is shown in Fig.   5. The calculated radiative lifetimes corresponding to the observed emissions increase slightly with the gamma dose. The calculated and measured radiative lifetime, at various γrays doses (D) are given in Table 5.   (Bitam et al., 2018) 0.88 (Patel et al., 1998) 0.39 (Deren and Mahiou, 2007) Fig. 6 shows the decay curves of the ( 2 H(2) 11/2 + 4 S 3/2 )→ 4 I 15/2 transition (green emission) and of the 4 F 9/2 → 4 I 15/2 transition (red emission). The behavior of the both decay luminescence is nonexponetial. They have been fitted by a double exponential function which give an effective lifetime of 1.005 ms for the green emission and of 2.46 ms for the red emission for the irradiated sample. These values are in a good agreement with those calculated using J-O formalism, but also with those obtained by other authors (Table 5) (Zhang et al., 2015;Bitam et al., 2018;Patel et al., 1998;Deren and Mahiou, 2007). The lifetime for the purple emission was not determined by the lack of equipment. Fig. 6. The decay curves of the (a) ( 2 H(2) 11/2 + 4 S 3/2 )→ 4 I 15/2 transition (green emission) and (b) of the 4 F 9/2 → 4 I 15/2 transition (red emission) before and after γ-rays irradiation.
The emission cross section corresponding to the observed emission was estimated using Füchtbauer-Ladenburg expression (Deren and Mahiou, 2007): where I(λ) is the emission intensity at each wavelength, τ is the radiative lifetime of the upper laser level and β is the branching ratio, n is the refractive index and c is the velocity of light. The emission cross section of the purple, green and red emission spectra of BaF 2 :0.2 mol% ErF 3 sample before and after γ-rays irradiation is shown in Fig. 7. The optical gain parameter for the main emission bands is also shown in the figures. The optical gain parameter, defined as . = $ τ /0 , is an important parameter in order to evaluate the laser performance of the material. The optical gain parameter The optical gain parameter, of the irradiated sample, is 3.42×10 -24 cm 2 ⋅s for green emission and 6.44×10 -24 cm 2 ⋅s for red emission. The optical gain parameter increases slightly with the gamma dose.
Therefore, the red emission is more attractive than the green emission for laser applications.

Conclusions
The influence of gamma rays irradiation on the optical absorption and luminescence of the ErF 3 doped BaF 2 crystals for a low dopant concentrantion (0.2 mol% ErF 3 ) was investigated.
The Judd-Ofelt analysis is performed in order to calculate the emission transition probabilities, the branching-ratios, the radiative lifetimes and the gain parameter for green and red emissions. The values of the J-O parameters are slightly influenced by gamma rays irradiation, even at maximum dose of 270 kGy. The calculated radiative lifetimes increase slightly with increasing the gamma rays dose. The measured lifetimes of the irradiated sample for the green and the red emissions are 1.005, respectively 2.46 ms. These values are in a good agreement with those calculated using the J-O formalism. Apart from the green and red emissions, arising under excitation by 378 nm, a new, intense UV emission band, centered at 321 nm (corresponding to the 2 P 3/2 → 4 I 15/2 transition) was observed under excitation by 290 nm. This emission was not reported before. The energy-level diagram of the excitation and emission mechanism was proposed. The optical gain parameter increases slightly with the gamma dose. The optical gain parameter resulted from this study is 3.3×10 -24 cm 2 ⋅s for the green emission and 6.4×10 -24 cm 2 ⋅s for the red emission. Therefore, the red emission of BaF 2 :0.2 mol% ErF 3 crystal is more attractive for laser applications than the green emission.