Postradiation accumulation of F4 centers in gamma-irradiated LiF crystals

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Tbilisi State University

E. Andronikashvili Institute of Physics

Postradiation accumulation of F4 centers in gamma-irradiated LiF crystals

PhD in Physics and Mathematics Nadareishvili M.M.

PhD in Physics and Mathematics Abramishvili M.G.

Doctor of Phyz. Math.Sciences Kvachadze V.G.

PhD in Physics and Mathematics Tsakadze S.J.

Abstract

radiation accumulation irradiated crystal

The postirradiation accumulation of F₄ centers in gamma-irradiated (1ч5•10⁶R)LiF crystals was studied. After gamma-irradiation, the LiF crystals were exposed (simultaneously or sequentially) to moderate temperature (140-160°C) and a static electric field (1,800-3,000V). The obtained results are discussed.

Keywords: gamma irradiation, color centers, F₄ centers

Аннотация

Исследовано пострадиационное накопление F₄ центров в гамма-облученных(1ч5Ч10⁶R) кристаллах LiF после комбинированного (одновременного или последовательного) воздействия умеренным теплом (температура отжига 140-160 °С) и постоянным электрическим полем (1800-3000 V). Проведен анализ полученных результатов.

Ключевые слова: гамма облучение, центры окраски, F₄ центры.

In the second half of the last century, by various investigation techniques, it was found that, in lithium fluoride monocrystals irradiated with high fluencies (>10¹⁸ N/cm²) in the reactor, metal particles 10³-10^{4 o}A formed. These values are proportional to nanoparticle sizes. Hence the investigation of the mechanisms of colloid particles aggregation will help us to produce metal particles in the medium of interest.

Of special note are the results of postirradiation annealing (400-500⁰C) of the LiF crystals irradiated in the reactor, particularly the color spots observed by using an optical transmission microscope [1]. By microspectrophotometric analyses of such crystals, absorption bands with maxima at wavelengths 462,487,515 and 525nm were recorded. According to K. Kubo, the above-mentioned color spots were due to the absorption of lithium particles of different sizes formed in the crystal.

At that time it was popular to study colloid particles and their precursors, quasicolloid ones, by annealing the samples irradiated in the reactor.However, the annealing failed to serve completely the function of selection for separating and investigating one or other process. First, the annealing was carried out at rather high temperature (>300⁰C) which could reduce the resources of color centers and hence limit the evolution of complex centers. At the same time, the particles formed were not uniform in size, etc.

Besides, the examination of colloid particles in the LIF monocrystals by the method of optical absorption is hindered by the fact that the optical absorption band of colloid particles (450-550nm [1]) is overlapped by that of complex color centers F₄(N₁,N₂) (N₁-517nm, N₂-549nm[2]). Moreover, when the mechanism of aggregation of the colloid centers is considered, the F₄ centers could be one of the elements in the formation of colloid centers. A comprehensive investigation of these centers has not been feasible so far, which is confirmed by the statement of the authors of work [2]. From the preceding, the necessity in the investigation of the above-mentioned problems by relatively novel methods is evident. We believe that the methods of the combined action of various external fields on colored alkaline halide crystals could be efficient in this field. The investigation results reported in work [3] argue in favor of this idea. In that investigation, colored crystals were exposed to UV radiation and the static electric field. In this manner, stable F₂₊centers, which usually disintegrate at rooms temperature, were obtained.

Thus, based on the previous experience, it is possible to create such a combination and conditions of external fields that will work in the given spectrum region. For instance, when a combination of UV radiation, for the ionization of neutral centers, and the static electric field is used for production of stable F₂₊ centers in preliminarily colored LiF crystals, moderate temperature (which will promote the diffusion of existing elemental centers and, at the same time, will not reduce the resources of color centers) and the electric field would be appropriate to use.

Samples and Methods. The test samples (0.5Ч1.0Ч1.0cm in dimensions) were cut out of the LiFmonocrystal block. Among the impurities, bivalent Mg ions prevailed (?2.6Ч10^-3 wt. %). The amount of other impurity ions (Fe, Pb, Si, Mn, Al and Cu) did not exceed ?10^-4 wt. % in total.

The gamma-irradiation was performed in an indium-gallium gamma loop allowing carry out the irradiation at room temperature, the radiation dose 8Ч10^-5R with the maximum energy of gamma-rays 2.1 MeV.

The optical absorption spectra were detected in the wavelength range from 200 to 900nm at room temperature with the help of spectrophotometer AvaSpec-2048.

Results and Discussion. Figure1 shows the optical absorption spectra of colored LiF (gamma-irradiation 5Ч10^-6 R) before the treatment (Curve1) and after the combined treatment (Curve2).

Fig.1. Optical absorption spectra of gamma-irradiation (5Ч10^-6 R) LiF crystals: Curve1- before treatment; Curve2- after combined treatment;

The static electric field was applied to the sample at 150 ⁰C (U= 3KV, exposure 4 hours). The spectrum recorded after switching-off of the electric field and cooling of the sample to room temperature showed a distinct increase in the number of aggregate centers F₃ (R₁-316 nm, R₂-374nm) and F₄ (N₁-505nm, N₂-549nm) (Fig.1, Curve 2). This occurred at the expense of F and F₂ centers:

ДK_F =K_OF - K_IF = 200 - 170 = 30 cm^-1; ДK_F2 =K_OF2 - K_IF2 = 133-98 = 41 cm^-1, where ДK_F and ДK_F2, and K_OF, K_IF, K_OF2 and K_IF2 are the absorption coefficients at maxima of the F and F₂ bands before the combined treatment and after it, respectively.

By selecting the conditions of the combined treatment, it must be possible to produce bigger quasicolloid Formations, i.e. to create such a medium depleted in fluoride that will promote the second-kind phase transition and the formation of the metal phase in the alkaline halide crystal.

For the analysis of the combined action fields, it was of interest to determine the share of each field separately.

Fig 2. Optical absorption spectra of gamma-irradiated LiF crystals (1.5Ч10⁶ R): Curve 1- before annealing; Curve 2 and 3-after annealing at 140 and 160⁰C, respectively; Curve 4- after combined treatment.

In order to clear up the action of each field, we took the crystals irradiated with a lower dose (<5Ч10⁶R). In Fig. 2, Curve 1 corresponds to the LiF sample after irradiation (1.5Ч10⁶R). Curve 2 and 3 to the irradiated sample after annealing at 140 and 160⁰C, respectively, for 3 hours. Curves 2 and 3 coincided in spite of temperature difference of 20⁰C. When, alongside with annealing, the electric field (U=1800 V) was applied to the sample, Curve 4 was recorded. The intensity of the main maximum (л=510nm) doubled. Besides, the Formation of F₂₊centers was observed as indicated by the increase in the absorption the wavelength range 600-700 nm. In this case, along with partial disintegration of F and F₂ centers, the disintegration of F₃₊ centers took place-the F₂ band shifted to the left by 10nm.

In our opinion, the post irradiation annealing of the colored crystal at moderate temperature increased the intensity of diffusion of elemental color centers (Fig 2, Curve 2and 3). Under the combined treatment, similar to [3], in addition the barrier reduced in a certain direction, which increases the probability of aggregation of elemental centers. That is why, under the combined action of temperature and the electric field, the accumulation of F₄ significantly increased.

The data given in the table below point to a dominant role of the electric field.

In the two experiments presented in the table, there was only one difference-the temperature differed by 10 ⁰C. Such a small difference turned out to be enough for a considerable difference in the accumulation of F₄ centers, while at U=0 even the 20⁰C difference was not pronounced (Fig.2, Curves 2and 3). Hence, the static electric field allows using more completely elemental and relatively simple resources for accumulation (aggregation) of F₄ centers.

Table 1. Data on the gamma-irradiated LiF crystals (5Ч10⁶R)

Conclusion

The low-temperature annealing (? 160 ⁰C) and the applied static electric field (1800-3000 V) caused intense postirradiation accumulation of aggregate color F₄ centers in gamma-irradiated LiF crystals. At the same time, the concentration of simpler centers (F_, F₂ and F₃₊) decreased. The efficiency of the electric field in the processes under study was revealed.

References

1. K. Kubo, J. Phys. Soc. Japan 10 (1963) 1703.

2. Vincenti, M.A.; Montereali, R.M.; Baldacchini, G.; Kalinov, V.S.; Voitovich, A.P. Opticalspectroscopy of F2 and F3+ color centers in gamma irradiated lithium fluoride crystals; Servizio Edizioni Scientific ENEA Centro Ricerche Frascati, 65 - 00044 Frascati, Rome, Italy, 2009

3. G.Abramishvili, M. Abramishvili, M. Galustashvili, T. Kalabegishvili, V. Kvatchadze, V. Tavkhelidze G Journal of Luminescence 140 (2013) 126-129

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