Melting properties of radiation-induced Na and Cl2 precipitates in ultra-heavily irradiated NaCl

Exposure of NaCl to high doses of ionizing radiation leads to the formation of nano-particles of metallic Na, very small chlorine precipitates, vacancy voids, and dislocations. A useful way to monitor the stage of the damage formation process is measuring the latent heat of melting of the Na-particles (~100°C) and chlorine precipitates (-101°C). In this paper we will present data, showing that for doses in the range of TRad (1010 Gy) the concentration of radiolytic Na may become very large. Even in pure samples, we have converted more than 20% of all NaCl molecules into metallic Na and chlorine, but often higher percentages can be achieved. In this paper we will present new data obtained for ultra-high irradiation doses and a first attempt will be made to understand the results.

Effect of radiation-induced emission of Schottky defects on the formation of colloids in alkali halides

Formation of vacancy clusters in irradiated crystals is considered taking into account radiation-induced Schottky defect emission (RSDE) from extended defects. RSDE acts in the opposite direction compared with Frenkel pair production, and it results in the radiation-induced recovery processes. In the case of alkali halides, Schottky defects can be produced as a result of the interaction of extended defects with excitons, as has been suggested previously. We consider a model that takes into account excitonic mechanisms for the creation of both Frenkel and Schottky defects, and which shows that although the contribution of the latter mechanism to the production of primary defects may be small, its role in the radiation-induced evolution of microstructure can be very significant. The model is applied to describe the evolution of sodium colloids and the formation of voids in NaCl, which is followed by a sudden fracture of the material, presenting a potential problem in rock salt-based nuclear waste repositories. The temperature, dose rate and dose dependence of colloid growth in NaCl doped with different types of impurities is analyzed. We have found that colloid growth may become negative below a threshold temperature (or above a threshold dose rate), or below a certain impurity concentration, which is determined by the RSDE, that depends strongly on the type and concentration of the impurities. The results obtained with the model are compared with experimental observations.

Investigation of inelastic light scattering in heavily electron-irradiated alkali halides

The properties of several unusual Raman scattering peaks observed in heavily irradiated NaCl with vast amounts of colloidal sodium are investigated. A new peak (at 3580 cm-1) has been observed far beyond the cut-off limit of the one-phonon spectrum. This scattering phenomenon has been explained with an electron quantum confinement model where the electronic excitations are quantized as expected for wire-like structures of nano-particles of metallic sodium.

Characterization of Na precipitates in electron irradiated NaCl crystals by wide angle X-ray scattering (WAXS)

Samples of synthetic NaCl crystals have been exposed to different doses of electron irradiation up to 1500 MGy (150 Grad) at elevated temperatures, and studied subsequently by X-ray diffraction. Our experimental results clearly show that there is a close correspondence between the geometrical properties (such as lattice distances and the crystal orientation) of the host crystal and the radiation-induced Na precipitates, which is referred to as the Kurdjumov–Sachs orientation relationship (K–S OR). The size of the precipitates has been estimated.

Melting of sodium clusters in electron irradiated NaCl

In this paper we present the results of the first systematic investigation of the geometrical properties of sodium nanoclusters in NaCl using the combined results of differential scanning calorimetry (DSC) and atomic force microscopy (AFM). The melting behaviour of the sodium nanoclusters which had been produced in NaCl by electron beam irradiation up to a dose of 200 MGy has been studied by means of DSC. The sodium colloids have been visualized by AFM and their size distribution has been evaluated for two irradiation doses. The DSC scans of the irradiated samples exhibit two melting peaks located at temperatures below the melting temperature of bulk sodium. A phenomenological model has been proposed which explains this behaviour by the existence of two different structural states in the population of sodium colloids. We believe that small sodium clusters inherit the FCC structure from the surrounding Na sublattice in the NaCl matrix. When a small colloid grows, its lattice structure eventually transforms into the BCC structure of bulk sodium. The model results have been compared with the DSC measurements and the transition radius for the two structural states has been estimated.

Nucleation and growth of sodium colloids in NaCl under irradiation: theory and experiment

A mechanism of radiation-induced emission of Schottky defects from extended defects proposed originally for metals has recently been applied to ionic crystals, where it is based on interactions of excitons with extended defects such as dislocations and colloids. Exciton trapping and decay at colloids may result in the emission of F centers and consequent shrinkage of the colloid. In the present paper, the radiation-induced emission of F centers is taken into account in the modeling of nucleation and growth of sodium colloids and chlorine bubbles in NaCl exposed to electron or gamma irradiation. The evolution of colloid and bubble number densities and volume fractions with increasing irradiation dose is modeled in the framework of a modified rate theory and compared with experimental data. Experimental values of the colloid volume fractions and number densities have been estimated on the basis of latent heat of melting of metallic Na obtained with combined differential scanning calorimetry experiments and atomic force microscopy investigations of metallic clusters.

Systematic UHV-AFM experiments on Na nano-particles and nano-structures in NaCl

Results of systematic AFM (atomic force microscopy) experiments on heavily and moderatly irradiated NaCl samples are presented. The sodium nanoparticles and structures of nanoparticles are poduced in sodium chloride during irradiation. The AFM images of the nanoparticles have been obtained in ultra high vacuum (UHV) in the non-contact mode with an Omicron UHV AFM/STM system. The sizes and arrangements of the observed particles depend on the irradiation conditions. The melting behaviour of the sodium nanoclusters has been studied for these samples by differential scanning calorimetry (DSC). A phenomenological model has been proposed which explains the observed behaviour by the existence of two different structural states in the population of sodium colloids. AFM measurements of heavily irradiated natural rock salt samples have revealed ring- and chain-like clusters (or agglomerates) of Na colloid particles.

Modeling of the radiation-induced microstructural evolution in ionic crystals

Results of experimental and theoretical investigations are presented on heavily irradiated natural and synthetic NaCl crystals in the temperature range where anion defects are mobile. They give a strong evidence for the formation of vacancy voids, which cannot be explained by the Jain-Lidiard model used up to date for description of metal colloids and dislocation loops formed in ionic crystals during earlier stages of irradiation. We consider an additional set of reactions between experimentally observed extended defects (metal colloids, gas bubbles and voids) and point defects. The latter include F and H centers that are the primary defects produced by irradiation, and cation vacancies (with a trapped hole) that are secondary defects, produced in the process of dislocation climb due to absorption of extra H centers. We show that highly overpressurized bubbles of fluid halogen are strongly biased for absorption of H centers, which makes them grow via punching out interstitial dislocation loops. The loops grow and produce cation vacancies that are subsequently trapped at the incoherent colloids together with extra F centers giving rise to the colloid-void transition. Elastic interaction between extended defects and point defects is shown to play a major role, since it determines the bias factors of extended defects, which is a major driving force of the microstructural evolution under irradiation. A quantitative comparison of the new model for radiation damage in NaCl with experimental data is presented. Mean sizes and volume fractions of all types of observed defects are calculated. It is shown that voids formed due to agglomeration of F centers and cation vacancies can grow to the dimensions exceeding the mean distance between colloids and bubbles, eventually absorbing them, hence, bringing the halogen gas and metal to a back reaction. Impurities play a major role in the void development with increasing irradiation dose, which strongly affects the radiation stability of NaCl. (C) 1999 Elsevier Science B.V. All rights reserved

AFM of metallic nano-particles and nano-structures in heavily irradiated NaCl

AFM investigations are reported for heavily, electron irradiated NaCl crystals in ultra high vacuum (UHV) in the non-contact mode with an UHV AFM/STM Omicron system. To avoid chemical reactions between the radiolytic Na and oxygen and water, the irradiated samples were cleaved and prepared for the experiments in UHV. At the surface of freshly cleaved samples, we have observed sodium nano-precipitates with shapes, which depend on the irradiation dose and the volume fraction of the radiolytic Na. It appears that the nano-structures consist of (i) isolated nano-particles, (ii) more or less random aggregates of these particles, (iii) fractally shaped networks and (iv) ‘‘fabrics’’ consisting of bundles of Quasi-1D arrays forming polymeric networks of nano-particles. Almost independent of the concentration of the metallic Na in the samples the size of the individual nano-particles is in the range 1–3 nm. Our new AFM results are fully in line with our CESR and previous Raman scattering results.

Investigation of the energy deposition profile in NaCl under electron irradiation

We have proposed a new model for the calculation of the absorbed dose profile in a thick target under 0.1…3 MeV electron irradiation. The build-up phenomenon is shown to increase the maximum of the energy deposition profile in thick samples by a factor of two in comparison with thin targets as a result of backscattered and multi-scattered electrons. The absorbed dose profile in NaCl for 0.5 MeV electron irradiation has been determined by measuring the stored energy with differential scanning calorimetry.Запропоновано нову модель для розрахунку профілів поглиненої енергії в товстих мішенях, що опромінюються пучком електронів з енергіями 0.1…3 МеВ. Показано, що внаслідок ефекту накопичування дози, зв'язаного з багаторазовим і зворотним розсіюванням електронів, максимальне значення поглиненої енергії у товстих мішенях збільшується вдвічі в порівнянні з тонкими мішенями. Вивчено профіль розподілу поглиненої енергії в кристалічній пластинці NaCl, опроміненої електронами з енергією 0.5 МеВ, шляхом вимірювання запасеної енергії методом диференціальної скануємої калориметрії.Предложена новая модель для расчета профилей поглощенной энергии в толстых мишенях, облучаемых пучком электронов с энергиями 0.1…3 МэВ. Показано, что вследствие эффекта накопления дозы, связанного с многократным и обратным рассеянием электронов, максимальное значение поглощенной энергии в толстых мишенях увеличивается вдвое по сравнению с тонкими мишенями. Изучен профиль распределения поглощенной энергии в кристаллической пластинке NaCl, облученной электронами с энергией 0.5 МэВ, путем измерения запасенной энергии методом дифференциальной сканирующей калориметрии