Norair E. Grigoryan

Radiation-modified natural zeolites for cleaning liquid nuclear waste (irradiation against radioactivity)

There have been comparatively few investigations reported of radiation effects in zeolites, although it is known that these materials may be modified substantially by exposure to ionizing radiation. Thus, by exposure to γ-rays or high-energy particles, the charge states of atoms may be changed so to create, and accumulate, lattice point defects, and to form structurally disordered regions. Such a technique may permit the creation, in a controlled fashion, of additionally useful properties of the material while preserving its essential stoichiometry and structure. Accordingly, we present an application, in which the cation-exchange capacity of a natural zeolite (clinoptilolite) is substantially enhanced, for the treatment/decontamination of water contaminated with radionuclides e.g. 134Cs, 137Cs and 90Sr, by its exposure to high-energy (8 MeV) electrons, and to different total doses.

Study of optical absorption of corundum in the vacuum ultraviolet region of synchrotron radiation

Abstract The optical system with a modified normal-incidence monochromator, installed in the synchrotron radiation channel, has been developed. Some changes in the optical system make it possible to use the monochromator as a double-beam spectrophotometer maintaining the polarization of synchrotron radiation at the exit. Experimental control and the recording of irradiation and spectra are automated by a computer. The high degree of polarization of synchrotron radiation, in contrast with other sources, makes it possible to detect experimentally the optical absorption bands at 4.8, 5.4, 6.1, 6.3 and 7 eV photon energies in corundum single crystals grown by the method of horizontally directed crystallization and irradiated with high-energy particles. It is proved that at the orientation of the electrical vector E of synchrotron radiation parallel to the optical axis ( C 3 ) of the crystal, the intensity of the 6.3 eV absorption band is larger than that at the orthogonal orientation. In the case of E ⊥ C 3 the optical absorption bands at 5.4 and 7 eV are more outlined. Optical bleaching in the F-band results in the reduction of F-center concentration and in the increase of the intensity of the 4.86, 5.4, 6.3 and 7 eV absorption bands. The results show that the 7 eV band is also due to electron transition in the F+-center.

Vacuum ultraviolet luminescence excitation spectra of α-Al2O3 single crystals

Abstract The luminesence excitation spectra of corundum single crystals irradiated with 50 MeV electrons were studied. In the region of fundamental absorption there appeared excitation bands (blue and ultraviolet) which correlated well with the optical absorption bands of the F + -centre in the absorption (reflection) spectra. The excitonic peaks observed in the excitation spectra of red luminescence were explained by the charge energy transfer by Cr 2+ and Cr 4+ ions at excitonic excitation. At hv > 9.5 eV sharp drops appeared which were interpreted as nonirradiative near-surface recombinations, the probability of which sharply increased at high absorption coefficients. The enhancement of the luminescence efficiency in the high-energy region was connected with the decay effect of electron excitations as well as with the influence of volumetric excitations leading to the enhancement of recombination glow.

The study of corundum single crystal reflection spectra in the corundum fundamental absorption region

Abstract With the use of the highly polarized synchrotron radiation the reflection spectra of a nominally pure corundum single crystal were measured. The electron irradiation is shown to lead to the formation of an F + -centre (6.3 and 7 eV) and to the shift of the maximum of the excitonic peak as well as to the display of anisotropy of high-energy bands in the reflection spectra depending on the orientation of the electric vector E to the optical axis C 3 . The observed effects in the reflection spectra in the fundamental absorption region are explained by the change in the density of 2p electron oxygen states, which is due to the increased concentration of defects in the near-surface layers owing to radiation effects.

Comparative analyses of physical properties of natural zeolites from Armenia and USA

Dielectric permittivity, specific conductivity, luminescence and infrared absorption properties of natural zeolites from Armenia and the US were studied. Optical spectra and dielectric permittivity including specific conductivity are compared for samples of both origins. It was shown that measured properties determine clinoptilolite content in the sample. In the sample of US origin there were more impurities than in Armenian ones. In the samples of Armenian origin clinoptilolite properties are more prominent.

The effect of electron irradiation on the optical properties of the natural armenian zeolite-clinoptilolite

Infrared (IR) absorption and luminescence in chemically and radiation-modified natural Armenian Zeolite (clinoptilolite) samples have been studied. The luminescence was studied in 390–450 nm and 620–710 nm wavelength bands, and the IR measurements were carried out in the 400–5400 cm−1 range. It is shown that the luminescence intensity depends on the content of pure clinoptilolite in the samples and, probably on the distribution of “passive” luminescence centers over Si and Al sites that became “active” under radiation or chemical treatment. The samples of electron irradiated clinoptilolite have higher luminescence intensity than the chemically and thermally treated ones. A decrease in the intensity of IR absorption bands at 3550 cm−1 and 3650 cm−1 was found after irradiation.

BUNCH TIME STRUCTURE DETECTOR WITH PICOSECOND RESOLUTION

We propose a device measuring bunch time structure of continuous wave beams, based on radio frequency (RF) analysis of low energy secondary electrons, (SEs). By using a currently developed 500 MHz RF deflector it is possible to scan circularly and detect the SEs, amplified in multi-channel plates (MCP). It is demonstrated that the noise induced by RF source is negligible and the signals, generated in MCP, can be processed event by event, without integration, by using available nanosecond-time electronics. Therefore, this new device can be operated alone as well as in combination with more complex setups, using common electronics and providing time resolution for single SE better than 20 ps.

Dielectric properties and specific conductivity of armenian natural clinoptilolite irradiated by electrons

The results of dielectric properties and direct current specific electric conductivity measurements in Armenian natural clinoptilolite samples are presented. Electron irradiation with energy 8 MeV and thermal treatment of samples are performed to elucidate possible enhancement mechanisms of clinoptilolite parameters. The results are discussed on the basis of new point structural defects formation and recombination of initial ones in samples. It was shown that the irradiation dose of 3·1016 el/cm2 is critical for natural zeolite structural change, which is manifested by significant changes in dielectric properties and other characteristics. The samples subjected to high temperature heating after electron irradiation, in comparison with an unquenched one, have a significantly higher (about an order of magnitude) value of specific conductivity.

Quasi-Chemical Reactions in Irradiated Silicon Crystals with Regard to Ultrafast Irradiation

This paper reports results from an investigation of the interaction of displaced Si-self atoms (I) and their vacancies (V), with impurities in crystalline silicon (Si), as induced by micro-second pulse duration irradiation with electrons at different energies: 3.5, 14, 25 and 50 MeV and pico-second pulse duration with energy 3.5 MeV. V-V, I-impurity atom and V-impurity atom interactions are analyzed both experimentally and as modeled using computer simulations. A process of divacancy (V2) accumulation in the dose-dependent linear region is investigated. The effect of impurities on recombination of correlated divacancies, and I-atoms that had become displaced from regular lattice points is estimated by computer modeling of an appropriate diffusion-controlled process. It is concluded that the experimental results can be interpreted quantitatively in terms of a strongly anisotropic quasi-one-dimensional diffusion of displaced I-atoms. In addition, a significant difference is found between the effects of pico-second duration electron beam irradiation, which causes the formation of A-centre (V + Oxygen) clusters, while when the beam is applied on a micro-second timescale, divacancies are created instead, although the electrons have the same energy in both cases.

The Use of Different Pulsed Electron Irradiation for the Formation of Radiation Defects in Silicon Crystals

This paper reports the formation of structural defects in the lattice of silicon (n-Si) single crystals, as a result of irradiation by different intensities and pulses of electrons. The samples were studied by means of Hall effect measurements of electro-physical parameters (specifically the concentration of the main charge carriers) as a function of temperature and radiation dose. The role of the radiation current density (pulse height) is discussed, which gives rise to a peculiar behavior in the electrical-physical properties of n-Si. In particular, thermal processes are found not to develop, due to the ultrafast (pulse duration in the range 10−12–10−13s) nature of the incident radiation, which causes an almost “pure” energy interaction to occur between the radiation and the atoms within the crystal, and the formation of cluster defects. A scheme for the time-scale of the formation of these radiation defects is presented. From the dose and temperature dependences of the concentration of main charge carriers, the radiation defects introduction rates were determined.