Neutron-irradiated Crystals Research Articles

Luminescence of neutron-irradiated quartz crystals

The effect of crystal structural inhomogeneities in the form of β-phases, metamict phases, as well as point defects and impurities on the kinetics of phase transformations and amorphization occurring in quartz crystals under neutron irradiation has not been sufficiently investigated.For this reason, the kinetics of gamma-luminescence (GL) and photoluminescence (FL) bands in quartz crystals grown on seeds irradiated with neutrons at a dose of 5*1018, 1019 and 5*1019 n/cm2 after additional neutron irradiation of 1016-8*1020n/cm2 in the temperature range of 77-300 K. Based on the analysis of luminescent properties, the nature of intrinsic defects in the structure of quartz crystals, their role in the formation of color and glow centers, has been studied. Based on the analysis of experimental data, the study of the luminescent properties of quartz crystals determined the properties of various centers of luminescence and the formation of color centers of crystalline quartz.

EPR-active dimer centers with S=1 in α-Al2O3 single crystals irradiated by fast neutrons

Corundum (α-Al2O3) possesses a number of unique properties, including high tolerance to harsh radiation environment. Material functionality is strongly affected by radiation-induced structural defects, single interstitial-vacancy Frenkel pairs and their aggregates. Three novel paramagnetic two-electron defects with total spin S = 1 have been revealed in fast neutron irradiated α-Al2O3 single crystals by means of the EPR method. Analysis of the EPR characteristics of these defects allows to conclude that the main novel D1 defect (initial concentration after irradiation about 2.8 × 1016 cm−3) is oriented along one of three equivalent b crystal axes, that connect two adjacent anion sites of oxygen triangles on the base corundum plane. The D1 defect is ascribed to the F22+ center (two adjacent single-charged oxygen vacancies), while the D2 and D3 have the same dimer structure with an additional defect in their vicinity. The suggested defect structure is supported by pulse annealing of the EPR signal of relevant defects: defect concentration dependence contains a rising stage at 500–630 K, while complete thermal destruction due to the recombination with complementary oxygen interstitials occurs by 720–750 K. Such behavior qualitatively coincides with earlier studied thermal annealing of the F22+ center optical absorption in neutron-irradiated corundum.

Detection of hidden oxygen interstitials in neutron-irradiated corundum crystals

An α-Al2O3 (corundum) possesses unique optical, electrical and mechanical properties, demonstrates high tolerance to heavy irradiation and, in particular, is in a short list of candidates for optical/diagnostics windows in advanced fission and forthcoming fusion (DEMO) reactors. However, material functionality is strongly affected by structural defects induced by radiation of different types. Optical and magnetic characteristics of Frenkel defects (interstitial-vacancy pairs) as well as the processes of their thermal annealing (radiation damage recovery) have been investigated by means of optical absorption and the EPR methods in α-Al2O3 single crystals exposed to fast neutrons with fluence of 6.9 × 1018n/cm2. For the first time in metal oxides, a single oxygen interstitial, which is not adjacent to any other imperfection, has been detected. The analysis of the EPR spectra/parameters testifies that this interstitial forms with a regular oxygen ion a superoxide ion O2−, stabilized by a trapped hole. The interstitial becomes mobile above 500 K and recombines with a complementary oxygen vacancy that is a part of electronic F+ or F centers. The thermal annealing kinetics of the F and F+ centers has been theoretically analyzed in terms of interrelated diffusion-controlled recombination reactions of radiation defects. The analysis indicates that both, negatively charged and neutral oxygen interstitials (relevant absorption bands at 5.6 and 6.5 eV, respectively) co-exist in similar concentrations.

Channeling efficiency reduction in high dose neutron irradiated silicon crystals for high energy and high intensity beam collimation and extraction

The channeling process in bent silicon crystals are used since '70s to manipulate beams of high energy particles. During the last decade, several studies and experiments carried out by the UA9 Collaboration at CERN demonstrated the possibility to use bent crystals for beam collimation, extraction, focusing and splitting in particle accelerators. These crystals are subject to deterioration due to the interaction of the particles with the crystal lattice, degrading the beam steering performance. For this reason, robustness tests are crucial to estimate their reliability and operational lifetime. A ∼8% of reduction in channeling efficiency on crystals irradiated with 2.5·1021/cm2 thermal neutrons was measured and reported in this manuscript. Extrapolations to possible operational scenarios in high energy accelerators are also discussed.

Structural and magnetic properties of swift heavy-ion irradiated SiC

Ferromagnetism has been observed in ion and neutron irradiated SiC single crystals. In this paper, we present a structural and magnetic investigation on 6H–SiC irradiated by swift heavy ions. The co-exist of paramagnetism, superparamagnetism and ferromagnetism is revealed by using different magnetometry methods. The ferromagnetic component persists well above room temperature. This study confirms the general existence of defect-induced magnetism in SiC.

Thermal annealing and transformation of dimer F centers in neutron-irradiated Al2O3 single crystals

The precise study of the thermal annealing of the F2-type dimer defects, being under discussion in the literature for a long time and responsible for the number of absorption bands below 4.5 eV, has been performed in corundum single crystals irradiated by fast neutrons with a fluence of 6.9 × 1018n/cm2. The Gaussian components of the radiation-induced optical absorption with the maxima at 4.08, 3.45 and 2.75 eV have been considered as a measure of the F2, F2+and F22+centers, respectively. In contrast to the F and F+ centers, the concentration of which continuously decreases at the sample heating up to 1100 K, the concentration of dimer defects with different charge states passes the increasing stages above 500 K starting from the F22+centers. The tentative mechanisms of such rise of the F22+centers as well as of the subsequent transformation/rise of dimer centers, F22+→F2+→F2at 650-800 K are considered. The possible sources of carriers needed for the recharging of dimer centers are also analysed on the basis of thermally stimulated luminescence measurements up to ~850 K.

Atomic, electronic and magnetic structure of an oxygen interstitial in neutron-irradiated Al2O3 single crystals

A single radiation-induced superoxide ion O_{2}^{ - } has been observed for the first time in metal oxides. This structural defect has been revealed in fast-neutron-irradiated (6.9×1018n/cm2) corundum (α-Al2O3) single crystals using the EPR method. Based on the angular dependence of the EPR lines at the magnetic field rotation in different planes and the determined g tensor components, it is shown that this hole-type O_{2}^{ - } center (i) incorporates one regular and one interstitial oxygen atoms being stabilized by a trapped hole (S = 1/2), (ii) occupies one oxygen site in the (0001) plane being oriented along the a axis, and (iii) does not contain any other imperfection/defect in its immediate vicinity. The thermal stepwise annealing (observed via the EPR signal and corresponding optical absorption bands) of the O_{2}^{ - } centers, caused by their destruction with release of a mobile ion (tentatively the oxygen ion with the formal charge −1), occurs at 500–750 K, simultaneously with the partial decay of single F-type centers (mostly with the EPR-active F+ centers). The obtained experimental results are in line with the superoxide defect configurations obtained via density functional theory (DFT) calculations employing the hybrid B3PW exchange-correlation functional. In particular, the DFT calculations confirm the O_{2}^{ - } center spin S = 1/2, its orientation along the a axis. The O_{2}^{ - } center is characterized by a short O–O bond length of 1.34 Å and different atomic charges and magnetic moments of the two oxygens. We emphasize the important role of atomic charges and magnetic moments analysis in order to identify the ground state configuration.

EPR and optical spectroscopy of neutron-irradiated Gd3Ga5O12 single crystals

In this paper, we have performed comparative analysis of EPR, optical absorption (OA) and luminescence spectra for a series of Gd3Ga5O12 (GGG) single crystals irradiated with fast neutrons with fluencies varied from 1016 to 1020n/cm2. In a crystal irradiated with the maximum neutron fluence, the EPR spectra demonstrated the formation of several paramagnetic defects. In particular, EPR spectrum shows a strong resonance at (effective) g ≈ 1.4 with practically isotropic behavior in the crystal rotation around the [1 1 1] direction (magnetic field being perpendicular to [1 1 1]) and several weaker lines in the g ≈ 1.1–2.6 region, which show more pronounced angular dependences. While the photoluminescence of non-irradiated GGG is characterized by uncontrolled impurities, in the case of neutron-irradiated GGG, a broad asymmetric luminescence band was observed with a peak at 725–733 nm, which increased with fluence. Consequently, this band can be associated with the formation of radiation defects.

Room temperature ferromagnetism in D-D neutron irradiated rutile TiO2 single crystals.

Room temperature ferromagnetism (RTFM) was observed in unirradiated rutile TiO2 single crystals prepared by the floating zone method due to oxygen vacancy (VO) defects. D–D neutrons mainly collide elastically with TiO2, producing VO, titanium vacancies (VTi) and other point defects; the density and kind of defect is related to the neutron irradiation fluence. D–D neutron irradiation is used to regulate the concentration and type of defect, avoiding impurity elements. As the irradiation fluence increases, the saturation magnetization (Ms) first increases, then decreases and then increases. To verify the origin of RTFM, the CASTEP module was used to calculate the magnetic and structural properties of point defects in TiO2. VO induces a 2.39 μB magnetic moment, Ti3+ and F+ induce 1.28 μB and 1.70 μB magnetic moments, respectively, while VTi induces a magnetic moment of ∼4 μB. Combining experimental and theoretical results, increases in VO concentration lead to Ms increases; more VO combine with electrons to form F+, inducing a smaller magnetic moment. VO and VTi play a key role and Ms changes accordingly with larger fluence. VO, F+ and VTi are the most likely origins of RTFM.

Comparison of the F-type center thermal annealing in heavy-ion and neutron irradiated Al2O3 single crystals

Abstract The optical absorption and thermally stimulated luminescence of Al2O3 (sapphire) single crystals irradiated with swift heavy ions (SHI) 238U with energy 2.4 GeV is studied with the focus on the thermal annealing of the F-type centers in a wide temperature range of 400–1500 K. Its theoretical analysis allows us to obtain activation energies and pre-exponentials of the interstitial oxygen ion migration, which recombine with both types of immobile electron centers (F and F+ centers). A comparison of these kinetics parameters with literature data for a neutron-irradiated sapphire shows their similarity and thus supports the use of SHI-irradiation for modeling the neutron irradiation.

Comparison of methods applied in photoinduced transient spectroscopy to determining the defect center parameters: The correlation procedure and the signal analysis based on inverse Laplace transformation.

The procedure for determination of trap parameters by photo-induced transient spectroscopy is based on the Arrhenius plot that illustrates a thermal dependence of the emission rate. In this paper, we show that the Arrhenius plot obtained by the correlation method is shifted toward lower temperatures as compared to the one obtained with the inverse Laplace transformation. This shift is caused by the model adequacy error of the correlation method and introduces errors to a calculation procedure of defect center parameters. The effect is exemplified by comparing the results of the determination of trap parameters with both methods based on photocurrent transients for defect centers observed in tin-doped neutron-irradiated silicon crystals and in gallium arsenide grown with the Vertical Gradient Freeze method.

Optical absorption and Raman studies of neutron-irradiated Gd3Ga5O12 single crystals

In this work we have performed a comparative analysis of optical absorption and micro-Raman spectra for series of Gd3Ga5O12 (GGG) single crystals irradiated by fast neutrons with fluences from 1016 n/cm2 to 1018 n/cm2. It was found that the optical absorption spectra of non-irradiated Czochralski grown GGG consist of the relatively narrow lines in the UV spectral range related to the 4f–4f transitions in Gd3+. Transitions from the 6S7/2 ground state to the 6P, 6J and 6D states in a Gd3+ cation are clearly detected. For a GGG crystal containing Ca impurity ions, additional absorption band at 350 nm is observed, and it is tentatively ascribed to oxygen vacancies associated with Ca impurities. Several neutron-induced optical absorption bands, including ones related to F+ centres, were observed in fast neutron-irradiated Gd3Ga5O12 single crystals. Furthermore, clear evolution of the Raman spectra with neutron dose is also found. In particular, fine structure observed between 1350 and 1550 cm−1 is completely disappeared at high fluences, while several significant changes and the emergence of new transitions have been ascertained in spectral range of 100–850 cm−1.

Creation and thermal annealing of structural defects in neutron-irradiated MgAl2O4 single crystals

Abstract Several novel hole-type defects (a hole localized at a regular oxygen ion near a negatively charged structural defect) have been revealed in fast neutron irradiated MgAl2O4 crystals using the EPR method. The pulse annealing of the EPR signal of these centers was compared to that of radiation induced optical absorption in the same crystals. Taking into account the determined models of V1, V2 and V22 paramagnetic centers, the tentative scenario of the thermal annealing process of neutron-induced defects (hole-type and complementary electron F-type ones) is proposed. In addition, one more paramagnetic hole center consisting of an Al|Mg as-grown antisite defect near an aluminum vacancy is created just by X-ray irradiation at room temperature.

Electronic structure of carbon antisite in SiC: localization versus delocalization

In paper (Baranov et al., 2008 [26]) the EPR spectrum with large 13C isotope hyperfine splitting was observed in neutron-irradiated 6H-SiC crystals and attributed to the positively charged carbon antisite CSi+. However up till now numerous ab initio calculations show that this defect is optically and electrically inactive and hence it cannot be detected in experiment. In the present paper we have considered carbon antisite in framework of DFT with self interaction correction. We show that small polaron-like ground state with broken bond configuration may be formed in the case of positively charged carbon antisite CSi+ while for negatively charged defect formation of such state is energy-unprofitable. For unambiguous identification of carbon antisite the parameters of hyperfine interaction with central carbon CSi as well as with four nearest carbons were calculated and compared with EPR experiment both for positively and negatively charged centers.

Point defects and magnetic properties of neutron irradiated MgO single crystal

(100)-oriented MgO single crystals were irradiated to introduce point defects with different neutron doses ranging from 1.0×1016 to 1.0×1020 cm-2. The point defect configurations were studied with X-ray diffuse scattering and UV-Vis absorption spectra. The isointensity profiles of X-ray diffuse scattering caused by the cubic and double-force point defects in MgO were theoretically calculated based on the Huang scattering theory. The magnetic properties at different temperature were measured with superconducting quantum interference device (SQUID). The reciprocal space mappings (RSMs) of irradiated MgO revealed notable diffuse scattering. The UV-Vis spectra indicated the presence of O Frenkel defects in irradiated MgO. Neutron-irradiated MgO was diamagnetic at room temperature and became ferromagnetic at low temperature due to O Frenkel defects induced by neutron-irradiation.

Radiation damage induced in Al2O3 single crystal sequentially irradiated with reactor neutrons and 90 MeV Xe ions

The present investigation reports the effect of 90MeV Xe ion irradiation on neutron irradiated Al2O3 single crystals. Three irradiation experiments were performed, with neutrons only, 90MeV Xe ions only and with neutrons followed by 90MeV Xe ions. Neutron and 90MeV Xe ion irradiations were performed at NUR research reactor, Algiers, Algeria and at GANIL accelerator, Caen, France respectively. After irradiation, the radiation damage was investigated by Raman spectroscopy, Fourier Transform Infrared Spectroscopy (FTIR), optical absorption measurements, and X-ray diffraction (XRD) techniques. Raman technique revealed that the concentration of the defects formed in Al2O3 samples subsequently irradiated with neutrons and 90MeV Xe ions is lower than that formed in Al2O3 samples which were irradiated only with neutrons. This reveals the occurrence of ionization-induced recovery of the neutron damage. Furthermore, as revealed by XRD analysis, a new peak is appeared at about 2θ=38.03° after irradiation at high fluence (>3×1013Xe/cm2). It can be assigned to the formation of new lattice plane.

Identification of the zinc-oxygen divacancy in ZnO crystals

An electron paramagnetic resonance (EPR) spectrum in neutron-irradiated ZnO crystals is assigned to the zinc-oxygen divacancy. These divacancies are observed in the bulk of both hydrothermally grown and seeded-chemical-vapor-transport-grown crystals after irradiations with fast neutrons. Neutral nonparamagnetic complexes consisting of adjacent zinc and oxygen vacancies are formed during the irradiation. Subsequent illumination below ∼150 K with 442 nm laser light converts these (VZn2− − VO2+)0 defects to their EPR-active state (VZn− − VO2+)+ as electrons are transferred to donors. The resulting photoinduced S = 1/2 spectrum of the divacancy is holelike and has a well-resolved angular dependence from which a complete g matrix is obtained. Principal values of the g matrix are 2.00796, 2.00480, and 2.00244. The unpaired spin resides primarily on one of the three remaining oxygen ions immediately adjacent to the zinc vacancy, thus making the electronic structure of the (VZn− − VO2+)+ ground state similar to the isolated singly ionized axial zinc vacancy. The neutral (VZn2− − VO2+)0 divacancies dissociate when the ZnO crystals are heated above 250 °C. After heating above this temperature, the divacancy EPR signal cannot be regenerated at low temperature with light.

Laser excited novel near-infrared photoluminescence bands in fast neutron-irradiated MgO·nAl2O3

Abstract New near-infrared photoluminescence bands were observed in neutron-irradiated spinel single crystal upon excitation by a 532 nm laser. The surface morphology of the unirradiated and fast neutron-irradiated samples was investigated using atomic force microscopy and scanning probe microscopy. Fast neutron-irradiated samples show a strong emission peak at 1685 nm along with weak bands at 1065 and 2365 nm. The temperature dependence of the photoluminescence intensity was also measured. At lower temperatures, the dominant peak at 1685 nm shifts toward lower energy whereas the other peaks remain fixed. Activation energies of luminescence quenching were estimated to be 5.7 and 54.6 meV for the lower and higher temperature regions respectively.

Luminescence and EPR spectroscopy of neutron-irradiated single crystals of magnesium aluminium spinel

Neutron irradiated single crystals of stoichiometric MgAl2O4 and MgAl2O4:Mn2+ (0.15 wt. %) were studied using the methods of luminescence spectroscopy and versions of electron paramagnetic resonance. In stoichiometric MgAl2O4:Mn2+ single crystals, the transition of a part of manganese impurity ions from tetrahedral to octahedral coordination, caused by irradiation with fast fission neutrons, is detected using photoluminescence spectra. This fact confirms the partial inversion of a normal spinel due to neutron-irradiation. Using pulse EPR technique it is shown that an electron in the field of oxygen vacancy (i.e. from neutron-irradiation induced F+ centres, which are spatially separated from each other) is in hyperfine interaction only with neighbouring 27Al nuclei with nuclear spin 5/2.

Plasticization of neutron-irradiated LiF crystals upon thermal stimulation of a single-system dislocation glide

It has been shown that post-radiation annealing of LiF crystals irradiated by high neutron fluences (1015–1018 neutrons/cm2) at comparative low temperatures (300–400°C) creates optimal conditions for single-system dislocation glide, which favors a complete recovery of the plasticity with conservation of a significant fraction of radiation hardening.