Characteristics of a stilbene scintillation crystal in a neutron spectrometer

Various organic scintillators are commonly used as the detecting material for neutrons, but these detectors are less sensitive to gamma rays. In particular, stilbene crystals and BC501A (NE213, EJ301) have good pulse-shape discrimination (PSD) between neutron and gamma-ray events, and have been selected as the media for fast-neutron detection among the organic, inorganic and plastic materials in a mixed radiation field. Although some of the scintillation characteristics of stilbene crystals have been studied, the detailed scintillation characteristics of the crystal are not completely understood. In this study, the light yield, decay time and pulse shape discrimination capability of a stilbene crystal were measured because this crystal is an optimized detector in a large flux of neutrons such as those might be found in cyclotron and charged particle accelerator facilities. The pulse-shape discrimination of neutrons and gamma rays with a stilbene crystal was measured using a ²⁵²Cf neutron source at room temperature. A neutron tagger module was used for the neutron and gamma separation using the charge comparison method in real time. The total pulse width for the charge integration and the delay from the peak-to tail start time were optimized for a better neutron and gamma separation. The relative light yield and decay time of the stilbene crystal scintillator were also measured.     

Sum-frequency mixing of optical vortices in nonlinear crystals

Results of theoretical and experimental investigation of the influence of walk-off on sum-frequency mixing of optical vortices in nonlinear crystals are presented. Various phenomena of vortex interaction such as decay of higher order vortices into singly charged vortices, formation of aligned arrays of vortices perpendicular to walk-off direction, particle-like pulling and pushing of vortices, and appearance of pairs of vortices having opposite charges were observed.     

Fluorescence decay time measurements in tetracene crystals excited with synchrotron radiation

Fluorescence decay times from tetracene single crystals excited at room temperature with synchrotron radiation have been recorded as a function of the excitation wavelength (in the 400–500 nm range). A non-exponential decay with two decay rates is observed. The analysis of our data shows that the first singlet exciton level of tetracene (single crystal) decays radiatively mainly through, as we call it, channel 1, with a lifetime of 0.200 ± 0.020 ns. About 10% of the emitted fluorescence transits through channel 2 with a lifetime of 1.7 ± 0.2 ns. These results do not agree with previously published decay data obtained when tetracene is excited by means of powerful lasers. Thus there is experimental evidence to believe that the decay properties of condensed materials can be very dependent on the excitation density. Because synchrotron radiation compared to lasers is a very weak source, and therefore secondary effects are minimized in our experimental conditions, the decay values reported in the present work are the true lifetimes of the tetracene single crystal.     

Tunable optical vortex arrays using spontaneous periodic pattern formation in nematic liquid crystal cells

Controllable liquid crystal (LC) defects can provide an effective approach to creating tunable optical vortices. We develop a method to create tunable matter vortex arrays in an LC cell, in which +1 and −1 defects are periodically arranged in a square grid lattice. Spontaneous formation of the periodic defect array is achieved using a spontaneous standing pressure wave without using any patterned electrode or patterned alignment layer. The +1 and −1 defects in the array can induce optical vortices with opposite handedness, and the matter vortex array produces a periodic optical vortex array with orbital angular momenta of −2ℏ and +2ℏ in the same grid lattice. Because the pitch of the grid can be controlled, the method can provide a useful pathway to producing tunable optical vortex arrays for various applications such as advanced optical communication and quantum computation.     

Visualization of interacting crossing vortex lattices in the presence of quenched disorder

We have imaged interacting crossing pancake vortex (PV) and Josephson vortex (JV) lattices in highly anisotropic Bi2Sr2CaCu2O(8+delta) single crystals under tilted magnetic fields. The dependence of vortex structures on in-plane field is in good quantitative agreement with theoretical predictions, yielding an almost temperature-independent anisotropy parameter of gamma=640+/-25. We directly confirm that the PV/JV attraction arises from small PV displacements in the presence of JV supercurrents and demonstrate how the existence of quenched disorder leads to indirect JV pinning and dynamic vortex fragmentation.     

Critical current density, magnetic relaxation and pinning distribution in Bi2Sr2CaCu2Ox superconductors

Critical current densities and magnetic relaxation have been measured for Bi₂Sr₂CaCu₂O_x (Bi-2212) single crystals and melt-processed ceramic samples. Strongly nonlogarithmic decay of the magnetization is observed down to 10 K in Bi-2212 single crystals over a time range of more than 3 decades. Different decay behaviours are found in Bi-2212 single crystals, bulk ceramics and powdered ceramics. The experimental results are discussed within the pinning distribution model of Hagen and Griessen. A pinning distribution function yielding the power law decay of magnetization as an analytical solution is found. A careful analysis of the pinning energy distribution reveals high energy tails which can hardly be associated with single vortex pinning mechanisms. These high pinning energies can, however, be explained by reinterpreting the Hagen-Griessen model in terms of collective pinning theory.     

Vortices in superconducting nano-networks with anti-dots array

Vortices (magnetic flux quanta) in the superconducting networks perforated with anti-dots (holes) arrays behave as electrons in atomic lattice of crystals. Repulsive and attractive interaction among vortices and anti-dots resemble to those among electrons and atoms in crystals. To confirm the variety of the vortex physics similar to the solid state physics, we have fabricated such superconducting networks with antidots array in metallic, inter-metallic and high-T _c superconductors (HTSCs), and have measured magneto-resistance of vortex-flow. In these materials, we have observed integer-matching at the matching fields and fractional-matching effect between them. Most of them are well explained by commensurability between Abrikosov vortex lattice and anti-dots array. Furthermore, the effect of the anti-dots array in HTSCs appears as another kind of phase transitions instead of to the first-order melting transition of vortex lattice in pristine samples.     

Yb∶YAG晶体的红外闪烁特性

用脉冲电子束激发测量了不同Yb3+掺杂浓度的Yb∶YAG晶体的红外(IR)闪烁发光性能。Yb∶YAG晶体的IR闪烁发光具有高的光产额和长的衰减时间,但存在浓度猝灭效应和温度依赖关系。Yb∶YAG晶体的IR闪烁性能还与晶体品质有关,相同掺杂浓度的Yb∶YAG晶体,品质优异的会获得更高的光产额。这一初步的研究成果表明,部分掺Yb3+晶体有可能用于医学成像装置。     

Bleaching of color centers in NaCl crystals with laser pulses

Abstract

Bleaching of F centers in gamma irradiated NaCl crystals at room temperature and elevated temperatures using laser pulses from pulsed laser systems is studied in this paper. The rate of decay of F centers is monitored and the decay constants are evaluated. A comparison on the decay kinetics is made for CW and pulsed bleaching with a He-Cd laser. F center to C center conversions using a thermo-optic scheme are carried out with nanosecond dye laser pulses in gamma irradiated NaCl crystals.     

Reflection of surface acoustic waves in the bulk of a hexagonal single crystal

It has been shown that surface acoustic waves in some hexagonal crystals can include not only the waves outgoing from the surface into the bulk of the crystal but also the reflected waves, i.e., those coming from the bulk of an infinite single crystal. The outgoing and reflected waves decay exponentially with the distance from the surface. It has been found that the reflected wave can exist if the velocity of its propagation is below the critical value, which does not exist for some crystals. The numerical calculations have shown in what real hexagonal crystals the reflected wave can exist. The values of the critical velocity have been found for a number of hexagonal crystals.     

Possibility of elastico-mechanoluminescence dosimetry using alkali halides and other crystals

The elastico-mechanoluminescence (EML) intensity of X or γ-irradiated alkali halide crystals can be used in radiation dosimetry. The EML intensity of X or γ-irradiated alkali halide crystals increases linearly with the strain of the crystals, and when the crosshead of the testing machine deforming an X or γ-irradiated crystal is stopped, then the EML intensity decreases with time. The semilog plot of the EML intensity versus (t − t_c) (where t_c is the time where the crosshead of the testing machine is stopped) indicates that, in the post-deformation region, the EML intensity initially decreases exponentially at a fast rate and later on it decreases exponentially at a slow rate. The EML intensity increases linearly with the density of the F-centres in the crystals. This fact indicates that elastico-ML can suitably be used for the radiation dosimetry. The EML spectra of X or γ-irradiated alkali halide crystals are similar to their thermoluminescence spectra. Based on the detrapping of electrons during the mechanical interaction between the dislocation segments and F-centres, an expression is derived, which indicates that the EML intensity should increase linearly with the density of F-centres in the crystals. The expression derived for the decay of EML indicates that the decay time for the fast decrease of EML should gives the pinning time of dislocation segments (lifetime of interacting F-centres), and the decay time for the slow decrease of EML intensity should gives the lifetime of electrons in the shallow traps. As the elastic deformation is non-destructive phenomenon and the EML intensity depends on the radiation dosage given to the alkali halide crystals, similar to the thermoluminescence and photo-stimulated luminescence, the EML of alkali halide crystals and other crystals may be used for the radiation dosimetry. In EML dosimetry, the same crystal can be used number of times because the elastic deformation does not cause permanent deformation in the crystals, and moreover, comparatively the devices needed for the EML measurements are of low cost and very simple. In recent years, a large number of elastico mechanoluminescent materials have been investigated, and the study of their suitability for the radiation dosimetry may be interesting.     

Transient vortex states in Bi2Sr2CaCu2O(8+delta) crystals

A high temporal resolution magneto-optical system is employed to observe the time evolution of the vortex structure in Bi(2)Sr(2)CaCu(2)O(8+delta) crystals after a sudden application of a magnetic field. The magneto-optical images reveal dynamic coexistence of two vortex phases: a quasiordered phase in the sample interior and a transient disordered phase near the sample edges. The border between these two phases, marked by an abrupt change in the gradient of the local induction, moves with time. This motion enables tracing the decay of the transient state and the concurrent growth of the thermodynamic vortex phases. The growth rate is sensitive to the location in the field-temperature phase diagram.     

Self assembly of inorganic nanocrystals in 3D supra crystals: Intrinsic properties

Here we describe how arrangements of nanocrystals can self-organize in 3D arrays called supra crystals. The 3D arrays can fall into the familiar categories of face centered cubic (fcc), hexagonal compact packing (hcp) crystals, and body centered (bcc) crystals. Intrinsic collective properties of these 3D arrangements are different from the properties of individual nanoparticles and from particles in bulk.We demonstrate by two various processes and with two types of nanocrystals (silver and cobalt) that when nanocrystals are self ordered in 3D superlattices, they exhibit a coherent breathing mode vibration of the supra crystal, analogous to a breathing mode vibration of atoms in a nanocrystal.Comparison between the approaches to saturation of the magnetic curve for supra crystals and disordered aggregates produced from the same batch of nanocrystals is similar to that observed with films or nanoparticles either highly crystallized or amorphous.     

Growth of LuAP/LuYAP single crystals for pet applications

The light yield and decay properties are the most crucial parameters of cerium-doped lutetium orthoaluminate (LuAlO 3 -Ce, LuAP) and yttrium containing solid solution (Lu 1 m x Y x AlO 3 -Ce, LuYAP), which will be used in Positron Emission Tomographs (PET). Both characteristics strongly depend on the optical quality and cerium concentration introduced into the crystal. Factors governing the crystal quality are commented on. A clear improvement of scintillation properties with increasing the doping concentration is demonstrated in a series of LuYAP (Lu=80 v at.%) crystals scanning the concentration range from 0.1 to 0.4 v at.%. Testing of relations is performed between the crystal doping level and low-temperature thermoluminescence spectra, as well as between the doping level and absorption spectra in UV range.     

Suppression and Acceleration Effects of Measurements on A-type Atomic Decay in Anisotropic Photonic Crystals

The measurement-induced suppression and acceleration effects on A-type atomic decay in three-dimensional anisotropic photonic crystals are investigated. We consider the two cases, only one or both of the two atomic resonant frequencies being near the band edge of a photonic crystal. Due to the deformation of the electromagnetic density of states, the repeated projection onto the excited state can lead to decay suppression or acceleration effect already at rather low repetition rates. Such measurement-induced effects are dependent on the frequency of measurements and the relative position of the atomic upper level from the band edge of photonic crystals. The behaviors of instant and effective decay rates are studied in detail, which are different from the case of isotropic photonic crystals.     

Generation of optical vortex array based on the fractional Talbot effect

A phase-only diffractive optical element (DOE) for generating array of optical vortices with high compression ratio is presented. Since it is designed according to the fractional Talbot effect, we name the DOE optical vortex Talbot array illuminator (OVTAI). As an example, an OVTAI for generating a hexagonal array of optical vortices is designed, and demonstrated through displaying the OVTAI on a programmable liquid crystal spatial light modulator. The diffraction properties of the vortex array generated by the OVTAI are observed and analyzed, and an optimal distance for generating sharp ringed vortex arrays is given.     

Vortex shedding and vorticity fluxes in the wake of cylinders within a random array

This work investigates the impact of the background turbulence generated by randomly placed cylinders on the vortex shedding regime and the mechanisms associated to vorticity fluxes. The goals are achieved by exploring velocity databases acquired with a two-dimensional particle image velocimetry system in two types of turbulent flow experiments: flow around a single infinite cylinder and flow within random array of infinite cylinders. Formation lengths, power spectral density functions and vortex distributions are employed to discuss the vortex shedding regime. The effects of background turbulence and vorticity cancellation, due to opposite sign vorticity, on the vorticity fluxes are discussed. The results show that the background turbulence reduces the formation length and consequently increase the shedding frequency. The stronger decay of longitudinal vorticity flux in denser arrays is not accompanied by an increase of the lateral flux of vorticity. Furthermore, it was concluded that the decay of longitudinal vorticity flux is mainly caused by the vorticity cancellation due to the vorticity of opposite sign of close downstream cylinders.     

Luminescence of different modifications of crystalline silicon dioxide: Stishovite and coesite

Luminescence of very small samples of single crystals of coesite and stishovite has been studied. The spectra were detected under ionizing radiation (X-ray and electron beam) and the decay kinetics of cathodoluminescence in the range of time from 10 ns to 3 ms was measured. The coesite luminescence possesses a broad band at 3 eV with exponential decay about 680 μs at 80 K. The nature of this luminescence was explained as a self-trapped exciton creation in tetrahedron framework. The stishovite luminescence possesses two bands—blue (2.8 eV) and UV (4.7 eV). The UV band intensity grows more than 20 times with irradiation dose from initial level. This shows that the corresponding luminescence centers could be induced by the radiation. The decay of the UV band possesses a fast and a slow component. The determination of the fast decay parameters is beyond the capabilities of our apparatus (less than 10 ns), whereas the slow decay of the UV is non-exponential and takes place in the range of hundreds of microsecond. The blue band decay kinetics can be well approximated by power law ∼t⁻², which may correspond to recombination of defects created by radiation. The stishovite single crystal luminescence is very similar to that of germanium dioxide single crystal of rutile structure. The nature of the stishovite luminescence is explained as recombination of defects created by irradiation in octahedron-structured lattice.     

Incommensurateness effects in a lattice Ginzburg-Landau model

This paper discusses the effect of magnetic translational symmetry on the vortex structure in superconducting crystals with a large basis in artificial Josephson media (regular lattices of superconducting clusters) prepared with opal as the base material. For external magnetic fields lower than the upper critical field, the lattice Ginzburg-Landau model reduces to the two-dimensional Frenkel’-Kontorova model which in some cases is exactly solvable, in which the crystal lattice plays the role of an “hard sublattice” while the deformable vortex lattice plays the role of a “soft sublattice.” It is shown that static shear waves in the vortex lattice are solutions to the two-dimensional sine-Gordon equation with an additional condition of incompressibility implied by flux quantization. The pinning energy is found as a function of the magnetic field, nearness to the transition line, and the crystal lattice constant. Fiz. Tverd. Tela (St. Petersburg) 39, 1158–1162 (July 1997)     

Space-charge grating kinetics in photorefractive crystals

Equations for the grating space-charge density and concentration of free carriers induced by a standing light wave in an anisotropic photorefractive crystal are derived and solved. The charge density and electric field are found to depend on the orientation of the graing relative to the crystal axes. It is shown that the grating kinetics is determined by the difference between two exponentials with characteristic times the sum of which can be treated as the decay constant of the grating. Three ranges of Maxwell relaxation times are distinguished, where this time constant behaves differently. The grating kinetics for a number of crystals is considered.