Plasticity of perfect crystals

A specific type of plasticity induced by complicated pointlike thermally-induced defects is proposed. In this case it is possible to derive a system of hydrodynamic equations describing bulk crystal dynamics and a set of boundary conditions for them for both crystal-crystal and crystal-liquid interfaces. A certain limit of those gives the boundary condition introduced by Herring.² The obtained system describes a phenomenon of equilibrium plasticity. The defect-induced motion of rigid bodies through the crystal is investigated on the basis of presented equations.     

Efficient temporal shaping of ultrashort pulses with birefringent crystals

We report on a simple and robust technique to temporally shape ultrashort pulses. A number of birefringent crystals with appropriate crystal length and orientation form a crystal set. When a short pulse propagates through the crystal set, the pulse is divided into numerous pulses, producing a desired temporal shape. Flexibility in the final pulse shape is achieved through varying initial pulse duration, divided-pulse number, the polarization-mode delay, and energy distribution of the divided pulses. The energy efficiency of the technique is near 100% for a pulse train of alternating polarizations, and 50% for a linearly polarized pulse train.     

Neutron transmission properties of perfect silicon crystals

Silicon crystals are of interest as filter material for thermal neutron beams. For this reason the total cross-section has been determined as a function of temperature for neutron energies ranging from 22 × 10^?4 eV to 50 eV. Whereas cooling the filter to 77 K considerably improves the transmittance for thermal neutrons a further decrease in temperature has only a minor effect. The observed cross-section is well described by a treatment of thermal diffuse scattering based on the Debye approximation. Precise values of σ_abs and σ_free are given and compared to literature.     

Measurement of d + T neutron pulses

A high efficiency detector is proposed for the measurement of short pulses of d + T neutrons. It is based on the activation of sodium in a sodium iodide scintillation detector via the ²³Na(n, α)²⁰F reaction and the immediate, postpulse measurements of the induced beta activity which has an 11 s half-life. The detector is insensitive to neutrons with energies below about 7 MeV, and may be calibrated directly against an absolute technique using a “steady current” d + T neutron generator.     

MAMBO II: neutron lifetime measurement with storage of ultra cold neutrons

Building on the experience that Mampe gained with his ultra-cold neutron (UCN) storage bottle MAMBO I (Phys Rev. Lett. 63 (1989) 593) the successor experiment MAMBO II has been designed and set up. The basic method of this experiment relies on the storage of UCN in a rectangular glass vessel of variable size with walls covered by Fomblin oil. A preliminary value for the lifetime of 881.0±3 s has been measured.     

Thermal neutron detection with Ce doped LiCaAlF6 single crystals

Cerium-doped LiCaAlF₆ (Ce:LiCAF) crystals have been studied as scintillators in application to thermal neutron detection. Three crystals: high-doping Ce:LiCAF, low-doping Ce:LiCAF with 50% enrichment of ⁶Li (both 10 mm×10 mm×2 mm, rectangular) and high-doping Ce:LiCAF with 95% enrichment of ⁶Li (Ø50.8 mm×2 mm, discus) coupled to Photonis XP5300B PMT, were tested. The response of these crystals to neutrons emitted from a paraffin moderated ²³⁸PuBe source has been investigated. Thermal neutron peaks have been found at a Gamma Equivalent Energy (GEE) of ∼2.5 MeV for high-doping Ce:LiCAF (50% ⁶Li), ∼2 MeV for low-doping Ce:LiCAF (50% ⁶Li) and ∼1.9 MeV for high-doping Ce:LiCAF (95% ⁶Li). The light output of Ce:LiCAF was also measured (175-250 phe/MeV from sample to sample). Lithium-6 glass GS20 from Saint Gobain was used as a reference scintillator (Ø50 mm×2 mm, circle). Relative neutron efficiency, normalized to that of GS20 lithium glass, as well as gamma-neutron intrinsic efficiency for all tested samples was calculated. Intrinsic efficiency on thermal neutron detection for small Ce:LiCAF samples was estimated at about 32-35% of that of GS20 and for large Ce:LiCAF sample as about 82% of that of GS20.     

Effect of neutron irradiation on optical spectra of sapphire crystals

We have studied the effects of neutron irradiation (fluences from 5 × 10¹⁵ to 5 × 10¹⁹ cm^?2) and thermal annealing on the optical spectra of sapphire crystals and the generation and annealing of visible luminescence and absorption centers in the crystals. The radiation-induced color centers with λ = 460, 570, 620, and 780 nm have been shown to be annealed in steps at temperatures from 70 to 1150°C. The process can be represented by two exponentials with activation energies of 0.05 and 0.30 eV. We have revealed antiannealing of the 460-, 570-, and 620-nm color centers at 200–300 and 300–450°C, and evaluated the thermal ionization potential U of the 400-and 460-nm color centers. The results suggest the possibility of charge transfer from the former center (U = 0.5 eV) to the latter (U = 0.2 eV). Increasing the neutron fluence from 2 × 10¹⁸ to 5 × 10¹⁹ cm^?2 is shown to increase the color center concentration by one order of magnitude. We have analyzed the nature of the radiation-induced centers and have shown that the concentrations of 460-nm absorption and 540-nm emission centers are power law functions of neutron fluence, with exponents of 0.79 and 0.81, respectively. These findings strongly suggest that the same mechanism underlies the generation of these centers.     

LiF crystals as high spatial resolution neutron imaging detectors

Neutron imaging by color center formation in LiF crystals was applied to a sensitivity indicator (SI) as a standard samples for neutron radiography. The SI was exposed to a 5 mm pinhole-collimated thermal neutron beam with an LiF crystal and a neutron imaging plate (NIP) for 120 min in the JRR-3M thermal neutron radiography facility. The image in the LiF crystal was read out using a laser confocal microscope. All gaps were clearly observed in images for both the LiF crystal and the NIP. The experimental results showed that LiF crystals have excellent characteristics as neutron imaging detectors in areas such as high spatial resolution.     

Behavior of optical spectra of leucosapphire crystals under neutron irradiation

The paper addresses the dose and temperature dependences of the formation and decoloration of visible-spectrum absorption bands for leucosapphire monocrystals subjected to treatment in a reactor core; they reveal special behavior of the material’s optical spectra as a function of flux and temperature. When taking readings of the above-mentioned functions during the high-temperature isochronal annealing, the author has identified the main stages and substages of decoloration at 460, 570, 620 nm and detected the negative annealing of these bands at some temperatures. The defect formation process has been found to proceed in stages; the activation energies and concentration of the respective centers have been determined; an assumption is made as to the nature of these centers. It has been established that the radiation kinetics of generation of a color center at 460 nm and photoluminescence at 540 nm is represented by a power law; it is assumed that there might be a single mechanism responsible for inducing these bands. The author discusses the factors that may affect the behavior of the dose dependence of the luminescence intensity with increasing neutron fluence.     

Scatter rejection in quantitative thermal and cold neutron imaging

The accuracy of quantitative neutron transmission radiography can be substantially decreased if highly scattering materials, such as water or plastics, exist in the sample. There are currently two main solutions to this problem: either performing experiments at a large distance between the detector and the sample or employ some numerical correction techniques. In the former case, the spatial resolution is substantially reduced by the limited beam divergence, while the latter correction requires a priori information about the sample and is limited to distances of above ∼2 cm. We demonstrate the feasibility of another technique, namely the possibility to remove the scattered neutron component from the transmitted neutron beam by a very compact polycapillary collimator. These ∼1 mm thick devices can be placed between the sample and the detector and remove most of the neutrons scattered at angles larger than the acceptance angle of the collimator (typically 1°). No image distortions above ∼10 μm scales are introduced by these collimators. The neutron transmission of highly scattering samples (water and plexiglass) is measured in our experiments with and without scatter rejection. In the latter case, the accuracy of measured transmission coefficient was substantially improved by our collimators.     

Investigation of superconducting crystals by neutron scattering and nuclear microanalysis

A combination of nuclear microanalysis (ion x-ray spectral analysis, Rutherford backscattering, and nuclear reactions) with low-angle neutron diffraction in the bulk of a Y-Ba-Cu-O ceramic rod, recrystallized by zone refining, was used to determine the elemental and phase composition of the initial ceramic, the crystal, and the transition sections. The degree of local homogeneity of the medium was also determined at scales of ∼10¹−10² nm. Pis’ma Zh. Tekh. Fiz. 24, 47–51 (February 26, 1998)     

Sophisticated equipment developed for growth and evaluation of perfection of nearly perfect crystals

This paper reviews four major equipment developed at the National Physical Laboratory for growth and perfection evaluation of single crystals, namely (i) a crystal puller for growth of nearly perfect crystals by the Czochralski method; (ii) a microfocus x-ray generator; (iii) an x-ray diffraction topography camera; and (iv) a triple crystal x-ray diffractometer. The crystal puller can provide smooth, uniform and variable pulling rates. The maximum length of pull is nearly 60cm. Efforts have been made to isolate vibrations. Nearly perfect single crystals of KCl, KBr and NaCl with maximum diameter of ∼ 60 mm have been grown. The crystals give diffraction curves with half width in the range of 10–30 sec of arc. In the projection topographs, dislocations can be resolved and characterized. The microfocus x-ray generator is a demountable continuously evacuated system with specially designed electron gun and anode assembly. The vacuum is continuously monitored for ease of maintenance. In the point focus mode the spot size is 40 μm on the anode. X-ray topography system is a versatile equipment used for projection and section topography. It can provide 360° rotations to the specimen disc around an axis perpendicular to it. Rotations of a few sec of arc can be given to the specimen around a vertical axis. Typical diffraction curves of a dislocation-free crystal and a crystal with boundaries are shown. Well-resolved images of dislocations are shown in a topograph as an illustration. In the triple crystal x-ray diffractometer a highly collimated and monochromated Kα₁ exploring x-ray beam is obtained by combining microfocus source, a special collimator and crystal monochromators of Bonse-Hart type. With this beam very narrow diffraction curves with half width of about 5 sec of arc can be recorded. Typical results of measurement of diffuse x-ray scattering (dxs) on nearly perfect silicon single crystals are discussed. It has been observed that the contribution of phonons to thedxs is negligible. Thedxs is mainly due to point defect aggregates.     

High resolution X-ray diffraction studies of real structure of nearly perfect single crystals

Real structure together with composition and elemental purity of single crystals controls their properties. This paper reviews recent work carried out at the National Physical Laboratory on application of high resolution X-ray diffractometry, topography and diffuse X-ray scattering for direct observation and characterization of real structure of single crystals of silicon, gallium arsenide, diamond and LiNbO₃. A series of six multicrystal X-ray diffractometers have been designed, developed and fabricated indigenously. The most versatile of these systems is a five crystal X-ray diffractometer with state-of-the-art level resolution. These techniques and equipments have been applied in studying several interesting problems. Even in dislocation-free crystals of silicon, remarkable differences in the defect structure have been observed if the growth method was changed from float zone to Czochralski. Study of effect of externally applied electric fields and ion implantation on real structure of crystals has yielded interesting results. Images of ‘filaments’ which show nonhomogeneous distribution of electric current through semiconductors and insulators have been recorded for the first time in high resolution traverse topographs. Diffracted X-ray intensities could be modified by externally applied electric fields. It has been shown that implantation of BF ₂ ⁺ ions in silicon for producing shallow junctions does not produce homogeneous distribution of boron. The impurity is partially in clustered form. Biaxial stress introduced by thin depositions in substrate crystals are of considerable applied concern. The value and nature of stress have been determined in a number of systems. Typical results obtained on GaAs: multilayer metallizations are described. Also, degradation of perfection of substrates has been monitored. This work has shown that the stress is not homogeneously distributed and is quite anisotropic. A new high resolution X-ray diffraction technique has been developed for direct observation and study of forward diffracted X-ray beam and anomalous transmission of X-rays through ‘thin’ diamond crystals of varying degrees of perfection.     

Characterization of a polychromatic neutron beam diffracted by pyrolytic graphite crystals

The beam spectrum for polychromatic neutrons diffracted by pyrolytic graphite crystals was characterized. The theoretical beam spectrum was obtained using the diffraction model for a mosaic crystal. The lattice vibration effects were included in the calculation using the reported vibration amplitude of the crystal and the measured time-of-flight spectra in the thermal region. The calculated beam spectrum was compared with the results obtained in the absence of thermal motion. The lattice vibration effects became more important for the higher diffraction orders and a large decrease in the neutron flux induced by the vibrations was identified in the epithermal region. The validity of the beam spectrum was estimated by comparing with the effective quantities determined from prompt γ-ray measurements and Cd-ratios measured both for 1/ν and non-1/ν nuclides.     

High peak-power single-frequency pulses using multiple stage, large core phosphate fibers and preshaped pulses

We have developed a monolithic high power pulsed fiber laser in a master oscillator power amplifier (MOPA) configuration, which is capable of reaching 0.38 mJ pulse energy and 128 kW peak power for 3 ns pulses at ~1550 nm while maintaining transform-limited linewidth. The fiber laser pulse seed was achieved by directly modulating a CW single-frequency fiber laser using an electro-optic modulator. We used an arbitrary waveform generator to preshape the fiber laser pulses before amplification to avoid pulse steepening and dynamic gain saturation. Single-mode, polarization maintaining highly Er/Yb codoped large core phosphate fibers were used in the power amplifier stages to scale the transform-limited fiber laser pulses, avoiding any nonlinearities.     

Growth of larger and more perfect single crystals of KClO4 in silica gel

As part of a continuing effort on crystal growth in gels, this paper describes the growth of large transparent single crystals of KClO₄ by the improved design. The effect of various anionic and cationic components on nucleation, growth and quality of these crystals has been studied. It has been found that a combination of KNO₃ and HClO₄ as the reactants resulted in the best crystals in terms of crystal size, quality and inter-crystalline separation. The effect of various impurities on nucleation, growth and quality of these crystals has been studied. It has been found that the impurities which enhance the solubility of KClO₄ have a positive effect on the size and quality of the crystals. A new etchant, consisting of concentrated HCl and H₂SO₄ in the volume ratio 2:3 has been found to reveal as-grown as well as freshly introduced dislocations.     

Cross-section model for cold neutron scattering in solid and liquid methane

Incoherent neutron scattering cross-sections for solid CH₄ in the temperature range of 20.4–90.7 K and liquid CH₄ at temperatures between 90.7 and 111.7 K are evaluated. A space–time correlation approach is used to describe a double-differential scattering cross-section which is basically expressed by a generalized frequency distribution. The cross-section model includes molecular translations and rotations as well as intramolecular vibrations. The former are concerned with very short-time free-gas like translation, short-lived vibration and long-time diffusion (only in liquid state). The latter consists of short-time free rotation and long-time isotropic rotational diffusion. Numerical calculations on double-differential and total cross-sections are carried out for incident neutron energies covered 0.1 μeV to 10 eV. Good agreement with experimental results at many different temperatures is found.