Natalia P. Zaitseva

Rapid growth of large-scale (40–55 cm) KH2PO4 crystals

KDP (KH2PO4) single crystals up to 45 cm in size have been grown by the rapid growth technique on the point seed in glass crystallizers of 1000 L in volume at growth rates of 10–20 mm/day in both the [0 0 1] and [1 0 0] directions.

The effect of impurities and supersaturation on the rapid growth of KDP crystals

Potassium orthophosphate (KDP) crystals were grown at the growth rates from 0.5 to 25 mm/day. The distribution of impurities connected with the vicinal and sectoral structure of the rapidly grown crystals is described. It has been shown that to decrease the effect of impurities and to obtain crystals of high homogeneity, rapid growth should be performed at high supersaturations outside the dead zone and the region of the sharp increase of the growth rate with supersaturation.

New Organic Crystals for Pulse Shape Discrimination

Efficient, readily-available, low-cost, high-energy neutron detectors can play a central role in detecting illicit nuclear weapons since neutrons are a strong indication for the presence of fissile material such as Plutonium and Highly-Enriched Uranium. The main challenge in detecting fast neutrons consists in the discrimination of the signal from the gamma radiation background. At present, the only well-investigated organic crystal scintillator for fast neutron detection, in a n/gamma mixed field, is stilbene, which while offering good pulse shape discrimination, is not widely used because of its limited availability and high cost.

The effect of dislocation cores on growth hillock vicinality and normal growth rates of KDP {1 0 1} surfaces

We present results from atomic force microscopy measurements on KDP {1 0 1} faces which show that over the range of supersaturations, 3% ≤ σ ≤ 30%, the terrace widths on vicinal growth hillocks formed by dislocations are nearly independent of both supersaturation and dislocation structure, in contradiction to the predictions of simple BCF models. The data also show that, for Burgers vectors in excess of one unit step height, the dislocations generate hollow cores in accordance with theoretical predictions. Both analytical and numerical analyses are presented, which show that a model that takes into account the effect of these cores on the period of step rotation predicts a dependence of slope on supersaturation and Burgers vector, which is in good agreement with the experimental results. These calculations also show that the effect of the core perimeter on the step transit time dominates the effect of reduced step velocity due to stresses near the core. Consequently, a simple analytical expression can be used to describe the slope even in the case of anistropic step kinetics. Finally, the results are used to explain the reproducible character of macroscopic growth rates and to rescale data on growth rate as a function of temperature and supersturation onto a single curve.

Sources of optical distortion in rapidly grown crystals of KH2PO4

Abstract We report the results of X-ray topographic and optical measurements on KH 2 PO 4 crystals grown at rates of 5–30 mm/day. We show that optical distortion in these crystals is caused primarily by three sources: dislocations, differences in composition between adjacent growth sectors of the crystal, and differences in composition between adjacent sectors of vicinal-growth hillocks within a single growth sector of the crystal. We find that the compositional heterogeneities cause spatial variations in the refractive index and induce a distortion of the transmitted-wave front, while large groups of dislocations are responsible for strain-induced birefringence which leads to beam depolarization.

Rapid growth of large-scale (40-55 cm) KDP crystals

KDP (KH2PO4) single crystals up to 47 cm in size have been grown by the rapid growth technique on the point seed in glass crystallizers of 1000 L in volume at growth rates of 10 to 25 mm/day in both the [001] and [100] directions. Measurement of the optical quality of 41 X 41 cm single crystal plates are presented.

Pulse Shape Discrimination in Impure and Mixed Single-Crystal Organic Scintillators

Neutron/gamma pulse shape discrimination (PSD) utilized for detection of high-energy neutrons with organic scintillators was investigated using a model system of mixed diphenylacetylene-stilbene single crystals of different compositions. The results of the studies, which include experimental tools of crystal growth and characterization combined with computer simulation, showed that the presence of impurities with lower bandgap energies can be a major factor influencing PSD properties of organic materials. Depending on the concentration, an impurity may suppress or increase the rate of excited triplet state interaction leading, respectively, to a complete disappearance or enhancement of PSD, consistent with a percolation threshold. The results are applied to produce novel materials with controlled decay characteristics. Single crystals with a large fraction of delayed light and enhanced PSD have been grown for high energy neutron detection, while crystals with suppressed delayed light were produced for use as low-afterglow scintillators for energetic neutron detection in time-of-flight experiments.

Unseeded growth of germanium nanowires by vapor-liquid-solid mechanism

Single-crystal germanium nanowires have been synthesized in the temperature range 380–430 °C without the use of any metal catalysts. Tetraethylgermane was used as a precursor in combination with different organic solvents to produce nanowires with controlled diameter ranging from 5 nm to 700 nm. The growth direction of nanowires is along direction. The method is based on the vapor-liquid-solid mechanism, with the classical liquid-metal catalyst replaced by droplets of high-boiling-point hydrocarbons.

Oriented liquid inclusions in KDP crystals

We describe a particular type of defect in KDP that consists of oriented chains of liquid inclusions. Using optical microscopy and X-ray topography we show that these inclusions are not directly formed by dislocations. The results of in situ atomic force microscopy show that hollow channels are stable during growth and that they can be caused by inclusion of foreign particles.

Thermal and laser conditioning of production and rapid growth KDP and KD*P crystals

Large solid state lasers such as Beamlet and the proposed National Ignition Facility require optical materials with extremely high damage thresholds. Potassium dihydrogen phosphate (KDP) and its deuterated analog (KD*P) both require some form of conditioning to reach the design fluence of these lasers. Both the bulk material and the crystal surfaces must have damage thresholds in excess of 16 J/cm2 at 1053 nm and 11 J/cm2 at 351 nm for 3- ns pulselengths. The use of ultrafiltration techniques has been demonstrated to produce bulk material with damage thresholds exceeding these requirements with the use of R:1 laser conditioning. More recent results at LLNL using large-area laser conditioning and thermal annealing are described for a variety of state-of-the-art KDP and KD*P crystals. Results on thermally annealed KD*P with a deuteration range of 60% to 80% are also presented, and compared to those of ordinary KDP.