Leslie Carman

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.

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.

Growth phenomena in the surface layer and step generation from the crystal edges

A possible mechanism of growth-step generation from the edges of faceted crystals obtained from experimental observations with KDP crystals is described. It suggests that growth from the crystal edges is initiated by the deviation of the edges from their crystallographic orientation and formation of incomplete shapes of singular facets. The conditions for formation of the incomplete faceted shapes during dislocation growth are considered. It is shown that the process of step generation from the edges can be determined by the mutual positions of the vicinal slopes on the adjacent faces.

Neutron detection with single crystal organic scintillators

Detection of high-energy neutrons in the presence of gamma radiation background utilizes pulse-shape discrimination (PSD) phenomena in organics studied previously only with limited number of materials, mostly liquid scintillators and single crystal stilbene. The current paper presents the results obtained with broader varieties of luminescent organic single crystals. The studies involve experimental tools of crystal growth and material characterization in combination with the advanced computer modeling, with the final goal of better understanding the relevance between the nature of the organic materials and their PSD properties. Special consideration is given to the factors that may diminish or even completely obscure the PSD properties in scintillating crystals. Among such factors are molecular and crystallographic structures that determine exchange coupling and exciton mobility in organic materials and the impurity effect discussed on the examples of trans-stilbene, bibenzyl, 9,10- diphenylanthracene and diphenylacetylene.

Solution-Grown Rubrene Crystals as Radiation Detecting Devices

There has been increased interest in organic semiconductors over the last decade because of their unique properties. Of these, 5, 6, 11, 12-tetraphenylnaphthacene (rubrene) has generated the most interest because of its high charge carrier mobility. In this work, large single crystals with a volume of ~1 cm3 were grown from solution by a temperature reduction technique. The faceted crystals had flat surfaces and cm-scale, visually defect-free areas suitable for physical characterization. X-ray diffraction analysis indicates that solvent does not incorporate into the crystals and photoluminescence spectra are consistent with pristine, high-crystallinity rubrene. Furthermore, the response curve to pulsed optical illumination indicates that the solution grown crystals are of similar quality to those grown by physical vapor transport, albeit larger. The good quality of these crystals in combination with the improvement of electrical contacts by application of conductive polymer on the graphite electrodes have led to the clear observation of alpha particles with these rubrene detectors. Preliminary results with a 252Cf source generate a small signal with the rubrene detector and may demonstrate that rubrene can also be used for detecting high-energy neutrons.