The two-photomultiplier method of studying scintillator decay has been adapted to measure over the time range from 0.1 ps to 3 ms. Results are given for the most commonly used inorganic scintillators with both a and y excitation. It is found that the decay is, in most cases, better described by a power law than by a series of exponentials. The results for caesium iodide are analysed to derive the optimum filter for use in pulse shape discrimination. Attention is drawn to the influence of the decay behaviour on the reported measurements of scintillator efficiency.to well over 100 ps, even though the decay time was quoted as being less than 1 ps. This was a serious disadvantage as it meant using a relatively long dead time, whereas it had been hoped to use the counters to detect a number of neutrons occurring within their diffusion lifetime.
Spherical Si nanocrystallites with a Ge core on ultrathin SiO 2 have been prepared by controlling the selective growth conditions of low-pressure chemical vapor deposition (LPCVD) using alternately pure SiH 4 and 5% GeH 4 diluted with He. Cross-sectional transmission electron microscopy (TEM) images show spherical Si nanocrystallites with a Ge core in contrast with hemispherical Si dots on SiO 2 , implying a high structural strain at the interface between the cladding Si and Ge core. Atomic force microscopy (AFM) observations and X-ray photoelectron spectroscopy (XPS) measurements confirm the highly selective growth of Ge on the pregrown Si dots and the subsequent complete coverage by Si selective growth on Ge/Si dots. Raman-scattering spectra of multiply stacked structures of dots with a Ge core show that the compositional mixing occurs partly at the Si-clad/Ge-core interface during the samples preparation. Additionally, Raman spectra have also shown that the compositional intermixing becomes significant at 850 C and the SiO 2 reduction by interdiffused Ge atoms is promoted at 900 C.
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