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4 citations found for Arzi, E.

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A variable-temperature sample container was developed for the neutron powder diffraction study of crystal structures and phase transitions in solidified gases. It consists of two coaxial tubes made of boron-free fused silica and fitted with heating coils and thermocouples. The exchange space between the tubes is filled with low-pressure helium gas controlled by two fine needle valves. Used in conjunction with a centre-stick-type liquid-helium cryostat and a proportional temperature controller, it maintains a powder sample of about 8 cm3 volume at a constant temperature anywhere in the 4.2 to 77.4 K temperature range. Its stability is not worse than ±0.15 K and the liquid-helium consumption is about 1 litre a day above 17 K, increasing up to 6 litres a day at lower temperatures. It can also be used for the preparation of fine-grained powder samples directly from the gas phase.

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A computer program is described which is suitable for calculating the angular positions of Bragg reflections compatible with a given type of unit cell, taking into account any systematic absences.

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The quality of powder specimens in diffraction experiments is very important. Any preferred orientation in the specimen under study affects the observed integrated intensities of the powder peaks and makes them unreliable; thus, results obtained from these intensities hardly appear to be trustworthy. The existing methods of powder preparation were considered to be unsuitable for samples such as isotopic methanes, nitrogen, oxygen, hydrogen, etc. which are gaseous at room temperature and have relatively low melting points. Therefore, a new method was developed for producing fine-grained powder specimens of solidified gases without preferred orientation directly from the gas phase using a specially designed liquid-helium cryostat which allowed visual inspection of the formation of powder in the specimen container. Subsequent neutron diffraction tests proved that the powder specimens prepared by this method were free of preferred orientations.

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