The analysis of a time-of-flight neutron diffractometer for amorphous materials: the structure of a molecule in a liquid

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Abstract
A comparison is made of the conventional method of using a continuous monochromatic slow neutron beam method of measuring the structure factor of an amorphous material (e.g. a liquid or a glass) and the method using a pulsed beam of heterochromatic neutrons and measurement of their time-of-flight. The Placzek [1] method has been adapted to the analysis of the time-of-flight experiment. The corrections required to convert the measured counting rates to static structure factors are explicitly evaluated for various detector laws and for any incident spectrum. It is shown that there is no particular virtue in equal-path time-of-flight arrangements. There is however a particular interest in a ‘1/v 2’ detector. The method of analysis is of particular value for the determination of the interference part of the structure factor at high momentum transfer. Some recent experiments on molecular liquids (Clarke and Dore [2 (a] are analysed and it is shown that certain features of the structure of a molecule in the liquid state can be determined. This is a novel result and is of particular importance for liquid water. It is suggested that the D2O molecule in liquid heavy water at 20°C has a form closer to that in the vapour than that found in ice.