A new approach to the determination of the uncertainty in neutron diffraction experiments with isotopic substitution method

摘要

Neutron diffraction experiment with isotopically substituted substances is a powerful approach claiming to yield unambiguous information about the local atomic structure in disordered materials. This information is expressed in the partial structure factors, and extracting them from a series of measurements requires solution of a set of linear equations that is affected by experimental errors. In this article, we suggest a method for the determination of the optimal set of H/D compositions with or without taking into account the experimental error. For the case of water, our investigations show that the selection of the isotope concentrations and the distribution of measurement time among the various samples have a critical role if one wants to utilize the limited neutron beam time efficiently. It is well known thatmeasurements of pure H_2O introduce fairly large errors in the partial structure factors due to its very strong incoherent scattering. In water and methanol as examples, we investigated the propagation of random errors to the partial structure factors using partial pair-correlation functions from molecular dynamics simulation. It is shown in the example of water that it is not worthwhile to measure pure H_2O.