Journal of Neutron Research

The effect of uncertainty in chopper phasing (jitter) has been investigated for the high-resolution time-of-flight spectrometer LET at the ISIS second target station. The investigation is carried out using virtual experiments, with the neutron simulation package McStas, where the chopper jitter is found to cause a Lorentzian tail in the resolution function. We find that jitter even up to the unrealistically high value of 2 μs can be tolerated without any noticeable degradation of resolution or incident intensity. The results are supported by simple analytical estimates and are believed to be general for chopper spectrometers.

In this paper we propose three experimental set-ups: one uses very cold neutrons (VCN) and two use thermal neutrons. These set-ups allow us to investigate two possible length scales in the interference pattern corresponding to total energy and potential energy, respectively. We propose that firstly the thermal neutron experiments are carried out in order to investigate the length scale corresponding to potential energy, and if successful, then the experiment requiring VCN be attempted in order to investigate the length scale corresponding to total energy.

Joining of pipes made of stainless steel (SS) and some other metals (Ti and Nb) still represents a serious technical problem. Recently reliable and hermetic joining of pipes from SS and Ti/Nb has been achieved by the explosive bonding technique at the Russian Federal Nuclear Center (Sarov). These adapters are supposed to be used at the future International Linear Collider with crossed electron–positron beams. The adapters have shown excellent behavior at room and liquid helium temperatures, during high pressure tests and thermal cycling. Here we describe the neutron diffraction experiments, in which the residual stresses in the SS–Ti and SS–Nb adapters were measured with the POLDI instrument at the SINQ spallation neutron source.

In a recent numerical optimization study we have found that liquid para-hydrogen coupled cold neutron moderators deliver 3–5 times higher cold neutron brightness at a spallation neutron source if they take the form of a flat, quasi 2-dimensional disc, in contrast to the conventional more voluminous shapes used by now. In the present paper we describe a simple theoretical explanation of this unexpected behavior, which is based on the large difference in para-hydrogen between the values of the scattering mean free path for thermal neutrons (in the range of 1 cm) and its much larger equivalent for cold neutrons. This model leads to the conclusions that the optimal shape for high brightness para-hydrogen neutron moderators is the quasi 1-dimensional tube and these low dimensional moderators can also deliver much enhanced cold neutron brightness in fission reactor neutron sources, compared to the much more voluminous liquid D₂ or H₂ moderators currently used. Neutronic simulation calculations confirm both of these theoretical conclusions.