A.V. Strelkov

Direct measurement of the neutron–neutron scattering cross section at the reactor YAGUAR

We propose to perform the first measurement of the neutron–neutron scattering cross section in the through-channel of the pulsed aperiodic reactor YAGUAR (Snezhinsk, Russia). Such a measurement directly determines the neutron–neutron scattering length, and by comparison with the proton–proton scattering length bears upon the issue of charge symmetry of the nuclear force. The proposed experimental set-up, as well as modelling of the neutron density and of the frequency of neutron–neutron collisions is described. Experimental results are reported on the formation and optimization of the thermal neutron field inside the through-channel of the reactor YAGUAR. The instantaneous value of 1.1 × 1018 cm−2 s−1 obtained for the thermal neutron flux density is large enough to perform the first direct neutron–neutron scattering length measurement.

Calculations of neutron spectra after neutron–neutron scattering

A direct neutron–neutron scattering length, ann, measurement with the goal of 3% accuracy (0.5 fm) is under preparation at the aperiodic pulsed reactor YAGUAR. A direct measurement of ann will not only help resolve conflicting results of ann by indirect means, but also in comparison to the proton–proton scattering length, app, shed light on the charge-symmetry of the nuclear force. We discuss in detail the analysis of the nn-scattering data in terms of a simple analytical expression. We also discuss calibration measurements using the time-of-flight spectra of neutrons scattered on He and Ar gases and the neutron activation technique. In particular, we calculate the neutron velocity and time-of-flight spectra after scattering neutrons on neutrons and after scattering neutrons on He and Ar atoms for the proposed experimental geometry, using a realistic neutron flux spectrum—Maxwellian plus epithermal tail. The shape of the neutron spectrum after scattering is appreciably different from the initial spectrum, due to collisions between thermal–thermal and thermal–epithermal neutrons. At the same time, the integral over the Maxwellian part of the realistic scattering spectrum differs by only about 6 per cent from that of a pure Maxwellian nn-scattering spectrum.

A Direct Measurement of the Neutron-Neutron Scattering Length

A direct measurement of nn-scattering by colliding free neutrons has never been performed. Indirect measurements continue to provide inconsistent results, leaving the issue of charge symmetry in the nuclear force unresolved. At present the Russian pulsed reactor YAGUAR is the best neutron source for such a measurement. A neutron moderator is installed in the central through channel and the scattered neutrons are detected at a distance of 12 m from the reactor. An instantaneous value of 1.1 × 1018/cm2s was obtained for the thermal neutron flux density. The experiment will be performed by the DIANNA Collaboration as ISTC project No. 2286.

Direct Measurement of Neutron-Neutron Scattering

In order to resolve long‐standing discrepancies in indirect measurements of the neutron‐neutron scattering length ann and contribute to solving the problem of the charge symmetry of the nuclear force, the collaboration DIANNA (Direct Investigation of ann Association) plans to measure the neutron‐neutron scattering cross section σnn. The key issue of our approach is the use of the through‐channel in the Russia reactor YAGUAR with a peak neutron flux of 1018 /cm2/s. The proposed experimental setup is described. Results of calculations are presented to connect σnn with the nn‐collision detector count rate and the neutron flux density in the reactor channel. Measurements of the thermal neutron fields inside polyethylene converters show excellent prospects for the realization of the direct nn‐experiment.

Current Status of the Experiment on Direct Measurement of Neutron‐Neutron Scattering Length at the Reactor YAGUAR

A new experiment was proposed in 2002 to perform the first direct measurement of neutron‐neutron scattering on the powerful pulsed reactor YAGUAR located at Snezhinsk, Ural region, Russia. Extensive efforts were made to model the background conditions and to optimize the set‐up design. To make the experiment feasible it was necessary to suppress the background from various origins by more than 16 orders of magnitude for thermal neutrons and 14 orders of magnitude for fast neutrons. In 2003 a channel was drilled under the reactor and equipped for time‐of‐flight measurements. During the next two years at this channel there were carried out a series of test experiments aimed at verifying the accuracy of the background modeling. Good agreement of the measured results with the calculated values enabled us to make the final design of the full scale set‐up. During 2005–2006 the experimental system was manufactured. After vacuum tests at JINR the set‐up was mounted at the YAGUAR reactor hall. In 2006–2007 calibra...