Direct nn-scattering at the YAGUAR reactor

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Abstract

The Direct Investigation of ann Association (DIANNA) is finalizing the design of a direct measurement of the nn-scattering length to be performed at the YAGUAR reactor in Snezhinsk, Russia. Extensive modeling of the neutron field, nn-scattering kinematics, and sources of detector background have verified the plan for a 3% measurement of ann. Measurements of the neutron flux support the neutron field modeling. Initial test measurements of the neutron field inside the underground channel have confirmed calculations of the thermal neutron background.

Introduction

Current understanding of charge-symmetry breaking in the nuclear force is limited by experimental uncertainty, in particular in the value of the neutron–neutron singlet scattering length, ann[1]. In order to improve the accuracy of the experimental value of ann and to help resolve conflicting results from indirect measurements [2], [3], a direct nn-scattering experiment is underway at the YAGUAR pulsed reactor in Snezhinsk, Russia, by the Direct Investigation of ann Association (DIANNA) [4]. The singlet scattering length can be related to the measured nn-cross section through effective range theory, which for low energy neutrons leads toσnn=πann2.

The current experiment has two important advantages over past proposals: the very high flux (1018 /cm2/s) of the YAGUAR reactor and the ability to thoroughly model the experiment using both analytic techniques and computer codes such as MCNP. Modeling the neutron field [4], [5], the nn-scattering kinematics [6], and the various sources of background [7] have verified the viability of the experiment (with an expected precision in ann of 3% or 0.5 fm) and have had a significant impact on experimental design.

Section snippets

The YAGUAR nn-experiment

The YAGUAR reactor has a 40 l cylindrical volume (40 cm diameter, 39 cm height, 15 cm through channel), is filled with a UO2SO4 solution, giving 465 g/l of 90% enriched 235U, and can be pulsed once per day. The instantaneous peak power during the approximately 0.68 ms wide YAGUAR pulse is 48.5 × 103 MW, producing a peak flux of 1018 neutrons cm−2s−1, which leads to about 107 nn-collisions per pulse in the through channel. Fig. 1 shows the experimental apparatus for the nn-scattering experiment. A

Modeling nn-scattering in the YAGUAR through channel

The first step in modeling the nn-scattering process is to accurately determine properties of the neutron field. Using the reactor geometry and fuel properties, MCNP was used to model the reactor fission neutrons and their subsequent thermalization in the polyethylene moderator. Tallies were taken of the neutron flux as a function of position, angle leaving the moderator surface, and energy. The spatial distribution of neutrons determined by MCNP was confirmed by activation measurements [4], [5]

Neutron background

The most pressing challenge facing the DIANNA experiment is reducing all sources of neutron background to a fraction of the neutron detector counts obtained from nn-collisions. Progress has been made in identifying the types of background, modeling their effects on the detector count rate, using modeling results to optimize the design of the experimental apparatus, and benchmarking the modeling results with measurement [7]. The calculated total background is expected to be ∼30 counts/pulse

Summary

The effort by the DIANNA collaboration to measure the nn-scattering length for the first time by direct nn-scattering at the YAGUAR reactor is underway and looks promising. Extensive modeling has been done to study the neutron field, the nn-scattering kinematics, and sources of background. Results indicate that background can be kept below 20% and that the nn-cross section can be reliably determined from the detector signal, leading to a 3% measurement of ann.

Acknowledgements

This work has been supported by the International Science and Technology Center (ISTC) under Project No. 2286, by the US DOE Office of High Energy and Nuclear Physics under grant numbers DE-FG02-97-ER41042 and DE-FG02-97-ER41033 and by the US NSF through an International Research Fellow Award No. 0107263.

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