N-N Scattering Cross Section Measurements at a Pulsed Reactor
Direct Measurements of the N-N Scattering Cross Section at the YAGUAR Pulsed Reactor
Tech Area / Field
- PHY-ANU/Atomic and Nuclear Physics/Physics
8 Project completed
Senior Project Manager
Kulikov G G
Joint Institute of Nuclear Research, Russia, Moscow reg., Dubna
- VNIITF, Russia, Chelyabinsk reg., Snezhinsk
- Duke University / Triangle Universities Nuclear Laboratory, USA, NH, Durham\nGettysburg College, USA, PA, Gettysburg
Project summaryThe nn-scattering experiments bear directly on the problem of the charge-dependence of nuclear forces. The thermal neutron scattering cross section measures the value of the nn-scattering length ann which can be compared with the pp- and np-scattering lengths to check the isospin symmetry. Scattering lengths are extremely sensitive to small differences in nucleon potentials. As reviewed in , indirect ann determinations from nuclear reactions with three particles in final states, two of them being neutrons, were contradictory for a long time. The situation has changed during the last two years after the new experiments at TUNL and LANL that provided consolidation of indirect ann results with values: –18.7±0.6 fm  from the D(n,p)2n reaction and -18.55±0.005 stat.±0.3syst fm  from the D(p,g)2n reaction.
These results show that the nn-interaction is stronger than the nuclear part of the pp-interaction. The charge symmetry breaking (CSB) is of the size DaСSB=app-ann=1.6±0.6 fm . There are theoretical explanations of the charge symmetry violation elaborated in terms of the meson-exchange theory of nuclear force and on the quarks level. Whereas in the meson-exchange picture, CSB originates mostly from - mixing, within QCD the charge symmetry breaking is due to different masses of u- and d-quarks. In both approaches, as reviewed in , predictions differ in the range of 0.5-2.5 fm, depending on the assumptions of the theory, which makes a new important argument for a direct measurement of the nn-scattering cross section.
Proposals for a direct measurement of the nn-scattering length have a long history, however, none of them has been performed so far. The given project is directed towards performing the direct nn-scattering cross section experiment at the reactor Yaguar (Snezhinsk, Russia) as suggested in . The Yaguar reactor provides optimal conditions for such an experiment. It has:
– A high, at the level of 1018 cm -2s -1, instantaneous thermal neutron flux density.
– A through-channel in active core with a cylindrical symmetry of the neutron field, which should make the experimental data treatment straightforward.
– A possibility of placing the neutron moderator inside the channel without the danger of a local overheating of active core near the moderator.
– A relatively short power pulse allowing the use of time-of-flight method for the separation of thermal neutrons from fast neutrons.
The use of the high instantaneous thermal neutron flux density reduces the role of background because the useful experimental effect depends quadratic on the flux density while background depends linearly.
The project goal is the building of the experimental setup at Yaguar and the first direct observation of the neutron scattering on neutron. This should answer the main question on the ann amplitude precision, which can be achieved in such an experiment. The expected precision of the first result is at the level of 5%. It is planned, in frame of the project, to solve the following problems:
– To form thermal neutron flux density optimal for the existing Yaguar’s channel geometry.
– To work out, by mathematical modeling, the procedure of obtaining the nn-cross section.
– To study background conditions by performing a test experiment.
– To accommodate reactor surroundings for the flight path and shielding installation.
– To make a specialized collimator for suppressing the multi-fold scattering background.
– To construct the experimental channel, the neutron detector and data acquisition system.
– To conduct the first direct measurement of the nn-cross section (obtaining and analyzing data).
The duration of the project is two years. The project combines efforts of scientists from JINR with an experience in the neutron study and experts from VNIITF with a vast experience in pulsed reactors (the Yaguar including) design and operation. Switching weapon scientists to this project and applying the pulsed reactor Yaguar conforms to the ISTC policy and goals. In frame of the project, the nn-collaboration will be established to conduct the nn-experiment and to help in integration of Russian scientists into the world scientific society, which will be represented by the Triangle Universities Nuclear Laboratory (TUNL), the U.S., with the world class experts in measuring the nn- and pp- scattering lengths.
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