Page 68 - ILLS Annual Report 2018

NUCLEAR AND PARTICLE PHYSICS
Victor A. Vesna (1938-2017). Russian PNPI, Gatchina, Russia
Viktor Vesna was the initiator, informal permanent leader and ‘driving force’ of the cycle of experiments on the search for, and measurement of, P-odd asymmetry coefficients in the emission of products of reactions of polarised cold neutrons with light nuclei.
First observation of P-odd
AUTHORS
V. Nezvizhevsky (ILL)
V. Vesna (PNPI, Gatchina, Russia)
ARTICLE FROM
Phys. Rev. B (2017)—doi: 10.1016/j.physletb.2017.03.035
REFERENCES
[1] V.A. Vesna, Yu.M. Gledenov, V.V. Nesvizhevsky et al., Phys. Rev. C 77 (2008) 035501
[2] V.A. Vesna, Yu.M. Gledenov, V.V. Nesvizhevsky et al., Eur. Phys. J. A 47 (2011) 43
[3] Yu.M. Gledenov, V.V. Nesvizhevsky, P.V. Sedyshev et al., Phys. Lett. B 769 (2017) 111
[4] D.R. Phillips, D. Samart and C. Schat, Phys. Rev. Lett. 114 (2015) 062301
In order to describe the contribution of weak interactions in nuclear processes, the problem is often parametrised
in terms of single (π, ρ, ω) and multiple meson exchange with one weak and one or more strong vertices. Within this parametrisation, neutral and charged weak currents are described with the coupling constants fπ and hρ0, respectively. Neutral weak currents were predicted by the unified theory of electromagnetic and weak interactions, and discovered in the scattering of muon neutrinos by electrons and the rotating polarisation of a laser beam passing through atomic bismuth vapours. The existence
of neutral currents in lepton-lepton and lepton-nucleon interactions led to numerous attempts to discover neutral currents in nucleon-nucleon interactions too, the reaction of neutrons with light nuclei being the obvious candidate. The principal difficulty for such experiments lies in the extreme smallness of the contribution of the weak interaction to the strong nuclear interaction (~10-7).
Reliable evidence of the existence of the non-zero P-odd asymmetry, -(8.8±2.1)·10-8, of the triton (3H) emission in the reaction 6Li(n,α)3H was obtained some years ago in experiments carried out at the ILL [1]. However, the result of an experiment searching for P-odd asymmetry in the γ-quantum channel in the reaction 10B(n,α)7Li*→vLi+γ (Eγ = 0.480 MeV) was compatible with zero [2]: +(0.8±3.9)·10-8 (figure 1 shows a plan of this experiment). The measurement of a non-zero effect in another neutron-induced reaction with light nucleus (10B) would allow us to estimate more accurately the constants of neutral and charged currents.
Figure 1
A plan of the measurement of the asymmetry of the γ-quantum emission in the reaction 10B(n,α)7Li*→7Li+γ, induced by polarised neutrons. Straight green arrows illustrate the direction of neutron spin polarisation, which is periodically flipped (light and dark green shades) by the adiabatic RF (radio frequency) spin flipper. Two γ-detectors are placed on opposite sides of the 10B target; trajectories of γ-quanta are illustrated with wavy arrows.
asymmetry of the alpha-particle 10 7
emission in the B(n,α) Li nuclear reaction and its consequences
Polarised cold neutron beam facility PF1B
The investigation of spatial parity in
the reactions of polarised neutrons
with light nuclei is an important topic associated with the fundamental problem of manifestations of weak interaction in nuclear processes and the evaluation of weak nucleon-nucleon (NN) potential.
ANNUAL REPORT 2018