Evidence for a Neutron Halo in Heavy Nuclei from Antiproton Absorption

Abstract

From a study of stopping antiprotons in a variety of elements located in a hydrogen bubble chamber, we find evidence for the existence of a neutron fringe in heavy nuclei.

Neutron density distributions from antiprotonicPb208andBi209

“…This interpolated value can be compared with the results of Bugg et al [42], where the idea of the neutron halo factor was introduced for the first time to interpret the ratio of charged pions generated by antiproton annihilation in various targets, including 208 Pb. In that work the halo factor, defined as f π halo = N(np) np) and N (pp) are the number ofp annihilations on peripheral neutrons and protons, respectively], was measured for the 208 Pb nucleus by detecting the charged pions emitted after antiproton capture in nuclei [42].…”

“…The more recent Obelix experiments determined R = 0.48(3) . The cross at 8 fm represents the halo factor from Bugg's experiment [42]; the cross at 9.1 fm is the interpolated halo factor (see Sec. IV F).…”

“…In that work the halo factor, defined as f π halo = N(np) np) and N (pp) are the number ofp annihilations on peripheral neutrons and protons, respectively], was measured for the 208 Pb nucleus by detecting the charged pions emitted after antiproton capture in nuclei [42]. The result obtained, f π halo = 2.34 (50), is based on the assumption that R = 0.63, with this latter number extracted from thep capture in carbon.…”

“…In the simplest nuclear optical potentials, this ratio is given by the ratio of the imaginary parts of the effective scattering lengths, R = Im b n 0 /Imb p 0 . The experimental determination of this quantity [42,43] gives R = 0.63.…”

Antiprotons for nuclear structure research

“…For stable nuclei, such experiments have been done at BNL, e.g. [7,8] and LEAR, e.g. [9][10][11][12].…”