Neutron Halo Isomers in Stable Nuclei and their Possible Application for the Production of Low Energy, Pulsed, Polarized Neutron Beams of High Intensity and High Brilliance

摘要

We propose to search for neutron halo isomers populated via $\gamma$-capturein stable nuclei with mass numbers of about A=140-180 or A=40-60, where the$4s_{1/2}$ or $3s_{1/2}$ neutron shell model state reaches zero binding energy.These halo nuclei can be produced for the first time with new $\gamma$-beams ofhigh intensity and small band width ($\le$ 0.1%) achievable via Comptonback-scattering off brilliant electron beams thus offering a promisingperspective to selectively populate these isomers with small separationenergies of 1 eV to a few keV. Similar to single-neutron halo states for verylight, extremely neutron-rich, radioactive nuclei\cite{hansen95,tanihata96,aumann00}, the low neutron separation energy andshort-range nuclear force allows the neutron to tunnel far out into free spacemuch beyond the nuclear core radius. This results in prolonged half lives ofthe isomers for the $\gamma$-decay back to the ground state in the 100ps-$\mu$s range. Similar to the treatment of photodisintegration of thedeuteron, the neutron release from the neutron halo isomer via a second,low-energy, intense photon beam has a known much larger cross section with atypical energy threshold behavior. In the second step, the neutrons can bereleased as a low-energy, pulsed, polarized neutron beam of high intensity andhigh brilliance, possibly being much superior to presently existing beams fromreactors or spallation neutron sources.