Structure and formation of deeply bound pionic atoms 深く束縛されたパイ中間子原子の構造及び生成

博士論文

We have calculated the structure of deeply-bound pionic atoms using the standard pion-nucleus optical potential. The widths of the deeply-bound states are found to be narrow and are considered as quasi-stable states even for the ls state in ^<208>Pb. These unexpected results are caused by the repulsive pion-nucleus optical potential which pushes pionic wave functions outwards so that pion absorption by a nucleus is weakened. This result remains relatively unchanged by the use of different optical potentials, as far as they reproduce shallow pionic atoms. A theoretical study is then made to see how the properties of deeply bound pionic atoms depend on the neutron skin. With increased neutron skin, defined as the difference between neutron and proton radii, the strong isovector part of the pion-nucleus optical potential helps developing a repulsive bump at the nuclear surface which pushes further out the pionic wave function and decreases the binding energy. With further increase of the neutron skin, the repulsive potential in the central part is reduced and an inner component of the wave function develops in the nuclear interior, and ultimately a pion bound state of broad width is formed in the deep interior. We propose to produce these deeply-bound pionic atoms, hitherto untouched region, by using the "pion transfer" (n,p) reaction, where π^- is transfered to the target nucleus. We have calculated the cross sections for the formation of various deeply bound states via the (n,p) reaction. The calculated cross sections increase rapidly with the nuclear charge and become quite large for heavy atoms. The distortion effect of the incident and outdoing nucleons are, however, found very large and the cross sections are reduced to small values . This is consistent to the experimental data which was carried out recently at TRIUMF. We comment also on the use of complex projectiles for the "pion transfer" reactions such as (d,^2He). We also study the (n,d) and (d,^3He) reactions leading to deeply bound pionic atoms in heavy nuclei of configuration (l_π ・j_n^<-1>)j theoretically. The cross sections for various pionic and neutron-hole configurations for ^<208>Pb and ^<136>Xe are calculated at incident energies 300-1000 MeV/u by using the effective number approach and the eikonal approximation for distortion. The DWIA cross section for (n,d) producing a pion in the ls orbit is found to be around 75 μb/sr in the case of the ^<136>Xe target. The (d,^3He) cross sections are calculated to be around 40 μb/sr for the pionic 2p state production with neutron p3/2 hole in ^<208>Pb. The (n,d) and (d,^3He) reactions are found to be suited for the production of deeply bound pionic atoms.

東京都立大学, 1991-03-25, 理学博士, 甲第262号