Search

The photon difference method is applied to the study of pion emission branches in the 12C(γ,π+) reaction. The π+ spectra at emission angles of 30°, 70°, 110° and 150° have been measured with 200 MeV and 205 MeV electron energy. The difference π+ spectra are analyzed for the known discrete states and highly excited region. The branch cross sections and angular distributions are compared with the results by the Fermi-gas model for quasi-free production and by the shell model. The π+ yields are found to be dominated by the branch to the individual discrete states and giant resonance states in the residual nucleus. The contribution of the quasi-free production seems small.

The energy and angular distributions of photopions in the 14N(e, π+)14C∗ reaction have been measured at Ee = 190.0 MeV in order to study spin-isospin flip transitions leaving the sum of low-lying 2+ states of residual nucleus 14C. Dominant contributions were found for the Ml transition around at 7–8 MeV in 14C. The transition strength has been deduced and compared with the results of the (e, e′), (p, γ), (π−, γ) reaction. The measured (γ, π+) cross section agrees with the theoretical result.

Energy spectra of Λ 12 C and Λ 56 Fe have been measured by means of (π+, K+) reactions at the KEK 12 GeV PS. The spectrometer PIK, which has large angular and momentum acceptance and medium energy resolution, has been constructed to study the reaction. The differential cross-section of the (π+, K+) reaction at forward angles θK∼(0÷6)o for the ground state of Λ 12 C and that for the effectively Λ-bound region of Λ 56 Fe agree well with the DWIA calculation using single-particle Λ wave functions obtained from a Woods-Saxon potential. The spectrum of Λ 56 Fe is consistent with the DWIA calculation using the realistic distribution of the hole-state strengths in55Fe.

Photopion energy distributions have been measured on 7 Li , 28 Si , 51 V and 93 Nb at θ π = 90° with 200 MeV electron. The logarithmic plot of the distributions shows a break at around 10 MeV of the residual energy. This is not explained by the quasi-free π + production. The (e, π + ) cross sections at θ π = 90° deduced by integrating the energy distribution. The result can be approximated by σ 0 Z 2 3 , where σ 0 is 0.13 times the elementary cross section of H(e, π + ) at θ π = 90°. The quasi-free π + production calculated by the Fermi-gas model with Pauli exclusion principle approximately reproduces the relative dependence on the charge number but its absolute value is about ten times as large as the experimental result. The present result for the charged photopion cross section in the threshold region is in contrast to the case in the Δ-resonance region where the cross section of π + + π − photoproduction is expressed by A 2 3 times the elementary cross sections.

The angular distribution of the 1 2 − → 1 2 − transition in 13 C (γ, π − ) 13 N g.s. has been measured using 165 MeV electrons The data were taken at 6 angles between 50° and 150° including the forward and backward regions around the deep minimum of the cross section. Comparison of the theory with a non-local calculation shows that the M1 component alone reproduces the experimental data. This implies some suppression of the E0 component