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It is now known that energetic photons are resonantly absorbed by nuclei, i.e. excitation to specific discrete levels of lightand medium-weight nuclei in an energy region of some keV–MeV, giant dipole resonance (GDR) around 20 MeV, and quasi-deuteron resonance (QDR) at 30 to 140 MeV. At energies above the pion threshold (140 MeV), the ∆ isobar is expected to be produced by the (3,3) resonance, P33 (1232), interaction of an incident photon with a single nucleon inside the target nucleus, whereas the nonresonant Born terms have been reported to be responsible for the strong background for pion production, which varies with photon energy. The isobar decays immediately (10 s) into a stable nucleon and a pion, and either one or both particles produced in these initial processes would usually develop a cascade-evaporation process in the same nucleus, resulting in a multiple nucleon emission (spallation) and/or fission in case of heavy nuclei. The pion would give excitation energy of 140 MeV when its absorption occurs in the cascade process. Emissions of light nuclei with mass numbers of A < 40 in high energy photonuclear reactions on mediumto heavy-weight targets have also been observed, but their formation mechanism during the cascade-evaporation process has not been well-understood as in high-energy hadron reactions. During the process in an early stage, one or both of the particles may escape from the nucleus. The probability of the escape may depend on the location of photoabsorption by a nucleon and nuclear transparency for the associated particles. If the pion were emitted in a forward direction at a small angle, the resulted nucleus would be left with an energy insufficient for developing the cascade-evaporation process. Especially, when photoabsorption occurs at the surface region of the target nucleus, the chance for an escape would be high and such simple reactions as (γ, n), (γ, p), and (γ, π) could result. 2 These simple reactions, especially on light nuclei, have received considerable attention since 1950s in order to gain valuable basic information concerning photonuclear interactions and nuclear structure. Hughes and March attempted for the first time to measure the residual nucleus of B(γ, π)C by observing its positron activity of 20 min half life with use of the Glasgow 330 MeV electron synchrotron as early as in 1957. Good agreement with theoretical results of Laing and Moorehouse was found if the surface production model was to be valid, and seemed to be consistent with pion observations performed since the first one by McMillan, Peterson and White in 1949. One problem found in photopion measurements was irregularly varying (π/π) yield ratios from complex nuclei irrespective of their neutron-to-proton ratios. Additional emissions of nucleon, mostly neutrons, will also occur if sufficient energy is left after the primary process. The neutron emission channels following pion emission, i.e. (γ, πxn) reactions, had been studied far less extensively before the works by activation method by the present author’s group, because of the overwhelming productions of other radionuclides due to spallation (see below) and of the experimental complexity regarding multiparticle coincidence in inclusive experiments. Most of the previous physical investigations were restricted to the inclusive measurements of one or two of the emitted particles from light nuclei. A series of double differential cross section measurements of pions and nucleons (2p and pn) from heavier targets was reported, and the dependence of the energy-integrated differential cross sections on nuclear size (SN), SN being the number of protons or neutrons in the target, was found to be described by dσ/dΩ∝SN, where the exponent α is 0.6 for π and 1.15 for a proton. The authors explained the results of the cross sections as a consequence of a combinaKoh Sakamoto*

Radiochemical Study on Photonuclear Reactions of Complex Nuclei at Intermediate Energies