E. J. Hoffman

On the role of γ-ray emission from nucleon unbound states in the mechanism of compound nuclear reactions

Abstract The compound statistical theory for nuclear reactions was employed to compute total cross sections for many levels in 61 Cu and 60 Ni populated via ( 4 He, pγ) and ( 4 He, 2pγ) reactions on 58 Ni. Angular momentum effects and photon emission from the continuum were included in the formalism. Decay to the well-known discrete states in 61 Cu and 60 Ni is treated separately and for higher excitations the level spectrum is represented by a level-density expression. The cross sections for many levels in 61 Cu and 60 Ni were calculated as a function of excitation energy in the 61 Cu compound nucleus from where the γ-cascades originate in competition with nucleon emission. Spectra of protons populating levels in 61 Cu and 60 Ni via ( 4 He, pγ) and ( 4 He, 2pγ) reactions were also calculated. Fermi-gas-model expressions for nuclear level densities were employed with nuclear moments of inertia approaching the rigid-rotor value with increasing excitation energy. The calculated level cross sections and particle spectra were compared with recent experimental results to determine the level-spacing parameter a as 1 10 A . By comparison with experimental independent yields ratios for many levels in 60 Cu the following quantities were deduced: (a) the moment of inertia as (0.7–0.8) I rig , for 8–12 MeV of excitation in 61 Cu and (b) the fraction of decay of the continuum states by quadrupole radiation. The γ-transition strength for E1 of M1 and E2 radiations were obtained from ( 4 He, pγ)/( 4 He, 2pγ) cross-section ratios and were found to correspond to E1 or M1 retardation factors of ≈ 350 or ≈ 7, respectively, and to E2 enhancement factors of ≈ 35 over the single-proton estimates. At the higher bombardment energies it is found that the 9 2 and 11 2 states in 61 Cu are populated via cascades through the yrast line with ≈ 25 % of the yield carried by dipole radiation. The states with J ≦ 7 2 are populated primarily by quadrupole transitions not involving the yrast line. The variation with bombardment energy of the independent yield ratios for the levels in 61 Cu and 60 Ni is well reproduced. The region in the EJ diagram for 61 Cu where γ-emission dominates is also deduced.

Structure of 61Cu via 58Ni(4He, pγ) high-resolution prompt p-γ coincidence spectrometry

Abstract The level structure and the detailed decay properties of the levels in 61 Cu have been studied via prompt γ-ray energy and intensity measurements utilizing a NaI(T1)-Ge(Li) anti-Compton spectrometer and pγ prompt coincidence techniques via the 58 Ni( 4 He, pγ) 61 Cu reaction with 4 He ++ energies in the range 4.6–12.5 MeV. Many of the γ-rays associated with levels in 61 Cu were identified with the ( 4 He, pγ) reaction by means of extensive on-beam pγ coincidence measurements employing a 41 cm 3 Ge(Li) γ-ray detector and several annular Si(Li) or surface-barrier charged-particle detectors. Characterization of many levels in 61 Cu was aided considerably by a high-resolution study of the proton spectrum from the 58 Ni( 4 He, p) reaction employing a tandem Van de Graaf accelerator and a split-pole Enge magnetic spectrograph. It was thus established that 50 levels up to 4.0 MeV in excitation are populated by this reaction in 61 Cu and that these accommodate 98γ-rays. Branching ratios for the γ-de-excitation of 48 levels in 61 Cu were obtained by averaging the measured angular correlation of each γ-ray about the beam direction. The directional correlation of 18 primary γ-rays in coincidence with protons detected at approximately 180° with respect to the beam direction were measured and the results were used to assign definite J -values for many levels in 61 Cu. A detailed level scheme for the de-excitation of levels in 61 Cu is offered.

An improved scheme for the decay of 86 sec 61Zn

Abstract The levels in 61 Cu populated in the decay of 86 sec 61 Zn have been reinvestigated using a 29 cm 3 Ge(Li) detector in an anti-Compton arrangement which employed a large NaI(Tl) annular detector. From extensive γ-ray energy and intensity measurements as a function of time ten new γ-rays were assigned to 61 Zn decay and levels at 475.0, 970.3, 1311.0, 1394.6, 1660.5, 1904.5, 1932.5, 2088.8, 2204.3, 2358.2, 2472.6, 2684.0, 2793.0, 2840.6, 2857.0, 2933.3, 3019.2, 3092.3 and 3521.1 keV were observed to be populated in the decay of 61 Zn. From log ƒ values determined in this work, from γ-ray intensity information, and from previously reported correlation data from nuclear reactions definite J π assignments have been made for six levels and limits have been placed for another six levels in 61 Cu.