A. Sandell

Photoinduced fission of the doubly even uranium isotopes 234U, 236U and 238U

Abstract A measurement of the angular distributions and yields of fission fragments in the photofission of 234U has been performed between 5.2 and 6.4 MeV. As γ-source, the bremsstrahlung from a microtron was used. For the detection of the fission fragments, solid-state track detectors were used. The present data for 234U have been analysed together with earlier obtained data for 236U and 238U. The values of the fission barrier parameters obtained are compared to results in theoretical macroscopic and microscopic fission potential energy calculations.

The (γ, p) reaction at Eγ ∼60 MeV

Abstract A study of the (γ, p) reaction on 12 C, 14 N, 27 Al and 51 V targets, at photon energies E γ ∼60 MeV, is reported. Angular distributions in the range t p = 60°-120° are presented for transitions leading to the ground state and several low-lying states in each residual nucleus. The relative population of residual nucleus slates is similar to those found in (e, ep) and (d, 3 He) experiments, apart from the well-known exception of the peak at 6.8 MeV in the 12 C(γ, p) reaction. Direct-knockout calculations fail to account for the magnitude of the (γ, p) results, which indicates the need for calculations that include effects such as meson exchange or short-range correlations. Strength seen at higher excitation energies is attributed to inclusive (γ, p) events. The excitation-energy spectra have been compared with quasi-deuteron model calculations which assume a (γ, np) reaction where the neutron goes undetected. The shape of the 51 V(γ, p) excitation spectrum is compared to a pre-equilibrium model calculation. The results appear to indicate that two-nucleon absorption of the photon is important.

Compton scattering by mesons in nuclei: Experiment on 208Pb☆

Abstract Using 58 and 73 MeV tagged photons and scattering angles from 60° to 150°, it is shown that it is possible to observe Compton scattering by “mesons in nuclei” through an incomplete cancellation of the mesonic (exchange-current) seagull amplitude by parts of the nuclear resonance amplitude related to the giant-dipole resonance of 208 Pb. This phenomenon is a property of an extended nucleus and, therefore, cannot be studied on the deuteron. Predictions of the exchange from factor which determines the angular distribution of the exchange seagull amplitude are compared with experimental data.

Photon-induced proton knockout from 208Pb

Abstract Cross sections have been measured for the reaction 208Pb(γ,p) leading to the low-lying ( 1 2 + , 3 2 + ) and ( 11 2 − , 5 2 + ) doublets and 7 2 + state in 207Tl using a high energy-resolution tagged photon beam of 41 to 57 MeV. The data are compared with results obtained with the reaction 208Pb(e,e′p) in the same recoil-momentum range. The recoil-momentum distributions derived from the (γ,p) data do not coincide with those obtained from the (e,e′p) data. Distorted-wave impulse-approximation (DWIA) calculations of which the input is constrained by the (e,e′p) data underestimate the (γ,p) data by typically one order of magnitude. An estimate of meson-exchange currents (MEC) effects using the Siegert theorem brings the DWIA-calculations close to the data. Random-phase approximation calculations also give a fair account of the data, but in this case MEC-effects are predicted to be less important.

Investigation of the (γ,p) reaction in 6Li and 12C at 61 and 77 MeV

The tagged photon technique has been used to measure the excitation energy spectra resulting from the (γ, p) reaction in 6Li and 12C at 90 degrees for incident photon energies around 61 and 77 MeV. Strong evidence is found for the quasideuteron absorption mechanism, not only in the high excitation energy regions, but also in bound states. Furthermore, for the 12C(γ, p) reaction the photon seems to be absorbed on pn-pairs in both T = 0 and T = 1 states.

Photon-induced proton knockout from 208Pb and 12C

Abstract Cross sections have been measured for the reactions 208Pb(γ,p) and 12C(γ,p) leading to the low-lying and continuum states in 207Tl and 11B using a high energy-resolution tagged photon beam of 41 to 57 MeV. The data are compared with results obtained with the (e,e′p) reaction in the same recoil-momentum range and with various theoretical calculations. A comparison of the (e,e′p) and (γ,p) data in terms of a reduced cross section versus momentum does not show a systematic overlap of the (e,e′p) and (γ,p) results, i.e. no scaling is observed. Distorted-wave impulse-approximation (DWIA) calculations, of which the input is constrained by the (e,e′p) data, and random-phase approximation calculations (RPA) underestimate the 12C(γ,p) data by typically a factor five depending on angle and missing energy. For 208Pb the difference between the calculations and the data for the low-lying states is much less, but an increasing discrepancy between the data and the calculations is found in the continuum. The inclusion of meson-exchange currents (MEC) brings the calculations for both nuclei on average closer to the data, but MEC effects are considerably less important for 208Pb as compared to 12C. Finally, a remarkable similarity is observed between the continuum response of the (γ,p) and (e,e′p) reactions on 208Pb despite the difference in the electromagnetic couplings involved.

Isospin selection rules for real photons observed in the6Li(γ,d) reaction

The reaction6Li (γ, d) with Eγ around 60 MeV has been investigated with monoenergetic photons using a Si-Ge ΔE-E telescope, allowing accurate identification of deuterons with good energy resolution. A strict obedience of the isospin selection rule for E1 absorption, i.e. ΔT=1, is observed for the ground state transition.