K. Hansen

The photon tagging facility at the max accelerator system in Lund

Abstract A description is given of the photon tagging facility at the MAX accelerator system in Lund for use in photonuclear experiments in the energy region between the giant delta resonances. The system enables experiments with high-energy resolution, 300 keV at 60 MeV photons, and a tagged rate of 5 × 10 6 /sMeV.

Surface science at MAX-LAB

The 550 MeV storage ring at MAX-LAB provides synchrotron radiation for a large community of users. An important part of the scientific program is photoelectron spectroscopy studies of semiconductor and metal surfaces and interfaces. Here is a brief review of some of the experimental efforts obtained in this area. Results are shown for the Ge(100)2×1:As, Si(111):Au, Mo(110):Yb and Mo(110):Sm systems.

A broad range tagging spectrometer for the MAX-laboratory

A broad range tagging spectrometer together with a new beam transport system for photonuclear experiments at the MAX-laboratory in Lund is described. The spectrometer consists of a quadrupole followed by an Elbek-type dipole and has a large momentum acceptance. It can produce both polarized and unpolarized tagged photons in the energy range 10–80 MeV with an energy resolution of about 300 keV.

Elastic scattering of 58 and 75 MeV photons by 12C and 16O and electromagnetic polarizabilities of the bound nucleon

Differential cross sections for elastic scattering of photons by C-12 and O-16 are measured at 58 and 75 MeV photon energies and 45 degrees to 150 degrees scattering angles using the tagged photon ...

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.

COMPTON-SCATTERING OF 87 MEV PHOTONS BY HE-4

Abstract Differential cross sections for Compton scattering by 4He have been measured at photon energies ranging from 83.7 to 93.8 MeV with a weighted average of 87 MeV using the bremsstrahlung beam of the MAX-LAB (Lund) nuclear-physics facility. The experimental differential cross sections show a pronounced increase at backward angles, strongly differing from predictions based on zero-angle dispersion relations and on the standard method used to calculate the differential cross section at nonzero angles. It is shown that the attempt to explain this effect in terms of quadrupole strength leads to unreasonable properties of the photoabsorption cross section. A parametrization in terms of a shift in the multipolarity ratio of the electromagnetic polarizabilities of the bound nucleon leads to Δβ = − Δα = 7 × 10−4 fm3.

Electromagnetic polarizabilities of nucleons bound in 40Ca, 16O and 4He

Differential cross sections for elastic scattering of photons have been measured for 40Ca at energies of 58 and 74 MeV and for 16O and 4He at 61 MeV, in the angular range from 45° to 150°. Evidence is obtained that there are no significant in-medium modifications of the electromagnetic polarizabilities except for those originating from meson exchange currents.

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.

Pion emission in H-2, C-12, Al-27(gamma, pi(+)) reactions at threshold

The first data from MAX-lab in Lund, Sweden on pion production in photonuclear reactions at threshold energies, is presented. The decrease of the total yield of pi+ in gamma + 12C, 27Al reactions below 200 MeV as well as differential, dsigma/dOmega, cross sections follow essentially predictions from an intranuclear cascade model with an attractive potential for pion-nucleus interaction in its simplest form. Double differential, d2sigma/dOmegadT, cross sections at 176 MeV show, however, deviations from the model, which call for refinements of nuclear and Coulomb potentials and possibly also for coherent pion production mechanisms.

Combined Bragg-PPAC ionization detector for light charged particles from photonuclear reactions

Abstract A combined Bragg-PPAC ionization detector has been developed for registration and identification of light charged particles from photonuclear reactions at tagged photon beams. The detector is described and also its performance, which was obtained in a test experiment at the tagged photon beam.