H. Ruijter

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.

Near-threshold measurement of the He-4(gamma,n) reaction

A near-threshold He-4(gamma, n) cross-section measurement has been performed at MAX-lab. Tagged photons from 23 < Ey < 42 MeV were directed toward a liquid He-4 target, and neutrons were detected by time-of-flight in two liquid-scintillator arrays. Seven-point angular distributions were measured for eight photon energies. The results are compared to experimental data measured at comparable energies and Recoil-Corrected Continuum Shell Model, Resonating Group Method, and recent Hyperspherical-Harmonic Expansion calculations. The angle-integrated cross-section data are peaked at a photon energy of about 28 MeV, in disagreement with the value recommended by Calarco, Berman, and Donnelly in 1983.

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.

Measurement of the He-4(gamma,n) reaction from 23 gamma <70 MeV

A comprehensive set of {sup 4}He({gamma},n) absolute cross-section measurements was performed at MAX-lab in Lund, Sweden. Tagged photons from 23<E{sub {gamma}}<70 MeV were directed toward a liquid {sup 4}He target, and neutrons were identified using pulse-shape discrimination and the time-of-flight technique in two liquid-scintillator detector arrays. Seven-point angular distributions have been measured for 14 photon energies. The results have been subjected to complementary transition-coefficient and Legendre-coefficient analyses. The results are also compared to experimental data measured at comparable photon energies as well as recoil-corrected continuum shell-model, resonating group method, and effective interaction hyperspherical-harmonic expansion calculations. For photon energies below 29 MeV, the angle-integrated data are significantly larger than the values recommended by Calarco, Berman, and Donnelly in 1983.

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.

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.

The 4He(γ,n) reaction: a potential testing ground for the alpha-particle wavefunction

Differential cross sections (σ(Eγ,θn)) for the 4He(γ,n) reaction have been measured at Eγ=50–71 MeV and θn=30–120°. These data are compared with theoretical predictions where a microscopic calculation of the 3He and 4He wavefunctions has been made within the Alt-Grassberger-Sandhas, integral-equation formalism.

The semi-inclusive 40Ca(γ, p) reaction

Abstract The experimentally determined semi-inclusive 40 Ca(γ, p) process at 60 MeV photon energy is interpreted in terms of two different approaches. Applying the hybrid model for pre-equilibrium reactions gives a fair description of the spectral shapes and the angular distributions. In a more microscopic approach, it is shown that a recently developed model for the 40 Ca(γ, pn) reaction yields at least half of the detected (γ, p) strength above the pn emission threshold.

Investigation of the40Ca(γ,N) reaction at 60 MeV

The values of the angle-integrated cross section for the decay to various low-lying states in39K following the40Ca(γ,π) reaction at a mean photon energy of 60 MeV are reported on, together with the values of the reduced Legendre coefficientsai. The40Ca(γ,gp) and40Ca(γ,n) reactions yield the same angular cross-section values and these agree with the corresponding RPA-calculations.

Measurement of theHe4(γ,n)reaction from23<Eγ<70MeV

A comprehensive set of {sup 4}He({gamma},n) absolute cross-section measurements was performed at MAX-lab in Lund, Sweden. Tagged photons from 23<E{sub {gamma}}<70 MeV were directed toward a liquid {sup 4}He target, and neutrons were identified using pulse-shape discrimination and the time-of-flight technique in two liquid-scintillator detector arrays. Seven-point angular distributions have been measured for 14 photon energies. The results have been subjected to complementary transition-coefficient and Legendre-coefficient analyses. The results are also compared to experimental data measured at comparable photon energies as well as recoil-corrected continuum shell-model, resonating group method, and effective interaction hyperspherical-harmonic expansion calculations. For photon energies below 29 MeV, the angle-integrated data are significantly larger than the values recommended by Calarco, Berman, and Donnelly in 1983.