C. Van den Abeele

The forward, backward and 90° deuteron photodisintegration between 7 and 19 MeV photon energy

Abstract The 0° and 180°, as well as the 90° differential cross sections for the 2 H(γ, p)n reaction have been measured at lab photon energies between 7 and 19 MeV. Special attention was paid to the accuracy, in particular by measuring the forward Compton electron yield. For the extreme angles, the statistical error on our results amounts to 4–5% (0°) and 7–8% (180°), respectively, while the systematic uncertainty is at most 3%. The data confirm the existence of a minimum in the forward cross section and indicate beyond any doubt that the fore/aft ratio is larger than unity. Comparison shows reasonable agreement with the results from recent “conventional” theoretical approaches, including meson exchange and relativistic corrections, although the description of the c -coefficient, appearing in the Partovi expansion of the differential cross section, remains unsatisfactory. A possible source for this discrepancy could be the inadequate description of the El and/or the E2 transition operator.

High-resolution 40Ca(γ, p) measurements and the photoproton reaction mechanism

Abstract High-resolution measurements for the 40 Ca ( γ , p ) 39 K reaction have been performed at E γ =60 MeV. Five groups of states in the residual nucleus were identified. The angular cross sections for the 2 s 1 2 and the 1 d 3 2 hole state are reasonably well predicted in an RPA model. Both the shape and the magnitude of the angular cross sections of the remaining groups of states suggest the excitation of two-hole-one-particle states evidencing a two-body photoabsorption mechanism.

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.

Search for the minimum in the forward2H(γ,p)n cross section

The forward2H(γ,p)n cross section was measured at 7.2 and 8.5 MeV photon energy. The new absolute values confirm the theoretically predicted presence of a minimum in this cross section. A discrimination is made between the results from various theoretical descriptions.

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.

Determination of the random particle energy spectrum in tagged photon experiments

A new, improved method to define the random particle energy spectrum in tagged photon experiments is described. This procedure yields the random spectrum with very good statistical accuracy, and can be applied when no hard coincidence with the focal plane is required in order to accept events.

Microstructure in the electron beam and the shape of tagging TDC spectra

Abstract The effect of fast electron beam intensity fluctuations and of the dead time of the electron detectors on the shape of TDC spectra in tagged photon experiments is explained. Some consequences for the data analysis and for the use of multi-hit TDCs are also discussed.

Absorption of intermediate-energy photons by multi-particle clusters in 6Li

Abstract The results of a study of the 6 Li (γ, cc) (c, c : charged particle ) reaction with the tagged-photon technique are presented. It is shown that the (γ, pt) reaction can be understood within the framework of a semi-phenomenological quasi-alpha reaction model. The total (γ, pt) cross section is found to be larger than the 6Li(γ, pn)α cross section at the same photon energy. No evidence is found for a three-body absorption mechanism, but the possible role of final-state interactions in the (γ, pd) reaction is outlined.

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.