H. Ferdinande

A general solid angle calculation by a Monte Carlo method

Abstract A procedure is described for calculating the solid angle in a general source-to-detector geommetry, by a Monte Carlo method.

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.

Total photoneutron cross sections for 12C and 19F

Abstract The total photoneutron cross sections for 12 C and 19 F were measured from threshold to 33 and 19 MeV respectively, with the use of bremsstrahlung from a linac and by direct detection of the neutrons. The yield curves were analysed using the variable-bin Penfold and Leiss unfolding technique, and the least-structure method of Cook. The 12 C data are in good agreement with available results; the 19 F ( γ , χ n) cross section shows considerable fine structure, not observed in neighbouring odd-mass nuclei.

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.

The absolute forward photodisintegration cross section of the deuteron at low excitation energies

Abstract The 2 H(γ, p)n cross section was measured absolutely at 0° for the outgoing proton, for photon energies between 8 and 18 MeV. The rising trend observed in the cross section in this energy region indicates the existence of the theoretically predicted minimum. Our results at E γ ≈10 MeV are well described by recent relativistic calculations, the data around 15 MeV, on the contrary, are commonly underestimated.

An absolute recoil-proton photoneutron spectrometer using a Monte Carlo calculated efficiency

Abstract The use of a recoil-proton spectrometer as a tool for the measurement of absolute differential photoneutron cross sections is described. The efficiency of the stilbene scintillator was calculated by means of a Monte Carlo method. The test measurements of the photoneutron spectra from 16 O and 12 C show that this simple device may prove very valuable in photonuclear research.

Fine structure in the208Pb(γ,n) cross section

The total photoneutron cross section of208Pb was measured between 8 and 13 MeV using the bremsstrahlung photon facility from a 35 MeV linac. Considerable resonance structure was observed in the cross section, of which the peak around 9 MeV, as well as the structure around 10.8 MeV may be due toE2 excitations.

Time-evolution of isospin mixing in the unified exciton model

Abstract The unified exciton model, based on a master-equation approach, is extended to describe also the coupling between exciton states of different isospin. This allows the determination of the time evolution of the isospin mixing coefficient μ 2 ( t ). The equilibration of the excited nucleus, both in exciton number and isospin space, is discussed. It is shown that the nucleus essentially decays before full equilibrium is established.

A general Monte Carlo calculation for the geometrical efficiency of a detection system

A general Monte Carlo procedure is described for calculating the geometrical efficiency of a detection system with regard to a beam of particles impinging on a gaseous target or on a plane parallel target plate with finite thickness, while the detected particles are emitted with an isotropic angular distribution or one of the form [1−0.5P2 (cos θ)], with P2 (cos θ) the second Legendre polynomial. This method was successfully applied to the analysis of the 16O(γ, P0)15N data.

Charged-particle decay of 28Si induced by quasi-monochromatic annihilation photons in the GDR

Abstract Spectra of protons and alpha-particles, emitted from the giant electric dipole resonance of 28 Si excited by quasi-monochromatic photons from in-flight annihilation of accelerated positrons, were measured. Cross sections for six charged-particle decay channels were extracted in the energy region from 16.8 to 25.7 MeV. While the ( γ , α 0 ) cross section is completely statistical in nature, the (γ, p) process was found to have a 65% semidirect component, mainly in the ( γ , p 0 ) and ( γ , p 1+2 ) channels.