(γ, n) cross sections for O¹⁶ and Si²⁸☆

The reaction ²⁸Si(p, γ)²⁹P has been studied for 7 < E_p < 24 MeV, viz. 10 < E_x < 26 MeV, using a large NaI spectrometer. The transitions γ₀, γ₁, and γ₅ are observed to be the strongest. From the (p, γ₀) yield curve, five new $\text{T =}\text{3}\text{2}$ levels in ²⁹P have been postulated for 9.7 < E_x < 13.6 MeV. The giant dipole resonance observed via ²⁸Si(p, γ₀)²⁹P is found to be centred at E_x = 18 MeV with a width Γ = 5 MeV, and exhibits much structure. The (p, γ₁) and (p, γ₅) yield curves also exhibit much structure and their giant dipole resonances are observed at E_x = 20 and 22 MeV, respectively. Angular distributions measured throughout the giant-resonance region indicate $\text{J}{}^{\mathrm{\pi }}\text{=}\text{3}\text{2}{}^{\mathrm{-}}$ for the GDR and yield a lower limit of 3% for the E2/E1 intensity.

The inclusive partial photoneutron cross section σ(γ, ln…) and σ(γ, 2n…) and the total photoneutron cross section σ⁽¹⁾(E_γ) = ∑_{i = 1}σ(γ, in…) for ¹⁶O have been measured with mono-energetic photons in the energy region above the giant resonance and below the meson thresholds. Comparison with other data shows the dominance of the (γ, pn) process in this energy region and thus underscores the importance of the mechanism of photon absorption by a correlated proton-neutron pair in the nucleus; thus, satisfactory agreement of modern versions of the quasideuteron theory with the data has been obtained. The measured integrated total photoneutron cross section up to 140 MeV is 0.67 ± 0.07 TRK sum-rule units.

Absolute (γ, xn) cross sections for ⁹Be were measured with monochromatic photons from positron annihilation in flight in the energy range from 17 to 37 MeV. No pronounced structure was found in the cross sections in contrast to recent experiments performed with bremsstrahlung sources. The results are compared with many-particle shell model calculations.

The method of the positron annihilation in flight is used to produce quasimonoenergetic photons of variable energy. The photon facility (8–40 MeV) at the Giessen accelerator is described. The positron production, the beam transport, and monitor system are discussed. The absolute calibrations and the 4π neutron detector used in (γ, xn)-reactions are explained The performance of the facilities is demonstrated by test measurements of the total photoneutron cross sections σ(γ, Tn) for ¹²C and ¹⁶O.

Partial photoneutron cross sections [σ(γ, n) + σ(γ, pn)] and σ(γ, 2n) of s-d shell nuclei were measured in the GDR region by means of a continuously variable monochromatic photon beam and a high efficiency neutron detecting system. In some cases, additional measurements of target associated β⁺ activities made a separation of the (γ, n) from the (γ, pn) reaction possible. It is shown that the mean energy position of the gross resonance shape of the GDR of s-d shell nuclei does not show a systematic dependence on A as is the case for heavy nuclei. Further, the average energy positions of the main strength concentrations of the observed “structure” in these GDR are shown to depend strongly on shell effects. An analysis of the competition between the (γ, n), (γ, pn) and (γ, 2n) exit channels is also carried out. The ratio of the integrated experimental photoneutron cross section σ₀ⁿ to the classical TRK dipole sum rule for A = 4N nuclei, is shown to be only half as big as the one obtained for the neighbouring A = 4N−1 nuclei. This ratio shows also a monotonie increase as a function of isospin T for all nuclei.

The ³⁹K(γ, n)^38mK cross section has been measured with moderate resolution (800 keV) by detecting annihilation radiation from the 0.95 s decay of the residual nucleus. Between threshold and 29 MeV, resonances are observed with widths of the order of 1 MeV. This structure is compared to predictions of a shell-model calculation in the Tamm-Dancoff approximation. A $\text{d}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ hole is coupled to the 1⁻ (T = 1) 1p-lh excitations of a ⁴⁰Ca core and the hole excitations, $\text{1}\text{p}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}{}^{\mathrm{-1}}$ and $\text{1}\text{p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}{}^{\mathrm{-1}}$, are allowed. Results for zero-range and finite-range residual interactions both describe the low-energy side of the giant resonance remarkably well but give too little strength at higher energies. The zero-range force provides some explanation for the strong resonance observed at 22.2 MeV. The theory is compared also with the O'Connell isospin sum rule.