Energieverteilung und Wirkungsquerschnitt der Photoprotonen aus N15

Abstract

The energy spectra of photo protons from highly enriched N15 were investigated with the bremsstrahlung from the Heidelberg betatron running at six different endpoint energies between 19 and 30.5 MeV. The protons were detected at 90° to theγ-beam by means of a CsJ-spectrometer with pulse shape discrimination. The energy spectra show pronounced maxima atE p =3.2; 4.6; 9.5 and 13.3 MeV. Proton yields are given as a function of endpoint energy, the yield value atE 0=30.5 MeV being (7.0±0.8) μb/MeV · ster. Because the first excited state in the daughter nucleus C14 lies 6.09 MeV above the groundstate, the cross section for groundstate transitions of the process N15(γ, p)C14 could be derived from the upper 6 MeV of the single proton spectra. The main contribution to the cross section comes from the region between 18 and 26 MeV excitation energy with maxima at 19.5; 20.4; 22.7 and 24.5 MeV. A “pygmy resonance” occurs at 15.2 MeV with further less pronounced structures at 13.6 and 17.0 MeV. The integrated cross section for groundstate transitions up to 30.5 MeV is (22±3) MeVmb assuming isotropic angular distribution. The ratios of protons from transitions to excited states and from the (γ, n p)-reaction to those of groundstate transitions rise from 0.45 atE 0=24.5 MeV to 0.70 atE 0=30.5 MeV.