Commissioning on 32S and First Results for 112Sn from the Low Energy Photon Tagger NEPTUN
The origin of the elements is a question which is still a field of active research. Especially for elements heavier than nickel, the explosive nucleosynthesis plays a key role. In order to describe this process, a reliable knowledge of the equation of state for nuclear matter is necessary. While the equation of state for symmetric matter is well known, this is not the case for asymmetric matter i.e. for the symmetry energy. Recently it has been shown that constraints on the symmetry energy can be drawn from the neutron skin thickness of nuclei which itself can be constrained from the dipole polarizability. This quantity can be extracted from a measurement of the complete dipole response.
This work describes the current setup and the commissioning of the low energy photon tagger NEPTUN which is located at the e--accelerator S-DALINAC and delivers a beam of tagged bremsstrahlung photons between ≈ 1 MeV and 20 MeV. This quasi-monoenergetic photon beam with an energy resolution of approximately 25 keV is used to study the dipole response of nuclei. The highly efficient LaBr3:Ce-based γ-spectrometer GALATEA detects not only the direct decays back to the ground state but also cascading decays with suitable efficiency. In contrast to similar experiments no calibration target for the photon flux is required.
For commissioning of NEPTUN the well known 8215.4 keV resonance of 32S is studied. The integrated cross section to the ground state and to the first excited state was measured and is consistent with literature values. It was possible to confirm the cascade decay via the first excited state by detecting the two coincident decay photons in GALATEA. Qualitative evidence of proton evaporation was also seen and nine levels in 31P were observed. While designed to measure a continuum and not single states, these results demonstrate that the NEPTUN experiment is well understood.
The key goal of NEPTUN in the near future is to measure the complete dipole response of the stable Sn isotopic chain and to provide a reliable value of the dipole polarizability. The first measurement was performed with photons in an energy range from 7.6 MeV to 9.6 MeV on a 112Sn-target. Within this range the integrated cross section and the B(E1) value have been measured. The values match recent (p, p’) scattering experiments and the extrapolation from the giant dipole resonance measured in (γ, n) experiments but deviate from previous (γ, γ’) experiments by more than an order of magnitude.
This indicates that most of the strength is hidden in small states which can only be measured in experiments like NEPTUN in which the incident photon energy is known.
To complete the measurements within a reasonable beam time, NEPTUN is currently undergoing a major upgrade which is supposed to speed up the measurements by more than two orders of magnitude. The design, purpose and expected performance of the new parts are described and new ideas for prospective measurements are provided.