Orateur
Description
The low-energy part of the electric-dipole response, commonly associated with the Pygmy Dipole Resonance (PDR), has been extensively investigated using a wide range of probes, including photons, alpha particles, protons, neutrons and heavy ions, as well as experimental techniques such as inelastic scattering, beta decay and fusion–evaporation reactions. Interest in PDR states is motivated by their potential influence on nucleosynthesis and by the ongoing debate concerning their microscopic structure. Complementary experiments employing isoscalar and isovector probes have revealed an energy-dependent isospin character of the PDR in several nuclei. These findings suggest the presence of a lower-energy, predominantly isoscalar surface mode and a higher-energy component containing an isovector admixture associated with the isovector giant dipole resonance (IVGDR). Recent experimental studies have indicated that PDR states may also be populated in the fission process.
This contribution will present some results from an innovative experiment performed at GANIL, in which the VAMOS++ heavy-ion spectrometer was coupled for the first time to the new-generation PARIS scintillator array [1]. Fusion–fission reactions were induced using a 238U beam at 5.8 MeV per nucleon incident on a 9Be target. VAMOS++ provided event-by-event identification of the mass and atomic number of the fission fragments, while PARIS measured coincident prompt gamma rays over a wide energy range and provided complementary information on coincident neutrons. This combination enabled the isotopic mapping of prompt fission gamma-ray emission.
Particular attention was devoted to the broad high-energy structure known as the fission gamma-ray bump and its likely connection with the Pygmy Dipole Resonance. This interpretation was supported by calculations employing microscopic nuclear level densities and gamma-ray strength functions. The results indicate that fission constitutes a promising new probe of low-energy dipole modes, complementary to conventional experimental approaches.
If time permits, the status and perspectives for the PARIS array will be also presented.
References
[1] N. Kumar, Ch. Schmitt, M. Ciemała, et al., "First experimental isotopic mapping of the fission “γ-bump” and its connection to the Pygmy dipole resonance", Phys. Lett. B 878 (2026) 140506