Photonuclear reactions near the threshold. Excitation of pygmy resonances by real and virtual photons

INR RAS - Photonuclear reactions near the threshold.
Excitation of pygmy resonances by real and virtual photons.

New experimental data on the excitation of spin isomers 111mCd, 113mIn, 115mIn by real and virtual photons near the threshold (in the pygmy resonance region) were obtained at the LUE-8-5 linear electron accelerator of the Institute for Nuclear Research RAS. The measurements were carried out using a low-background camera with a high-resolution germanium detector. For the first time, it has been established that, in contrast to theoretical predictions, the ratio of cross sections for excitation of nuclei by real and virtual photons drops sharply at low energies, which indicates a change in the multipolarity of photoabsorption in the pygmy resonance region. This may be due to collective excitations of nuclei of an exotic nature (toroidal, scissor, compression vibration modes) predicted within the framework of existing models.

Currently, experiments are being carried out by measuring the yields of isomeric states on Cd and In isotopes in the reaction (γ,γ')m at an electron energy of 7.5 MeV using a low-background camera with a high-resolution germanium detector. The data analysis uses the calculated spectra of virtual photons in both the plane-wave PWBA and the distorted-wave DWBA approximation.

The interpretation of the results is based on the Weizsäcker-Williams model, which predicts a sharp increase in the multipole absorption of virtual photons by nuclei near the threshold.

Fig.2. Spectra of virtual photons for heavy nuclei. On the left - E1 excitations in the DWBA approximation for electrons and positrons with an energy of 27.5 MeV. On the right - E1 and E2 excitations in the PWBA approximation for heavy nuclei for electrons with an energy of 10 MeV.