Search for the origin of the enhanced low energy gamma emission

The gamma decay of nuclei is governed by the so-called gamma-ray strength function, which is poorly known in the region below the particle binding energy. Normaly one uses the extrapolation of the tail of the Giant Electric Dipole Resonance (GEDR) to desc ribe the strength function at low energy. Recent results from experiments done in Oslo and analyzed with the Oslo-method revealed an enhancement of the gamma-strength function at low energy for Fe and Mo isotopes. We found that the probability to decay w ith low energy gammas was 10-100 times larger than current theories predict. This projects main aim is to understand the origin of this enhanced low-energy gamma emission, and why is it not observed in the heavier rare earth nuclei. The enhancement of l ow energy gamma emission is expected to help understanding the formation of the elements in stellar environments (nucleosynthesis). We will perform several experiments on different Sn isotopes at the Oslo Cyclotron, using the powerful CACTUS detector arr ay for measuring particle-gamma coincidences. The project involves people from several countries and will also include complementary neutron capture experiments abroad. ISOLDE is a radioactive beam facility offering unique possibilities. In addition to ex periments in Oslo there will be a series of experiments at ISOLDE investigating nuclear shapes and shape co-existence, starting with 96Sr and 182,184Hg isotopes next year. There the radioactive beams nuclei will be studied via coulomb exitation and the hi ghly efficient and highly segmented MINIBALL detector array will be used to measure the decay of the nucleus. From the intensity of the different transitions, combined with lifetime information, one can use the GOSIA code to calculate matrix elements and get the quadrupole moments and thus the shape of the different states.