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The complex shapes of atomic nuclei

In an atomic nucleus, the interplay between single-particle motions and pair and multi-particle interactions produces a rich variety of coexisting nuclear shapes and exotic excitations.
The complex shapes of atomic nuclei
To understand nuclear structure and phenomena, in-beam gamma ray (γ-ray) and electron spectrometers have been the most important tools until now. However, a γ-ray or an electron spectrometer alone can provide only partial information on nuclear de-excitation processes. Electron spectroscopy used in conjunction with radioactive ion beams (RIBs) has the potential to provide missing parts of the picture.

The SPEDE (Shape coexistence and collectivity in atomic nuclei) project initiated an electron spectroscopy campaign to perform simultaneous in-beam conversion electron-γ-ray spectroscopy in the Accelerator Laboratory of University of Jyväskylä (JYFL). New in-beam conversion electron-γ-ray spectroscopy techniques have been developed for complementary experiments with RIBs at REX-ISOLDE (and in the future with HIE-ISOLDE).

To understand the shape coexistence phenomenon of nuclei it is important to measure the transition probabilities between the coexisting nuclear states. The scientific aim of SPEDE was to employ novel methods of in-beam conversion electron spectroscopy to investigate electric monopole transitions and other highly converted transitions. Such studies have been carried out for nuclei close to the stability line, but until now few attempts have been made to investigate exotic nuclei close to the drip lines, especially while employing RIBs.

The SPEDE spectrometer has been developed from a design table to a working device. This effort included mechanical and electronics design, construction of all the required parts, assembly of the spectrometer, and thorough testing of vacuum, electronics, cryogenics, safety and operational performance.

The main result has been the construction of a working in-beam conversion electron spectrometer employing original concepts. The SPEDE spectrometer has been tested in-beam under realistic operational conditions, and conversion electron spectroscopy has been demonstrated at the target position, where the background radiation is high.

When combined with the MINIBALL spectrometer at HIE-ISOLDE, the European Organization for Nuclear Research (CERN), the SPEDE spectrometer will provide essential data to model the shape coexistence, collectivity and mixing of different proton-neutron configurations in the neutron-deficient Pb region.

Related information

Keywords

Atomic nuclei, in-beam, gamma ray, electron spectroscopy, radioactive ion beams, SPEDE
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