One of the fascinating quantum phenomena in Nature is the occurrence of neutron halos and neutron skins in atomic nuclei. Thick neutron skins and halos have not yet been seen in medium mass nuclei, i.e. with more than 20 nucleons, and would be unique low-density neutron matter accessible in the laboratory. Their study should greatly enhance our knowledge on nuclear structure and the nuclear equation of state, intimately linked to the structure of neutron stars. Nuclear shell structure is known to change with the number of protons and neutrons: shell closures may disappear and new ones may emerge along an isotopic chain. The nuclear structure of very heavy nuclei at and above Z=100 is barely known, and the existence of new long-lived heavy isotopes is still an open question. The above fundamental phenomena related to the unbalance of neutron and protons in unstable nuclei are essential to understand the complex nature of nuclei, nuclear matter and related astrophysical processes.
We propose a new physics program to determine the neutron over proton densities at the nuclear surface for the most exotic nuclei that can be produced today, to evidence and to characterise neutron halos and skins in medium and heavy mass nuclei. PUMA will also allow the spectroscopy of single-particle states in heavy nuclei above Z=100 where nuclear stability is only due to shell effects and will offer a new insight into the unknown shell structure at the top of the nuclear landscape. To address these two questions of nuclear structure, PUMA explores a new way to study radioactive nuclei produced at very low kinetic energy: the interaction of antiprotons with unstable nuclei. This approach has never been developed anywhere thus far.
PUMA is based on a new apparatus: a transportable magnetic trap to store antiprotons and maximise their interaction with slow rare isotopes in order to trigger annihilations and measure the resulting radiation. The PUMA methodology involves two steps. (i) The storage of antiprotons will be performed at the new AD/ELENA facility of CERN in collaboration with the GBAR collaboration. (ii) The PUMA physics program is to take place at CERN/ISOLDE.
PUMA will open new horizons for nuclear structure research.
