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Article on 92Pd published in Nature

Together with a research group at KTH, the nuclear structure group at the department has identified the basic low-energy structure of the extremely neutron deficient nucleus lying 92Pd. The results of the discovery has been published as a "Letter" in the journal Nature.

The article concerns the experimental discovery and theoretical interpretation of the three lowest lying excited states in the exotic nucleus 92Pd. This nucleus has an equal number of neutrons and protons (46) and a large deficit of neutrons compared to the stable palladium isotopes (102Pd with 56 neutrons is the lightest one of these).

An important property of atomic nuclei is the formation of pairs between protons and neutrons. According to the Pauli principle, one of the fundamental principles of quantum mechanics, pairs of identical particles can only couple their spins anti-parallel, while non-identical particles can couple their spins either parallel or anti-parallel. The atomic nucleus, which consists of non-identical particles (neutrons and protons), is unique in this respect and a fantastic "laboratory" for studies of quantum mechanical systems in which pairs of particle can couple their spins in parallel.

According to the interpretation presented in the Nature article, the relative distance between the energy levels indicate that neutron-proton pairs with parallel spins have a crucial importance for the structure of 92Pd. In other nuclei, the coupling of neutron-neutron or proton-proton pairs with anti-parallel spins dominates, which gives properties similar to the ones observed for electron-electron pairs in atomic systems, for example in superconducting materials and superfluid helium.

Nuclear structure scientists have since long searched for evidence for this type of pairing correlations between neutrons and protons. The results of the current study of 92Pd contribute to the understanding of the strong force that binds the particles in the nucleus and to the development of better and more reliable nuclear models.

Exotic nuclei like 92Pd are probably also created in nature. According to present theoretical astrophysical models this can take place in the rp process (rapid-proton process), which is predicted to occur in binary star systems in which one of the stars is a neutron star and the other a red giant. An increased knowledge of the structure of nuclei like 92Pd is therefore also important for the understanding of the nucleosynthesis of a number of isotopes of the elements with atomic numbers between iron and tin.

The Neutron Wall and EXOGAM detector systems at the experimental site at GANILThe Neutron Wall and EXOGAM detector systems at the experimental site at GANIL

The experiment that led to the discovery was performed at GANIL, an accelerator laboratory which is located in Normandy in France. The 92Pd nuclei were created in nuclear reactions in which a beam of 36Ar ions from one of the GANIL accelerators bombarded a thin foil of 58Ni. In this reaction, the 36Ar and 58Ni nuclei fuse together to form the compound nucleus 94Pd which decays to 92Pd after emission of two neutrons. Only a few 92Pd nuclei are created per million fusion reactions. After the emission of the neutrons the 92Pd nucleus returns to its ground state by emitting a cascade of gamma rays. The detection of these gamma rays lead to the wanted information regarding the energy levels of the nucleus. The gamma rays were detected by the EXOGAM gamma-ray spectrometer and the two neutrons by the Neutron Wall neutron detector system, which to a large extend is financed by the Swedish Research Council and for which the Uppsala group has the main responsibility. To identify the extremely rare 92Pd nuclei that are produced in the nuclear reactions, an effective neutron detection system was crucial for the success of the experiment.

One of the long term goals of the nuclear structure group is to produce and study energy levels of the doubly magic nucleus 100Sn with 50 neutrons and 50 protons. The nucleus 100Sn is often called the "Holy Grail" of nuclear structure physics. It is located at the proton dripline, which determines the limit for how many protons the strong force is able to bind in the nucleus for a fixed number of neutrons. In order to succeed in the studies of 100Sn radioactive ion beams of high intensity are required. Such beams are presently being developed at the future accelerator facilities FAIR in Germany and SPIRAL2 in France. The instruments for detecting gamma rays and neutrons must also be considerably improved. The nuclear structure group in Uppsala is actively involved in several research projects aiming at developing the next generation of such instruments, for example the gamma-ray spectrometer AGATA and the neutron detector NEDA.

The research project regarding 92Pd was led by professor Bo Cederwall at the Royal Institute of Technologies in Stockholm and it was carried out in collaboration with scientists from several other universities and research institutes.

For further information and questions regarding this work, please contact Johan Nyberg.

Evidence for a spin-aligned neutron–proton paired phase from the level structure of 92Pd, B. Cederwall et al., Nature 469 (06 January 2011) 68-71.
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