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Evolution of magicity

One of the most important phenomena that has arisen during the study of nuclei far from stability is the observation of the disappearance of magic numbers, although these are well established for near-stable nuclei. Accompanying this has been the emergence of new, local, regions of magicity far from stability. Though a lot of progress has been made over the last decade, the issue in the coming years remains one of identifying the origin of these changes in the different terms of the nuclear interaction, including the tensor, the spin-orbit, and the 3-body terms, the end goal being to put forward a global description of shell evolutions and come up with a reliable way of predicting them. Most of the recent breakthroughs in this field have been on nuclei far from stability in mass regions below 80. Recent progress in techniques for producing exotic nuclei, either with the ISOL machines (REX-ISOLDE, ALTO, SPIRAL2, etc.) or with projectile fragmentation machines (GSI, RIKEN, etc.) will open up previously inaccessible mass ranges : this mainly involves very neutron rich nuclei that are of intermediate mass.

In tackling these issues, the IPN holds a privileged position. Most notably, the nuclear structure group has taken the lead in the study of the evolution of the magic neutron number N=50 in the 78Ni region, particularly through ground-breaking experiments using the PARRNe separator, but also through on-line gamma spectroscopy experiments conducted at GANIL (projectile fragmentation), and at Legnaro National Laboratories (multinucleon transfer reactions in collisions ranging from near elastic to deep inelastic). Despite this, nuclei that are immediate neighbours of 78Ni have so far remained inaccessible to structural study. One of the most promising solutions for finally reaching this very exotic region will be provided by the availability of intense fission fragment beams post-accelerated at SPIRAL2, initially at CIME energies, using deep inelastic multinucleon transfer mechanisms, then on a longer horizon, fragmenting these beams at an energy in the order of 150AMeV as soon as such levels of post-acceleration become available. The nuclear structure group notably intends to use an 81Ga beam post-accelerated by CIME, whose expected intensity is in the order of 106 pps. In these conditions, the best tools available for performing on-line gamma spectroscopy will be the AGATA detector coupled with the VAMOS spectrometer. In addition, experiments intended to measure the lifetime of nuclear states using the so-called “plunger” technique will be implemented. This technique has already been used successfully by the nuclear structure group in the scope of the AGATA campaign in Legnaro or at LISE GANIL. Alongside its on-line spectroscopy experiments, this group also intends to pursue its rewarding programme of study on the structure of fission products in the 80 and 130 mass regions, at very low energy, with the PARRNe separator operating in line with ALTO. The priority is to exploit the full potential of this installation for radioactive decay gamma spectroscopy, for collinear laser spectroscopy, and for measuring nuclear orientation. This will be achieved by completing the construction then using the BEDO, SPECOLOR, and POLAREX facilities. The radioactive decay and collinear laser spectroscopy programmes will naturally continue with the DESIR installation at SPIRAL2, where beams of 500 times the intensity will be available.

Furthermore, the IPN, as one of the world’s leading-edge laboratories, is ideally placed for studying these direct reactions with inverse kinematics, both because of the richness of the physics programme sustained by its physicists, and because of its initiative in developing the MUST detector followed by the MUST2. The energy developed by SPIRAL2, determined by the CIME cyclotron, is especially favourable to the study of direct reactions such as transfer reactions. The MUST and MUST2 detectors have enabled IPN teams to conduct many studies using direct reactions over the last fifteen years and bring them to a successful conclusion. These studies concern practically all magic numbers of light and medium nuclei, such as N,Z=8, N=20,28, and more recently, N=40. At SPIRAL2, the evolution of gaps can be studied for a large number of medium-heavy/heavy nuclei thanks to an array of direct reactions using various probes (H, He, Li, etc.). Also related to this problem is the quest of “giant halo” nuclei that could comprise up to six neutrons in a peripheral orbit around the core. Recent mean field calculations predict that the p3/2 and p1/2 orbits play a determinative role in this phenomenon for very neutron rich isotopes of Zr (A>122). Even though these nuclei cannot be produced at SPIRAL2, studies of the evolution of these shells with their numbers of neutrons and protons, carried out using transfer reactions on a series of nuclei, will provide crucial information on the possibility of such structures developing.

Lastly, on a longer timescale, the usefulness of post-accelerating the beams from SPIRAL2, which will enable their fragmentation in excellent conditions, must be studied in depth. With this rapid two-stage process, a range of hitherto unseen “terra incognita” nuclei will become accessible, for example very neutron-rich Ca and Ni isotopes. In addition to its skills in accelerator technology and its expertise in studying the evolution of magicity in very neutron-rich nuclei, the IPN must actively support the project aimed at post accelerating intense beams from SPIRAL2 up to energies of 150MeV/u.

The nuclear structure group’s roadmap on the subject of magicity is therefore very clear for the next five years : optimum exploitation of the ALTO beams, allowing them to prepare the physics to be performed after SPIRAL2 DESIR ; the use of S3 for the study of neutron-deficient nuclei in the region N≈Z, first in the focal plane then redirected toward DESIR using gas cell and laser ionization techniques ; full exploitation of beams post-accelerated with the CIME of SPIRAL2 phase 2, and alongside this, study and design of the post-acceleration of intense neutron-rich beams at fragmentation energies for the heaviest elements in a longer term perspective of the EURISOL type.



Institut de Physique Nucléaire Orsay - 15 rue Georges CLEMENCEAU - 91406 ORSAY (FRANCE)
UMR 8608 - CNRS/IN2P3

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