New Physics: Sae Mulli, Vol. 66, No. 12, December 2016, pp. 1537∼1542 http://dx.doi.org/10.3938/NPSM.66.1537 Study of Nuclear Structure with Rare Isotope’s Seonho Choi∗ Department of Physics & Astronomy, Seoul National University, Seoul 08826, Korea (Received 16 November 2016 : revised 7 December 2016 : accepted 7 December 2016) With the advent of next generation rare isotope (RI) accelerators, nuclear physics is receiving new attention worldwide. The RAON of Korea will play an important role in international research activities. Isotopes far from the valley of stability provide new, interesting phenomena, which have never been observed in ordinary, stable nuclei. At the same time, the study of these unstable nuclei will provide deep insight into understanding the nucleus, a strongly-bound system of protons and neutrons. The new knowledge obtained from the study of rare, unstable isotopes will play an important role in nuclear structure, nuclear reaction dynamics, nuclear astrophysics, etc. This review will focus on the aspects of the study of nuclear structure that can be accressed at next- generation RI accelerators, including RAON. PACS numbers: 81.05.Ea, 85.30.Tv Keywords: Neutron-rich isotopes, Halo nucleus, Evolution of shell structure, Beta-delayed gamma ray spec- troscopy, Nuclear structure I. NUCLEAR PHYSICS AND NUCLEI FAR 1960’s, thanks to the development of heavy ion accelera- FROM THE STABILITY LINE tors, it has been possible to produce various nuclei in ex- treme conditions: high temperature, high density and/or Nuclear physics can be stated as the study of sys- high spin states and our knowledge on nuclei has been tem of strongly interacting particles, although over- enlarged. Since 1980’s, using fragmentation reaction of simplified. Nuclei are bound system of protons andneu- heavy ions, isotopes have been produced with quite dif- trons (nucleons in general) via strong interaction, while ferent number of protons and neutrons which then de- hadrons, which include nucleons are another strongly cays in various way [1–3]. In another words, although bound system of quarks and gluons. The fundamental still limited, we have a way to freely control the number law for strong interaction is Quantum Chromo-Dynamics of protons and neutrons in nuclei, which has opened a (QCD) which has quarks and gluons as the basic de- new era of RI physics. grees of freedom. Recently, there has been considerable Although there are only about 300 nuclear species progress in understanding hadrons in terms of these ba- found in nature, depending on the theoretical model, it is sic degrees of freedom of QCD. However, until now, it estimated that about 10,000 isotopes can exist as bound is not straightforward to explain nuclei starting from the nuclei under strong interaction. Up to now, about 3,000 QCD. On the other hand, nuclei have been understood isotopes have been produced in laboratories. As we en- quite successfully in terms of effective degrees of freedom large the area of research from the stability line toward of nucleons, pions, etc. the drip lines, many new phenomena have been observed In the beginning, most of the study of nuclear physics even for relatively light nuclei. A few examples include: has been carried out with respect to the ground or ex- • low density neutron matter covering the surface of cited states of stable nuclei found in nature. Around the nucleus such as neutron halo or neutron skin ∗E-mail: [email protected] [4–6] This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited. 1538 New Physics: Sae Mulli, Vol. 66, No. 12, December 2016 • modification of the magic numbers in neutron-rich core nucleus and easily break up during nuclear reactions isotopes [7–10] [15–17]. Especially, the 9Li core and the two neutrons form a Borromean system, where any two pair of three • deformation of nuclear shapes from the usual systems (10Li or di-neutron) are unbound while all three spherical shape and even coexistence of multiple together are weakly bound. When 11Li beam breaks up shapes in single nuclei [11] into 9Li and two neutrons, the transverse momentum dis- • evolution of nuclear shell structure leading to the tribution of 9Li fragment represents the transverse mo- so-called island of inversion [12] mentum of the two neutrons. Experimental measure- ment shows relatively narrow transverse momentum dis- • new oscillation mode in nuclei such as pygmy res- tribution and using Heisenberg’s uncertainty principle onance [13] between momentum and space, it can be interpreted that • new decay mode of unstable nuclei such as emission the spatial distribution of the two neutrons are relatively of single or multi protons [14] large around the core nuclei, confirming once more the halo nature of these two neutrons [18–20]. Until now, In addition to experimental efforts to clarify these in- several more halo nuclei were found in relatively light teresting new phenomena, theoretical understanding of mass nuclei. There are possibilities that more than two nuclei should be enlarged to include them within con- neutrons can form a halo in medium heavy isotopes. It text. Eventually, successful nuclear model should be ap- might be also possible to have halo states with the de- plicable to all nuclei in (N; Z) plane of isotopes including formation of nuclear shape. The next generation RI ac- traditional nuclear model valid near the stability valley celerator should be able to answer to these interesting as a special case when N and Z are similar. questions. With the advent of new generation of RI accelerators, we are entering into new, exciting renaissance of nuclear physics. 2. Evolution of Nuclear Shell Structure Single particle motion in nuclei is the most fundamen- II. STUDY OF NUCLEAR STRUCTURE tal in the study of nuclear structure and is the basis for FOR RI’S understanding various properties of the nuclei [21]. For 1. Halo Nuclei stable nuclei where the numbers of protons and neutrons are almost symmetric, we have pretty good understand- Up to now, almost of all the nuclei we have known ing of the single particle motion deduced from those ex- have protons and neutrons quite well mixed inside the isting in nature. The mean potential, where the single nuclei. In other words, the density distributions for pro- particle motion occurs is made from the nuclear inter- tons and neutrons are almost identical and their radii actions of protons and neutrons in a self-consistent way are the same. In halo nuclei, a completely new form [22]. For stable nuclei, this mean potential has well de- of nuclei has been observed [5]. There is a central core fined surface and combined with strong spin-orbital in- which resemble our typical nucleus and extra neutrons teraction, produces well known magic numbers N; Z = surround this core. For example, 11Li has a central core 2, 8, 20, 28, 50, 82, 126 [7,8]. Those nuclei with these of 9Li and the extra two neutrons form a halo around magic numbers have relatively stable compared to neigh- this core. Due to this interesting structure, the “size” of boring isotopes. For example, oxygen with Z = 8 and halo nuclei appears larger than non-halo nuclei with the lead with Z = 82 have many stable isotopes with varying same mass number. number of neutrons, are spherical in shape and the exci- The rms radius of 11Li is 3.16 fm which is the size tation energies to the first excited states are quite large. of normal nucleus with about 32 nucleons, such as 32S. However, in neutron rich isotopes, where N and Z are The two halo neutrons of 11Li are loosely bound to the quite asymmetric, even though N is one of the magic ≪ Review Article ≫ Study of Nuclear Structure with Rare Isotope’s ··· – Seonho Choi 1539 numbers, the excitation energy to the first excited states [31]. As a result, it is possible to observe new defor- can be low [23]. At the same time, E2 transition strength mations different from those observed in stable isotopes. can be large indicating possible deformation from spher- For example, it might be possible to observer very ex- ical shape [24,25]. For sd shell nuclei, the magic number otic deformation such as an octupole deformation which 20 shows up as the large energy gap between d and the have banana or tetrahedra shape, or for heavy nuclei, higher f shell. However, as the number of neutrons in- a nucleus with a hole at the center. Another interest- creases, the energy level of d shell increases while those ing feature is the coexistence of different shapes in the for f and s shell decreases, creating new, large energy same nucleus and already some hints have been found gap between s snd d shell while removing the original for several nuclei. It is even possible to have transition gap between df shell [26]. In this case, instead of 20, between several different shapes via quantum tunneling, new magic number of 16 appears. This region is called thus producing changing shapes with time. Probing nu- the island of inversion. For example, recently, it has been clear shapes in wider domain of (N; Z) and excitation energy E is another interesting physics to study with RI shown that 24O has new magic number of N = 16 and accelerator. that the shape is close to spherical [27]. The study of the evolution of the shell structure in the enlarged plane of (N; Z) is one of the key goal of the RI physics.