arXiv1601 issue. Proceedings of the 5th International Conference on -Emitting Nuclei, PROCON2015. Lanzhou. July 2015

Nuclear Physics Programs for the Future Rare Beams Accelerator Facility in Korea

MOON Chang-Bum*

Hoseo University, Asan, Chung-Nam 336-795, Korea

30 August, 2015

Abstract

We present programs based on the planned experiments using rare isotope beams (RIBs) for the future Korean Rare Isotope Beams Accelerator facility; RAON. This ambitious facility has both an Isotope Separation On Line (ISOL) and fragmentation capability for producing RIBs and accelerating beams of wide range mass of with energies of a few to hundreds MeV per . Low energy

RIBs at Elab = 5 to 20 MeV per nucleon are for the study of and toward and beyond the drip lines while higher energy RIBs produced by in-flight fragmentation with the re-accelerated ions from the ISOL enable to explore the drip lines in intermediate mass regions. The planned programs have goals for investigating nuclear structures of the exotic nuclei toward and beyond the nucleon drip lines by addressing the following issues: how the shell structure evolves in areas of extreme proton to neutron imbalance; whether the isospin symmetry maintains in isobaric at and beyond the drip lines, how two-proton radioactivity affects abundances of the elements, what the role of the continuum states including resonant states above the neutron- and proton-decay threshold in exotic nuclei is in astrophysical processes, and how the nuclear reaction rates triggered by unbound proton-rich nuclei make an effect on rapid processes in a very hot stellar plasma.

PACS: 21.10.-k, 21.60.-n, 23.20.-g, 23.50.+z.

Keywords: Rare Isotope Beams (RIBs), , magic numbers, mirror nuclei, Isotope Separation On Line (ISOL), In-Flight Fragmentation (IFF), rp-processes, r-processes, two proton capture reactions, two proton radioactivity, transfer reactions, one and two nucleon removal (knockout) reactions, isomers.

* [email protected]

1 arXiv1601 issue. Proceedings of the 5th International Conference on Proton-Emitting Nuclei, PROCON2015. Lanzhou. July 2015

1 Introduction

Atomic nuclei are complex quantum-mechanical many-body systems with a finite number of ; , Z, and , N. As a result, shell structure of the nuclei reveals discretely according to the Z and N numbers. The question of how shell structure develops in the finite quantum many-body systems has been a common problem among various disciplines; nuclear physics, atomic physics, condensed matter physics, molecular physics, and biophysics. The shell structure of the is one of the cornerstones for a comprehensive understanding of the many-body quantum mesoscopic system. The new isotope beams offer an opportunity for approaching and mapping regions of the drip lines, and helps the study of nuclear many-body quantum stability toward the proton and neutron drip lines. The study of nuclear structure including nuclear astrophysics is going through a new era owing to the development of rare isotope beams (RIBs) accelerators and a new generation of sophisticated detector systems, World-class accelerator facilities to produce RIBs being operated and planned in the world are: SPIRAL I and II(Système de Production d'Ions Radioactifs en Ligne) at GANIL(Grand Accélérateur National d'Ions Lourds) in France [1], RIBF (Radioactive Ion Beams Factory) at RIKEN in Japan [2], NSCL(National Super Conducting Cyclotron Laboratory) at Michigan State University (MSU) in USA [3], which will become the FRIB (Facility for RIBs) at MSU [4], FAIR at GSI (Gesellschaft fur Schwerionenforschung mbH) in Germany [5], ISAC I and II (Isotope Separator and ACelerator) at TRIUMF(Tri-University Meson Facility) in Canada [6], and REX and HIE-ISOLDE at CERN [7]. The future of nuclear physics depends to a large extent on the planning of the new facilities. In this respect, the Korean Rare Isotope Beams Accelerator facility to be built (RAON; Rare isotope beams AcceleratOr National facility)* stands on the heart of the future of nuclear physics in the world. We report on nuclear physics programs at the RAON facility with a goal for exploring new areas of the nuclei and the extreme areas of astrophysical nuclear reaction processes.

2 An overview of the shell structure evolution

To observe the behavior related to a fundamental characteristic of nuclear structure, it is better to draw a systematics of experimental observables across the nuclear chart [8, 9, 10, 11]. Especially, illuminating are the systematics of the first 2+ excited states of isotopic and isotonic chains which span the major shell closure. Figure 1 describes a nuclear shell structure development; (1) upper part shows the nuclear energy level sequences based on the shell model theory and (2) lower part shows the systematics of the first excited 2+ states experimentally observed in even-even nuclei in the vicinity of shell closures [11]. We notice that large energy gaps are observed at 40Ca, 48Ca, 56Ni, 68Ni, and 90Zr, which are known as a doubly-magic nucleus. Dynamical structural changes along the lines of both Z and N = 20, 28, and 40 isotones and have been main subjects of the studies in nuclear structure physics. Excluding characteristics of the doubly magic nuclei, we notice the following features revealed in the first excited 2+ states: First, shell gap at N = 20 is more reinforced with Z = 14 and 16 than that with Z = 18, and weakened below Z = 14 and above Z = 20. Second, the shell closure at Z = 20 (Ca isotopes) is more enhanced with N = 16 and 32. It is worthwhile to know that the shell gap at Z = 8 (O isotopes) shows a semi-doubly shell closure at N = 14 and 16,

respectively. According to the level sequences as shown in Fig. 1, N = 14 and 16 occupy sequentially 1d5/2 and 2s1/2,

leading to a sub-shell gap. Similarly, N = 32 and 34 form a sub-shell gap by occupying 2p3/2 and 2p1/2 orbitals, respectively. If we follow this tendency for a shell gap, we expect that 34Ca, with N = 14, and 54Ca, with N = 34, would have a semi-doubly magic character. The study of 34Ca, known as a nucleus with two proton radioactivity, is one of the subjects for our plans [11]. Third, the shell gap at N = 40 is enhanced only at Z = 28. This implies that the nuclei with N = 40 favor collective rather than spherical character for ground states. Fourth, proton number 40 (Zr isotopes) develops a semi-doubly magic character at N = 56 and at N = 58 that are also the sub-shell gap numbers. By focusing on the harmonic oscillator shell closure numbers; 8, 20, 40, and 70, we notice the following particular responses to the sub-shell gaps: Z = 8 responds to N = 14 and 16; Z = 20 does to N = 32 and 34; and Z = 40 does to N = 56 and 58, which lead to a semi-doubly shell closure. For N = 40, we raise a question, 'why does not it respond to Z = 32 and 34 for developing a semi-doubly shell closure?'. This problem may be intimately connected with concepts of the shape coexistence in the 72Ge (Z = 32) and 74Se (Z = 34) nuclei. In contrast, both of N = 70 and Z = 70 shell numbers have no shell gap partners leading to a semi-doubly shell closure. For instance, 120Sn (Z = 50 and N = 70) shows no

evidence for a semi-double shell closure, proving that 1h11/2 orbital remains in a strong intruder state. In this regard, it is questionable that 110Z (Z = 40 and N = 70) would reveal a semi-magic character.

2 arXiv1601 issue. Proceedings of the 5th International Conference on Proton-Emitting Nuclei, PROCON2015. Lanzhou. July 2015

(a) (b) (c) (d) harmonic no spin orbit strong spin orbit modified spin orbit oscillator 1i13/2 1i13/2 112 112 2f 126 126 3p1/2 5/2 3p 3p1/2 106 1/2 110 3p 3p 3p 2f 3p3/2 3/2 1/2 1i 5/2 120 106 104 13/2 2f5/2 1h9/2 2f 3p 114 2f5/2 94 3/2 100 N = 5 1i13/2 2f 3p 90 1h9/2 7/2 90 3/2 2f7/2 1h 1h 2f7/2 1h 9/2 2f7/2 9/2 82 82 82 82 1h11/2 1h 11/2 70 3s 2d3/2 1/2 70 3s 70 3s 1h 2d3/2 1/2 1g 11/2 1h 68 7/2 2d 64 3s1/2 11/2 2d3/2 2d 64 58 3/2 1g7/2 64 N = 4 56 2d5/2 2d5/2 3s1/2 2d5/2 1g 56 7/2 1g 1g 7/2 2d5/2 50 50 50 50 1g 40 1g9/2 9/2 1g 1g 2p 2p 409/2 40 9/2 2p 1/2 1/2 1f 2p1/2 1f5/2 N = 3 1f5/2 32 5/2 38 34 32 2p 2p 2p 2p 1f 3/2 3/2 343/2 32 1/2 2p 28 1f5/2 3/2 28 28 28 1f 1f 20 7/2 7/2 1f7/2 1f7/2 2s 20 20 20 20 N = 2 1d 1d3/2 1d3/2 1d3/2 16 3/2 16 16 16 1d 2s 2s 2s 2s1/2 1/2 1/2 1/2 14 14 14 14 1d 1d 5/21d 5/21d 5/2 5/2 8 8 N = 1 888 8 1p 1p1/2 1p1/2 1p1/21p 1/2 6 6 6 6 1p 1p 1p 1p 3/2 3/2 3/2 3/2 proton neutron CBM@2012 neutron proton

O Ne Mg Si SArCaTi Cr FeNi Ar Ca TiCr Fe Ni Zn Ge 5000 4000 E(2+) ; Mirror States E(2 + ) N=28 ; Unknown 3500 N=30 0 4000 N = 16 2 N=32 6 = 1 3000

= N=34 N 34 54 N 20Ca14 Ca N=26 4 2500 20 34 3000 1 N ? =

= N

1 2000 4 N = 28 2000 Energy (keV) Energy N = 22 1500

N = 18 (keV) Energy 1000 1000 N = 26 N = 24 500 8 10121416182022242628 18 20 22 24 26 28 30 32 Proton Number Proton Number Zn 3000 Ti Cr FeNi Ge Se Kr Sr Zr Mo Ru Ni Zn Ge Se KrSr Zr Mo Ru Pd Cd Sn + N=48 E(2 ) N=36 + E(2 ) N=50 2500 N=38 2000 N=52 N=40 N=54 2000 N=42 N=56 N=44 1500 N=58 1500 N=46

( )? 1000 1000 Energy (keV) Energy Energy (keV) Energy 500 500 0 22 24 26 28 30 32 34 36 38 40 42 44 28 30 32 34 36 38 40 42 44 46 48 50 Proton Number Proton Number

Fig. 1. (Upper panel) (a) The single particle energies of a harmonic oscillator potential as a function of the oscillator quantum number N. (b) A schematic representation of the single-particle energies of a Woods-Saxon potential. (c) A schematic illustration of the level splitting due to the spin-orbit coupling term. The numbers at the energy gaps are the subtotals of the number of particles represented by Nj = 2j + 1 of identical particles that can occupy each state. Note that the levels are numbered serially by a given orbital quantum number. (d) Modified energy levels according to the existing experimental data for neutron-rich region. The level pattern given represents only qualitative features. The arrows represent possible combinations of the proton and neutron (sub) shell gap numbers for developing a semi-double shell closure. (Below) energy level systematics for the first excited 2+ states in even-even nuclei as a function of proton numbers [11]. Data are taken primarily from NNDC [12].

3 arXiv1601 issue. Proceedings of the 5th International Conference on Proton-Emitting Nuclei, PROCON2015. Lanzhou. July 2015

3 A brief overview of nuclear experiments

The general experimental technique for nuclear physics includes mass measurements, stopped beam beta()-ray measurements, and nuclear reactions [13]. The mass measurements allow us to obtain basic information on nuclear structure, nuclear astrophysics, and fundamental interactions [13]. The nuclear masses are a direct reflection of the energies of the nuclei. In equilibrium, a system trends toward the lowest energy states and the transition to lower energy states releases energy, providing a source to power and to explode stars [14]. The stability of the nuclei against the various modes of can easily be understood in terms of the liquid drop model mass formula: M(Z, A) = 2/3 2 2 1/3 (A – Z)mn + Z(mp+me) – a1A + a2A + a3(A/2-Z) /A + a4Z /A + a5(A), here a1, a2, a3, a4, and a5 are coefficients due to volume, surface, asymmetric, Coulomb, and pairing energy terms, respectively. The pairing term (A) corresponds to 1/A3/4 for odd-odd nuclei, 0 for odd-even nuclei, and –1/A3/4 for even-even nuclei [15]. The stopped beam - spectroscopy allows us to measure gamma rays following beta-decay of excited states into the daughter nuclei. Besides, the stopped beam spectroscopy with sophisticated detector systems offers information on exotic decay modes such as -delayed proton(s) or neutron(s) emission in the nuclei toward the drip lines [16]. Data on - delayed plays a key role in understanding of the abundances of the elements as it affects the pathways of the rapid processes (r-process) [17]. Both the detailed stopped beam and the in-beam spectroscopic techniques provide complementary data on the location and the ordering of single particle states for exotic nuclei of interest. They also enable us to deduce radiative neutron capture on the very neutron-rich nuclei impossible to access in direct measurements. It is difficult and/or impossible to directly measure the thermal nuclear reaction rates based on (p, ), (p, ), and (, p) with RIBs such as; 14O, 15O, 17F, 18Ne, 22Mg, 23Mg, 26Al, 25Si, and 44Ti,, because secondary beams are limited to both intensity and production from ISOL. Instead, indirect measurements based on elastic resonance scatterings, transfer reactions, and Coulomb excitations provide information on the reaction rates for a given reaction system. The cross sections for the capture reactions can be determined by Coulomb dissociation based on the inverse photo-dissociation reactions [18]. The elastic scattering reactions are needed to study resonant states at higher excitation energies and provide information on the Coulomb amplitude and the nuclear amplitude [13, 19]. The inelastic scatterings offer the properties of states in a compound nucleus where decay by particle emission to an excited state is possible. Among direct nuclear reactions, the single-nucleon removal (knockout or breakup) reactions with heavy projectile ions at intermediate energies (100~300 MeV per nucleon) have become a specific and quantitative tool for studying single-particle occupancies and correlation effects in the . Charge exchange reactions are another method to measure Gamow-Teller strength compared to the usual –decay study. While the measurements of the –decay are limited with Q values, the charge exchange reactions do not have such a limit. The charge exchange reactions will play another important role in studying the properties of exotic nuclei. For nuclear astrophysics, the proposed experiments are mainly focused on the rapid proton capture reactions (rp- process) in a very hot stellar plasma. The rp-process has been known to be considerably complex due to the interplay of proton captures, decays, possibly photo-disintegrations, and particle induced reactions [20]. The important factors that influence the rp-process nuclear reaction networks above Z ≥ 32 include the proton-capture reaction rates and their inverse photo-disintegration rates, and the -decay and electron-capture rates [20]. We should remember that nuclear deformations significantly affect both the r-process and the rp-process reactions.

4 The characteristics of the RAON facility

To produce radioactive ion beams, there are two main approaches; ISOL production [19, 21] and In-Flight production [22]. The RAON facility has both an ISOL and fragmentation capability. The efforts to create, separate, and study radioactive nuclides are being undertaken for the coming decade. This is an ambitious project to build a multi-beam facility capable of producing and accelerating beams of a wide mass range of nuclides with energies of a few to hundreds MeV per nucleon. See Fig. 2. The KOBRA (KOrea Broad acceptance Recoil spectrometer and Apparatus) and the IFFS (In-Flight Fragmentation Separator) facilities will provide the selection and identification of the beam-like particles as well as target-like ones, and with the combination of -ray detectors array offer spectroscopic information on internal structures of the nuclei to be investigated.

4 arXiv1601 issue. Proceedings of the 5th International Conference on Proton-Emitting Nuclei, PROCON2015. Lanzhou. July 2015

SC ECR IS (10A keV, 12 p A) LEBT RFQ (300A keV, 9.5 p A) MEBT The RAON facility SCL 1 m (18.5 A MeV,9.5 p A) 28.5 28.5 100 m 100 1 Driver Linac

(200 A MeV, 8.3 p A for U+75 ) SCL2 (600 MeV, 660 p A for p) 375 m 100 m SR Bio-Medical MEBT RFQ CB HRMS ISOLtarget RF 250 m Cooler Cyclotron SCL3 ECR-IS KOBRA Post Accelerator Atom/Ion Trap Gas Catcher 110 m In-Flight Experimental Experiments at KOBRA target using RIBs with Experimental Area 1 In-Flight (Low Energy) E=1~20A MeV Experiments at IFF using Area 2 (High Energy) Fragment RIBs with Separator Nuclear Physics E=100~250 A MeV Condensed Matter Physics LAMPS

( RAON;Rare isotope beams AcceleratOr National facility) CBM@2015 Fig. 2. Layout of the RAON facility [23].

The KOBRA facility is divided into two stages by consisting of a series of magnetic dipoles, quadrupoles, and Wien filters as shown in Fig. 3 [23]. The stage 1 is utilized to produce the low energy RIBs via multi-nucleon transfer reactions at about 20 MeV per nucleon or via direct one or two nucleon transfer reactions at a few MeV per nucleon [24]. It is worthwhile to know that it is designed to reject the primary beams for the studies of thermonuclear, namely direct capture reactions under high temperature plasma conditions in hot stars. The stage 2 placed at down-stream of the stage 1 is employed to separate the fragments following the reactions of RIBs on a reaction target, or to reject the primary beam in the same manner as for the stage 1.

beam

F0 KOBRA (Korea Broad accelptance Recoil Spectromer and Apparatus)

Curved-edge bending magnet (D1)

F1 (dispersive focal plane)

Wien filter 1 Wien filter 2 D2 F3 F4

F2 (ach ro m atic focal p lan e)

F5 (dispersive focal plane) Fig. 3. Layout of KOBRA. This facility is divided into a stage 1 (F0 - F3) and a stage 2 (F3 – F5). Curved-edge shape bending magnets are utilized to minimize the high order aberration [23, 24].

At the KOBRA facility, the lower energy re-accelerated ISOL rare isotope beams with energies 5 to 15 MeV per nucleon can be separated and identified. Such low energy reaccelerated RIBs are suitable for producing the nuclear reactions such as; Coulomb excitations, -evaporation reactions, elastic resonance scatterings, inelastic scatterings, one or two nucleon transfer reactions, and direct capture reactions. The higher energy RIBs with energies of 100 up to 250 MeV per nucleon will be produced by nuclear fragmentation at the IFFS. High quality and intense RIBs combined with re-accelerated ISOL beams including high efficient detector systems will provide unique experimental possibilities to study the very neutron-rich and proton-rich nuclei toward and beyond the drip lines.

5 arXiv1601 issue. Proceedings of the 5th International Conference on Proton-Emitting Nuclei, PROCON2015. Lanzhou. July 2015

The KOBRA and the IFFS have two complex detector systems: One is located at the target position for detection of reaction products including light-charged particles and  rays; and the second one is positioned at focal plane for detection and identification of heavy recoiled nuclei as well as for spectroscopic study of delayed activity. Conjunction of detector systems with a polarized spin target (or polarized beam) is desirable for studying spin-orbital interactions.

5 The planned experiments

In the following we describe briefly some proposed experiments aiming at the RAON facility. More detailed descriptions are found in [11].

A Study on two proton capture reactions. The proton drip line imposes a constraint on the reaction path of the rp-process. As shown in Fig. 4, many proton-rich nuclides near the Z = N line are unbound to proton decay. If we assume an immediate proton-decay for these proton unbound nuclei, we expect that no further proton-capture proceeds. Thus further processing depends on the -decay of the last proton stable isotone. This bottleneck is called a waiting point. However, if the lifetime of a proton unstable nucleus is appreciably long due to a high Coulomb barrier, 2p-capture reactions on the last proton bound isotone would be possible [20]. The 2p-capture reactions allow to bridge the single proton unstable nucleus to a proton bound nucleus. This plan aims at measuring the 2p capture reaction rates combined with the proton resonance scatterings for the associated unbound nuclei. The 2p-capture reactions to be studied are summarized in Table I. We introduce, as an example in this field, a proposal for investigating the unbound states of 73Rb. This proposal aims to measure the unbound states of the proton radioactive 73Rb through proton elastic resonance scatterings with 72Kr on a hydrogen thick target. 72Kr is a waiting point where the proton capture is followed by the instantaneous emission of a proton by the proton unbound 73Rb nucleus. This waiting point, however, as already pointed out, can be bypassed by a two-proton radiative capture reaction if the nucleus 74Sr is bound. Such alternative paths can be estimated by the calculations using the properties of the intermediate unbound nucleus, 73Rb.

Z = N

52 Te 104105106 10710810911 0 111 11 2 113 114 Sb 2p capture path 103104105 106107108109110 111 112 11 3 50 Sn 99 100 101102103104105106107108 10911 0 111 11 2 In 98 99 100101102103104105106107108109110 111 rp-process paths Cd 95 96 97 98 99 100101102 103104 105106 107108109110 Ag 94 95 96 97 98 99 100101 102103104105106107108109 46 Pd 90 91 92 93 94 95 96 97 98 99 100 101102103104105106107108 Rh 89 90 91 92 93 94 95 96 97 98 99 100101102103104105106107 Ru 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100101102 103104105106 Tc 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100101102103104105 Mo 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100101102103104 Nb 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100101102103 40 Zr 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100101102 Y 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100101 Sr 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 Rb 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 36 Kr 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 Br 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 34Se 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 As 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 Ge 60 61 62 6364 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 60 30 32 34 36 40 46 50 56

Fig. 4. The chart of nuclides in the mass region of Ge to Te.

6 arXiv1601 issue. Proceedings of the 5th International Conference on Proton-Emitting Nuclei, PROCON2015. Lanzhou. July 2015

Table I. Candidates for the two-proton capture reactions and their intermediate proton-unbound nuclei.

Elastic resonance Intermediate proton- Two proton capture Spin-parity and half-life for the scatterings unbound nuclei reactions final nucleus 15O + p 16F 15O (2p, ) 17Ne 1/2- ;109 ms 18Ne + p 19Na 18Ne (2p, ) 20Mg 0+ ; 90.8 ms 20Mg + p 21Al 20Mg (2p, ) 22Si 0+ ; 9 ms 29S + p 30Cl 29S (2p, ) 31Ar 5/2+ ; 14.4 ms 37Ca + p 38Sc 37Ca (2p, ) 39Ti (3/2+) ; 31(+6/-4) ms 38Ca + p 39Sc 38Ca (2p, ) 40Ti 0+ ; 52.4 ms 41Ti + p 42V 41Ti (2p, ) 43Cr (3/2+) ; 20.6 ms 58Zn + p 59Ga 58Zn (2p, ) 60Ge 0+; > 110 ns 62Ge + p 63As 62Ge (2p, ) 64Se 0+ ; > 180 ns 68Se + p 69Br 68Se (2p, ) 70Kr 0+ ; > 0.05 s 72Kr + p 73Rb 72Kr (2p, ) 74Sr 0+, > 1.2 s

As for nuclear structure studies, we investigate the shape coexistence in the proton-rich Kr nuclei with Z ≤ N. For this purpose, the proposed experiments using 70,72Kr RIBs employ Coulomb excitation measurements which provide information on the nuclear shape built on the ground state as well as built on the first excited 0+ state. It is known that the ground state of 72Kr would be of oblate in shape while 74Kr and 76Kr are known to be prolate in their ground states [25- 27]. The proposed experimental methods that would require conversion electron spectroscopy are shown in Fig. 5.

(a) Resonance scatterings scheme (b) Coulomb excitations scheme

Ge detectors Ge detectors  72 72 208  Kr Hydrogen gas E:DSSSD Kror 70 Kr Pb E:DSSSD E: CsI(Tl) E: CsI(Tl) 5 A MeV 8-10 A MeV  KOBRA  KOBRA Ge detectors Ge detectors

Fig. 5. Experimental set-up schemes for measurements of (a) unbound states in 73Rb and (b) excited states of 72Kr and 70Kr.

In addition, we propose invariant mass measurements for the proton unbound nuclei, such as 82Mo, 86Ru, and 90Pd. They provide information on isospin symmetry, as well as rp-processes beyond the proton drip lines.

B Study on shell structure evolution along a chain of N = 19, 20, 21 isotones with Z = 22 and 24; 41Ti, 44Cr, and 45Cr. This proposal aims at measuring internal structures of the proton-rich 41Ti, 44Cr, and 45Cr nuclei to test the symmetry of mirror states in isobaric A = 41, 44 and 45 by comparing to the structures of 41K, 44Ca, and 45Sc. The nuclei of interest are produced through one or two neutron removal out of secondary beams, knockout reactions, on a thick 9Be target. The secondary beams are produced by the fragmentations of a 250 MeV per nucleon 78Kr primary beam on a thin 9Be production target. The proposed reactions are as follows: 9Be (42Ti, 41Ti + n) X for 41Ti, 9Be (46Cr, 44Cr + 2n) X for 44Cr, and 9Be (46Cr, 45Cr + n) X for 45Cr. The -ray spectroscopy employs to measure the internal electromagnetic transitions in the nuclei of interest.

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33 35 41 49 51 37 53 55 K K 19 K22 K K Sc Sc Sc

5 / 45 _ proton rich K (Z=19) 3000 2 Sc Sc(Z=21) 3000 + 21 24 3/2

proton rich( 1 neutron rich neutron rich 3 d / / 5 2 + 2 / 2 + 2500 2500 + 1/2 (s ) ) 1/2 ( ( d s Inversion

1 _ 3 + 6.3 ns / / 2 3/2 (d ) 2000 2000 2 ) 3/2 3/2 ) )

/2 ) 7 _ f 1500 2 1500 ( / 3 2 p 7.35 ns / _ ( 10.4 ns 7 _ 2 1000 3.2 ns 1000 / 3 5/2 Energy (keV) Energy Energy (keV) Energy

738 ns Re-Inversion?

Inversion + 500 1.1  s 500 3/2 (d ) Isomers ? + radioactivityOneproton 3/2

One proton radioactivity ? radioactivity proton One _ + 1/2 (s1/2 ) ? Re-Inversion 438 s 3/2 (d3/2 ) 8 ns 318 ms7/2 (f 7/2 ) 0 0

14 16 18 20 22 24 26 28 30 32 16 18 20 22 24 26 28 30 32 34 Neutron Number Mirror Nuclei Mirror Nuclei Ne Mg Si S Ar Ca Ti Cr Ne Mg Si S Ar Ca Ti Cr Fe 41 45 N = 21 Cr 3000 N = 19 22Ti19 3000 24 21 _ 62 ps 3/2 (p ) neutron rich 47Fe 2500 3/2 2500 26 21 neutron rich proton rich proton rich eV) )1.02 ns 2000 k 2000 /2 ( _ f 7 4.38 ns ( y /2 1500 g 1500 7 3 r 10.2 ns / ) 2 e + + Isomer /2 + 1/2 (s ) 5/2 (d 5/2 )

n 3 1/2 1000 p ( 1000 _ ( d

E ? (keV) Energy 3 deformed deformed ) 2 / 2 / 2 / ) 3 3 10.5 ns 500 d 500 ( _ 2.6 s + ? + 2 7/2 (f ) 16 ns 3 /2 (d 3/2 ) / 7/2 Isomer 0 0 3

10 12 14 16 18 20 22 24 10 12 14 16 18 20 22 24 26 Proton Number Proton Number

Fig. 6. Systematics of the ground and some low-lying excited states in Z = 19, 21 (upper) as a function of neutron numbers and in N = 19, 21 (lower) as proton numbers [11]. For discussion, the associated level sequences are included as designating their relations by the arrows with the corresponding shell structure evolution.

The goals of the proposal are: (1) Identification of E(2+) and E(4+) in even-even 44Cr for investigating the magic N = 20 shell evolution like an emergence of collectivity; (2) measurements for the low-lying excited states in odd-Z 41Ti and 45Cr for testing charge symmetry between mirror nuclei, Z , N = 19 and 21 with A = 41 and 45; and (3) search for isomers in the nuclei of interest to study the single-particle and collective features based on isomerism. Figure 6 illustrates the energy level systematics for the ground and low-lying single-particle excited states in Z = 19 and 21 nuclei as a function neutron numbers and in N = 19 and 21 as a function of proton numbers. This proposal is focused on the proton rich side. The neutron-rich side will be discussed in plan C. In the region of Z, N = 19 and 21, the

2s1/2, 1d3/2, 1f7/2, and 2p3/2 orbitlas play a critical role in the strength of the shell gaps as shown in Fig. 6. We find the + following distinctive features: First, for Z = 19 isotopes, the 1/2 level due to 2s1/2 subshell decreases rapidly with increasing neutron numbers from N = 20 and finally becomes the ground state at N = 28. For N = 19 isotones, the subshell also decreases in energy toward ground level with decreasing proton numbers from Z = 20 and finally becomes the ground state at Z = 12. We notice that the nuclei at and below Z = 12 turned out to be deformed in their ground states - at magic neutron number 20 [10]. Second, for Z or N = 19, the 7/2 level due to the 1f7/2 subshell forms isomers with half- lives of a few nanoseconds across the 20 magic gap region as an intruder state. Third, for Z and N = 21, the 3/2+ level due

to 1d3/2 subshell changes dramatically with neutron and proton numbers and forms isomers near the ground states at N = 22, 24 and Z = 22. This proposal aims at proving the charge symmetries by observing low-lying states in isobars with Z, N = 19 and 21 toward Z > 20 and N < 18. Besides, we are interested in searching for isomers in 41Ti and 45Cr as denoted in Fig. 6.

C Study on shell structure evolution based on the proton single-particle orbital changes in the vicinity of 54Ca. This plan aims at exploring single-particle shell migrations in the vicinity of 52Ca and 54Ca. The experiments are based on the nucleon removal (knockout) reactions using the IFFS. The proposed one-proton removal reactions are as follows: 9Be (50Ca, 49K + p) X, 9Be (52Ca, 51K + p) X, 9Be (54Ti, 53Sc + p) X, and 9Be (56Ti, 55Sc + p) X. This proposal has a goal for studying nuclear structures of the very neutron rich nuclei with Z = 19 and 21 and N ≥ 30; 49K, 51K, 53Sc, and 55Sc. The measurements involve the thick reaction target 9Be and the -ray spectroscopy of the projectile-like residual nuclei

8 arXiv1601 issue. Proceedings of the 5th International Conference on Proton-Emitting Nuclei, PROCON2015. Lanzhou. July 2015 for the final state resolution. The secondary rare isotopes beams produce from the fragmentation between a 86Kr primary beam with 250 MeV per nucleon and a 9Be production target.

As shown in Fig. 6, the 2s1/2, 1d3/2 and 1f7/2 subshells contribute dominantly to shell structure change in proton-single levels. In contrast, neutrons begin to occupy the 2p3/2, 2p1/2 and/or 1f5/2 orbitals cross over the 1f7/2 orbital, which + separates the N = 20 gap and the N = 28 gap. For Z = 19 isotopes, the 1/2 level due to 2s1/2 subshell decreases remarkably with increasing neutron numbers and finally becomes ground state at and beyond N = 28. For Z = 21 isotopes, + the 3/2 level due to 1d3/2 subshell changes dramatically as it goes down near to the ground state at N = 24. It is interesting to notice that the levels of 3/2+ and 1/2+ are inverted at N = 28 in Z = 19 (47K) and Z = 21 (49Sc). The inversion of the 3/2+ and 1/2+ levels, however, is uncertain in 49K and unknown in 53Sc and 55Sc above N = 28. It is worthwhile to remind that N = 32 and 34 develop the semi-double shell gaps at Z = 20. We address how the distinctive quasiparticle levels, such as 2s1/2, 1d3/2, and 1f7/2 migrate and make an impact on the formation of the semi-double shell 54 closure of Ca. The migration of the proton 2s1/2, d3/2, f5/2 levels as well as the neutron p1/2, f5/2 levels with respect to neutron numbers may indicate an underlying physics not well accounted for in the present shell model interactions. In this regard, the present proposal is an important step toward extremely neutron-rich region where more exotic phenomena including halos or skins are expected.

2000 (a) Tl(Z=81) _ 1500 /2 13 _ eV

k 11/2 (h 11/2 ) n i

1000 y

g _ r 9/2 (h 9/2 )

Ene 500 + ? 3/2 (d 3/2 ) ? + Isomers(beta-decay) 1/2 (s 1/2 ) 0

98 102 106 110 114 118 122 126 130 Neutron Number

2500 (b) Bi(Z=83) + 3/2 (d 3/2 ) 2000 + 1/2 (s 1/2 ) + eV 13/2 (i 13/2 ) k 1500

n _ i 5/2 (f 5/2 ) y _ + g 13/2 (2 *h 9/2 ) r 1000 _

Ene + 1/2 (s 1/2 ) 11/2 (h 11/2 ) _ 500 7/2 (f 7/2 ) ? _ _ 7/2 (f 7/2 ) 9/2 (h 9/2 ) ? 0 102 106 110 114 118 122 126 130 134 Neutron Number

(c) At(Z=85) 1000 1 3 /2 _

( 2 + * h 9 / _ 2 ) 13/2 + (i ) 11/2 (h ) 500 13/2 11/2 5 + _ / 2 _ 1/2 (s 1/2 ) Energy in keV in Energy 7/2 (f 7/2 ) (f ? 5 /2 ) ? _ 9/2 (h ) 0 9/2

110 114 118 122 126 130 134 Neutron Number Fig. 7. Systematics of ground and low-lying states in the odd-mass nuclei; (a) Thallium, (b) , and (c) Astatine as a function of neutron numbers. The expected level crossings on the basis of the systematic trends are denoted by the arrows. Data are from NNDC [12].

D Shell structure evolution and shape transitions in the vicinity of proton , Z = 82. This region is a section of the nuclear chart that is not easy to access under current experimental condition. In particular, both the north-eastern area and the south area of the Z = 82 and N =126 point remain mostly unexplored. As a starting point to study shell structure evolution based on the single particle configurations over Z = 82 and N = 126, we focus on investigating the level structures of Tl (Z = 81) and Bi (Z = 83) since the intruder states and the associated

9 arXiv1601 issue. Proceedings of the 5th International Conference on Proton-Emitting Nuclei, PROCON2015. Lanzhou. July 2015 isomers in odd Z nuclei with ±1 outside doubly magic core provide critical information on the shell structure evolution. Figure 7 demonstrates the single-particle energy level systematics based on the low-lying excited states for odd-Z nuclei 208 175 177 179 lying close to Pb. By considering the currently known nuclei, we focus on the following nuclei: Tl94, Tl96, Tl98, 211 213 183 185 217 219 191 193 219 221 Tl130, Tl132, Bi100, Bi102, Bi134, Bi136, At106, At108, At134, and At136. An interesting feature is that the levels at J = 1/2+, 7/2–, as well as 13/2+ decrease down to the ground level as the neutron numbers decrease in Bi and At nuclei. It is also interesting to know whether the 7/2– level would reach the ground state beyond N = 132 in the said nuclei. The shell migration of these orbitals is closely connected to concepts of the triplet shape coexistence in the region of neutron deficient Pt, Os, Hg, and Pb. The nuclei of interest can be investigated by the multi-transfer reactions at E ~ 10 MeV per nucleon. For example the 136 208 219 217 reaction of Xe + Pb at Ecm = 526 MeV is estimated to be 0.2 b for the production of At and 3 b for the Bi, respectively. However, the role of transfer channels is less clear due to the difficulty of making qualitative calculations for both the multi-nucleon transfer and the sub-barrier fusion simultaneously. The very neutron-rich RIBs, for example 144Xe, with the high intensity and high quality with a few MeV per nucleon are essential for producing exotic nuclei far from the Z = 82 and N = 126 point, such as 196Yb and 220Pt. It is noteworthy to mention that the measurements of half- lives of the nuclei with Z < 82 and N = 126 provide decisive information on the r-process paths along waiting points with N = 126, while the -delayed neutron emission schemes in these neutron rich-nuclei play an important role in determining abundances of Au and Pt elements.

6 Summary and outlook

We have introduced and discussed the planned experiments using RIBs for the Korean Rare Isotope Beams Accelerator facility, RAON to be built in a decade. This facility is one of the world-class multi-beam facilities capable of producing and accelerating beams of wide range mass of nuclides with energies of a few to hundreds MeV per nucleon. The low energy RIBs at Elab = 5 to 20 MeV per nucleon are used for the study of nuclear structures and nuclear astrophysics toward and beyond the drip lines while the higher energy RIBs produced by the in-flight fragmentation with the reaccelerated ions from the ISOL enable us to explore the neutron drip lines in intermediate mass regions. Beam specifications for the KOBRA and ones for the IFFS spectrometers are complementary, which allow scientists to extent the investigations toward the very neutron-rich and proton-rich nuclei.

124Xe + 208Pb => 179,181 Tl, 187 Bi 9Be ( 181 Tl, 179 Tl+2n) X 9Be ( 179 Tl, 177 Tl+2n) X 9Be ( 187 Bi, 185 Bi+2n) X

124 9 105,107 109,111 208 Xe + Be => Sb, I 82Pb 126

9 105,107 103,105 Be ( Sb, Sb+2n) X Pb 82 136 208 217,219 9 109,111 107,109 Xe + Pb => At 96866 Be ( I, I+2n) X Be ( Se, Se+2n) X 9Be ( 217 At, 215 Tl+2p) X 97068 Be ( Kr, Kr+2n) X 9219217 Be ( At, Bi+2p) X

9Be (46Cr, 44Cr+2n) X 100 (stopped beam spectroscopy by fission-like 9Be (46Cr, 45V + p )X 50Sn50 or multi-transfer reactions) Sn 50 93634 Be ( Ca, Ca+2n) X 140,142 140,142,144 142,144,146 Sn Sb Te 144,146,148I 36 34 종 132 p ( Ca, t) Ca 핵 Sn82 146,148,150126 146,148,150 152,154,156 56 56 50 Xe Fe seed 정 Cs Ba 니켈 28Ni28 안 92423 불 Be ( Si, Si+n) X Ni 28 92523 Be ( Si, Si+2n) X 79,80,81 80,81,82,84 82,83,84,85,86 칼슘 Cu Zn Ga Ca 20 78 Ni 82 84,85,86,87,88 86,87,88,89,90 85,86,87,88,90 28 50 Ge Se As

O 8 50 95049 28 62 63 He 2 Be ( Ca, K+p) X d ( Cr,p) Cr 20 95251 64 65 2 8 Be ( Ca, K+p) X d ( Cr,p) Cr

32 33 95453 92725 d ( Mg, p) Mg 66 67 Be ( Na, F+2p) X Be ( Ti, Sc+p) X d ( Fe,p) Fe 50 51 92625 d ( Ca, p) Ca 95655Be ( Ti, Sc+p) X d (68 Fe,p) 69 Fe Be ( Ne, F+p) X

Fig. 8. Summary of the proposed experiments aiming at the rare isotope beams accelerator facility [11], RAON.

10 arXiv1601 issue. Proceedings of the 5th International Conference on Proton-Emitting Nuclei, PROCON2015. Lanzhou. July 2015

The experiments are planned to investigate the internal structures of exotic nuclei toward and beyond the nucleon drip lines as addressing the following questions: how the shell structure evolves in areas of extreme proton to neutron imbalance; whether the isospin symmetry maintains in isobaric mirror nuclei at and beyond the drip lines, how two- proton or two-neutron radioactivity affects abundances of the elements, what the role of the continuum states including resonant states above particle-decay threshold in exotic nuclei is in astrophysical nuclear reaction processes, and how the nuclear reaction rates triggered by unbound proton-rich nuclei make an effect on rapid proton capture processes in a very hot stellar plasma. To summarize, we show a set of the planned experiments with the associated nuclei to be investigated over the nuclear mass chart in Fig. 8. We hope that the proposed experiments will play an important role in developing a nuclear physics program for the rare isotope beams accelerator facilities, as well as in promoting the creation of next generation of nuclear scientists in the world.

* This is not an official abbreviation. The Rare Isotope Beams Accelerator Facility to be built in Korea has been named officially, regardless of an abbreviation for the facility, as 'RAON' which is said to mean 'joyful, delightful' in old Korean language.

References

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