Evidence of halo structure inMg37observed via reaction cross sections and intruder orbitals beyond the island of inversion

Abstract

Precise reaction cross sections (oR) for 24_38M g on C targets at energies around 240 M eV /nucleon have been measured at the Radioactive Isotope Beam Factory at RIKEN. The oR for 36-38 Mg have been measured for the first time. An enhancement o f oR compared to the systematics for spherical stable nuclei has been observed, especially in the neutron-rich region, which reflects the deformation of those isotopes. In the vicinity of the drip line the aR for 37Mg is especially large. It is shown by analysis using a recently developed theoretical method that this prominent enhancement of oR for 37Mg should come from the p-orbital halo formation breaking the N = 28 shell gap.Since the early years of the study of atomic nuclei, the nuclear shell model has been the basic framework for understanding nuclear structure. The high stability of nuclei with certain numbers of neutrons (or protons) observed in stable nuclei indicates the existence of the shells filled at certain so-called "magic numbers." Studies in the last few decades have revealed that those magic numbers are sometimes broken or changed in unstable nuclei [1], The breakdown of the N = 20 shell gap between the sd and f p shells has been extensively studied since the irregularities in binding energies and 2+ excitation energies were observed in neutron-rich nuclei around N = 20 [2-6]. The term "island of inversion" was applied to this region [6] and deformed nuclear structures related to the changing of shell structures have been reported in this region [7]. The vanishing of the N = 28 shell closure has been also extensively studied, starting from neutronrich S-Ar isotopes [8][9][10][11][12][13][14]. The development of deformation observed in those nuclei could be interpreted as degeneracy of the f p shell, which induces strong quadrupole deformation [9][10][11][13][14][15][16][17][18]. Such deformation has been reported also for Si isotopes [19,20], and studies have recently indicated that this * takechi @ np.gs .niigata-u. ac .jp PACS number(s): 21.10.Gv, 25.60.Dz phenomenon could be seen even in a very neutron-rich Mg region [21].The purpose of our present study is to elucidate the changes of nuclear structures, such as a development of deformation, a breakdown of the magic numbers and possible halo formation in Mg isotopes, from the stability line to the vicinity of the neutron drip line. For this purpose, precise measurements of reaction cross sections for 24_38Mg have been performed at the Radioactive Isotope Beam Factory (RIBF) at RIKEN. The reaction cross section aR or interaction cross section ay reflects the nuclear size, and has been a powerful probe in searching for halo formation since the first study by Tanihata et al. [22], Recently, measurements of o, for Ne isotopes performed at RIBF [23] have successfully revealed the halo structure of 3lNe in which the sd-pf shell inversion associated with nuclear deformation causes the formation of a halo [23][24][25]. Moreover, theoretical studies on those data have shown that a precise data set on crR is v...

Examination of the C22 radius determination with interaction cross sections

“…IV A, the validity of our calculations is confirmed in comparison with available experimental data of 12 C + 12 C and 12 C + 1 H systems. Then, we further confirm the reliability of our calculations in the reactions involving 20 C and 12 C. Section IV B presents our main results: We describe the 22 C + 12 C and 22 C + 1 H reactions in a consistent manner and discuss the possible uncertainties in the radius determination using the interaction cross section. The sensitivity of the halo tail of 22 C to the interaction cross sections is also discussed.…”

“…In fact, the two-neutron separation energy observed is very small: 0.42 ± 0.94 MeV [3] and 0.14 ± 0.46 MeV [4]. The s-wave two-neutron halo structure is further confirmed by the 20 C fragment momentum distribution measurement of the two-neutron removal reaction from 22 C [5]. This nucleus has attracted much attention not only in nuclear physics but also in atomic physics in connection to the Efimov physics [6,7].…”

“…In the present work, we have considered the three nuclei, 12 C, 20 C, and 22 C. A configuration of the 12 C wave function is assumed to be (0s 1/2 ) 2 (0p 3/2 ) 4 for both proton and neutron with the harmonic-oscillator (HO) single-particle wave functions. Since the charge radius of 12 C is well known, the HO length parameter can be fixed in such a way to reproduce the pointproton radius, 2.33 fm, extracted from the charge radius [39].…”

“…This nucleus is a key to the understanding of the shell evolution along the neutron dripline with the magicity at the neutron numbers 14 ( 20 C) and 16 ( 22 C). The three-body 20 C + n + n system is the so-called Borromean, in which neither of the subsystems, 20 C-n and n-n, are bound, leading to an extended twoneutron wave function with s-wave dominance predicted by the earlier three-body calculation [2]. In fact, the two-neutron separation energy observed is very small: 0.42 ± 0.94 MeV [3] and 0.14 ± 0.46 MeV [4].…”

“…Use of such inclusive observables has some advantages: The theory of describing the cross section is well established, the cross sections can be measured for almost all nuclei as long as the beam intensity is sufficient, and the different sensitivity to the nuclear density profile can be controlled by a choice of a target nucleus and an incident energy. Systematic analyses of nuclear matter radii with the total reaction cross sections on 12 C target incident at 200 MeV/nucleon have revealed structure changes and the role of excess neutrons of light neutron-rich unstable nuclei [14][15][16][17][18][19][20][21]. We remark that the total reaction cross sections on 1 H target is also useful because the probe has different sensitivity to protons and neutrons in the projectile nucleus depending on incident energies that can be used to extract the neutron-skin thickness of unstable nuclei [22,23].…”