Shape Coexistence and the N=28 Shell Closure Far from Stability

A mass measurement experiment by a time of flight method with the SPEG spectrometer at GANIL has been performed to investigate the N=20 and N=28 shell closures far from stability. The masses of 31 neutron-rich nuclei in the range A=29–47 have been measured. The precision of 19 masses has been significantly improved and 12 masses were measured for the first time. The neutron-rich Cl, S and P isotopes are seen to exhibit a change in shell structure around N=28. Comparison with shell model and relativistic mean field calculations demonstrate that the observed effects arise from deformed prolate ground state configurations associated with shape coexistence. The evidence of an isomeric state in the ⁴³S and its interpretation by a shell model calculation confirm the analysis of the masses and constitutes the first evidence of the predicted shape coexistence around N=28. This revised version was published online in July 2006 with corrections to the Cover Date.     

Evolution of the N=50 shell gap energy towards 78Ni

Atomic masses of the neutron-rich isotopes (76-80)Zn, (78-83)Ga, (80-85)Ge, (81-87)As, and (84-89)Se have been measured with high precision using the Penning trap mass spectrometer JYFLTRAP at the IGISOL facility. The masses of (82,83)Ga, (83-85)Ge, (84-87)As, and 89Se were measured for the first time. These new data represent a major improvement in the knowledge of the masses in this neutron-rich region. Two-neutron separation energies provide evidence for the reduction of the N=50 shell gap energy towards germanium (Z=32) and a subsequent increase at gallium (Z=31). The data are compared with a number of theoretical models. An indication of the persistent rigidity of the shell gap towards nickel (Z=28) is obtained.     

放射性核束的库仑激发

综述了利用中、高能放射性核束的库仑激发方法研究位于N＝20和28主壳隙附近的丰中子核结构所取得的进展．系统的实验结果清晰地表明,在离开β稳定线区域时N＝20兰壳隙突然消失和N＝28主壳隙的减弱过程．提出了利用兰州放射性束流线开展双幻核Ni50附近核素的低位能级核结构研究的构想． The study of coulomb excitation of the neutron-rich nuclei around N=20 and 28 shell closure with radioactive ion beam at intermediate energy is reviewed. The systematics of the measured energy of the 2+1 state shows that the N=20 shell closure in neutron-rich isotopes with Z≤12 disappears suddenly and N=28 shell elosure appears to be weak for 44S.The coulomb excitation studies of the exotic nuclei around the double magic 7828Ni50 at RIBLL are proposed.     

Probing shell structure and shape changes in neutron-rich sulfur isotopes through transient-field g-factor measurements on fast radioactive beams of 38S and 40S

The shell structure underlying shape changes in neutron-rich nuclei near N = 28 has been investigated by a novel application of the transient-field technique to measure the first-excited-state g factors in 38S and 40S produced as fast radioactive beams. There is a fine balance between proton and neutron contributions to the magnetic moments in both nuclei. The g factor of deformed 40S does not resemble that of a conventional collective nucleus because spin contributions are more important than usual.     

Collapse of the N=28 shell closure in (42)Si

The energies of the excited states in very neutron-rich (42)Si and (41,43)P have been measured using in-beam gamma-ray spectroscopy from the fragmentation of secondary beams of (42,44)S at 39A MeV. The low 2(+) energy of (42)Si, 770(19) keV, together with the level schemes of (41,43)P, provides evidence for the disappearance of the Z=14 and N=28 spherical shell closures, which is ascribed mainly to the action of proton-neutron tensor forces. New shell model calculations indicate that (42)Si is best described as a well-deformed oblate rotor.     

Mass prediction of proton-rich nuclides with the Coulomb displacement energies in the relativistic point-coupling model

The masses,one-and two-proton separation energies of proton-rich nuclei with Z = 20-55,are computed using the measured masses of mirror neutron-rich nuclei and the Coulomb displacement energies calculated from the relativistic point-coupling model.The implications for the proton drip lines,candidates for two-proton emitters,as well as the impact on the astrophysical rp-process are discussed.     

Mass measurement of neutron-rich isotopes from51Ca to72Ni

The ground state masses of thirty-nine neutronrich nuclei from⁵¹Ca to⁷²Ni have been measured using the Time-of-Flight Isochronous (TOFI) spectrometer. Eight of these masses have been measured for the first time and thirty-one are remeasurements of neutron-rich nuclei previously reported. Good agreement between these results and a previous TOFI experiment was observed except for the most neutron-rich isotopes of vanadium through iron with the present results being more bound and in better agreement with theory. The low binding energy of⁶⁸Ni, as indicated by an unreasonably low two-neutron separation energy, suggests the presence of a high-lying, long-lived isomeric state in this nucleus.     

Precision Mass Measurements beyond ^{132}Sn: Anomalous Behavior of Odd-Even Staggering of Binding Energies

Atomic masses of the neutron-rich isotopes ^{121-128}Cd, ^{129,131}In, ^{130-135}Sn, ^{131-136}Sb, and ^{132-140}Te have been measured with high precision (10?ppb) using the Penning-trap mass spectrometer JYFLTRAP. Among these, the masses of four r-process nuclei ^{135}Sn, ^{136}Sb, and ^{139,140}Te were measured for the first time. An empirical neutron pairing gap expressed as the odd-even staggering of isotopic masses shows a strong quenching across N=82 for Sn, with a Z dependence that is unexplainable by the current theoretical models.     

丰中子锰同位素的壳模型计算

利用球形壳模型和投影壳模型两种方法对Z≤28,N=40附近pf壳的丰中子核素的结构进行了一系列的研究。以丰中子的锰同位素为例讨论了对奇奇核58,60Mn的两种壳模型计算结果。结果显示,两种理论方法都很好地再现了58,60Mn实验上观测到的从低激发到高自旋态的能级。在对这两个同位素由中子g_9/2轨道闯入所产生的负宇称转动带进行描述时,两种理论计算也得到了一致的结果。通过对比,阐明了两种壳模型方法及其采用的有效相互作用在该丰中子核区的适用性,特别强调了中子g_9/2轨道的激发对于pf壳Z≤28丰中子核素结构的重要性。Recently,we have carried out a series of studies on the structures of pf shell neutron-rich nuclei around N=40 using the spherical shell model and the projected shell model respectively.As an example,these two types of shell model calculations for the neutron-rich odd-odd isotopes 58,60Mn are discussed in this paper.The results show that both the calculations reproduce the observed experimental energy levels from the lowexcitation states to the high-spin ones in 58,60Mn.Consistent results are also obtained by these two theoretical calculations when describing the negative-parity rotational band derived from the intruder neutron g_9/2 orbital in both isotopes.Through comparison,we show the applicability of these two shell model methods and the adopted effective interactions in the present neutron-rich mass region.The significance of the excitations from neutron g_9/2 orbital to the structures of the neutron-rich nuclei in pf shell with Z≤28 are especially emphasized in this paper.     

B(E2) upward arrow measurements for radioactive neutron-rich ge isotopes: reaching the N=50 closed shell

The B(E2;0(+)(1)-->2(+)(1)) values for the radioactive neutron-rich germanium isotopes (78,80)Ge and the closed neutron shell nucleus 82Ge were measured at the HRIBF using Coulomb excitation in inverse kinematics. These data allow a study of the systematic trend between the subshell closures at N=40 and 50. The B(E2) behavior approaching N=50 is similar to the trend observed for heavier isotopic chains. A comparison of the experimental results with a shell model calculation demonstrates persistence of the N=50 shell gap and a strong sensitivity of the B(E2) values to the effective interaction.     

Spectroscopy of nuclei far from stability at GANIL: Recent experiments

The structure of neutron-rich light nuclei around N = 20, 28 has been investigated at GANIL by means of in-beam gamma spectroscopy using fragmentation reactions of ³⁶S and ⁴⁸Ca beams on a Be target. Gamma decay of relatively high-lying excited states have been measured for the first time in nuclei around ³²Mg and ⁴⁴S. Level schemes are proposed and discussed for a large number of these neutron-rich nuclei around N = 20 and N = 28. Received: 1 May 2001 / Accepted: 4 December 2001     

First observation of medium-spin excitations in the <Superscript>138</Superscript>Cs nucleus

The Penning trap mass spectrometer JYFLTRAP, coupled to the Ion Guide Isotope Separator On-Line (IGISOL) facility at Jyv?skyl?, was employed to measure the atomic masses of neutron-rich ^70-73Ni and ^{73, 75}Cu isotopes with a typical accuracy less than 5keV. The mass of ⁷³Ni was measured for the first time. Comparisons with the previous data are discussed. Two-neutron separation energies show a weak subshell closure at ⁶⁸ ₂₈Ni₄₀ . A well established proton shell gap is observed at Z = 28 .     

Inverse Quasifission in <Superscript>156,160</Superscript>Gd + <Superscript>186</Superscript>W Reactions

To investigate the impact shell effects have in the formation of neutron-rich fragments in multinucleon transfer reactions, a series of experiments to explore the binary channel in ^156,160Gd + ¹⁸⁶W reactions at energies near and above the Coulomb barrier is performed at the Flerov Laboratory’s U-400 accelerator using the CORSET setup. These experiments are aimed mainly at obtaining the production cross sections of leadlike fragments in the process of inverse quasifission. The mass, energy, and angular distributions of the binary reaction products are measured at energies of 860 and 935 MeV of ¹⁶⁰Gd ions and 878MeV in the case of ¹⁵⁶Gd ions. The excitation energies of primary fragments are estimated using their measured mass–energy distributions. Enhanced yields of products with masses of 200–215 amu are observed for both reactions. At energies above the barrier for side-to-side collisions (935 MeV), the yield of lead-like fragments is an order of magnitude larger than at energies near the Coulomb barrier, due possibly to the influence of orientation effects. The enhancement observed in the yield of reaction products with masses heavier than the target mass confirms that multinucleon transfer reactions can be used to obtain new neutron-rich isotopes, and to synthesize new superheavy elements.     

Reactions with fast radioactive beams of neutron-rich nuclei

The neutron dripline has presently been reached only for the lightest nuclei up to the element oxygen. In this region of light neutron-rich nuclei, scattering experiments are feasible even for dripline nuclei by utilizing high-energy secondary beams produced by fragmentation. In the present article, reactions of high-energy radioactive beams will be exemplified using recent experimental results mainly derived from measurements of breakup reactions performed at the LAND and FRS facilities at GSI and at the S800 spectrometer at the NSCL. Nuclear and electromagnetically induced reactions allow probing different aspects of nuclear structure at the limits of stability related to the neutron-proton asymmetry and the weak binding close to the dripline. Properties of the valence-neutron wave functions are studied in the one-neutron knockout reaction, revealing the changes of shell structure when going from the beta-stability line to more asymmetric loosely bound neutron-rich systems. The vanishing of the N = 8 shell gap for neutron-rich systems like ¹¹Li and ¹²Be, or the new closed N = 14, 16 shells for the oxygen isotopes are examples. The continuum of weakly bound nuclei and halo states can be studied by inelastic scattering. The dipole response, for instance, is found to change dramatically when going away from the valley of stability. A redistribution of the dipole strength towards lower excitation energies is observed for neutron-rich nuclei, which partly might be due to a new collective excitation mode related to the neutron-proton asymmetry. Halo nuclei, in particular, show strong dipole transitions to the continuum at the threshold, being directly related to the ground-state properties of the projectile. Finally, an outlook on future experimental prospects is given.     

Direct mass measurements of the neutron-rich isotopes of fluorine through chlorine

The masses of 34 neutron-rich isotopes of fluorine through chlorine are reported. These measurements more fully delineate the mass surface in the region of deformed nuclei centered around ³¹Na and, in addition, provide the first mass values of several silicon through sulfur nuclei. We compare our data to recent shell model and mass model calculations     

Laser spectroscopy of the bismuth isotopes

Isotope shifts and hyperfine structures of the bismuth isotope chain have been studied on the 306.7 nm line in off-line measurements using gas cell laser spectroscopy and atomic beam spectroscopy. The changes in nuclear mean square charge radii and the nuclear magnetic and spectroscopic quadrupole moments have been deduced. The neutron-rich isotopes are the first isotones of Pb to be measured immediately above the N=126 shell closure. A remarkable correspondence between the nuclear charge radii of the Bi and Pb isotope chains is demonstrated by a King Plot analysis. The relationship between nuclear shapes and the charge radii can be understood in the framework of the spherical shell model using few-nucleon configurations. This revised version was published online in July 2006 with corrections to the Cover Date.     

Shell Evolution Study for New Magic Number N =32 via Isochronous Mass Spectrometry

Recent results and progress of mass measurements of neutron-rich nuclei utilizing Isochronous Mass Spectrometry (IMS) based on the HIRFL-CSR complex at Lanzhou are reported. The nuclei of interest were produced through projectile fragmentation of primary 86Kr ions at a realistic energy of 460.65 MeV/u. After in-flight separation by the fragment separator RIBLL2, the fragments were injected and stored in the experimental storage ring CSRe, and their masses were determined from measurements of their revolution times. The re-determined masses were compared and evaluated with other mass measurements, and the impact of these evaluated masses on the shell evolution study is discussed.     

Shell evolution in neutron-rich nuclei: the single particle perspective

The isospin dependence of spin-orbit(SO)splitting becomes increasingly important as N/Z increases in neutron-rich nuclei.Following the initial independent-particle strategy toward explaining the occurrence of magic numbers,we systematically investigated the isospin effect on the shell evolution in neutron-rich nuclei within the Woods-Saxon mean-field potential and the SO term.It is found that new magic numbers N=14 and N=16 may emerge in neutron-rich nuclei if one changes the sign of the isospin-dependent term in the SO coupling,whereas the traditional magic number,N=20,may disappear.The magic number N=28 is expected to be destroyed despite the sign choice of the isospin part in the SO splitting,corresponding to the strength of the SO coupling term.Meanwhile,the N=50 and 82 shells may persist within the single particle scheme,although there is a decreasing trend of their gaps toward extreme proton-deficient nuclei.Besides,an appreciable energy gap appears at N=32 and 34 in neutron-rich Ca isotopes.All these results are more consistent with those of the interacting shell model when enhancing the strength of the SO potential in the independent particle model.The present study may provide a more reasonable starting point than the existing one for not only the interacting shell model but also other nuclear many-body calculations toward the neutron-dripline of the Segrèchart.     

Fusion hindrance and synthesis of superheavy elements

A mechanism for fusion hindrance is clarified, based on the observation that the sticking configuration of projectile and target is located outside of the conditional saddle point. Accordingly, the fusion process is described by two sequential steps of passing over the Coulomb barrier and shape evolution toward the spherical compound nucleus. The latter one is indispensable in massive systems. With the use of a two-step model, excitation functions of fusion reaction are calculated for various combinations of projectiles and targets which lead to superheavy elements. The hindered fusion excitation measured is reproduced precisely without any adjustable parameter. Combined with survival probabilities calculated by the statistical theory of decay, excitation functions for residues of superheavy elements are calculated to compare with the systematic data measured for the cold fusion path. The peak positions and the widths are correctly reproduced, though it is necessary to reduce the shell correction energies of the compound nuclei predicted by the structure calculations in order to reproduce their absolute values. Predictions are made for a few unknown heavier elements. Furthermore, a preliminary attempt toward the shell closure N = 184 is also presented using a neutron-rich secondary beam. The text was submitted by the authors in English.