There is strong circumstantial evidence that the shape of atomic nuclei with particular values of Z and N prefers to assume octupole deformation, in which the nucleus is distorted into a pear shape that loses the reflection symmetry of a quadrupole-deformed (rugby ball) shape prevalent in nuclei. Recently, useable intensities of accelerated beams of heavy, radioactive ions have become available at the REX-ISOLDE facility at CERN. This has allowed electric octupole transition strengths, a direct measure of octupole correlations, to be determined for short-lived isotopes of radon and radium expected to be unstable to pear-like distortions. The data are used to discriminate differing theoretical approaches to the description of the octupole phenomena, and also help restrict the choice of candidates for studies of atomic electric-dipole moments, that provide stringent tests of extensions to the Standard Model.
The ''island of inversion'' nucleus 32 Mg has been studied by a (t, p) two neutron transfer reaction in inverse kinematics at REX-ISOLDE. The shape coexistent excited 0 þ state in 32 Mg has been identified by the characteristic angular distribution of the protons of the ÁL ¼ 0 transfer. The excitation energy of 1058 keV is much lower than predicted by any theoretical model. The low-ray intensity observed for the decay of this 0 þ state indicates a lifetime of more than 10 ns. Deduced spectroscopic amplitudes are compared with occupation numbers from shell-model calculations. The evolution of shell structure in exotic nuclei as a function of the proton (Z) and neutron (N) number is currently at the center of many theoretical and experimental investigations [1,2]. It has been realized that the interaction of the last valence protons and neutrons, in particular, the monopole component of the residual interaction between those nucleons, can lead to significant shifts in the single-particle energies, leading to the disappearance of classic shell closures and the appearance of new shell gaps [3]. A prominent example is the collapse of the N ¼ 20 shell gap in the neutron-rich oxygen isotopes where instead a new magic shell gap appears for 24 O at N ¼ 16 [4,5]. Recent work showed that the disappearance of the N ¼ 20 shell can be attributed to the monopole effect of the tensor force [3,6,7]. The reduced strength of the attractive interaction between the proton d 5=2 and the neutron d 3=2 orbitals causes the d 3=2 orbital to rise in energy and come closer to the f 7=2 orbital. In regions without pronounced shell closures correlations between the valence nucleons may become as large as the spacing of the single-particle energies. This can thus lead to particle-hole excitations to higher-lying single-particle states enabling deformed configurations to be lowered in energy. This may result in low-lying collective excitations, the coexistence of different shapes at low energies or even the deformation of the ground state for nuclei with the conventional magic number N ¼ 20. Such an effect occurs in the ''island of inversion'', one of most studied regions of exotic nuclei in the nuclear chart. In this region of neutron-rich nuclei around the magic number N ¼ 20 strongly deformed ground states in Ne, Na, and Mg isotopes have been observed [8-11]. Because of the reduction of the N ¼ 20 shell gap, quadrupole correlations can enable low-lying deformed 2p-2h intruder states from the fp shell to compete with spherical normal neutron 0p-0h states of the sd shell. In this situation the promotion of a neutron pair across the N ¼ 20 gap can result in deformed intruder ground states. Consequentially, the competition of two configurations can lead to the coexistence of spherical and deformed 0 þ states in the neutron-rich 30;32 Mg nuclei [12]. Coulomb excitation experiments have shown that 30 Mg has a rather small BðE2Þ value for the 0 þ gs ! 2 þ 1 transition [13,14] placing this nucleus outside the island of inversion. The excited deform...
The SwissFEL X-ray Free Electron Laser (XFEL) facility started construction at the Paul Scherrer Institute (Villigen, Switzerland) in 2013 and will be ready to accept its first users in 2018 on the Aramis hard X-ray branch. In the following sections we will summarize the various aspects of the project, including the design of the soft and hard X-ray branches of the accelerator, the results of SwissFEL performance simulations, details of the photon beamlines and experimental stations, and our first commissioning results.
The reduced transition probabilities, B E2; 0 gs ! 2 1 , have been measured in the radioactive isotopes 108;106 Sn using subbarrier Coulomb excitation at the REX-ISOLDE facility at CERN. Deexcitation rays were detected by the highly segmented MINIBALL Ge-detector array. The results, B E2; 0 gs ! 2 1 0:222 19 e 2 b 2 for 108 Sn and B E2; 0 gs ! 2 1 0:195 39 e 2 b 2 for 106 Sn were determined relative to a stable 58 Ni target. The resulting B E2 values are 30% larger than shell-model predictions and deviate from the generalized seniority model. This experimental result may point towards a weakening of the N Z 50 shell closure. DOI: 10.1103/PhysRevLett.101.012502 PACS numbers: 23.20.Js, 21.60.Cs, 25.70.De, 27.60.+j Precision measurements in unstable nuclei together with recently developed models of the nucleon-nucleon interaction, stemming from many-body techniques and QCD, show promise to improve our understanding of the finer aspects of the dynamics of the atomic nucleus. One approach to this question is to measure reduced transition probabilities -B E2; 0 gs ! 2 1 -for specific nuclei in the vicinity of a shell closure and to compare these results with calculations based on such models. In particular, one of the pressing questions in nuclear physics today is whether the shell closures, that are well established close to stability, remain so also for isotopes with a more extreme proton-toneutron ratio. Intuitive models, such as the generalized seniority scheme [1], predict that these B E2 values follow a parabolic trend, that peaks at midshell, for a sequence of isotopes between two shell closures. In the following we address the 100 Sn shell closure and consequently present results from measurements in the sequence of neutron-deficient even-mass Sn isotopes. This approach has been made possible by newly developed facilities that produce high-quality radioactive ion beams. Recent measurements in 110;108 Sn [2 -4] consistently deviate from the broken-pair model as given by the generalized seniority scheme and from current large-scale shell-model calculations [2]. Parallel work [4], using intermediate energy Coulomb excitation, suggests a constant trend of the reduced transition probabilities extending to 106 Sn. In this Letter we report results from the first measurements of 108;106 Sn using subbarrier Coulomb excitation. This is the only experiment so far for 106 Sn that has permitted for complete control of the scattering process and thus explicitly fulfills the conditions for safe Coulomb excitation. Our result still deviates significantly from theoretical predictions but indicates a decreasing trend of the B E2 with a decreasing number of valence particles outside of the 100 Sn core. Note that with this Letter three different isotopes have been used for normalization as 112 Sn [2] and 197 Au [4] have been used previously. All three experiments yield similar PRL 101, 012502 (2008)
Collisions induced by 9;10;11 Be on a 64 Zn target at the same c.m. energy were studied. For the first time, strong effects of the 11 Be halo structure on elastic-scattering and reaction mechanisms at energies near the Coulomb barrier are evidenced experimentally. The elastic-scattering cross section of the 11 Be halo nucleus shows unusual behavior in the Coulomb-nuclear interference peak angular region. The extracted total-reaction cross section for the 11 Be collision is more than double the ones measured in the collisions induced by 9;10 Be. It is shown that such a strong enhancement of the total-reaction cross section with 11 Be is due to transfer and breakup processes. A hundred years after Rutherford's scattering experiment [1], heavy-ion-elastic-scattering angular distributions (AD) are usually plotted as a ratio of the Rutherford cross section (i.e., pure Coulomb scattering). Such representation usually shows a decrease of the elastic cross section with the angle due to absorption at small impact parameters by nonelastic processes, and an oscillatory behavior. The latter, using the language of optics, is described in terms of refraction by nonabsorbing lenses (Coulomb rainbow model) or diffraction by sharp-edged, nonrefracting apertures (Fraunhofer or Fresnel diffraction model). However, the refraction or diffraction descriptions are oversimplifications of the realistic process; rather, the nucleus behaves as a ''cloudy crystal ball.'' The elasticscattering AD may show a peak resulting from the interference between the Coulomb and nuclear amplitudes (Coulomb-nuclear interference peak) [2], which, in analogy with the Coulomb rainbow model, is sometimes called the rainbow peak. Since elastic scattering is a peripheral process, it does not give information on the interior region of nuclei. It probes the tail of the wave function, and hence one can learn about surface properties, such as size of nuclei and surface diffuseness. Therefore, elastic scattering is an ideal tool to study peculiar nuclear structures as, for example, the nuclear halo. Such structure originates when very weakly bound nucleon(s) can tunnel into the classically forbidden region, giving rise to a diffuse tail surrounding a well-bound core. The behavior of the system in nuclear reactions is mostly determined by the tail of the wave function [3]. The reaction mechanisms may also be affected by the weak binding: at energies around the Coulomb barrier, couplings between the entrance channel and the continuum [4][5][6][7][8], as well as to the various reaction channels [9][10][11][12], are expected to be very important. Direct reactions, such as breakup or transfer, may be favored owing to the low binding energy, the extended tail of halo nuclei, and the large Q values for selected transfer channels.Almost all elastic-scattering and reaction mechanism studies around the barrier with halo nuclei have been performed with the 2n halo nucleus 6 He. All authors agree that, due to the 6 He structure, one has an enhancement of the total-reaction (T...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.