Systematic trends in the beta minus strength functions of neutron-rich rubidium isotopes

Experimental β^?-strength functions (S_β) are presented for the odd-mass Rb isotope sequence A=89to97. As expected from general nuclear structure considerations, but in contrast to present purely statistical concepts, these strength functions exhibit well separated “pygmy” resonances and a systematic behaviour as a function of mass number and β-decay energy.     

The structure and spectroscopy of neutron-rich nuclei

Our knowledge of the structure and spectroscopy of neutron-rich nuclei has greatly increased due to two important developments in nuclear physics: the construction of large γ-ray arrays to investigate prompt γ-rays from fission and deep-inelastic reactions; and the availability of radioactive nuclei from fragmentation and spallation reactions. In this review examples will be given of the advances that have been made in our understanding of the properties of neutron-rich nuclei. The examples are necessarily selective, given the limitations of space and time. Received: 21 March 2002 / Accepted: 16 May 2002 / Published online: 31 October 2002 RID="a" ID="a"e-mail: nsd@mags.ph.man.ac.uk     

Exotic cluster structure in light neutron-rich nuclei

The cluster structure of C isotopes is investigated using a microscopic α+α+α+n+n… model based on the molecular orbit (MO) model. The stability of the linear chain of 3α with respect to the breathing mode and the bending mode for various neutron configurations is investigated. The combination of the valence neutrons in the π- and σ-orbits is promising to stabilize for these modes, and the excited states of ¹⁶C with the (3/2 _π ^- )²(1/2 _σ ^- )² configuration for the four valence neutrons are one of the most promising candidates for such structure. Furthermore, the equilateral-triangular shape of 3α surrounded by valence neutrons is suggested for ¹⁴C. The 3^-, 4^-, and 5^- members of this rotational band appear around the ¹⁰Be+α threshold, and these calculated states correspond to the experimentally observed 3^- state (9.80 MeV) and 4^- state (11.67 MeV). A positive-parity rotational band (0⁺, 2⁺, 4⁺) also arises around this threshold energy, and these results suggest that the picture of inversion doublet structure works also in neutron-rich nuclei.     

Strong and decoupling structure in transitional odd-odd mass nuclei

The well known decoupling and strong coupling features of the high spin states in the transitional odd mass nuclei are found to persist also in the odd-odd mass nuclei. The simple structure of the levels borne out by the present calculations provides a straightforward understanding of the nature of the electromagnetic transitions.     

Shell-model based study of the direct capture in neutron-rich nuclei

The European Physical Journal A - This report is an outcome of the workshop AI for Nuclear Physics held at Thomas Jefferson National Accelerator Facility on March 4–6, 2020     

Disturbance of the shell structure of neutron-rich nuclei in the oxygen-magnesium region

The effects related to nuclear deformation and their influence on the shell structure and nuclear properties have been investigated. Calculations were performed within the self-consistent theory of finite Fermi systems. The possibility of existence of strongly deformed nuclei at the neutron drip line and behind it, such as ²⁸O, ³⁹Na, and ^41–43Na, is discussed.     

丰中子奇异核17B的实验研究

在兰州重离子加速器国家实验室(HIRFL)放射性次级束流线(RIBLL)上,用束流透射法测量了丰中子奇异核17B与C靶反应的总截面.假定17B具有15B(核芯) 2n结构,采用Gauss HO形式的密度分布和零力程Glauber模型进行计算的结果可以很好地拟合实验数据,并得出17B的密度分布有一个很大的弥散,表明17B是双中子晕核.     

Quasi-rotation-aligned bands in the odd-odd Au nuclei

Newly observed negative parity bands of the low lying high spin states in the odd-odd Au nuclei (^190,192,194 Au) are explained in the framework wherein the two odd quasi-particles ($\text{π}\text{h}{}_{\mathrm{<mtext>1</mtext><mtext>1</mtext><mtext>2</mtext>}}\text{?v}\text{i}{}_{\mathrm{<mtext>1</mtext><mtext>3</mtext><mtext>2</mtext>}}$) are coupled to the gamma deformed core. Interesting physical situations leading to different kinds of level structures and electromagnetic properties are pointed out. The present data are shown to represent the dynamical situation which for the high spin states, due to the energy considerations, does not prefer the mixing of various intrinsic angular momentum states of the two quasi-particle configuration. This property in turn allows the formation of rotational-like bands on these intrinsic states.     

Neutrino-induced fission of neutron-rich nuclei

We calculate neutrino-induced fission cross sections for selected nuclei with Z=84-92. We show that these reactions populate the daughter nucleus at excitation energies where shell effects are significantly washed out, effectively reducing the fission barrier. If the r process occurs in the presence of a strong neutrino fluence, and electron neutrino average energies are sufficiently high, perhaps as a result of matter-enhanced neutrino flavor transformation, then neutrino-induced fission could lead to significant alteration in the r-process flow in slow outflow scenarios.     

Magnetic moments of odd-odd spherical nuclei

We analyze the time-like processes $ \gamma\gamma\rightarrow B\overline{B}$ and $ p\bar{p}\rightarrow\gamma M$ at large Mandelstam variables within the handbag approach, for which the process amplitudes factorize in hard partonic subprocesses and annihilation form factors. The latter represent moments of baryon-antibaryon generalized parton distributions (GPDs). Symmetry relations restrict the number of independent annihilation form factors for the ground state baryons drastically. We determine these form factors from the present BELLE data on $ \gamma\gamma\rightarrow B\overline{B}$ with the help of simplifying assumptions. The knowledge of these form factors allow for predictions of $ p\bar{p}\rightarrow\gamma M$ for various mesons which may be probed at FAIR.     

Magnetic properties of odd-odd deformed nuclei

Magnetic moments and Ml transition probabilities depend on the parameters $\text{g}{}_{\mathrm{R}}\text{, g}{}_{\mathrm{\Omega }}{}_{\mathrm{<mtext>p</mtext>}}\text{,}\text{and}\text{g}{}_{\mathrm{\Omega }}{}_{\mathrm{<mtext>n</mtext>}}$ in odd-odd deformed nuclei. The parameter g_R can be obtained from an intraband branching ratio and a magnetic moment. Available data are analyzed and compared with predictions of the unified model.     

Study of transitional odd-odd mass nuclei

The low lying high spin states in the transitional odd-odd mass nuclei are studied systematically in the framework of gamma deformed rotor model. The two odd particles, a proton and a neutron, are treated as the BCS quasi particles moving in a triaxially deformed field. The calculations are confined to smallβ-deformation which is typical of the transitional nuclei. Moreover, we consider only the case where two odd particles are in singlej-shells. Results for the excitation energy covering the various physical situations are presented as a function of theβ-deformation, the asymmetryγ, and the Fermi energiesλ _p andλ _n . Electromagnetic transitions and moments are also calculated. Certain features well known in the odd mass nuclei are shown to persist also in the odd-odd mass system. Trends in the level systematics and the electromagnetic properties are predicted for the case where both particles decouple or remain strongly coupled, and for the case where one decouples and the other couples strongly with the core. Measurements which are getting now available show encouraging agreement with the theoretical results.     

Triaxial projected shell model study of chiral rotation in odd-odd nuclei

Chiral rotation observed in ¹²⁸Cs is studied using the newly developed microscopic triaxial projected shell model (TPSM) approach. The observed energy levels and the electromagnetic transition probabilities of the nearly degenerate chiral dipole bands in this isotope are well reproduced by the present model. This demonstrates the broad applicability of the TPSM approach, based on a schematic interaction and angular-momentum projection technique, to explain a variety of low- and high-spin phenomena in triaxial rotating nuclei.     

Nuclear structure of neutron-rich nuclei near closed shells from excited-state g-factor measurements

New techniques to measure the g factors of picosecond-lived excited states of neutron-rich nuclei produced as radioactive beams are discussed along with their applications to study nuclear structure near ¹³² ₅₀Sn₈₂ and in the region between N=20 and N=28.     

Beta-decay properties of strongly neutron-rich nuclei

Experimental β-strength functions for about 50 neutron-rich mass-separated fission products with half-lives between 0.8 s and 30 s have been obtained at the OSIRIS facility. The β-strength to excited states depopulating by delayed-neutron emission is evaluated separately. The β^? strength functions are found to increase strongly with excitation energy, which is in contrast to the approximately energy-independent β-strength found for EC decay. By using semiempirical values for the level density, the average transition rate per energy level was evaluated under the assumption that only allowed transitions contribute to the decay. This alternative way of analyzing the data gives a more uniform picture of the β-decay to highly excited states since the transition rates are found to be roughly independent of excitation energy (above the pairing energy). A model of constant transition rate to each final level is introduced and its systematic behaviour is studied. Its use for estimating half-lives of unmeasured nuclides is of value for calculations on nucleosynthesis by the “r-process”. (A listing of β-feed and β-strength functions is available on request.)     

Structure of neutron-rich s-d shell nuclei

States in neutron-rich s-d shell nuclei were populated in the reaction of a ¹⁴C beam at E _lab=22 MeV on a ¹⁴C target. Coincidences between γ rays and either other γ rays or light charged particles were measured. γ rays in coincidence with protons established levels at 66.8, 1730, 1823, and 2219 keV in ²⁷Na. The states are compared with calculations based on the s-d shell model and the cranked Nilsson-Strutinsky model. A number of levels in ²⁴Ne were observed in both α-γ and α-γ-γ coincidences and are compared with shell-model calculations.     

Surface and curvature properties of neutron-rich nuclei

We use the extended Thomas-Fermi approximation and Skyrme-type interactions to describe the energy density of a semi-infinite slab of neutron-rich nuclear matter at zero temperature. We allow for the existence of a drip phase at low proton fractions in addition to the more dense nuclear phase. We determine various bulk properties of both phases when the system is in equilibrium. We extend the usual definition of the surface energy to apply to the case where drip is present. Assuming the density profile has the form of a Fermi function to a power, we perform a constrained variational calculation to determine the parameters of the density profile. The surface and curvature energies are calculated for proton fractions ranging from 0.5 (symmetric nuclear matter) to 0 (pure neutron matter) for typical Skyrme-type interactions. We find significantly different asymmetry dependences for different interactions. For proton fractions close to 0.5, our results are in close agreement with the predictions of the droplet model. We also present results of calculations for fission barrier properties and phase transitions between nuclei and bubbles to highlight the role of surface and curvature energies in the neutron-rich regime.