Neutron-rich Region Research Articles

Statistical and dynamical aspects in the decay of hot neutron-rich nuclei

A signal of isospin-asymmetric phase transition in the evolution of the chemical potential was observed in hot quasi-projectiles produced in the reactions Ca 40 , 48 + Al 27 confirming an analogous observation in the lighter quasi-projectiles observed in the reaction Si 28 + Sn 112 , 124 (Veselsky et al., 2004 [1]). With increasing mass, the properties of hot quasi-projectiles become increasingly influenced by secondary emission. Thermodynamical observables exhibit no sensitivity to the different number of missing (undetected) neutrons in the two reactions Ca 40 , 48 + Al 27 and provide a signal of dynamical emission of neutrons, which can be related to the formation and disintegration (rupture) of a very neutron-rich low-density region (neck) between the projectile and target.

New effective interaction forf5pg9-shell nuclei

We present a new effective interaction for shell-model calculations in the model space consisting of the single-particle orbits $1{p}_{3/2}$, $0{f}_{5/2}$, $1{p}_{1/2}$, and $0{g}_{9/2}$. Starting with a realistic interaction based on the Bonn-C potential, 133 two-body matrix elements and four single-particle energies are modified empirically so as to fit $400$ experimental energy data out of $69$ nuclei with mass numbers $A=63~96$. The systematics of binding energies, electromagnetic moments and transitions, and low-lying energy levels are described. The soft $Z=28$ closed core is observed, in contrast to the stable $N=50$ shell closure. The new interaction is applied to systematic studies of three different chains of nuclei, Ge isotopes around $N=40$, $N=Z$ nuclei with $A=64~70$, and $N=49$ odd-odd nuclei, focusing especially on the role of the ${g}_{9/2}$ orbit. The irregular behavior of the ${0}_{2}^{+}$ state in Ge isotopes is understood as a result of detailed balance between the $N=40$ single-particle energy gap and the collective effects. The development of the band structure in $N=Z$ nuclei is interpreted in terms of successive excitations of nucleons into the ${g}_{9/2}$ orbit. The triaxial/$\ensuremath{\gamma}$-soft structure in $^{64}\mathrm{Ge}$ and the prolate/oblate shape coexistence in $^{68}\mathrm{Se}$ are predicted, showing a good correspondence with the experimental data. The isomeric states in $^{66}\mathrm{As}$ and $^{70}\mathrm{Br}$ are obtained with the structure of an aligned proton-neutron pair in the ${g}_{9/2}$ orbit. Low-lying energy levels in $N=49$ odd-odd nuclei can be classified as proton-neutron pair multiplets, implying that the obtained single-particle structure in this neutron-rich region appears to be appropriate. These results demonstrate that, in spite of the modest model space, the new interaction turns out to describe rather well properties related to the ${g}_{9/2}$ orbit in various cases, including moderately deformed nuclei.

Nuclear masses near the proton drip-line and their impact on nucleosynthesis in explosive stars

The interplay of the fundamental forces (here mainly the strong force and electromagnetic force) holds nucleons together to form atomic nuclei. However, in nature only less than ten percent of the total number of nuclei is stable. The majority of nuclei are unstable; that is, after having lived for a certain time, they change into the daughter nuclei by radioactive decay. The familiar decay types that can be found in most parts of the chart of nuclide are α-decay and β-decay. There are other important processes in which unstable nuclei can change into other ones. These occur, for example, in very neutronrich or proton-rich regions of the chart of nuclide. There are boundaries called drip-lines, which are the lines on the Z (proton)-N (neutron) plane where the nucleon separation energy is zero. The nuclear binding becomes weaker and weaker when moving from the stable region towards the drip-lines, and eventually no nucleus can exist beyond the drip-lines. Taking the proton-rich region as an example, near the proton drip-line, protons are only weakly bound and a proton radioactivity can occur. Also because of the small binding, nuclei near the proton drip-line may capture additional protons and become new, heavier nuclides if the capture conditions are present. Detailed knowledge on the nucleon-capture processes lies at the heart of the understanding of nucleosynthesis in the Universe——a very fundamental but unanswered question about how the heavy elements (i.e. those heavier than iron) are created. In order for nuclei to capture protons, a high temperature environment (usually above 1×10 9 K) is needed so that they can overcome the large coulomb barrier for charged particle reactions. A hydrogen rich

Shape ofAr44: Onset of deformation in neutron-rich nuclei nearCa48

The development of deformation and shape coexistence in the vicinity of doubly magic $^{48}\mathrm{Ca}$, related to the weakening of the $N=28$ shell closure, was addressed in a low-energy Coulomb excitation experiment using a radioactive $^{44}\mathrm{Ar}$ beam from the SPIRAL facility at GANIL. The ${2}_{1}^{+}$ and ${2}_{2}^{+}$ states in $^{44}\mathrm{Ar}$ were excited on $^{208}\mathrm{Pb}$ and $^{109}\mathrm{Ag}$ targets at two different beam energies. $B(E2)$ values between all observed states and the spectroscopic quadrupole moment of the ${2}_{1}^{+}$ state were extracted from the differential Coulomb excitation cross sections, indicating a prolate shape of the $^{44}\mathrm{Ar}$ nucleus and giving evidence of an onset of deformation already two protons and two neutrons away from doubly magic $^{48}\mathrm{Ca}$. New Hartree-Fock-Bogoliubov based configuration mixing calculations have been performed with the Gogny D1S interaction for $^{44}\mathrm{Ar}$ and neighboring nuclei using two different approaches: the angular momentum projected generator coordinate method considering axial quadrupole deformations and a five-dimensional approach including the triaxial degree of freedom. The experimental values and new calculations are furthermore compared to shell-model calculations and to relativistic mean-field calculations. The new results give insight into the weakening of the $N=28$ shell closure and the development of deformation in this neutron-rich region of the nuclear chart.

Mean-field and RPA approaches to stable and unstable nuclei with semi-realistic interactions

We have developed semi-realistic NN interactions by modifying the M3Y interaction that was derived from the G -matrix. The modification has been made so that the saturation and the spin-orbit splittings should be reproduced. After viewing nuclear-matter properties, the semi-realistic M3Y-P5 interaction, which includes realistic tensor channels, is applied to finite nuclei in the mean-field and the random-phase approximations. In comparison with the results with widely used phenomenological interactions, we find that notable interaction dependence could remain in several Landau-Migdal parameters in the nuclear matter, in the N -dependence of proton single-particle levels in the Sn isotopes, and in the M1 strength distribution in 208Pb . Having microscopic origin in part, the semi-realistic interaction describes all these properties to reasonable accuracy. We confirm that the tensor force plays a crucial role in the single-particle levels and in the M1 distribution. Applying the semi-realistic interaction, we give prediction for a ground-state property of 60Ca , for the magic nature of N = 32 and 34 in the neutron-rich region, and for the property of the first excited state of 24O .

A systematic study on nuclear pairing energy under the relativistic mean-field model

Nuclear pairing energy of dozens of atomic nuclei in nuclear period chart are calculated along the stable line by extracting the swatch at a proton or neutron number interval of 4. Especially, we have studied the rule of pairing energy of the even-even nuclei of O isotopes changing with nucleon number under the relativistic mean-field model, and found, when energy gap Δ is fixed, the magnitude of nuclear pairing energy is related to nuclear shell structure. Thereof we propose a simple method to test closed shell, and further, we conclude that the neutron number N=6 is a new magic number not only in neutron-rich region, but also in proton-rich region of light nuclei.

Evolution of theN=50Shell Gap Energy towardsNi78

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.

Study of N = 16 for Ne isotopes

27Ne has been investigated through the one neutron transfer reaction 26Ne(d,p)27Ne in inverse kinematics at 9.7 MeV/nucleon. The results support the existence of a low lying negative parity state in 27Ne which is a signature of a reduced sd-fp shell gap in the N = 16 neutron rich region, at variance with stable nuclei.

STRUCTURE OF THE DRIP LINE NUCLEI PROBED BY SEPARATION ENERGIES

The derivation of two-neutron separation energies from the direct mass measurement of the neutron-rich nuclei at GANIL has enabled us to establish new neutron magic numbers N=6 and 16 in neutron-rich region for the first time instead of normal 8 and 20 and to confirm the existence of new doubly magic nuclei 8 He and 24 O . Adjunction of a proton to the new doubly magic nuclei 8 He and 24 O enable to accept two or more neutrons by these nuclei and form, respectively, the neutron halo 11 Li nucleus and create a very neutron-rich set of 27,29,31 F isotopes. The connection between doubly magic nuclei and the neutron halo and/or very neutron-rich odd-Z nuclei is studied by analysis of triton separation energies St of light neutron-rich nuclei.

Nuclear shape and structure in neutron-richTc110,111

The high-spin nuclear structure of Tc isotopes is extended to more neutron-rich regions based on the measurements of prompt \ensuremath{\gamma} rays from the spontaneous fission of $^{252}\mathrm{Cf}$ at the Gammasphere. The high-spin level scheme of $N=67$ neutron-rich $^{110}\mathrm{Tc}$ ($Z=43$) is established for the first time, and that of $^{111}\mathrm{Tc}$ is extended and expanded. The ground band of $^{111}\mathrm{Tc}$ reaches the band-crossing region, and the new observation of the weakly populated $\ensuremath{\alpha}=\ensuremath{-}1/2$ member of the band provides important information on signature splitting. The systematics of band crossings in the isotopic and isotonic chains and a CSM calculation suggest that the band crossing of the ground band of $^{111}\mathrm{Tc}$ is due to alignment of a pair of ${h}_{11/2}$ neutrons. The best fit to signature splitting, branching ratios, and excitations of the ground band of $^{111}\mathrm{Tc}$ by the rigid triaxial rotor plus particle model calculations result in a shape of ${\ensuremath{\varepsilon}}_{2}=0.32$ and $\ensuremath{\gamma}=\ensuremath{-}{26}^{\ifmmode^\circ\else\textdegree\fi{}}$ for this nucleus. Its triaxiality is larger than that of $^{107,109}\mathrm{Tc}$, which indicates increasing triaxiality in Tc isotopes with increasing neutron number. The identification of the weakly populated $K+2$ satellite band provides strong evidence for the large triaxiality of $^{111}\mathrm{Tc}$. In $^{110}\mathrm{Tc}$, the four lowest-lying levels observed are very similar to those in $^{108}\mathrm{Tc}$. At an excitation of 478.9 keV above the lowest state observed, ten states of a $\ensuremath{\Delta}I=1$ band are observed. This band of $^{110}\mathrm{Tc}$ is very analogous to the $\ensuremath{\Delta}I=1$ bands in $^{106,108}\mathrm{Tc}$, but it has greater and reversal signature splitting at higher spins.

Mixed representation RPA calculation for negative-parity excitations built on superdeformed states in the 40Ca and neutron-rich sulfur regions

Low-lying negative-parity excitations on superdeformed states in the 40Ca and neutron-rich sulfur regions were calculated using fully self-consistent random phase approximation calculations with Skyrme interaction.

Prediction and possible observation of an oblate shape isomer in 190W

Energy-surface calculations for nuclides in the neutron-rich 19074W116 region show consistently that the collective rotation of a deformed oblate shape is an energetically favoured mode, especially for angular momenta I>12ℏ. By comparing the calculations with existing experimental data, it is concluded that the reported t1/2∼1 ms isomer in 190W may be an oblate shape isomer.

Mixed representation RPA calculation for octupole excitations on superdeformed states in the 40Ca and neutron-rich sulfur regions

By means of the mixed representation RPA based on the Skyrme–Hartree–Fock mean field, we investigate low-frequency octupole excitations built on the superdeformed (SD) states in the N = Z nuclei around 40Ca and the neutron-rich sulfur isotopes. The RPA calculation is carried out fully self-consistently on the three-dimensional Cartesian mesh in a box, and yields a number of low-frequency octupole vibrations built on the SD states in 32S, 36Ar, 40Ca and 44Ti. In particular, a strongly collective K π = 1 − octupole vibration is suggested to appear on top of the SD state in 40Ca. For 48,50S close to the neutron drip line, we find that the low-lying state created by the excitation of a single neutron from a loosely bound low Ω level to a high Ω resonance level acquires an extremely strong octupole transition strength due to the spatially very extended structure of the particle–hole wave functions.

Shell gap reduction in neutron-rich [formula omitted] nuclei

The spectroscopy of 27Ne has been investigated through the one-neutron transfer reaction 26Ne(d,p)27Ne in inverse kinematics at 9.7 MeV/nucleon. The results strongly support the existence of a low-lying negative parity state in 27Ne, which is a signature of a reduced sd–fp shell gap in the N=16 neutron-rich region, at variance with stable nuclei.

Microscopic HFB Fission Barriers for r-Process Nucleosynthesis Calculations

New systematic calculations of fission barriers are performed in the 3-dimensional deformation space using the constrained Skyrme-Hartree-Fock-Bogoliubov (SHFB) plus Particle-Number Projection method. The BSk8 Skyrme force, fitted to the full set of more than 2000 experimentally known masses, is used. The energy surfaces are analysed using the flooding method, a necessary tool if more than two dimensions are spanned. The SHFB outer fission barriers are compared with experimental data. The reflection asymmetry is shown to play a significant role in the Z ≈ 94 open-shell region. Extrapolations in the neutron-rich region can differ with the previous compilations of fission barriers.

Heavy element synthesis through disk winds in a collapsar

We investigate heavy element synthesis through thermally driven winds launched from an inner neutron-rich region of an accretion disk in a collapsar. Various heavy elements are found to be produced through the wind. There exist three types of winds characterized by their abundant nuclei: (i) 56 Ni, (ii) light ( Z = 30 − 45 ) neutron-rich nuclei, and (iii) light ( Z ⩽ 44 ) p-nuclei. We find that Y e , NSE > 0.49 , Y e , NSE ⩽ 0.45 , and 0.45 Y e , NSE ⩽ 0.49 for winds of types (i), (ii), and (iii), respectively, where Y e , NSE is the electron fraction of the wind at T = 9 × 10 9 K . The collasar winds may be a lighter element primary process, suggested from Galactic evolution of Sr, Y, and Zr. The p-nuclei, 92 Mo and 94 Mo, which are underproduced in oxygen layers in a supernova, can be abundantly synthesized in the collapsar winds.

Systematic study of charge form factors of elastic electron-nucleus scattering with the relativistic eikonal approximation

We systematically investigate the elastic electron scattering on both stable and unstable nuclei with the relativistic eikonal approximation, where the charge density distributions of nuclei are from the self-consistent relativistic mean field model. Calculations show that the relativistic eikonal approximation can reproduce the experimental data of electron scattering on nuclei ranging from the light region, such as $^{12}\mathrm{C}$, to the heavy region, such as $^{208}\mathrm{Pb}$. This is the systematic test of the relativistic eikonal approximation for elastic electron scattering for both light and heavy nuclei. With this method, further studies are made of the variation of charge form factors along isotopic chains and the sensitivity of charge form factors to the changes of charge distribution. For isotopic chains with Z = 20 and 28, we find that the charge form factors vary significantly with neutron number. The charge form factors shift outward and downward when the target nucleus moves from the neutron-rich region to the proton-rich region along the isotopic chain. The significant shift shows that the charge form factor is very sensitive to a change of neutron number. If the isotopic shifts of the charge form factor are measured on the next-generation electron-nucleus collider (at RIKEN and GSI), the charge size and charge distribution can be determined for unstable nuclei. The calculations will provide a reference for future experiments.

Evolution of collectivity in a ground–γ-band mixing scheme for even–even transitional nuclei

We propose an extended band-mixing formalism capable of describing the structure of the ground- and γ-bands in a wide range of collective spectra beyond the regions of well-deformed nuclei. We apply it to explain the odd–even staggering effect observed in the γ-bands of Mo, Ru and Pd nuclei and to obtain on this basis a consistent interpretation of new experimental data in the neutron rich region. As a result the systematic behaviour of the staggering effect, together with the mutual ground–γ-band disposition, interband mixing and intraband level spacing are explained as the manifestation of respective changes in nuclear collectivity.

Negative-parity excitations built on superdeformed states in neutron-rich Calcium region(Poster presentation,YITP Workshop on New Developments in Nuclear Self-Consistent Mean-Field Theories (MF05))

Negative-parity excitations built on superdeformed states in neutron-rich Calcium region(Poster presentation,YITP Workshop on New Developments in Nuclear Self-Consistent Mean-Field Theories (MF05))

QRPA study of low-lying 2 + states of even-even nuclei in neutron-rich Sn and Ni region

We calculate energies E 2 + and transition strengths B ( E 2 ) ↑ of the lowest 2 + states of even Te, Sn and Ni isotopes in a neutron-rich region, using quasiparticle random phase approximation with simple separable type interactions. With those calculations reproducing the experimental systematics very well, we discuss, first, an unusual relation of E 2 + and B ( E 2 ) ↑ of 132, 136 Te isotopes, which does not follow an empirical formula B ( E 2 ) ↑ ∝ 1 / E 2 + . Second, B ( E 2 ) ↑ of 132 Sn is discussed which is larger than those of the neighboring even Sn. This is a phenomenon confirmed by an experiment very recently. Third, we point out that the small B ( E 2 ) ↑ of 68 Ni is due to fragmentation of the transition strength into some excited states in a low-energy region.