Proton Shell Research Articles

Polarization Effects in the Quasielastic (p,2p) Reaction with the Nuclear S-Shell Protons at 1 GeV

The polarization of the secondary protons in the (p,2p) reaction with the S - shell protons of nuclei 4He, 6Li, 12C, 28Si, 40Ca was measured at 1 GeV unpolarized proton beam. The spin correlation parameters Cij for the 4He and 12C targets also were for the first time obtained as well. The polarization measurements were performed by means of a two - arm magnetic spectrometer, each arm of which was equipped with the multiwire proportional chamber polarimeter. This experiment was aimed to study a modification of the proton - proton scattering matrix in the nuclear medium.

Spectroscopic study of the64,66,68Ni isotopes populated in64Ni + 238U collisions

Excited states in ${}^{64}$Ni, ${}^{66}$Ni, and ${}^{68}$Ni were populated in quasielastic and deep-inelastic reactions of a 430-MeV ${}^{64}$Ni beam on a thick ${}^{238}$U target. Level schemes including many nonyrast states were established up to respective excitation energies of 6.8, 8.2, and 7.8 MeV on the basis of \ensuremath{\gamma}-ray coincidence events measured with the Gammasphere array. Spin-parity assignments were deduced from an angular-correlation analysis and from observed \ensuremath{\gamma}-decay patterns, but information from earlier \ensuremath{\gamma}-spectroscopy and nuclear-reaction studies was used as well. The spin assignments for nonyrast states were supported further by their observed population pattern in quasielastic reactions selected through a cross-coincidence technique. Previously established isomeric-state decays in ${}^{66}$Ni and ${}^{68}$Ni were verified and delineated more extensively through a delayed-coincidence analysis. A number of new states located above these long-lived states were identified. Shell-model calculations were carried out in the ${p}_{3/2}{f}_{5/2}{p}_{1/2}{g}_{9/2}$ model space with two effective interactions using a ${}^{56}$Ni core. Satisfactory agreement between experimental and computed level energies was achieved, even though the calculations indicate that all the states are associated with rather complex configurations. This complexity is illustrated through the discussion of the structure of the negative-parity states and of the M1 decays between them. The best agreement between data and calculations was achieved for ${}^{68}$Ni, the nucleus where the calculated states have the simplest structure. In this nucleus, the existence of two low-spin states reported recently was confirmed as well. Results of the present study do not indicate any involvement of collective degrees of freedom and confirm the validity of a shell-model description in terms of neutron excitations combined with a closed $Z$ $=$ 28 proton shell. Further improvements to the calculations are desirable.

γ-ray spectroscopy of one-proton knockout from45Cl

The role of proton shell effects in the structure of the N=28 isotones 45Cl and 44S has been studied via one-proton knockout from 45Cl. We compare measured γ-ray intensities, inclusive and partial knockout cross sections, and the inclusive momentum distribution of outgoing 44S particles with shell-model and reaction-theory predictions. The strong population in this reaction of the recently identified 41+ state in 44S, identified through its subsequent γ-ray decay energy, makes a compelling case for a Jπ=3/2+ ground state in 45Cl.

Missing mass spectroscopy on oxygen isotopes beyond the proton-drip line: mirror symmetry of nuclear shell evolution

Missing mass spectroscopy of the proton-rich 12O and 13O nuclei was performed via the neutron transfer (p, t and (p, d reactions, respectively, using a 14O beam of 51 MeV/u at GANIL (Grand Accelerateur National d’Ions Lourds). In addition to the ground states, an excited state of 12O at 1.8(4) MeV and two excited states of 13O at 2.8(3) and 4.2(3) MeV were observed. Spin-parity and/or spectroscopic factors were obtained from the comparison of the differential cross-sections to distorted-wave calculations. The excited state of 12O with a suggested spin-parity of 0+ or 2+ has a significantly low excitation energy, indicating that the proton shell closure at Z = 8 vanishes in 12O. The spin-parity of 1/2+ was suggested for the 2.8MeV state, which implies that the proton shell gap is weak in 13O, whereas the large spectroscopic factor extracted from the 14O(p , d reaction indicates that the ground 3/2− state remains dominated by normal p-shell configurations. These features of 12O and 13O have marked similarities with their neutron-rich mirror partners 12Be and 13B, respectively, demonstrating mirror symmetry in the fading of the shell closure at magic numbers 8 near the drip lines.

Binding energies, α-decay energies, and α-decay half-lives for heavy and superheavy nuclei

Combined with the new table of atomic mass evaluation (AME) 2011, the binding energies and alpha-decay energies for heavy and superheavy nuclei are investigated based on the previous studies. First, we use the new data from the AME2011 to check the reliability of the previously proposed formulas. It is found that there is good agreement between the calculated results and new experimental data, showing the good predictive power of the formulas. Then, a new formula with fewer adjustable parameters is presented for binding energies, and it shows better agreement with the available experimental data. However, it is still hard to use it to accurately calculate the alpha-decay energies of superheavy nuclei with N >= 160 and Z >= 106. A new formula of alpha-decay energies is proposed as well, where the shell corrections are better described and the Z = 108 proton shell effect is added. The experimental Q alpha values are reproduced with improved accuracy especially for heavier nuclei. This improvement is of great importance for precise predictions of alpha-decay half-lives for unknown superheavy nuclei. Moreover, the new Q alpha formula and the theoretical FRDM95 and KTUY05 masses are used to determine the Q alpha values of superheavy nuclei with Z = 108-120, and their alpha-decay half-lives are calculated using the generalized density-dependent cluster model (GDDCM). The agreement between theoretical results and known experimental data is discussed, together with the deformed shell effects in the superheavy mass region. (c) 2012 Elsevier B.V. All rights reserved.

The impact of the properties of the heaviest elements on the chemical and physical sciences

Abstract The unique role of the heaviest elements in chemical and physical sciences is discussed. With the actinide series (Z = 90-103) and the superactinide series (Z = 122-155), the heaviest elements have significantly shaped the architecture of the Periodic Table of the elements. Relativistic effects in the electron shells of the heaviest elements change the chemical properties in a given group in a non-linear fashion. Relativistically stabilized sub-shell closures give rise to a new category of elements in the Periodic Table: volatile metals. The prototype for this property is element 114 which, due to the relativistic stabilization of its 7s2 7p2 1/2 electron configuration, is volatile in its elementary state, but, in contrast to a noble gas, exhibits a marked metalmetal interaction with a gold surface at room temperature. Nuclear shell effects dominate the physical properties of the transuranium elements. These give rise to superdeformed shape isomers (fission isomers) in the actinides (U-Bk). Superheavy elements (Z ≥ 104) owe their existence solely to nuclear shell effects at N = 152, 162, and 184. At this time, a building lot is the location of the next spherical proton shell closure as there is evidence that the center of the “island of stability” is not at Z = 114. This needs urgently further theoretical and experimental efforts. The cross sections for the syntheses of elements 119 and 120 will give us important information on the “upper end of the Periodic Table of the elements”.

Cluster decay half-lives of trans-lead nuclei within the Coulomb and proximity potential model

Within the Coulomb and proximity potential model (CPPM) the cluster decay process in {199-226}^Fr, {206-232}^Ac, {209-237}^Th, {212-238}^Pa, {217-241}^U, {225-242}^Np, {225-244}^Pu, {231-246}^Am, {202-230}^Ra and {233-249}^Cm isotopes leading to the doubly magic 208^Pb and neighbouring nuclei are studied. The computed half lives are compared with available experimental data and are in good agreement with each other. The half lives are also computed using the Universal formula for cluster decay (UNIV) of Poenaru et al, Universal decay law (UDL) and the scaling law of Horoi et al, and their comparisons with CPPM values are found to be in agreement. The calculations for the emission of 22^O, 20^O, 20^O from the parents {209-237}^Th, {202-230}^Ra and {217-240}^U respectively were the experimental values are not available are also done. It is found that most of the decay modes are favourable for measurement, and this observation will serve as a guide to the future experiments. The odd-even staggering (OES) are found to be more prominent in the emission of odd mass clusters. The Geiger - Nuttall plots of log_10(T_1/2) vs. Q^{-1/2} for various clusters ranging from 14^C to 34^Si from different isotopes of heavy parent nuclei with atomic numbers within the range 87 \leq Z \leq 96 have been studied and are found to be linear. Our study reveals the role of doubly magic 208^Pb daughter nuclei in cluster decay process and also reveal the fact that the role of neutron shell closure is crucial than proton shell closure.

High-Spin States in Transuranium Nuclei 242,244Pu

We investigate the structure of yrast bands in the transuranium nuclei 242Pu and 244Pu in the framework of the projected shell model, which is a fully quantum mechanical and microscopic approach. It is found that an appropriate modification of the standard Nilsson spin-orbital parameters in the N = 6 proton shell is necessary to correctly describe the high-spin backbending phenomenon in nucleus 244Pu. In order to test whether this modification is correct, the same modified parameters are used to calculate the yrast band of its neighboring isotope 242Pu. It is found that without this modification, a backbending will occur at spin I = 20, which is not supported by the experimental data.

Searching critical-point nuclei in Te- and Xe-isotopic chains using sextic oscillator potential

We have identified the nuclei in the Te- and Xe-isotopic chains lying close to the critical point, through which the shape phase transition occurs, by using the sextic oscillator potential formalism. It has been found that 110Te, 124Te, and 124Xe isotopes are the most promising candidates for the critical-point nuclei slightly above the Z = 50 proton shell closure.

Influence of proton shell closure on production and identification of new superheavy nuclei

The shell structure of heavy nuclei with $Zg104$, which can be produced in the actinide-based complete fusion reactions, is studied with a modified two-center shell model. Using the microscopic-macroscopic approach, the mass excesses and ${Q}_{\ensuremath{\alpha}}$-values are calculated and compared with available experimental data. The predicted properties of superheavy nuclei show that the next doubly magic nucleus beyond ${}^{208}$Pb is at $Z\ensuremath{\ge}120$. It is shown that the production cross sections of new superheavy nuclei decisively depend on the position of proton shell closure.

Superheavy Elements – A Probe for Nuclear Matter at the Extremes

The spherical shell stabilised superheavy elements (SHE) predicted at the extreme of high Z and A are a nuclear structure phenomenon. They owe their existence to shell effects, an energy contribution of quantum mechanical origin to the nuclear potential, without which they would not be bound. Experimental activities in this field, apart from attempts to directly synthesise new elements, have to investigate reaction mechanism studies and, in particular, they have to pursue nuclear structure investigations to study the development of single particle levels towards the expected gaps for the proton (at Z = 114, 120 or 126) and neutron (at N = 184) shell closures in the region of spherical SHE. A number of exciting results in terms of the synthesis of new elements have reached the border of that region. In particular, the results obtained at the Flerov Laboratory of Nuclear Reactions (FLNR) for a rich number of decay patterns for 48 Ca induced reactions on actinide targets have by now been confirmed for reactions on 238 U, 244 Pu and 248 Cm at GSI, and on 242 Pu at LBNL. In recent years the development of efficient experimental set-ups, including separators and advanced particle and photon detection arrangements, allowed for more detailed nuclear structure studies for nuclei at and beyond Z = 100. Among the most interesting features is the observation of K-isomeric states. The heaviest example for such a structure feature was found in 270 Ds. In a recent experiment the knowledge on this nucleus and its decay products could be largely extended.

Influence of proton shell closure on production of new superheavy nuclei

The shell structure of nuclei with Z > 104 is studied with modified two-center shell model. Using macroscopic-microscopic approach, the mass excesses and Qα-values are calculated and compared with available experimental data. Our calculations reveal quite strong shell effects at Z = 120 −126 as well as at N = 184. If our predictions for the heaviest nuclei are correct then one can expect the production of evaporation residues with Z ∼ 120 in the reactions 50 Ti+ 249 Cf and 54 Cr+ 248 Cm with the cross sections 23 and 10 fb, respectively. Nuclei with Z = 120 and N = 178 − 182 are expected to have Qα about 11 MeV and life-time of 0.17 ms - 0.16 s.

Collective structural evolution in neutron-rich Yb, Hf, W, Os, and Pt isotopes

An interacting-boson-model Hamiltonian determined from Hartree-Fock-Bogoliubov calculations with the microscopic Gogny energy density functional D1M is applied to the spectroscopic analysis of neutron-rich Yb, Hf, W, Os, and Pt isotopes with mass $A\ensuremath{\sim}180$--200. Excitation energies and transition rates for the relevant low-lying quadrupole collective states are calculated by this method. Transitions from prolate to oblate ground-state shapes are analyzed as a function of neutron number $N$ in a given isotopic chain by calculating excitation energies, $B(E2)$ ratios, and correlation energies in the ground state. It is shown that such transitions tend to occur more rapidly for the isotopes with lower proton number $Z$ when departing from the proton shell closure $Z=82$. The triaxial degrees of freedom turn out to play an important role in describing the considered mass region. Predicted low-lying spectra for the neutron-rich exotic Hf and Yb isotopes are presented. The approximations used in the model and the possibilities to refine its predictive power are addressed.

Collectivity of Exotic Heavy Fe Isotopes

The properties of exotic neutron-rich nuclei between the proton shell closures Z = 20 and Z = 28 are of particular interest for the understanding of the shell structure for large neutron excess. Effects related to the energy gap between the neutron fp and 1g9/2 shells lead to a strong variation of collectivity for nuclei around N = 40. Whereas 68Ni was found to have doubly magic properties, this was not observed in neighbouring nuclei. Recent shell model calculations for the neutron rich iron isotopes clearly reveal the difficulty to describe nuclei in this mass region and resulted in large deviations of the predicted collectivity depending on the valence space. However, no experimental data on the transition strength existed for the very exotic nucleus 66Fe at N = 40. Here we present the newest results on absolute transition strengths of the lowest excited states in 62,64,66Fe measured model independently using the recoil distance Doppler-shift (RDDS) method. The experiments were performed at NSCL at Michigan State University with the Cologne/NSCL plunger device using Coulomb excitation in inverse kinematics at energies of 80 MeV/u. Our results yield a much higher collectivity for 64,66Fe than expected and allow tests of new calculations.

THE INFLUENCE OF PICK-UP IONS ON SPACE PLASMA DISTRIBUTIONS

The addition and incorporation of highly ordered distributions of pickup ions can increase the ordering of space plasmas, decreasing their entropy, and driving them away from equilibrium. In this study, we present a model for describing the entropy decrease of heliospheric plasmas caused by the pick-up ions and their highly ordered arrangement. We consider a plasma flow consisting of two proton populations: (1) a solar wind distribution that is in quasi-equilibrium and (2) a spherical shell distribution of pick-up protons. Because all protons are indistinguishable, they equally share a hybrid state, where the hybrid probability distribution is given by the normalized sum of the distributions of the solar wind and pick-up protons. We derive an analytical formulation of the thickness of the pick-up proton shell distribution. Then we show that the entropy of the hybrid state depends on a dimensionless fluctuation number, {Theta}, which is a measure of the organizing role of pick-up protons. This number characterizes and compares two types of fluctuations, the thermal motion of solar wind protons and the bounded motion of pick-up protons that forms the spherical shell distribution. We find that the entropy variation between the hybrid and original states is proportional to the negativemore » logarithm of {Theta}. When the two competing fluctuations balance each other, {Theta} {approx} 1, the pick-up protons have no effect on the entropy of solar wind protons. Remarkably, while sunward of the termination shock pick-up ions act to slightly increase entropy, throughout the inner heliosheath their effect is to decrease the entropy dramatically.« less

Exact Coulomb exchange calculations in the Skyrme-Hartree-Fock-BCS framework and tests of the Slater approximation

An exact treatment of the Coulomb interaction is performed within the Skyrme-Hartree-Fock/Bardeen-Cooper-Schrieffer approach for even-even nuclei ranging from light to superheavy nuclei. A test of the usual Slater approximation for the exchange part is carried out. The error made on the exchange term of the Coulomb interaction when using this approximation follows two schematic patterns. Beyond a decreasing behavior when increasing the mass number A, a more important structural effect has been found. The relative error ranges roughly from 0 to 8% being maximal for light closed proton (sub-)shell nuclei and minimal for open proton (sub-)shell nuclei.

Excitation of magnetosonic waves in the terrestrial magnetosphere: Particle-in-cell simulations

[1] Two-dimensional electromagnetic particle-in-cell simulations are performed to study the temporal development of an ion Bernstein instability driven by a proton velocity distribution with positive slope in the perpendicular velocity distribution fp(v⊥), where ⊥ denotes directions perpendicular to the background magnetic field B0. A subtracted Maxwellian distribution is first used to construct the positive slope in fp(v⊥), and linear kinetic dispersion analysis is performed. The results of a simulation using such an initial proton distribution agree well with the linear kinetic analysis. The simulation results demonstrate that the ion Bernstein instability grows at propagation angles nearly perpendicular to B0 and at frequencies close to the harmonics of the proton cyclotron frequency. The distribution in the simulation is further generalized to contain a proton shell with a finite thermal spread and a relatively cold ion background. The simulation results show that the presence of the cold background protons and the increase of the shell velocity shift the excited waves close to the cold plasma dispersion relation for magnetosonic waves, i.e., ωr = kvA, where ωr is the wave frequency, k is the wave number, and vA is the Alfven velocity. The general features of the simulated field fluctuations resemble observations of fast magnetosonic waves near the geomagnetic equator in the terrestrial magnetosphere. A test particle computation of energetic electrons interacting with the simulated electromagnetic fluctuations suggests that this growing mode may play an important role in the acceleration of radiation belt relativistic electrons.

Ultrasensitive search for long-lived superheavy nuclides in the mass rangeA=288toA=300in natural Pt, Pb, and Bi

Theoretical models of superheavy elements (SHEs) predict a region of increased stability around the proton and neutron shell closures of $Z$ $=$ 114 and $N$ $=$ 184. Therefore a sensitive search for nuclides in the mass range from $A$ $=$ 288 to $A$ $=$ 300 was performed in natural platinum, lead, and bismuth, covering long-lived isotopes of Eka-Pt (Ds, $Z$ $=$ 110), Eka-Pb ($Z$ $=$ 114), and Eka-Bi ($Z$ $=$ 115). Measurements with accelerator mass spectrometry (AMS) at the Vienna Environmental Research Accelerator (VERA) established upper limits for these SHE isotopes in Pt, Pb, and Bi with abundances of 2\ifmmode\times\else\texttimes\fi{}10${}^{\ensuremath{-}15}$, 5\ifmmode\times\else\texttimes\fi{}10${}^{\ensuremath{-}14}$, and 5\ifmmode\times\else\texttimes\fi{}10${}^{\ensuremath{-}13}$, respectively. These results complement earlier searches for SHEs with AMS at VERA in natural thorium and gold, which now amounts to a total number of 37 SHE nuclides having been explored with AMS. In none of our measurements was evidence found for the existence of SHEs in nature at the reported sensitivity level. Even though a few events were observed in the window for $^{293}\mathrm{Ek}$a-Bi, a particularly strong pileup background did not allow a definite SHE isotope identification. The present result sets limits on nuclides around the center of the island of stability, essentially ruling out the existence of SHE nuclides with half-lives longer than \ensuremath{\sim}150 million years.

Interactions of Poly(amidoamine) Dendrimers with Human Serum Albumin: Binding Constants and Mechanisms

The interactions of nanomaterials with plasma proteins have a significant impact on their in vivo transport and fate in biological fluids. This article discusses the binding of human serum albumin (HSA) to poly(amidoamine) [PAMAM] dendrimers. We use protein-coated silica particles to measure the HSA binding constants (K(b)) of a homologous series of 19 PAMAM dendrimers in aqueous solutions at physiological pH (7.4) as a function of dendrimer generation, terminal group, and core chemistry. To gain insight into the mechanisms of HSA binding to PAMAM dendrimers, we combined (1)H NMR, saturation transfer difference (STD) NMR, and NMR diffusion ordered spectroscopy (DOSY) of dendrimer-HSA complexes with atomistic molecular dynamics (MD) simulations of dendrimer conformation in aqueous solutions. The binding measurements show that the HSA binding constants (K(b)) of PAMAM dendrimers depend on dendrimer size and terminal group chemistry. The NMR (1)H and DOSY experiments indicate that the interactions between HSA and PAMAM dendrimers are relatively weak. The (1)H NMR STD experiments and MD simulations suggest that the inner shell protons of the dendrimers groups interact more strongly with HSA proteins. These interactions, which are consistently observed for different dendrimer generations (G0-NH(2)vs G4-NH(2)) and terminal groups (G4-NH(2)vs G4-OH with amidoethanol groups), suggest that PAMAM dendrimers adopt backfolded configurations as they form weak complexes with HSA proteins in aqueous solutions at physiological pH (7.4).

Coulombic effect and renormalization in nuclear pairing

We investigate effects of the Coulomb force on the nuclear pairing properties by performing the Gogny Hartree-Fock-Bogolyubov calculations for the $N=20$, $28$, $50$, $82$ and $126$ nuclei. The Coulomb force reduces the proton pair energy and the even-odd mass difference by about 25%, except for nuclei at and around the proton shell or subshell closure. We then propose a renormalization scheme via a reduction factor $\gamma_p$ for the proton pairing channel. It is found that a single value of $\gamma_p(=0.90)$ well takes account of the Coulombic effect, for nuclei covering wide range of the mass number and the neutron excess including the nuclei around the shell or subshell closure.