Aspects Of Nuclear Structure Research Articles

Nonadiabatic quasiparticle approach for deformed odd-odd nuclei and the proton emitter130Eu

Proton emission from deformed nuclei near the drip line is regarded as a versatile tool in nuclear structure physics and such a study in odd-odd nuclei provides us with an excellent opportunity to understand several interesting features including the interaction between the valence proton and the valence neutron. We present a detailed formalism for a full microscopic calculation of such proton emitters, where the nuclear structure and decay aspects are taken into account exactly. This formalism is based on the nonadiabatic approach for the two quasiparticle plus rotor model, where the residual neutron-proton interactions are considered in the mean field represented by a deformed Woods-Saxon potential. We demonstrate a systematic way of unambiguously identifying several parameters involved in the calculations with the aid of the experimental data. The quality of results in the case of ${}^{180}$Ta justifies the approach which is further extended to discuss in detail the proton emission from ${}^{130}$Eu. ${I}^{\ensuremath{\pi}}={1}^{+}$ state in ${}^{130}$Eu still remains to be the proton emitting state irrespective of the choice of several parameters. We also note that the decay widths could be quite sensitive to the residual $np$ interaction in case of proton emission from isomeric states.

Low-lying [formula omitted] states in the odd–odd [formula omitted] nucleus 62Ga

New, low-lying levels in the odd–odd, N=Z nucleus 62Ga have been identified using a sensitive technique, where in-beam γ rays from short-lived nuclei are tagged with β decays following recoil mass identification. A comparison of the results with shell-model and IBM-4 calculations demonstrates good agreement between theory and experiment, with the majority of predicted low-lying, low-spin T=0 states now identified. There is a dramatic change in the level density at low excitation energies for the N=Z nucleus 62Ga when compared with neighbouring odd–odd Ga isotopes where, in contrast, the low-lying level structure is dominated by configurations with T=1 pairing interactions between excess neutrons. This illustrates the distinctively different aspects of nuclear structure exhibited by nuclei with N=Z.

Optimized Chiral Nucleon-Nucleon Interaction at Next-to-Next-to-Leading Order

We optimize the nucleon-nucleon interaction from chiral effective field theory at next-to-next-to-leading order (NNLO). The resulting new chiral force NNLO(opt) yields χ(2)≈1 per degree of freedom for laboratory energies below approximately 125 MeV. In the A=3, 4 nucleon systems, the contributions of three-nucleon forces are smaller than for previous parametrizations of chiral interactions. We use NNLO(opt) to study properties of key nuclei and neutron matter, and we demonstrate that many aspects of nuclear structure can be understood in terms of this nucleon-nucleon interaction, without explicitly invoking three-nucleon forces.

Summary talk – from a theoretical point of view

This summary is based on the theoretical talks presented at the Tenth International Conference on Clustering Aspects of Nuclear Structure and Dynamics, Debrecen, Hungary, 24–28 September, 2012.

Q values of radioactive decay: Examples from nuclear physics and related fields

The measurement of the total energy or Q value released in a radioactive decay or involved in a nuclear reaction is of interest for interpreting the nuclear transition between mother and daughter state and thus gaining nuclear-structure information. The Q values are deduced by investigating the energy of the decay products or studying the corresponding inverse nuclear reaction, respectively. Moreover, the Q values represent “mass links” between the atomic mass of the initial and final nucleus involved in the transmutation and can thus be used to determine the atomic mass of the initial or final state, provided one of them is known. In this way the mass-energy surface can be probed in comparison with theoretical predictions. While paying comparatively less attention to the latter aspect, we shall focus in this review on examples of studies of α, proton, β+/EC and β− decay, in which the respective Q values play an important role. The examples deal mainly with nuclear-structure aspects but are also related astrophysics, fundamental physics and other research fields.

Constraining the 0ν2β matrix elements by nuclear structure observables

The discovery that neutrinos have finite rest mass has led to renewed interest in neutrinoless double beta decay. The development of large-scale experiments to search for neutrinoless double beta decay has increased the probability of a credible observation of the process in the near future. The reliability of calculations of the associated nuclear matrix elements is likely soon to become a critical issue. In this paper experimental techniques that access properties of the ground-state wave functions of double-beta-decay candidates, the occupancies of valence single-particle orbitals and pairing correlations, are summarized and the experimental data for candidate nuclei are reviewed. The results are discussed in relation to questions concerning which aspects of nuclear structure may play an important role in determining the nuclear matrix elements for neutrinoless double beta decay.

Characterization of myosin light chain gene up-regulated in the large yellow croaker immunity by interaction with RanGTPase

RanGTPases are highly conserved in eukaryotes from yeast to human and have been implicated in many aspects of nuclear structure and function. In our previous study, it was revealed that the RanGTPase was up-regulated in large yellow croaker challenged by pathogen. However, the mechanism of RanGTPase in immunity remains unclear. In this investigation, on the basis of protein interaction, it was found that RanGTPase interacted with myosin light chain (designated as LycMLC), a crucial protein in the process of phagocytosis. Furthermore, it was found and characterized in this marine fish for the first time. The full-length cDNA of LycMLC was 771bp, including a 5′-terminal untranslated region (UTR) of 36bp, 3′-terminal UTR of 279bp and an open reading frame (ORF) of 456bp encoding a polypeptide of 151 amino acids. RT-PCR analysis indicated that LycMLC gene was constitutively expressed in the 9 tissues examined, including kidney, liver, gill, muscle, spleen, skin, heart, intestine and blood. The result of quantitative real-time PCR analysis revealed the highest expression in muscle and the weakest expression in skin. Time course analysis showed that LycMLC expression was obviously up-regulated in blood after immunization with either poly I:C or formalin-inactive Gram-negative bacteria Vibrio parahaemolyticus. It indicated that the highest expression was 4.5 times (at 24h) as much as that in the control (P<0.05) challenged by poly I:C and 5.0 times (at 24h) challenged by bacteria. These results suggested that LycMLC might play an important role in large yellow croaker defense against the pathogen infection. Therefore our study revealed a novel pathway concerning immunity of RanGTPase by the direct interaction with the cytoskeleton protein, which would help to better understand the molecular events in immune response against pathogen infection in fish.

Meson Production in Antinucleon Annihilation on Nuclei

Pion production in coherent antiproton annihilation reactions on nuclei is studied in a fully quantum mechanical description. The elementary elastic NN and the NN! n annihilation vertices are obtained from a t-matrix accounting for coupled chan- nels e ects. The theoretical amplitudes are used to derive a microscopic NA optical model potential and the meson production vertices. Pion nucleus final state interaction are taken into account by adding higher pion-nucleon resonances beyond the -resonance. Nuclear structure aspects are described by HFB-theory. Results for pA, A and meson production cross sections are presented.

Structural Transitions in Higher Order Chromatin Assembly and Nuclear Plasticity during Stem Cell Differentiation

Undifferentiated cells integrate physico-chemical cues from the local microenvironment to elicit lineage specific gene expression programs. However the underlying mechanisms of this physical plasticity and how it impinges on gene expression is still unclear. In this study, using high resolution live-cell fluorescence polarization imaging, we analyze the spatio-temporal aspects of nuclear organization and chromatin structure in mouse embryonic stem (ES) cells and contrast them to a primary embryonic fibroblasts (PMEF). Higher-order chromatin compaction states exhibit unique features in ES cells, marked by homogeneous chromatin compaction but heterogeneity at the population level, but PMEFs evidence an inverse correlation. In addition, the nuclear lamina and actin cytoskeleton is highly flexible in ES cells but are frozen in PMEFs. This transition in nuclear architecture resembles that of fluid-like to solid-like transitions. Further the temporal evolution of nuclear plasticity is studied by differentiating ES cells on gelatin coated dishes. Taken together these results suggest that ES cells exhibit a broad epigenetic free energy landscape transitioning into a frozen configuration in higher-order chromatin assembly as lineage specific gene expression programs emerge.

Influence of transport variables on isospin transport ratios

The symmetry energy in the nuclear equation of state affects many aspects of nuclear astrophysics, nuclear structure, and nuclear reactions. Recent constraints from heavy-ion collisions, including isospin diffusion observables, have started to put constraints on the symmetry energy below nuclear saturation density, but these constraints depend on the employed transport model and input physics other than the symmetry energy. To understand these dependencies, we study the influence of the symmetry energy, isoscalar mean-field compressibility and momentum dependence, in-medium nucleon-nucleon cross sections, and light cluster production on isospin diffusion within the pBUU transport code. In addition to the symmetry energy, several uncertain issues strongly affect isospin diffusion, most notably the cross sections and cluster production. In addition, there is a difference in the calculated isospin transport ratios, depending on whether they are computed using the isospin asymmetry either of the residue or of all forward-moving fragments. Measurements that compare the isospin transport ratios of these two quantities would help place constraints on the input physics, such as the density dependence of the symmetry energy.

Feedback regulation of Ran gene expression by Ran protein

Ran, an abundant GTPase that is highly conserved in eukaryotes from yeast to human, has been implicated in many aspects of nuclear structure and function. Recently it is revealed that the Ran GTPase can regulate the hemocytic phagocytosis of shrimp by interaction with myosin. However the regulation of Ran gene expression remains unknown. In this study, the promoter of shrimp Ran gene was identified which contained a typical TATA box. The results showed that the shrimp Ran protein was bound with the Ran promoter in Drosophila S2 cells. Based on luciferase assays, our study indicated that the transcription of Ran gene could be regulated by the interaction between the Ran promoter and the Ran protein, suggesting the existence of a feedback regulation in Ran gene expression. Therefore our study presented a novel finding on the feedback regulation of gene transcription.

Extremes of the nuclear landscape: experimental studies.

Increasingly more intense beams of radioactive isotopes allow moving into unknown areas of the nuclear chart and exploring the limits in nuclear binding and proton-to-neutron ratio. New aspects of nuclear structure and important results for nuclear astrophysics are obtained. The paper provides some overview of experimental developments, facilities and research results; and is intended to set the stage for the many exciting examples of research presented in these proceedings.

Novel nuclear structure aspects of the Oνββ-decay

We explore the influence of the deformation on the nuclear matrix elements of the neutrinoless double beta decay (NME), concluding that the difference in deformation -or more generally in the amount of quadrupole correlations- between parent and grand daughter nuclei quenches strongly the decay. We correlate these differences with the seniority structure of the nuclear wave functions. In this context, we examine the present discrepancies between the NME's obtained in the framework of the Interacting Shell Model and the Quasiparticle RPA. In our view, part of the discrepancy can be due to the limitations of the spherical QRPA in treating nuclei which have strong quadrupole correlations. We surmise that the NME's in a basis of generalized seniority are approximately model independent, i. e. they are "universal".

THE HELIOS SPECTROMETER AND THE RADIOACTIVE BEAM PROGRAM AT ARGONNE

The near-term radioactive beam capabilities of ATLAS include radioactive beams produced in flight in a gas cell, or starting in the fall of 2009, re-accelerated beams of 252 Cf fission fragments provided by the new CARIBU injector. The availability of such exotic beams will allow for detailed studies of the single-particle aspects of nuclear structure in neutron-rich nuclei reaching out to the astrophysical r-process path by employing light-ion reactions in inverse kinematics. The HELIOS spectrometer is based on a new concept that is especially well suited for such studies. This concept was recently demonstrated using the reactions D (28 Si , p )29 Si with a (stable) 168 MeV 28 Si beam. Since then D (12 B , p )13 B , D (17 O , p )18 O , and D (15 C , p )16 C have been studied successfully. The combination of neutron-rich beams from CARIBU and the HELIOS spectrometer opens a fertile research area of precision studies of the single particle strengths and collective excitations in exotic nuclei, and is likely to have applications in other reactions as well.

Weak nonmesonic decay spectra of hypernuclei

We compute one- and two-nucleon kinetic-energy spectra and opening-angle distributions for the nonmesonic weak decay of several hypernuclei, and compare our results with some recent data. The decay dynamics is described by transition potentials of the one-meson-exchange type, and the nuclear structure aspects by two versions of the independent-particle shell model (IPSM). In version IPSM-a, the hole states are treated as stationary, while in version IPSM-b the deep-hole ones are considered to be quasi-stationary and are described by Breit-Wigner distributions.

Nuclear structure aspects of single and double beta decays for neutrino mass

Single β -decay of 187Re and double β -decay are discussed for probing absolute masses of neutrinos. It is shown that 187Re decay is dominated by p -wave emission of the electron over s -wave. Double β -decay nuclear matrix elements relevant to the neutrino mass mechanism are discussed in the view point of nuclear structure and the underlying NN interaction.

Search for low-energy induced depletion of 178Hfm2 at the SPring-8 synchrotron

Electromagnetic transitions within nuclei reflect specific aspects of nuclear structure. This is particularly true for metastable excited states, or isomers, like 178Hfm2 (T1/2=31 years, excitation energy 2446 keV). The interaction of external radiation with isomers can be used to study atomic and nuclear properties and, perhaps, to induce a release of the stored energy. Some experiments indicated that low-energy photons near the L3 edge (9.561 keV) of hafnium could cause this to occur for 178Hfm2, but the lack of a viable physical model and null experiments by other groups have left these claims in doubt. The present work describes a new experiment to examine this process by closely duplicating the irradiation conditions in positive studies, but using a more advanced multi-detector γ array. No support for an induced depletion of 178Hfm2 by low-energy photons was obtained, with an upper limit for the integral cross section that is eight orders-of-magnitude below the reported value.

Sizing up the nucleus: nuclear shape, size and nuclear-envelope assembly

The nucleus is one of the most prominent cellular organelles, yet surprisingly little is known about how it is formed, what determines its shape and what defines its size. As the nuclear envelope (NE) disassembles in each and every cell cycle in metazoans, the process of rebuilding the nucleus is crucial for proper development and cell proliferation. In this Commentary, we summarize what is known about the regulation of nuclear shape and size, and highlight recent findings that shed light on the process of building a nucleus, including new discoveries related to NE assembly and the relationship between the NE and the endoplasmic reticulum (ER). Throughout our discussion, we note interesting aspects of nuclear structure that have yet to be resolved. Finally, we present an idea - which we refer to as ;the limited flat membrane hypothesis' - to explain the formation of a single nucleus that encompasses of all of the cell's chromosomes following mitosis.

Nuclear Schiff moment and soft vibrational modes

The atomic electric dipole moment (EDM) currently searched by a number of experimental groups requires that both parity and time-reversal invariance be violated, According to current theoretical understanding, the EDM is induced by the nuclear Schiff moment, The enhancement of the Schiff moment by the combination of static quadrupole and octupole deformation was predicted earlier. Here we study a further idea of the possible enhancement in the absence of static deformation but in a nuclear system with soft collective vibrations of two types. Both analytical approximation and numerical solution of the simplified problem confirm the presence of the enhancement, We discuss related aspects of nuclear structure which should be studied beyond mean-field and random phase approximations.

Coupling gamma-ray arrays to spectrometers: The way to better exploit radioactive ion beams

The coupling of very efficient gamma-ray arrays with large acceptance (or highly selective) spectrometers in the context of radioactive beams, has pushed away the limits for the exploration of the nuclear chart and opened up new possibilities in nuclear physics studies. At GANIL the EXOGAM array (a large efficiency gamma-ray array) has been coupled to VAMOS (a large acceptance spectrometer) and to SPEG (a highly selective spectrometer) to study many different aspects of nuclear structure and reactions covering the whole nuclear chart. Indeed, this combination has been used to study neutron-rich nuclei toward 54Ca, neutron deficient tin isotopes around A=103, heavy elements around Md and No, reaction mechanism using the borromean light He isotopes, the nuclear structure in the island of inversion around 32Mg, and many other examples in various mass regions and energy regimes. This is also the case for the very efficient large BaF2 array Chateau de Cristal which is frequently coupled to SPEG to study nuclear structure far from stability. In this paper, the various possibilities offered at GANIL with such couplings will be given. In the future, AGATA will be installed at GANIL to exploit the beams produced by the SPIRAL2 facility. Again, it is anticipated that this new generation array will be coupled to spectrometers to better exploit the future beams.