Search

Background: At energies above the capture barrier, coupled-channels (CC) calculations with a standard nuclear potential diffuseness (0.65 fm) increasingly overestimate experimental capture cross section as the charge product (${Z}_{1}{Z}_{2}$) of the colliding nuclei increases. It has been suggested this may be linked to energy dissipation outside the capture barrier.Purpose: We investigate quantitatively the role of energy dissipation in suppressing capture in heavy ion fusion reactions.Method: The yields of sequential fission, including that resulting from deep inelastic collisions, and of fission following capture were determined simultaneously for collisions of $^{18}\mathrm{O}, ^{30}\mathrm{Si}, ^{34}\mathrm{S}$, and $^{40}\mathrm{Ca}+^{232}\mathrm{Th}$ at a range of energies around the respective capture barriers.Results: The ratio of experimental to CC capture cross sections was found to decrease with increasing ${Z}_{1}{Z}_{2}$. Conversely, the ratio of sequential fission to capture-fission increased with increasing ${Z}_{1}{Z}_{2}$. The sum of sequential and capture fission agrees quite well with the CC cross sections.Conclusions: The experimental capture fission and sequential fission cross sections, and their comparison with CC calculations, give a consistent picture that the increase in density overlap at the capture barrier with increasing ${Z}_{1}{Z}_{2}$ of the colliding nuclei is correlated with increasing energy dissipative processes. These compete increasingly strongly with capture as the ${Z}_{1}{Z}_{2}$ of the reaction increases. For the $^{40}\mathrm{Ca}$ reaction, the total fission yield exceeds expectations from capture model calculations, indicating that deep inelastic processes occur both from trajectories that would have led to capture and also from more peripheral trajectories.

The nucleolar surveillance pathway (NSP) monitors nucleolar fidelity and responds to nucleolar stresses (i.e., inactivation of ribosome biogenesis) by mediating the inhibitory binding of ribosomal proteins (RPs) to mouse double minute 2 homolog (MDM2), a nuclear-localised E3 ubiquitin ligase, which results in p53 accumulation. Inappropriate activation of the NSP has been implicated in the pathogenesis of collection of human diseases termed “ribosomopathies”, while drugs that selectively activate the NSP are now in trials for cancer. Despite the clinical significance, the precise molecular mechanism(s) regulating the NSP remain poorly understood. Using genome-wide loss of function screens, we demonstrate the ribosome biogenesis (RiBi) axis as the most potent class of genes whose disruption stabilises p53. Furthermore, we identified a novel suite of genes critical for the NSP, including a novel mammalian protein implicated in 5S ribonucleoprotein particle (5S-RNP) biogenesis, HEATR3. By selectively disabling the NSP, we unexpectedly demonstrate that a functional NSP is required for the ability of all nuclear acting stresses tested, including DNA damage, to robustly induce p53 accumulation. Together, our data demonstrates that the NSP has evolved as the dominant central integrator of stresses that regulate nuclear p53 abundance, thus ensuring RiBi is hardwired to cellular proliferative capacity.

The three-dimensional organization of the genome contributes to its maintenance and regulation. While chromosomal regions associate with nucleolar ribosomal RNA genes (rDNA), the biological significance of rDNA-genome interactions and whether they are dynamically regulated during disease remain unclear. rDNA chromatin exists in multiple inactive and active states and their transition is regulated by the RNA polymerase I transcription factor UBTF. Here, using a MYC-driven lymphoma model, we demonstrate that during malignant progression the rDNA chromatin converts to the open state, which is required for tumor cell survival. Moreover, this rDNA transition co-occurs with a reorganization of rDNA-genome contacts which correlate with gene expression changes at associated loci, impacting gene ontologies including B-cell differentiation, cell growth and metabolism. We propose that UBTF-mediated conversion to open rDNA chromatin during malignant transformation contributes to the regulation of specific gene pathways that regulate growth and differentiation through reformed long-range physical interactions with the rDNA., Jeannine Diesch et al. report the changes in rDNA chromatin state associated with cell transition into malignancy. They show that a specific transcription factor regulates this transition by altering rDNA chromatin, resulting in the reorganization of contacts between rDNA and the genome.

A search for production of the superheavy elements with atomic numbers 119 and 120 was performed in the 50Ti+249Bk and 50Ti+249Cf fusion-evaporation reactions, respectively, at the gas-filled recoil separator TASCA at GSI Darmstadt, Germany. Over four months of irradiation, the 249Bk target partially decayed into 249Cf, which allowed for a simultaneous search for both elements. Neither was detected at cross-section sensitivity levels of 65 and 200 fb for the 50Ti+249Bk and 50Ti+249Cf reactions, respectively, at a midtarget beam energy of Elab=281.5 MeV. The nonobservation of elements 119 and 120 is discussed within the concept of fusion-evaporation reactions including various theoretical predictions on the fission-barrier heights of superheavy nuclei in the region of the island of stability. peerReviewed

Psi, the sole FUSE Binding Protein (FUBP) family single stranded DNA/RNA binding protein inDrosophila, is essential for proper cell and tissue growth, however its mechanism of function remains unclear. Here we use Targeted DamID combined with RNA-sequencing to generate the first genome-wide binding and expression profiles for Psi. Surprisingly, we demonstrate Psi drives growth in theDrosophilawing through transcriptional repression of key developmental pathways (e.g. Wnt, Notch and TGFβ). Thus, Psi patterns tissue growth by directly repressing transcription of developmental growth suppressors. Analysis of direct Psi targets identified novel growth inhibitors, includingTolkin(Zinc metallopeptidase implicated in TGFβ signalling),Ephexin(Rho-GEF) andemp(CD36 scavenger receptor-related protein). Their depletion not only suppressed impaired growth associated with Psi knockdown, but alone was sufficient to drive wing overgrowth. Thus, Psi drives wing growth twofold, through direct activation ofMycand through transcriptional repression of growth inhibitors comprising core developmental pathways.

The nucleolus is a dynamic subnuclear compartment that has a number of different functions, but its primary role is to coordinate the production and assembly of ribosomes. For well over 100 years, pathologists have used changes in nucleolar number and size to stage diseases such as cancer. New information about the nucleolus' broader role within the cell is leading to the development of drugs which directly target its structure as therapies for disease. Traditionally, it has been difficult to develop high-throughput image analysis pipelines to measure nucleolar changes due to the broad range of morphologies observed. In this study, we describe a simple high-content image analysis algorithm using Harmony software (PerkinElmer), with a PhenoLOGIC™ machine-learning component, that can measure and classify three different nucleolar morphologies based on nucleolin and fibrillarin staining ("normal," "peri-nucleolar rings" and "dispersed"). We have utilized this algorithm to determine the changes in these classes of nucleolar morphologies over time with drugs known to alter nucleolar structure. This approach could be further adapted to include other parameters required for the identification of new therapies that directly target the nucleolus.

The heaviest currently known nuclei, which have up to 118 protons, have been produced in 48Ca induced reactions with actinide targets. Among them, the element tennessine (Ts), which has 117 protons, has been synthesized by fusing 48Ca with the radioactive target 249Bk, which has a half-life of 327 d. The experiment was performed at the gas-filled recoil separator TASCA. Two long and two short α decay chains were observed. The long chains were attributed to the decay of 294Ts. The possible origin of the short-decay chains is discussed in comparison with the known experimental data. They are found to fit with the decay chain patterns attributed to 293Ts. The present experimental results confirm the previous findings at the Dubna Gas-Filled Recoil Separator on the decay chains originating from the nuclei assigned to Ts.

miRNAs play critical roles in heart disease. In addition to differential miRNA expression, miRNA-mediated control is also affected by variable miRNA processing or alternative 3'-end cleavage and polyadenylation (APA) of their mRNA targets. To what extent these phenomena play a role in the heart remains unclear. We sought to explore miRNA processing and mRNA APA in cardiomyocytes, and whether these change during cardiac hypertrophy. Thoracic aortic constriction (TAC) was performed to induce hypertrophy in C57BL/6J mice. RNA extracted from cardiomyocytes of sham-treated, pre-hypertrophic (2 days post-TAC), and hypertrophic (7 days post-TAC) mice was subjected to small RNA- and poly(A)-test sequencing (PAT-Seq). Differential expression analysis matched expectations; nevertheless we identified ~400 mRNAs and hundreds of noncoding RNA loci as altered with hypertrophy for the first time. Although multiple processing variants were observed for many miRNAs, there was little change in their relative proportions during hypertrophy. PAT-Seq mapped ~48,000 mRNA 3'-ends, identifying novel 3' untranslated regions (3'UTRs) for over 7000 genes. Importantly, hypertrophy was associated with marked changes in APA with a net shift from distal to more proximal mRNA 3'-ends, which is predicted to decrease overall miRNA repression strength. We independently validated several examples of 3'UTR proportion change and showed that alternative 3'UTRs associate with differences in mRNA translation. Our work suggests that APA contributes to altered gene expression with the development of cardiomyocyte hypertrophy and provides a rich resource for a systems-level understanding of miRNA-mediated regulation in physiological and pathological states of the heart.

// Alexandra Schrader 1, 2, 10, 11 , Katharina Meyer 3, 6 , Neele Walther 1 , Ailine Stolz 4 , Maren Feist 1, 7 , Elisabeth Hand 1, 6 , Frederike von Bonin 1 , Maurits Evers 3, 6, 13 , Christian Kohler 3, 6 , Katayoon Shirneshan 1 , Martina Vockerodt 5, 8, 10, 12 , Wolfram Klapper 5, 6, 7, 9 , Monika Szczepanowski 5, 6, 7, 9 , Paul G. Murray 8 , Holger Bastians 4 , Lorenz Trumper 1, 2, 5, 7 , Rainer Spang 3, 5, 6, 7 , Dieter Kube 1, 2, 5, 6, 7 1 Department of Haematology and Medical Oncology, University Medical Centre of the Georg-August University Gottingen, Gottingen, Germany 2 GRK1034 of the Deutsche Forschungsgemeinschaft, Georg-August University Gottingen, Gottingen, Germany 3 Department of Statistical Bioinformatics, Institute for Functional Genomics, University of Regensburg, Regensburg, Germany 4 Goettingen Center for Molecular Biosciences (GZMB) and University Medical Center, Institute of Molecular Oncology, Section for Cellular Oncology, Gottingen, Germany 5 Network Molecular Mechanism of Malignant Lymphoma (MMML) of the Deutsche Krebshilfe, Germany 6 BMBF-Network HamatoSys, Germany 7 BMBF-Network Myc-Sys, Germany 8 School of Cancer Sciences, University of Birmingham, Birmingham, UK 9 University-Hospital Schleswig-Holstein, Hematopathology Section and Lymph Node Registry Kiel, Kiel, Germany 10 Department of Anatomy, University Medical Centre of the Georg-August University Gottingen, Gottingen, Germany 11 Present address: Laboratory of Lymphocyte Signaling and Oncoproteome, Department I of Internal Medicine, University Hospital Cologne, Center for Integrated Oncology (CIO) Koln-Bonn, Cologne, Germany 12 Present address: Department of Anatomy, University Medical Centre of the Georg-August University Gottingen, Gottingen, Germany 13 Current address: The John Curtin School of Medical Research the Australian National University Canberra, Australia Correspondence to: Dieter Kube, e-mail: dieter.kube@med.uni-goettingen.de Keywords: lymphoma, B cell receptor signaling, guided clustering, cell cycle delay, chromosomal aberrations Received: September 23, 2015 Accepted: March 31, 2016 Published: May 7, 2016 ABSTRACT To discover new regulatory pathways in B lymphoma cells, we performed a combined analysis of experimental, clinical and global gene expression data. We identified a specific cluster of genes that was coherently expressed in primary lymphoma samples and suppressed by activation of the B cell receptor (BCR) through αIgM treatment of lymphoma cells in vitro . This gene cluster, which we called BCR.1, includes numerous cell cycle regulators. A reduced expression of BCR.1 genes after BCR activation was observed in different cell lines and also in CD10 + germinal center B cells. We found that BCR activation led to a delayed entry to and progression of mitosis and defects in metaphase. Cytogenetic changes were detected upon long-term αIgM treatment. Furthermore, an inverse correlation of BCR.1 genes with c-Myc co-regulated genes in distinct groups of lymphoma patients was observed. Finally, we showed that the BCR.1 index discriminates activated B cell-like and germinal centre B cell-like diffuse large B cell lymphoma supporting the functional relevance of this new regulatory circuit and the power of guided clustering for biomarker discovery.

SummarySmall RNA pathway components and small RNA profiles of flowering plant egg cells are largely unexplored, mainly because they are not easily accessible but deeply buried inside the ovary.We describe here the utilization of proliferating callus tissue that adopted transcriptome features of Arabidopsis egg cell as a tool to explore small RNA pathway components and small RNA profiles in egg cells. We furthermore complement our studies with mRNA-Seq data from isolated Arabidopsis egg cells and provide data validation by promoter-reporter studies and whole mount in situ hybridization.Sequencing of small RNA libraries demonstrate the predominance of TE-derived siRNAs in the egg cell-related callus. TE-features and expression profiles suggest post-transcriptional silencing of activated Gypsy-like LTR retrotransposons, whereas the majority of class II DNA transposons belonging to Copia, CACTA, hAT-like and Mutator superfamilies are subjected to transcriptional silencing.Small RNA-seq furthermore led to the identification of differentially expressed known and novel miRNAs whose expression in the egg cell was verified by small RNA whole mount in situ hybridization. Both the strong expression of miRNAs in the egg-cell-adjoining synergids and the secretion of miRNAs into the micropyle suggest hitherto undescribed roles for these accessory cells in intercellular communication with the egg cell and the arriving pollen tube.In conclusion, our datasets provide valuable and comprehensive resources to study small RNA pathways and small-RNA-mediated epigenetic reprogramming during egg cell differentiation and the onset of plant embryogenesis.

Background: The formation of superheavy elements (SHEs) by fusion of two massive nuclei is severely inhibited by the competing quasifission process. Lowexcitation energies favor SHE survival agains ...

RNA Polymerase I (PolI)-mediated transcription of ribosomal RNAs is the limiting factor of ribosomal biogenesis, is often dysregulated in cancer 1 and is a new target for cancer therapy 2 , 3 . Decreased levels of PolI transcription is also associated with cell differentiation 4 suggesting that high levels of PolI transcription are required for stemness. Mammalian cells contain over 200 copies of the ribosomal genes (rDNA) clustered in specific chromosomal regions that aggregate in the nucleolus, where only a proportion acquires an active chromatic configuration allowing PolI transcription to occur. Malignant transformation is associated with an increase in the portion of active rDNA loci in the nucleolus. We showed that during malignant progression of B-cell lymphoma, the rDNA chromatin acquires an open state and rDNA-genome contacts are reorganized, which correlate with the repression of genes involved in B-cell differentiation 5 . These data support the intriguing hypothesis that dynamic changes in rDNA chromatin and long-range rDNA-genomic interactions regulate distinct programs of gene expression required for stemness, differentiation and malignant transformation. To address this hypothesis we are; (i) examining the molecular mechanisms by which nucleolar-associated chromatin domains dynamically regulate spatial organisation of the genome; (ii) testing whether the concomitant changes in distinct patterns of gene expression actively drive developmental programs such as differentiation and contribute to malignant transformations; and (iii) examining whether the therapeutic efficacy of Pol I inhibitors is mediated in part through the disruption of long-range rDNA genomic interaction.

The pervasive transcription of genomes into long noncoding RNAs has been amply demonstrated in recent years and garnered much attention. Similarly, emerging 'epitranscriptomics' research has shown that chemically modified nucleosides, thought to be largely the domain of tRNAs and other infrastructural RNAs, are far more widespread and can exert unexpected influence on RNA utilization. Both areas are characterized by the often-ephemeral nature of the subject matter in that few individual examples have been fully assessed for their molecular or cellular function, and effects might often be subtle and cumulative. Here we review available information at the intersection of these two exciting areas of biology, by focusing on four RNA modifications that have been mapped transcriptome-wide: 5-methylcytidine, N6-methyladenosine, pseudouridine as well as adenosine to inosine (A-to-I) editing, and their incidence and function in long noncoding RNAs. This article is part of a Special Issue entitled: Clues to long noncoding RNA taxonomy, edited by Dr. Tetsuro Hirose and Dr. Shinichi Nakagawa.

MotivationResearch in the emerging field of epitranscriptomics is increasingly generating comprehensive maps of chemical modifications in messenger RNAs (mRNAs). A computational framework allowing a reproducible and standardised analysis of these mRNA modification data is missing, but will be crucial for reliable functional meta-gene analyses and cross-study comparisons.ResultsWe have developed RNAModR, an open-source and R-based set of methods, to analyse and visualise the transcriptome-wide distribution of mRNA modifications. RNAModR allows the statistical evaluation of the mRNA modification site distribution relative to null sites on a meta-gene level, providing insight into the functional role of these mRNA modifications on e.g. mRNA structure and stability.Availability and implementationRNAModR is available under the GNU General Public License (GPL) as an R-package from https://github.com/mevers/RNAModR.Contactmaurits.evers@anu.edu.au

In this study, the presence of neutron transfer channels with positive Q values can enhance sub-barrier fusion cross sections. Recent measurements of the fusion excitation functions for 58Ni+132,124Sn found that the fusion enhancement due to the influence of neutron transfer is smaller than that in 40Ca +132,124Sn although the Q values for multineutron transfer are comparable. The purpose of this study is to investigate the differences observed between the fusion of Sn + Ni and Sn + Ca. Methods: Fusion excitation functions for 46,50Ti +124Sn have been measured at energies near the Coulomb barrier. As a result, a comparison of the barrier distributions for 46Ti+124Sn and 40Ca+124Sn shows that the 40Ca+124Sn system has a barrier strength resulting from the coupling to the very collective octupole state in 40Ca at an energy significantly lower than the uncoupled barrier. In conclusion, the large sub-barrier fusion enhancement in 40Ca induced reactions is attributed to both couplings to neutron transfer and inelastic excitation, with the octupole vibration of 40Ca playing a major role.

RNA binding proteins (RBPs) control the fate and expression of a transcriptome. Despite this fundamental importance, our understanding of plant RBPs is rudimentary, being mainly derived via bioinformatic extrapolation from other kingdoms. Here, we adapted the mRNA-protein interactome capture method to investigate the RNA binding proteome in planta. From Arabidopsis thaliana etiolated seedlings, we captured more than 700 proteins, including 300 with high confidence that we have defined as the At-RBP set. Approximately 75% of these At-RBPs are bioinformatically linked with RNA biology, containing a diversity of canonical RNA binding domains (RBDs). As no prior experimental RNA binding evidence exists for the majority of these proteins, their capture now authenticates them as RBPs. Moreover, we identified protein families harboring emerging and potentially novel RBDs, including WHIRLY, LIM, ALBA, DUF1296, and YTH domain-containing proteins, the latter being homologous to animal RNA methylation readers. Other At-RBP set proteins include major signaling proteins, cytoskeleton-associated proteins, membrane transporters, and enzymes, suggesting the scope and function of RNA-protein interactions within a plant cell is much broader than previously appreciated. Therefore, our foundation data set has provided an unbiased insight into the RNA binding proteome of plants, on which future investigations into plant RBPs can be based.

The authors acknowledge support from the Australian Research Council through Grants No. FL110100098, No. FT120100760, No. DP130101569, No. DP140101337, and No. DE140100784. Support for accelerator operations through the NCRIS program is acknowledged.

The SOLEROO ( sol enoidal e xotic r are is o t o pe) separator is under development for the production of light radioactive ion beams at the Australian National University. The separator is based on a single superconducting solenoid followed by two tracking detectors. Cross-section and beam purity measurements have been made to optimise the production of 6 He, 8 Li, 10 Be and 12 B beams. Having validated the trajectory simulation code SoliX, the optimal geometry for utilisation of the tracking detectors has also been explored.

A low-mass radioactive ion beam capability in Australia has been developed using a 6.5 T superconducting solenoid as the separator element. The separator, called SOLEROO , separates the large background of primary-beam particles from the radioactive species of interest. A further rejection of remnant unwanted nuclear species leaving the solenoid is achieved by tracking each emerging particle and identifying them event-by-event using a pair of position sensitive parallel plate avalanche counters. With primary 7 Li beam current of 1 e μ A , a 6 He production rate of 1.2 × 10 5 s − 1 has been achieved. The tagged secondary beam will be combined with a high efficiency 512 pixel silicon detector array for nuclear experiments.

Using a back-angle detector array covering 117° to 167°, coincidence measurements of breakup fragments at sub-barrier energies have enabled the complete characterisation of the breakup processes in the reactions of 6,7 Li with 208 Pb. Those breakup processes fast enough ( ∼ 10 − 22 s ) to affect fusion are identified through the measured relative energy of the two breakup fragments. The majority of these prompt breakup events are triggered by transfer of a neutron from 6 Li, and of a proton to 7 Li. These mechanisms, rather than breakup following direct projectile excitation, should thus be responsible for the majority of the ∼ 30 % suppression of complete fusion observed at above-barrier energies. Breakup characteristics thus depend both on the properties of the initial nucleus and its neighbours. Quantitative modelling of this two-step process will require development of a complete reactions model, relevant for reactions involving both α -cluster nuclei, and exotic nuclei near the neutron and proton drip-lines.

The successful coupled-channels model of peripheral reactions has been widely used to model fusion. Following measurements of inhibition of fusion at deep sub-barrier energies, it has been proposed that to properly describe the nuclear fusion process, quantum decoherence and energy dissipation should be treated simultaneously with coherent quantum superpositions, the latter being the basis of the coupled-channels model. The necessity for a simultaneous inclusion of quantum coherence and decoherence is discussed, and the first implementation through a density matrix model is described.

Objective : Previous studies have confirmed Slug as a key player in regulating phenotypic changes in several cell models, however, its role in smooth muscle cells (SMC) has never been assessed. The purpose of this study was to evaluate the expression of Slug during the phenotypic switch of SMC in vitro and throughout the development of vascular remodeling. Methods and Results : Slug expression was decreased during both cell-to-cell contact and TGFβ1 induced SMC differentiation. Tumor necrosis factor-α (TNFα), a known inductor of a proliferative/dedifferentiated SMC phenotype, induces the expression of Slug in SMC. Slug knockdown blocked TNFα-induced SMC phenotypic change and significantly reduced both SMC proliferation and migration, while its overexpression blocked the TGFβ1-induced SMC differentiation and induced proliferation and migration. Genome-wide transcriptomic analysis showed that in SMC, Slug knockdown induced changes mainly in genes related to proliferation and migration, indicating that Slug controls these processes in SMC. Notably, Slug expression was significantly up-regulated in lungs of mice using a model of pulmonary hypertension-related vascular remodeling. Highly remodeled human pulmonary arteries also showed an increase of Slug expression compared to less remodeled arteries. Conclusions : Slug emerges as a key transcription factor driving SMC towards a proliferative phenotype. The increased Slug expression observed in vivo in highly remodeled arteries of mice and human suggests a role of Slug in the pathogenesis of pulmonary vascular diseases. Fil: Coll Bonfill, Núria. Universidad de Barcelona; España Fil: Peinado, Victor I.. Universidad de Barcelona; España. CIBER Enfermedades Respiratorias; España Fil: Pisano, María Victoria. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Investigación Médica Mercedes y Martín Ferreyra. Universidad Nacional de Córdoba. Instituto de Investigación Médica Mercedes y Martín Ferreyra; Argentina Fil: Párrizas, Marcelina. CIBER Diabetes y Enfermedades Metabólicas Asociadas; España Fil: Blanco, Isabel. Universidad de Barcelona; España Fil: Evers, Maurits. Universitat Regensburg; Alemania Fil: Engelmann, Julia C.. Universitat Regensburg; Alemania Fil: García Lucio, Jessica. Universidad de Barcelona; España Fil: Tura Ceide, Olga. CIBER Enfermedades Respiratorias; España Fil: Meister, Gunter. Universitat Regensburg; Alemania Fil: Barberà, Joan Albert. Universidad de Barcelona; España Fil: Musri, Melina Mara. Universidad de Barcelona; España. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Investigación Médica Mercedes y Martín Ferreyra. Universidad Nacional de Córdoba. Instituto de Investigación Médica Mercedes y Martín Ferreyra; Argentina

Background: The fusion-evaporation reaction at energies around the Coulomb barrier is presently the only way to produce the heaviest elements. However, formation of evaporation residues is strongly hindered due to the competing fusion-fission and quasifission processes. Presently, a full understanding of these processes and their relationships has not been reached.Purpose: This work aims to use new fission measurements and existing evaporation residue and fission excitation function data for reactions forming Cf isotopes to investigate the dependence of the quasifission probability and characteristics on the identities of the two colliding nuclei in heavy element formation reactions.Method: Using the Australian National University's 14UD electrostatic accelerator and CUBE detector array, fission fragments from the ${}^{12}\mathrm{C}+{}^{235}\mathrm{U}$, ${}^{34}\mathrm{S}+{}^{208}\mathrm{Pb}$, ${}^{36}\mathrm{S}+{}^{206}\mathrm{Pb}$, ${}^{36}\mathrm{S}+{}^{208}\mathrm{Pb}$, and ${}^{44}\mathrm{Ca}+{}^{198}\mathrm{Pt}$ reactions were measured. Mass and angle distributions of fission fragments were extracted and compared to investigate the presence and characteristics of quasifission.Results: Mass-angle-correlated fission fragments were observed for the ${}^{44}\mathrm{Ca}+{}^{198}\mathrm{Pt}$ reaction; no correlation was observed in the other reactions measured. Flat-topped fission-fragment mass distributions were observed for ${}^{12}\mathrm{C}+{}^{235}\mathrm{U}$ at compound-nucleus excitation energies from 28 to 52 MeV. Less pronounced flat-topped distributions were observed, with very similar shapes, for all three sulfur-induced reactions at excitation energies lower than 45 MeV.Conclusions: A high probability of long-time-scale quasifission seems necessary to explain both the fission and evaporation residue data for the ${}^{34}\mathrm{S}+{}^{208}\mathrm{Pb}$ and ${}^{36}\mathrm{S}+{}^{206}\mathrm{Pb}$ reactions. Flat-topped mass distributions observed for ${}^{12}\mathrm{C}$- and ${}^{34,36}\mathrm{S}$-induced reactions are suggested to originate both from late-chance fusion-fission at low excitation energies and the persistence of shell effects at the higher energies associated with quasifission.

Deep inelastic collisions (DICs) can compete strongly with fusion in collisions of heavy nuclei. However, standard coupled-channels calculations do not take DIC processes into account. As a result, calculations have been shown to overestimate the fusion cross-sections, resulting in a discrepancy between experimental data and theoretical calculations, particularly at energies above the fusion barrier. To investigate this discrepancy, we conducted a series of experiments using the ANU 14UD tandem accelerator and the CUBE 2-body fission spectrometer to examine the competition between transfer/DIC and fusion. In particular, fusion-fission and 3-body fission yields have been extracted for 34 S + 232 Th and 40 Ca + 232 Th systems. This work shows that the transfer-fission probability is enhanced relative to fusion-fission for 40 Ca + 232 Th, when compared to 34 S+ 232 Th. It is suggested that the enhancement of this DIC process in 40 Ca + 232 Th is linked to an increase in the density overlap of the colliding nuclei as a function of the charge product and contributes to fusion hindrance.

We examined the 40 Ca + 238 U reaction experimentally using the Mass-Angle Distribution (MAD) technique and within the Time Dependent Hartree Fock (TDHF) theory, using the TDHF3D code. A new, TDHF based approach has been developed to construct mass distributions as well. The results revealed that the orientation of the heavy deformed prolate nucleus plays a major role in the reaction outcome. It was found that aligned collisions lead to quasifission and short contact times of 5-10 zs, whilst anti-aligned collisions lead to longer contact times (> 23 zs). TDHF accurately predicted the presence of quasifission and the average mass splits in this reaction. The influence of shell effects around 208 Pb in the calculated quasifission characteristics was observed in both experiment and theory.

The quasi-fission mechanism hinders fusion in heavy systems through breakup within zeptoseconds into two fragments with partial mass equilibration. Its dependence on the structure of both the collision partners and the final fragments is a key question. Our original approach is to combine an experimental measurement of the fragments' mass-angle correlations in $^{40}$Ca$+^{238}$U with microscopic quantum calculations. We demonstrate an unexpected interplay between the orientation of the prolate deformed $^{238}$U with quantum shell effects in the fragments. In particular, calculations show that only collisions with the tip of $^{238}$U produce quasi-fission fragments in the magic $Z=82$ region, whilst collisions with the side are the only one which may result in fusion., 5 pages, 4 figures; Accepted for publication in Physical Review Letters

The superheavy element with atomic number Z=117 was produced as an evaporation residue in the 48Ca+249Bk fusion reaction at the gas-filled recoil separator TASCA at GSI Darmstadt, Germany. The radioactive decay of evaporation residues and their α-decay products was studied using a detection setup that allowed measuring decays of single atomic nuclei with half-lives between sub-μs and a few days. Two decay chains comprising seven α decays and a spontaneous fission each were identified and are assigned to the isotope 294-117 and its decay products. A hitherto unknown α-decay branch in 270Db (Z=105) was observed, which populated the new isotope 266Lr (Z=103). The identification of the long-lived (T1/2=1.0+1.9−0.4 h) α-emitter 270Db marks an important step towards the observation of even more long-lived nuclei of superheavy elements located on an “island of stability.” (Less)

This contribution presents an overview of the research program in nuclear reaction dynamics at the Australian National University.

The formation of a compound nucleus in different projectile and target combinations is a powerful method for investigating the fusion process. Recently, the dominance of quasi-fission over fusion-fission has been inferred for 34 S+208 Pb in comparison to 36 S+206 Pb; both reactions lead to the compound nucleus 242 Cf* .The mass and angle distributions of the fission fragments from these reactions were studied in order to further investigate the presence of quasi-fission.

Background: The formation of superheavy elements by fusion of two massive nuclei is severely inhibited by the competing quasifission process.Purpose: Through extensive mass-angle distribution measurements, we map out the systematic dependence of quasifission characteristics as a function of the identity of the colliding nuclei.Methods: The Australian National University's Heavy Ion Accelerator Facility and CUBE spectrometer have been used to measure mass-angle distributions for 42 reactions forming heavy elements. Beam energies above their respective capture barriers were used to minimize the known influence of nuclear structure effects.Results: Different mappings of mass-angle distribution characteristics (including timescales) over the nuclear landscape show a systematic dependence on entrance channel and compound nucleus fissilities.Conclusions: The results provide an empirical baseline to assess effects of nuclear structure at lower beam energies, and motivate the testing and validation of complete dynamical models of heavy element fusion through comparison of mass-angle distributions.

Background: Quasifission, a fission-like reaction outcome in which no compound nucleus forms, is an important competitor to fusion in reactions leading to superheavy elements. The precise mechanisms driving the competition between quasifission and fusion are not well understood.Purpose: To understand the influence reaction parameters have on quasifission probabilities, an investigation into the evolution of quasifission signatures as a function of entrance channel parameters is required.Methods: Using the Australian National University's (ANU) CUBE detector for two-body fission studies, measurements were made for a wide range of reactions forming isotopes of curium. Important quasifission signatures---namely, mass-ratio spectra, mass-angle distributions, and angular anisotropies---were extracted.Results: Evidence of quasifission was observed in all reactions, even for those using the lightest projectile (${}^{12}\text{C}+{}^{232}\text{Th}$). But the observables showing evidence of quasifission were not the same for all reactions. In all cases, mass distributions provided some evidence of the possible presence of quasifission but were not sufficient in most cases to clearly identify reactions for which quasifission was important. For reactions using light projectiles (${}^{12}$C, ${}^{28,30}$Si, ${}^{32}$S), experimental angular anisotropies provided the clearest signature of quasifission. For reactions using heavier projectiles (${}^{48}$Ti, ${}^{64}$Ni), the presence of mass-angle correlations in the mass-angle distributions provided strong evidence of quasifission and also provided information about quasifission timescales.Conclusions: The observable offering the clearest signature of quasifission differs depending on the reaction timescale.

Understanding the mechanisms leading to the inhibition of fusion seen at suband above-barrier energies has been one of the prime questions in nuclear physics. The complementary process to fusion is scattering, and an analysis of the scattered flux in heavy-ion collisions may give insight into answering this question. An analysis of the projectile-like fragments detected at backward angles in the reactions 16O+208Pb and 32S+208Pb at energies below the fusion barrier is presented. Excitation functions corresponding to nucleon transfer with ∆Z = 1 and ∆Z = 2 were extracted, indicating surprisingly large absolute transfer probabilities already at sub-barrier energies. Excitation energies in the projectile-like fragments up to ∼ 15MeV for the 16O and ∼ 25MeV for 32S-induced reactions show the population of highly excited states in the residual nuclei, and indicate substantial dissipation of kinetic energy into nucleonic degrees of freedom. A new phenomenological framework explores the effect of energy dissipation through these highly inelastic (large excitation energies) and complex (few-nucleon transfer) processes on the probability for fusion in the reaction 16O+208Pb. Calculations combining both couplings to low-lying excited states within the coherent coupled channels framework and energy dissipation show that the suppression of fusion at energies above the barrier in this reaction can be explained within the new framework. Results furthermore suggest that in reactions relevant to astrophysical scenarios such as 12C+12C, fusion hindrance may not be present.

A detailed analysis of the projectile-like fragments detected at backward angles in the reactions 16 O, 32 S+ 208 Pb at energies below the fusion barrier is presented. Excitation functions corresponding to nucleon transfer with Δ Z = 1 and Δ Z = 2 were extracted, indicating surprisingly large absolute probabilities at sub-barrier energies. A comparison of 2 p transfer probabilities with time-dependent Hartree-Fock calculations suggests strong pairing correlations between the two protons. Excitation energies in the projectile-like fragments up to ~ 15MeV for the 16 O and ~ 25MeV for 32 S-induced reactions demonstrate the population of highly excited states in the residual nuclei, indicating substantial dissipation of kinetic energy. These highly inelastic (large excitation energies) and complex (correlated few-nucleon transfer) processes may be closely related to the depletion of fusion through tunnelling at sub-barrier energies.

The mass distributions of the fission fragments were measured in reactions induced by ${}^{48}$Ti beams on ${}^{144,154}$Sm, ${}^{162}$Dy, ${}^{174}$Yb, ${}^{186}$W, ${}^{192}$Os, ${}^{196}$Pt, ${}^{200}$Hg, and ${}^{208}$Pb targets at near-barrier energies. The systematic tendencies of the mass widths have been explored. For the reactions with the lighter or spherical target nuclei, the mass widths increase with increasing bombarding energy, a behavior expected for fusion-fission. For the heavier and statically deformed nuclei, the mass widths increase rapidly as energies fall close to and below the capture barriers. Well above the barrier energies, the mass widths and estimated minimum quasifission contributions increase smoothly with increasing fissility of the composite nucleus. The observed behavior is a complex function of the fissility, deformation, and energy with respect to the capture barrier. Comparisons with dynamical quasifission model calculations for these reactions will be a good test of models of fusion in heavy-element formation reactions.

Coincidence measurements of breakup of 6,7Li and 9Be incident on various high-Z targets have provided a complete picture of the physical mechanisms triggering breakup. The measurements identify those breakup processes that are fast enough (10−22) to affect fusion. The majority of these prompt breakup events are triggered by transfer of a neutron from 6Li, 9Be, and of a proton to 7Li. The dominance of breakup following transfer demonstrates that the reaction outcomes involving weakly bound nuclei depend not only on the properties of the two colliding nuclei, but also on the ground-state and excited state properties of their neighbours. This is a key insight for understanding and predicting reactions of weakly-bound nuclei near the limits of nuclear existence.

The effects of nuclear structure can be manifested at three stages of the reaction dynamics: through the effect of channel couplings in overcoming the entrance channel potential barrier; in damping of the relative motion leading to capture; and in diffusion over the potential energy surface resulting in fusion or quasifission outcomes. This sequence of coupled processes offers many and subtle opportunities for nuclear structure to play a role in affecting the dynamics, and thus the outcomes. Recent experimental results, in particular mass-angle distributions (MAD), have isolated different ways in which nuclear structure has been shown to play a significant role in determining reaction outcomes.

The role of spherical quantum shells in the competition between fusion and quasi-fission is studied for reactions forming heavy elements. Measurements of fission fragment mass distributions for different reactions leading to similar compound nuclei have been made near the fusion barrier. In general, more quasi-fission is observed for reactions with non-magic nuclei. However, the $^{40}$Ca+$^{208}$Pb reaction is an exception, showing strong evidence for quasi-fission, though both nuclei are doubly magic. Time-dependent Hartree-Fock calculations predict fast equilibration of $N/Z$ in the two fragments early in the collision. This transfer of nucleons breaks the shell effect, causing this reaction to behave more like a non-magic one in the competition between fusion and quasi-fission. Future measurements of fission in reactions with exotic beams should be able to test this idea with larger $N/Z$ asymmetries., accepted for publication in Physics Letters B

The reaction ${}^{16}$O $+$ ${}^{208}$Pb is a benchmark in nuclear reaction studies as it involves two doubly magic nuclei. New measurements of back-scattered projectile-like fragments at sub-barrier energies show that the probability of two-proton ($2p$) transfer is much larger than that of $\ensuremath{\alpha}$-particle transfer. At energies around the fusion barrier the probability for $2p$ transfer is $\ensuremath{\sim}10%$, similar to that for one-proton transfer. The $2p$ transfer probabilities are enhanced by up to an order of magnitude compared to calculations based on an independent particle picture as simulated by the fully microscopic time-dependent Hartree-Fock model (TDHF). Since beyond mean-field correlations like nucleon pairing are not included in the TDHF model, the enhancement indicates strong pairing correlations between the transferred protons. $2p$ transfer leads to excitation energies (most likely in the target-like nucleus) up to $\ensuremath{\sim}13$ MeV, indicating that it may represent an effective doorway for the dissipation of energy and thus provide a microscopic mechanism toward understanding the inhibition of fusion and energies both above and below the barrier.

A detailed analysis of the projectile-like fragments detected at backward angles in the reactions 16 O,32 S+208 Pb at energies below the fusion barrier is presented. Excitation functions corresponding to nucleon transfer with ∆Z = 1 and ∆Z = 2 were extracted, indicating surprisingly large absolute probabilities at subbarrier energies. A comparison of 2p transfer probabilities with time-dependent Hartree-Fock calculations suggests strong pairing correlations between the two protons. Excitation energies in the projectile-like fragments ~15 MeV and ~25 MeV for the 16 O and 32 S-induced reactions, respectively, indicate the population of highly excited states in the residual nuclei. A comparison with expected optimum Q -values suggests large losses in kinetic energy of the projectile-like fragments. These highly inelastic (large excitation energies) and complex (correlated few-nucleon transfer) processes may be closely related to the depletion of fusion through tunnelling at sub-barrier energies.

Below-barrier no-capture breakup measurements of the weakly bound $^{9}\mathrm{Be}$ nucleus, incident on targets ranging in atomic number from 62 to 83, have been carried out using a large-area high-resolution back-angle detector array. It is shown that the three-body reconstructed reaction $Q$-value and relative energy of the breakup fragments together reveal the full dynamics of the breakup mechanism, identifying all physical processes that lead to the breakup of the projectile-like nucleus. Contrasting with the simple expectation of direct breakup into the most energetically favored clusters, the data show that breakup following $n$-transfer dominates the total breakup yield. Breakup from long-lived states in the projectile-like nucleus, which on the reaction time scale may be considered stable, has been isolated from the prompt breakup yield. It has been shown that the prompt breakup probability essentially depends on the surface separation of the interacting nuclei. The measured prompt breakup probability functions for each target have been used together with a classical trajectory model to predict the above-barrier suppression of complete fusion. The suppression factor, expressed as the fraction of incomplete fusion, is nearly independent of target charge.

The sub-barrier excitation function of the octupole vibrational state at 2.615 MeV in {sup 208}Pb is analyzed within the coupled-channels framework. It is shown that the position of the minimum in the excitation function, which is due to the destructive interference of the Coulomb and nuclear scattering amplitudes, is very sensitive to the nuclear potential for both the ground state and the octupole state. A different nuclear potential for the 3{sup -} state may arise from changes in the matter distribution of {sup 208}Pb due to the particle-hole excitations. The present analysis places a strong limit on the 3{sup -} nuclear potential diffuseness, giving a difference of only {Delta}a{sub 0}=0.011+-0.004 fm between the ground state and 3{sup -} state diffuseness.

High precision measurements for back-angle quasi-elastic scattering of $^{16}\mathrm{O}$ from various target nuclei have been made, and are compared with coupled channels calculations to determine the diffuseness of the real part of the Woods-Saxon nuclear potential. The extracted diffuseness parameters are in the range of $0.60$ to $0.69$ fm, and agree well with previous results using a $^{32}\mathrm{S}$ projectile. The measured quasi-elastic energy spectra close to the barrier show both discrete peaks and a broad continuous structure with $Q$-values down to $\ensuremath{-}25$ MeV. These suggest the inadequacy of coupled channels calculations that include couplings only to low energy, discrete states of the colliding nuclei.

The formation of new, superheavy elements [1] requires the fusion of two massive nuclei. After contact of the two colliding nuclei, the dinuclear system is pictured as moving in deformation space over a potential energy surface (PES). The system may reach compact (mononuclear) shapes inside the fission barrier, and thus have the possibility of forming a heavy element in its ground state. Alternatively, before reaching a compact shape, the system may reseparate into two heavy fragments, intermediate in mass between the projectile and target nuclei. This is called quasifission [2,3]. The probability of fusion (forming a new nucleus with a compact near-equilibrium shape) is determined by many variables. Those believed to play a major role include the mass-asymmetry of the two colliding nuclei, the charge of the heavy element being formed, and the neutron and proton shell structure encountered during the fusion process [4]. The detailed role and significance of shell structure is the most complex and open question. Shell effects after contact can be categorized as dynamic or static. Nonequilibrium dynamic effects, associated with the diabatic motion of the nucleons at avoided crossings of their molecular orbitals, may effectively result in timedependent shell structures [5] that can influence the early collective motion of the dinuclear system. After thermal equilibration, the well-known static shell structures affect the PES, and also the inertia and the strength of energy dissipation (and thus the magnitude of shape fluctuations), which together determine the motion over the PES. In spontaneous and low energy fission of heavy nuclei, the role of (static) shell structures is well known. Fission mass yields and total kinetic energy distributions, and the existence and properties of fission isomers, can be understood as resulting from local structure in the PES caused by energetically favorable shell structures in the nascent fragments, both for spherical and deformed shapes [6]. It is expected (and observed) that as the excitation energy Ex increases, these shell effects are exponentially attenuated. This strong dependence on Ex may be a key to isolating their role in quasifission dynamics. However, there are uncertainties regarding the effective energy scale of the attenuation [7], which is critical to predict the influence of shell structure on quasifission, where Ex is often >20 MeV. In collisions of heavy nuclei, a further complication often arises, resulting from nuclear structure effects in the entrance channel. The heavy reaction partner may have a large static deformation. This is caused by unfavored shell effects for a spherical shape, compared with a gain in energy (shell stability) for a deformed shape. This deformation affects the entry point into the PES. For nuclear collisions involving prolate nuclei, when the deformation axis is aligned with the projectile nucleus, the dinucleus is very elongated at contact, whereas it is more compact if antialigned. In experiments, by choice of the bombarding energy, only the aligned orientation (low beam energy) or all orientations (higher energy) can be selected [8–10]. Measurements have consistently shown that the lower energies are associated with the largest deviation of fission properties from the expectations for fusion-fission [10 –14], as well as showing suppression of the yield of heavy elements following capture [15,16]. Both observables should be associated with an increased probability of quasifission [10,11].

Mass-angle correlations of binary fragments produced in $^{16}\mathrm{O}+^{204}\mathrm{Pb}$, $^{34}\mathrm{S}+^{186}\mathrm{W}$, and $^{48,50}\mathrm{Ti}+^{166,170}\mathrm{Er}$ reactions have been measured for a range of bombarding energies around their Coulomb barriers. At above-barrier energies, the width of the mass distributions for the fission-like fragments in the $^{50}\mathrm{Ti}+^{170}\mathrm{Er}$ reaction are found to be higher than those from the same compound system at similar excitation energies populated via the more mass asymmetric entrance channel reaction $^{34}\mathrm{S}+^{186}\mathrm{W}$, which in turn is higher than those for the $^{16}\mathrm{O}+^{204}\mathrm{Pb}$ system. The width of the mass distributions of the $\mathrm{Ti}+\mathrm{Er}$ systems is found to increase with decreasing bombarding energies, in contrast with those of the $^{16}\mathrm{O}+^{204}\mathrm{Pb}$ and $^{34}\mathrm{S}+^{186}\mathrm{W}$ systems, which show a monotonic reduction in mass widths. This may be associated with the elongated contact configuration at sub-barrier energies.