• Volume 75, Issue 1

July 2010,   pages  1-392a

• Foreword

• Gamma and electron spectroscopy of transfermium isotopes at Dubna: Results and plans

Detailed spectroscopic information of excited nuclear states in deformed transfermium nuclei is scarce. Most of the information available today has been obtained from investigations of fine-structure 𝛼-decay. Although 𝛼 decay gives access to hindrance factors and lifetimes which are strongly correlated to shell/subshell closures and the presence of isomers, only the combined use of 𝛾 and conversion electron spectroscopy allows the precise determination of excitation energy, spin and parity of nuclear levels.

In the years 2004–2009 using the GABRIELA set-up [Hauschild et al, Nucl. Instrum. Methods A560, 388 (2006)] at the focal plane of VASSILISSA separator [Malyshev et al, Nucl. Instrum. Methods A440, 86 (2000); A516, 529 (2004)] experiments with the aim of 𝛾 and electron spectroscopy of the isotopes from Fm to Lr, formed by complete fusion reactions with accelerated heavy ions were performed. In the following, the pre- liminary results of decay studies using $\alpha-\gamma$ and $\alpha-\beta$ coincidences at the focal plane of the VASSILISSA recoil separator are presented.

Accumulated experience allowed us to perform ion optical calculations and to design the new experimental set-up, which will collect the base and best parameters of the existing separators and complex detector systems used at the focal planes of these installations. In the near future it is planned to study neutron-rich isotopes of the Rf–Sg in the ‘hot’ fusion reactions with 22Ne incident projectiles and 242Pu, 243Am and 248Cm targets.

• Nuclei in the vicinity of `island of inversion' through the fusion reaction

The level structures of $N \sim 19$ nuclei such as 32,34P have been investigated using the 18O(18O,xnyp) and 18O(16O,xnyp) reactions at an incident beam energy of about 34 MeV. The de-exciting 𝛾-transitions were recorded using an array of clover detectors. These detectors have the dual advantage of higher efficiency at $E_{\gamma} \geq 2$ MeV, and are capable of providing information on the linear polarization of the observed 𝛾-transitions. These polarization measurements when coupled with the angular correlations help us to assign uniquely the spin parity for the observed levels. The experimental results have been compared with the predictions of the spherical shell model. The shell-model calculations are able to reproduce the observed energy levels to a reasonable degree. However, the observed transition probabilities are not reproduced by the calculations. Hence there is a need to re-visit these calculations using more detailed and microscopic effective interactions.

• Structure of $A \sim130$ nuclei in La–Ce region

The variety of shapes and structures, observed in light rare earth $A \sim 130$ nuclei, have been discussed in view of different angular momentum coupling schemes and their interplay that comes into effect at high spin. The $N = 79$ and 80 isotopes in La–Ce region, produced via fusion evaporation reaction, have been studied using the Indian National Gamma Array (INGA) consisting of 18 clover HPGe detectors. Two nearly degenerate $\Delta I = 1$ bands have been observed at high spin of 137Ce and a triaxial deformation of $\gamma = \pm 30^{\circ}$ has been assigned to the bands, from the total Routhian surface (TRS) calculations. The high-spin candidates of the yrast band of 138Ce show signature splitting both in energy and $B(M1)/B(E2)$ values. The bandcrossing due to the alignment of a pair of $h_{11}/2$ proton particles has been conjectured at $\hbar ω \sim 0.3$ MeV, from the single-particle Routhians obtained from TRS calculations. Lifetime measurements by Doppler shift attenuation method (DSAM) has been carried out and from the estimated reduced transition probability $B(M 1)$, the $\Delta I = 1$ band in 138Ce has been characterized as a magnetic rotation (MR) band. The rise in the values of $B(M1)$, for the higher spin candidates of the band, has been conjectured as the reopening of a different shear at the top of the Band B1. The characteristic of the MR bands in $A \sim 130$ region has been discussed in the light of a phenomenological calculation and compared to the MR bands in other mass regions.

• Ab-initio Hamiltonian approach to light nuclei and to quantum field theory

Nuclear structure physics is on the threshold of confronting several long-standing problems such as the origin of shell structure from basic nucleon–nucleon and three-nucleon interactions. At the same time those interactions are being developed with increasing contact to QCD, the underlying theory of the strong interactions, using effective field theory. The motivation is clear – QCD offers the promise of great predictive power spanning phenomena on multiple scales from quarks and gluons to nuclear structure. However, new tools that involve non-perturbative methods are required to build bridges from one scale to the next. We present an overview of recent theoretical and computational progress with a Hamiltonian approach to build these bridges and provide illustrative results for the nuclear structure of light nuclei and quantum field theory.

• Magnetic rotation – past, present and future

Magnetic-dipole rotational (MR) bands were discovered about 15 years ago without any theoretical prediction in contrast to the super-deformed (SD) bands which were predicted long ago. First identification of a quasirotational structure as MR band occurred around 1992 although Kr isotopes probably have the first set of data having the signatures of MR bands as shown by us. Our first compilation of MR bands listed 120 MR bands in 56 nuclides which have now grown to more than 180 bands in 80 nuclides. We have observed new MR bands in the $A = 130$ mass region in 137Pr,139Nd and 135Ba nuclei. This led to the observation of the smallest MR bands in 137Pr, multiple minima in the 𝛾 deformation in 135Ba, coexistence of band structure based on these minima and band crossing of MR bands in $A = 130$ region. Some of these results have been reviewed in this paper along with theoretical calculations. There are still a number of questions related to MR bands which have not been fully resolved. The role of neutrons/protons in magnetic rotation still needs to be delineated. Do the MR bands follow the $I(I + 1)$ behaviour? Are these structures as regular as normal rotational bands? How important is the existence of deformation for MR bands? We address some of these questions in this paper.

• Pygmy dipole resonance in stable nuclei

Two examples of recent work on the structure of low-energy electric dipole modes are presented. The first part discusses the systematics of the pygmy dipole resonance (PDR) in stable tin isotopes deduced from high-resolution ($\gamma, \gamma'$) experiments. These help to distinguish between microscopic QRPA calculations based on either a relativistic or a non-relativistic mean-field description, predicting significantly different properties of the PDR. The second part presents a novel approach to measure the complete electric dipole strength distribution from excitation energies starting at about 5 MeV across the giant dipole resonance (GDR) with high-resolution inelastic proton scattering under 0° at energies of a few 100 MeV/nucleon. The case of 208Pb is discussed in detail and first result from a recent experiment on 120Sn is presented.

• Excitation of pygmy dipole resonance in neutron-rich nuclei via Coulomb and nuclear fields

We study the nature of the low-lying dipole strength in neutron-rich nuclei, often associated with the pygmy dipole resonance. The states are described within the Hartree–Fock plus RPA formalism, using different parametrizations of the Skyrme inter-action. We show how the information from combined reaction processes involving the Coulomb and different mixtures of isoscalar and isovector nuclear interactions can provide a clue to reveal the characteristic features of these states.

• Efimov effect in 2-neutron halo nuclei

This paper presents an overview of our theoretical investigations in search of Efimov states in light 2-neutron halo nuclei. The calculations have been carried out within a three-body formalism, assuming a compact core and two valence neutrons forming the halo. The calculations provide strong evidence for the occurrence of at least two Efimov states in 20C nucleus. These excited states move into the continuum as the two-body (core-neutron) binding energy is increased and show up as asymmetric resonances in the elastic scattering cross-section of the n- 19C system. The Fano mechanism is invoked to explain the asymmetry. The calculations have been extended to 38Mg, 32Ne and a hypothetical case of a very heavy core ($A = 100$) with two valence neutrons. In all these cases the Efimov states show up as resonances as the two-body energy is increased. However, in sharp contrast, the Efimov states, for a system of three equal masses, show up as virtual states beyond a certain value of the two-body interaction.

• Heavy ion transfer reactions: Status and perspectives

With the large solid angle magnetic spectrometer (PRISMA) coupled to the 𝛾-array (CLARA), extensive investigations of nuclear structure and reaction dynamics have been carried out. In the present paper aspects of these studies will be presented, focussing more closely on the reaction mechanism, in particular on the properties of quasielastic and deep inelastic processes and on measurements at energies far below the Coulomb barrier.

• Influence of projectile breakup on complete fusion

Complete fusion excitation functions for 11,10B+159Tb and 6,7Li+159Tb have been reported at energies around the respective Coulomb barriers. The measurements show significant suppression of complete fusion cross-sections at energies above the barrier for 10B+159Tb and 6,7Li+159Tb reactions, when compared to those for 11B+159Tb. The comparison shows that the extent of suppression of complete fusion cross-sections is correlated with the 𝛼-separation energies of the projectiles. Also, the measured incomplete fusion cross-sections show that the 𝛼-particle emanating channel is the favoured incomplete fusion process. Inclusive measurement of the 𝛼-particles produced in 6Li+159Tb reaction has been carried out. Preliminary CDCC calculations carried out to estimate the 𝛼- yield following 6Li breaking up into $\alpha + d$ fail to explain the measured 𝛼-yield. Transfer processes seem to be important contributors.

• Fusion using radioactive ion beams

The capture-fission cross-section is measured for the collision of the massive nucleus 132Sn with 96Zr at near-barrier energies and compared with the collision of 124Sn with 96Zr. This study gives insight into fusion enhancement and hindrance in systems involving neutron-rich nuclei. The dinuclear system model (DNS) calculations describe the excitation function reasonably well and if we use the barrier heights predicted by this model we can conclude that fusion hindrance (represented by extra push energy) is greater for the more neutron-rich systems.

The fusion excitation function for 9Li+70Zn and 9Li+208Pb systems are measured for near-barrier energies using ISAC1 and ISAC2 Facilities at TRIUMF. The 𝛼-emitting evaporation residues (211−214At) are stopped in the 208Pb target and their decay is measured. The measured excitation function shows evidence for large enhancements in the sub-barrier energies, which is not accounted by current theoretical models. Suppression of the above-barrier cross-section with respect to these theoretical models are also seen.

• Unusual features of proton and 𝛼-spectra from low-energy heavy-ion reactions

Proton and 𝛼-particle spectra have been measured in low-energy 12C and 16O-induced reactions on Nb and Y targets with the primary aim of measuring the excitation energy and angular momentum dependence of nuclear level density. In the 𝛾-multiplicity gated spectra, an unusual feature of a broad structure at high particle energies is observed in all the cases. In the case of proton spectra, the structures have compound nuclear origin and point towards an excitation energy and angular momentum-dependent enhancement which is beyond the conventional level density prescription. The broad structures in the 𝛼-spectra cannot be fully explained within the statistical model even with the enhanced level density. In this case, other reaction mechanisms like the transfer of 𝛼 or 8Be to the target could also be important.

• Progress in all-order breakup reaction theories

Progress in breakup reaction theories, like the distorted wave Born approximation, the continuum discretized coupled channels method and the dynamical eikonal approximation, is brought into focus. The need to calculate exclusive reaction observables and the utility of benchmark tests as arbitrators of theoretical models are discussed.

• Low-energy nuclear reactions with double-solenoid- based radioactive nuclear beam

The University of Notre Dame, USA (Becchetti et al, Nucl. Instrum. Methods Res. A505, 377 (2003)) and later the University of São Paulo, Brazil (Lichtenthaler et al, Eur. Phys. J. A25, S-01, 733 (2005)) adopted a system based on superconducting solenoids to produce low-energy radioactive nuclear beams. In these systems the solenoids act as thick lenses to collect, select, and focus the secondary beam into a scattering chamber. Many experiments with radioactive light particle beams (RNB) such as 6He, 7Be, 8Li, 8B have been performed at these two facilities. These low-energy RNB have been used to investigate low-energy reactions such as elastic scattering, transfer and breakup, providing useful information on the structure of light nuclei near the drip line and on astrophysics. Total reaction cross-sections, derived from elastic scattering analysis, have also been investigated for light system as a function of energy and the role of breakup of weakly bound or exotic nuclei is discussed.

• Testing fundamental symmetries using radioactive ion beams at TRIUMF-ISAC

The ISAC Facility at TRIUMF, Canada’s national laboratory for particle and nuclear physics, provides rare isotope beams for a diverse research program. In this paper we summarize some recent experimental developments at TRIUMF pertaining to fundamental symmetry tests. These tests use the atomic nucleus as a probe to search for physics beyond the Standard Model. Some recent results and future plans are discussed.

• 𝛽-Decay and the electric dipole moment: Searches for time-reversal violation in radioactive nuclei and atoms

One of the greatest successes of the Standard Model of particle physics is the explanation of time-reversal violation (TRV) in heavy mesons. It also implies that TRV is immeasurably small in normal nuclear matter. However, unifying models beyond the Standard Model predict TRV to be within reach of measurement in nuclei and atoms, thus opening an important window to search for new physics. We will discuss two complementary experiments sensitive to TRV: Correlations in the 𝛽-decay of 21Na and the search for an electric dipole moment (EDM) in radium.

• List of participants

• # Pramana – Journal of Physics

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