Lifetime measurement of the 2(1)+ state in 20C

Establishing how and when large N/Z values require modified or new theoretical tools is a major quest in nuclear physics. Here we report the first measurement of the lifetime of the 2(1)+ state in the near-dripline nucleus 20C. The deduced value of τ(2(1)+)=9.8±2.8(stat)(-1.1)(+0.5)(syst)  ps gives a reduced transition probability of B(E2; 2(1)+→0(g.s.)+)=7.5(-1.7)(+3.0)(stat)(-0.4)(+1.0)(syst)  e2  fm4 in good agreement with a shell model calculation using isospin-dependent effective charges.

Knockout reactions from p-shell nuclei: tests of ab initio structure models

Absolute cross sections have been determined following single neutron knockout reactions from 10Be and 10C at intermediate energy. Nucleon density distributions and bound-state wave function overlaps obtained from both variational Monte Carlo (VMC) and no core shell model (NCSM) ab initio calculations have been incorporated into the theoretical description of knockout reactions. Comparison to experimental cross sections demonstrates that the VMC approach, with the inclusion of 3-body forces, provides the best overall agreement while the NCSM and conventional shell-model calculations both overpredict the cross sections by 20% to 30% for 10Be and by 40% to 50% for 10C, respectively. This study gains new insight into the importance of 3-body forces and continuum effects in light nuclei and provides a sensitive technique to assess the accuracy of ab initio calculations for describing these effects.

Enhanced quadrupole collectivity at N = 40: the case of neutron-rich Fe isotopes

The transition rates for the 2(1)+ states in (62,64,66)Fe were studied using the recoil distance Doppler-shift technique applied to projectile Coulomb excitation reactions. The deduced E2 strengths illustrate the enhanced collectivity of the neutron-rich Fe isotopes up to N = 40. The results are interpreted using the generalized concept of valence proton symmetry which describes the evolution of nuclear structure around N = 40 as governed by the number of valence protons with respect to Z ≈ 30. The trend of collectivity suggested by the experimental data is described by state-of-the-art shell-model calculations with a new effective interaction developed for the fpgd valence space.

¹⁵C(d,p)¹⁶C reaction and exotic behavior in ¹⁶C

We have studied the ¹⁵C(d,p)¹⁶C reaction in inverse kinematics using the Helical Orbit Spectrometer at Argonne National Laboratory. Prior studies of electromagnetic-transition rates in ¹⁶C suggested an exotic decoupling of the valence neutrons from the core in that nucleus. Neutron-adding spectroscopic factors give a different probe of the wave functions of the relevant states in ¹⁶C. Shell-model calculations reproduce both the present transfer data and the previously measured transition rates, suggesting that ¹⁶C may be described without invoking very exotic phenomena.

Nuclear spins and moments of Ga isotopes reveal sudden structural changes between N=40 and N=50

Collinear laser spectroscopy was performed on Ga (Z=31) isotopes at ISOLDE, CERN. A gas-filled linear Paul trap (ISCOOL) was used to extend measurements towards very neutron-rich isotopes (N=36-50). A ground state (g.s.) spin I=1/2 is measured for 73Ga, being near degenerate with a 3/2{-} isomer (75  eV≲E{ex}≲1  keV). The 79Ga g.s., with I=3/2, is dominated by protons in the πf{5/2} orbital and in 81Ga the 5/2{-} level becomes the g.s. The data are compared to shell-model calculations in the f{5/2}pg{9/2} model space, calling for further theoretical developments and new experiments.

34P(7Li,7Be+γ) reaction at 100A MeV in inverse kinematics

We report on the first successful extraction of a β+ Gamow-Teller strength distribution from a radioactive isotope in an intermediate-energy charge-exchange experiment in inverse kinematics. The (7Li,7Be+γ(429  keV)) reaction at 100A  MeV was used to measure Gamow-Teller transition strengths from 34P to states in 34Si. The results show that little mixing occurs between sd and pf shell configurations for the low-lying 0+ and 2+ states even though 34Si neighbors the island of inversion and low-lying 2ℏω intruder states exist. Shell-model calculations in the sdpf model space are consistent with these findings.

First experiment with HELIOS: the structure of 13B

A first experiment is reported that makes use of a new kind of spectrometer uniquely suited to the study of reactions with radioactive beams in inverse kinematics, the helical orbit spectrometer, HELIOS. The properties of some low-lying states in the neutron-rich N=8 nucleus 13B were studied with good resolution. From the measured angular distributions of the (d,p) reaction and the relative spectroscopic factors, spin and configuration assignments of the first- and third-excited states of this nucleus can be constrained.

In-beam gamma-ray spectroscopy of very neutron-rich nuclei: excited states in 46S and 48Ar

We report on the first in-beam gamma-ray spectroscopy study of the very neutron-rich nucleus 46S. The N=30 isotones 46S and 48Ar were produced in a novel way in two steps that both necessarily involve nucleon exchange and neutron pickup reactions 9Be(48Ca,48K)X followed by 9Be(48K,48Ar+gamma)X at 85.7 MeV/u midtarget energy and 9Be(48Ca,46Cl)X followed by 9Be(46Cl,46S+gamma)X at 87.0 MeV/u midtarget energy, respectively. The results are compared to large-scale shell-model calculations in the sd-pf shell using the SDPF-NR effective interaction and Z-dependent modifications.

Two-nucleon knockout spectroscopy at the limits of nuclear stability

Sudden single-nucleon removal reactions from fast radioactive beams are now key to studies of the structure of rare isotopes. The sensitivity of the heavy residue's parallel momentum distribution to the orbital angular momentum of the removed nucleon is a crucial feature with a high spectroscopic value. Two-nucleon removal reactions provide experimental reach toward the rarest nuclear species. We show that the residue parallel momentum distributions in these reactions offer a clear spectroscopic signal of the angular momentum of the pair of nucleons removed, and thus of the residue final state spins and spectroscopy. Our formalism is applied successfully to new final-state-inclusive measurements of like-nucleon pair removal reactions to states in neutron-rich 36Mg and neutron-deficient 20Mg. We also confront a new final-state-exclusive decomposition of two-proton knockout data to states in neutron-rich 26Ne.

Resolving ambiguities in genetic typing of human enterovirus species C clinical isolates and identification of enterovirus 96, 99 and 102

Molecular methods, based on sequencing the region encoding the VP1 major capsid protein, have recently become the gold standard for enterovirus typing. In the most commonly used scheme, sequences more than 75% identical (>85% amino acid identity) in complete or partial VP1 sequence are considered to represent the same type. However, as sequence data have accumulated, it has become clear that the '75%/85% rule' may not be universally applicable. To address this issue, we have determined nucleotide sequences for the complete P1 capsid region of a collection of 53 isolates from the species Human enterovirus C (HEV-C), comparing them with each other and with those of 20 reference strains. Pairwise identities, similarity plots and phylogenetic reconstructions identified three potential new enterovirus types, EV96, EV99 and EV102. When pairwise sequence comparisons were considered in aggregate, there was overlap in percentage identity between comparisons of homotypic strains and heterotypic strains. In particular, the differences between coxsackievirus (CV) A13 and CVA17, CVA24 and EV99, and CVA20 and EV102 were difficult to discern, largely because of intratypic sequence diversity. Closer inspection revealed the minimum intratypic values and maximum intratypic values varied by type, suggesting that the rules were at least consistent within a type. By plotting VP1 amino acid identity vs nucleotide identity for each sequence pair and considering each type separately, members of each type were fully resolved from those of other types. This study suggests that a more stringent value of 88% VP1 amino acid identity is more appropriate for routine typing and that other criteria may need to be applied, on a case by case basis, where lower values are seen.

Daclizumab in treatment of multiple sclerosis patients

BACKGROUND

Daclizumab is a humanized monoclonal antibody (mAb) that blocks the interleukin-2 receptor alpha subunit (IL-2R-alpha chain; CD25) expressed on activated T cells leading to the inhibition of T-cell expansion, thus strongly reduces brain inflammation in patients with multiple sclerosis (MS). Another mechanism is significant expansion of CD56 (bright) natural killer (NK) cells that in turn inhibit T-cell survival.

OBJECTIVE

At the Partners MS center, we have been using Daclizumab in an open-label fashion in patients who fail first line therapy or non-standard immunosuppressive treatment. Our aim was to assess its safety and tolerability in our patient population.

Coupled-cluster and configuration-interaction calculations for odd-A heavy nuclei

We compare coupled-cluster (CC) and configuration-interaction (CI) results for 55Ni and 57Ni obtained in the pf-shell basis, focusing on the practical equation-of-motion (EOM) CC approximations that can be applied to systems with dozens of correlated fermions. The weight of the reference state and the strength of correlation effects are controlled by the gap between the f7/2 orbit and the f5/2, p3/2, p1/2 orbits. Independent of the gap, the CC methods with up to 2p-2h components in the cluster operator and 3p-2h/3h-2p components in the EOMCC excitation operator are more accurate than the computationally more demanding CI approach with up to 3p-3h excitations and almost as accurate as the even more demanding CI approach truncated at 4p-4h excitations.

Extraction of weak transition strengths via the (3He, t) reaction at 420 MeV

Differential cross sections for transitions of known weak strength were measured with the (3He, t) reaction at 420 MeV on targets of 12C, 13C, 18O, 26Mg, 58Ni, 60Ni, 90Zr, 118Sn, 120Sn, and 208Pb. Using these data, it is shown that the proportionalities between strengths and cross sections for this probe follow simple trends as a function of mass number. These trends can be used to confidently determine Gamow-Teller strength distributions in nuclei for which the proportionality cannot be calibrated via beta-decay strengths. Although theoretical calculations in the distorted-wave Born approximation overestimate the data, they allow one to understand the main experimental features and to predict deviations from the simple trends observed in some of the transitions.

Z = 50 shell gap near 100Sn from intermediate-energy Coulomb excitations in even-mass 106-112Sn isotopes

Rare isotope beams of neutron-deficient 106,108,110Sn from the fragmentation of 124Xe were employed in an intermediate-energy Coulomb excitation experiment. The measured B(E2,0(1)(+)-->2(1)(+)) values for 108Sn and 110Sn and the results obtained for the 106Sn show that the transition strengths for these nuclei are larger than predicted by current state-of-the-art shell-model calculations. This discrepancy might be explained by contributions of the protons from within the Z = 50 shell to the structure of low-energy excited states in this region.

Selective population and neutron decay of an excited state of 23O

We have observed a resonance in neutron-fragment coincidence measurements that is presumably the first excited state of 23O at 2.8(1) MeV excitation energy which decays into the ground state of 22O. This interpretation is consistent with theory. The reaction mechanism supports the assignment of the observed state as the 5/2+ hole state. This assignment and the recently observed 3/2+ particle state advance the understanding of 23O.

Spectroscopy of 36Mg: interplay of normal and intruder configurations at the neutron-rich boundary of the "island of inversion"

We report on the first spectroscopy study of the very neutron-rich nucleus (36)(12)Mg24 using the direct two-proton knockout reaction 9Be(38Si,36Mg+gamma)X at 83 MeV/nucleon. The energy of the first excited 2+ state of 36Mg, E(2+(1)=660(6) keV, was measured. The magnitude of the partial cross sections to the ground state and the 2+(1) state is indicative of strong intruder admixtures in the lowest-lying states as suggested by Monte Carlo shell-model calculations.

Shape and structure of N=Z 64Ge: electromagnetic transition rates from the application of the recoil distance method to a knockout reaction

Transition rate measurements are reported for the 2(1)+ and 2(2)+ states in N=Z 64Ge. The experimental results are in excellent agreement with large-scale shell-model calculations applying the recently developed GXPF1A interactions. The measurement was done using the recoil distance method (RDM) and a unique combination of state-of-the-art instruments at the National Superconducting Cyclotron Laboratory (NSCL). States of interest were populated via an intermediate-energy single-neutron knockout reaction. RDM studies of knockout and fragmentation reaction products hold the promise of reaching far from stability and providing lifetime information for excited states in a wide range of nuclei.

Coupled-cluster and configuration-interaction calculations for heavy nuclei

We compare coupled-cluster (CC) and configuration-interaction (CI) results for (56)Ni obtained in the pf-shell basis, focusing on practical CC approximations that can be applied to systems with dozens or hundreds of correlated fermions. The weight of the reference state and the strength of correlation effects are controlled by the gap between the f(7/2) orbit and the f(5/2), p(3/2), p(1/2) orbits. Independent of the gap, the CC method with 1p-1h and 2p-2h clusters and a noniterative treatment of 3p-3h clusters is as accurate as the more demanding CI approach truncated at the 4p-4h level.

Measurement of excited states in (40)Si and evidence for weakening of the N=28 shell gap

Excited states in (40)Si have been established by detecting gamma rays coincident with inelastic scattering and nucleon removal reactions on a liquid hydrogen target. The low excitation energy, 986(5) keV, of the 2(1)(+) state provides evidence of a weakening in the N=28 shell closure in a neutron-rich nucleus devoid of deformation-driving proton collectivity.

Probing shell structure and shape changes in neutron-rich sulfur isotopes through transient-field g-factor measurements on fast radioactive beams of 38S and 40S

The shell structure underlying shape changes in neutron-rich nuclei near N = 28 has been investigated by a novel application of the transient-field technique to measure the first-excited-state g factors in 38S and 40S produced as fast radioactive beams. There is a fine balance between proton and neutron contributions to the magnetic moments in both nuclei. The g factor of deformed 40S does not resemble that of a conventional collective nucleus because spin contributions are more important than usual.

Species-specific RT-PCR amplification of human enteroviruses: a tool for rapid species identification of uncharacterized enteroviruses

The 65 serotypes of human enteroviruses are classified into four species, Human enterovirus (HEV) A to D, based largely on phylogenetic relationships in multiple genome regions. The 3'-non-translated region of enteroviruses is highly conserved within a species but highly divergent between species. From this information, species-specific RT-PCR primers were developed that can be used to rapidly screen collections of enterovirus isolates to identify species of interest. The four primer pairs were 100 % specific when tested against enterovirus prototype strains and panels of isolates of known serotype (a total of 193 isolates). For evaluation in a typical application, the species-specific primers were used to screen 186 previously uncharacterized non-polio enterovirus isolates. The HEV-B primers amplified 68.3 % of isolates, while the HEV-A and HEV-C primers accounted for 9.7 and 11.3 % of isolates, respectively; no isolates were amplified with the HEV-D primers. Twelve isolates (6.5 %) were amplified by more than one primer set and eight isolates (4.3 %) were not amplified by any of the four primer pairs. Serotypes were identified by partial sequencing of the VP1 capsid gene, and in every case sequencing confirmed that the species-specific PCR result was correct; the isolates that were amplified by more than one species-specific primer pair were mixtures of two (11 isolates) or three (one isolate) species of viruses. The eight isolates that were not amplified by the species-specific primers comprised four new serotypes (EV76, EV89, EV90 and EV91) that appear to be unique members of HEV-A based on VP1, 3D and 3'-non-translated region sequences.

Proton reaction cross sections as measures of the spatial distibutions of neutrons in exotic nuclei

Proton and neutron densities from Skyrme-Hartree-Fock calculations of a number of nuclei with masses ranging from 28 to 58 have been used to generate optical potentials for proton elastic scattering. Those potentials, generated by folding the structure functions with effective in-medium nucleon-nucleon (NN) interactions, have been used to evaluate proton total reaction cross sections; cross sections that reveal signatures of the structures.

First observation of 54Zn and its decay by two-proton emission

The nucleus 54Zn has been observed for the first time in an experiment at the SISSI/LISE3 facility of GANIL in the quasifragmentation of a 58Ni beam at 74.5 MeV/nucleon in a (nat)Ni target. The fragments were analyzed by means of the ALPHA-LISE3 separator and implanted in a silicon-strip detector where correlations in space and time between implantation and subsequent decay events allowed us to generate almost background free decay spectra for about 25 different nuclei at the same time. Eight 54Zn implantation events were observed. From the correlated decay events, the half-life of 54Zn is determined to be 3.2(+1.8)(-0.8) ms. Seven of the eight implantations are followed by two-proton emission with a decay energy of 1.48(2) MeV. The decay energy and the partial half-life are compared to model predictions and allow for a test of these two-proton decay models.

‘Magic’ nucleus 42Si

Nuclear shell structures--the distribution of the quantum states of individual protons and neutrons--provide one of our most important guides for understanding the stability of atomic nuclei. Nuclei with 'magic numbers' of protons and/or neutrons (corresponding to closed shells of strongly bound nucleons) are particularly stable. Whether the major shell closures and magic numbers change in very neutron-rich nuclei (potentially causing shape deformations) is a fundamental, and at present open, question. A unique opportunity to study these shell effects is offered by the 42Si nucleus, which has 28 neutrons--a magic number in stable nuclei--and 14 protons. This nucleus has a 12-neutron excess over the heaviest stable silicon nuclide, and has only one neutron fewer than the heaviest silicon nuclide observed so far. Here we report measurements of 42Si and two neighbouring nuclei using a technique involving one- and two-nucleon knockout from beams of exotic nuclei. We present strong evidence for a well-developed proton subshell closure at Z = 14 (14 protons), the near degeneracy of two different (s(1/2) and d(3/2)) proton orbits in the vicinity of 42Si, and a nearly spherical shape for 42Si.