Search

We present mass excesses (ME) of neutron-rich isotopes of Ar through Fe, obtained via TOF-$B\rho$ mass spectrometry at the National Superconducting Cyclotron Laboratory. Our new results have significantly reduced systematic uncertainties relative to a prior analysis, enabling the first determination of ME for $^{58,59}{\rm Ti}$, $^{62}{\rm V}$, $^{65}{\rm Cr}$, $^{67,68}{\rm Mn}$, and $^{69,70}{\rm Fe}$. Our results show the $N=34$ subshell weaken at Sc and vanish at Ti, along with the absence of an $N=40$ subshell at Mn. This leads to a cooler accreted neutron star crust, highlighting the connection between the structure of nuclei and neutron stars., Comment: Accepted for publication as a Rapid Communication in Physical Review C

We present a symmetry-based approach for prolate-oblate and spherical-prolate-oblate shape coexistence, in the framework of the interacting boson model of nuclei. The proposed Hamiltonian conserves the SU(3) and $\overline{\rm SU(3)}$ symmetry for the prolate and oblate ground bands and the U(5) symmetry for selected spherical states. Analytic expressions for quadrupole moments and $E2$ rates involving these states are derived and isomeric states are identified by means of selection rules., Comment: 5 pages, 3 figures, published version

We present the mass excesses of 59-64Cr, obtained from recent time-of-flight nuclear mass measurements at the National Superconducting Cyclotron Laboratory at Michigan State University. The mass of 64Cr is determined for the first time, with an atomic mass excess of -33.48(44) MeV. We find a significantly different two-neutron separation energy S2n trend for neutron-rich isotopes of chromium, removing the previously observed enhancement in binding at N=38. Additionally, we extend the S2n trend for chromium to N=40, revealing behavior consistent with the previously identified island of inversion in this region. We compare our results to state-of-the-art shell-model calculations performed with a modified Lenzi-Nowacki-Poves-Sieja interaction in the fp shell, including the g9/2 and d5/2 orbits for the neutron valence space. We employ our result for the mass of 64Cr in accreted neutron star crust network calculations and find a reduction in the strength and depth of electron-capture heating from the A=64 isobaric chain, resulting in a cooler than expected accreted neutron star crust. This reduced heating is found to be due to the >1-MeV reduction in binding for 64Cr with respect to values from commonly used global mass models., Comment: Accepted to Physical Review C

We present the mass excesses of 52-57Sc, obtained from recent time-of-flight nuclear mass measurements at the National Superconducting Cyclotron Laboratory at Michigan State University. The masses of 56Sc and 57Sc were determined for the first time with atomic mass excesses of -24.85(59)(+0 -54) MeV and -21.0(1.3) MeV, respectively, where the asymmetric uncertainty for 56Sc was included due to possible contamination from a long-lived isomer. The 56Sc mass indicates a small odd-even mass staggering in the A = 56 mass-chain towards the neutron drip line, significantly deviating from trends predicted by the global FRDM mass model and favoring trends predicted by the UNEDF0 and UNEDF1 density functional calculations. Together with new shell-model calculations of the electron-capture strength function of 56Sc, our results strongly reduce uncertainties in model calculations of the heating and cooling at the 56Ti electron-capture layer in the outer crust of accreting neutron stars. We found that, in contrast to previous studies, neither strong neutrino cooling nor strong heating occurs in this layer. We conclude that Urca cooling in the outer crusts of accreting neutron stars that exhibit superbursts or high temperature steady-state burning, which are predicted to be rich in A=56 nuclei, is considerably weaker than predicted. Urca cooling must instead be dominated by electron capture on the small amounts of adjacent odd-A nuclei contained in the superburst and high temperature steady-state burning ashes. This may explain the absence of strong crust Urca cooling inferred from the observed cooling light curve of the transiently accreting x-ray source MAXI J0556-332., Comment: Accepted to Physical Review Letters

We present results from recent time-of-flight nuclear mass measurements at the National Superconducting Cyclotron Laboratory at Michigan State University. We report the first mass measurements of 48Ar and 49Ar and find atomic mass excesses of -22.28(31) MeV and -17.8(1.1) MeV, respectively. These masses provide strong evidence for the closed shell nature of neutron number N=28 in argon, which is therefore the lowest even-Z element exhibiting the N=28 closed shell. The resulting trend in binding-energy differences, which probes the strength of the N=28 shell, compares favorably with shellmodel calculations in the sd-pf shell using SDPF-U and SDPF-MU Hamiltonians., Comment: Published in Physical Review Letters

Spectroscopic information has been extracted on the hole-states of $^{55}$Ni, the least known of the quartet of nuclei ($^{55}$Ni, $^{57}$Ni, $^{55}$Co and $^{57}$Co), one neutron away from $^{56}$Ni, the N=Z=28 double magic nucleus. Using the $^{1}$H($^{56}$Ni,d)$^{55}$Ni transfer reaction in inverse kinematics, neutron spectroscopic factors, spins and parities have been extracted for the f$_{7/2}$, p$_{3/2}$ and the s$_{1/2}$ hole-states of $^{55}$Ni. This new data provides a benchmark for large basis calculations that include nucleonic orbits in both the sd and pf shells. State of the art calculations have been performed to describe the excitation energies and spectroscopic factors of the s$_{1/2}$ hole-state below Fermi energy., Comment: Submitted to PLB

The extraction of detailed nuclear structure information from transfer reactions requires reliable, well-normalized data as well as optical potentials and a theoretical framework demonstrated to work well in the relevant mass and beam energy ranges. It is rare that the theoretical ingredients can be tested well for exotic nuclei owing to the paucity of data. The halo nucleus Be-11 has been examined through the 10Be(d,p) reaction in inverse kinematics at equivalent deuteron energies of 12,15,18, and 21.4 MeV. Elastic scattering of Be-10 on protons was used to select optical potentials for the analysis of the transfer data. Additionally, data from the elastic and inelastic scattering of Be-10 on deuterons was used to fit optical potentials at the four measured energies. Transfers to the two bound states and the first resonance in Be-11 were analyzed using the Finite Range ADiabatic Wave Approximation (FR-ADWA). Consistent values of the spectroscopic factor of both the ground and first excited states were extracted from the four measurements, with average values of 0.71(5) and 0.62(4) respectively. The calculations for transfer to the first resonance were found to be sensitive to the size of the energy bin used and therefore could not be used to extract a spectroscopic factor., Comment: 16 Pages, 10 figures

A system of two microchannel-plate detectors has been successfully implemented for tracking projectile-fragmentation beams. The detectors provide interaction positions, angles, and arrival times of ions at the reaction target. The current design is an adaptation of an assembly used for low-energy beams ($\sim$1.4 MeV/nucleon). In order to improve resolution in tracking high-energy heavy-ion beams, the magnetic field strength between the secondary-electron accelerating foil and the microchannel plate had to be increased substantially. Results from an experiment using a 37-MeV/nucleon ${}^{56}$Ni beam show that the tracking system can achieve sub-nanosecond timing resolution and a position resolution of $\sim$1 mm for beam intensities up to $5\times10^{5}$ pps.

The fusion cross sections of radioactive $^{134}$Te + $^{40}$Ca were measured at energies above and below the Coulomb barrier. The evaporation residues produced in the reaction were detected in a zero-degree ionization chamber providing high efficiency for inverse kinematics. Both coupled-channel calculations and comparison with similar Sn+Ca systems indicate an increased sub-barrier fusion probability that is correlated with the presence of positive Q-value neutron transfer channels. In comparison, the measured fusion excitation functions of $^{130}$Te + $^{58,64}$Ni, which have positive Q-value neutron transfer channels, were accurately reproduced by coupled-channel calculations including only inelastic excitations. The results demonstrate that the coupling of transfer channels can lead to enhanced sub-barrier fusion but this is not directly correlated with positive Q-value neutron transfer channels in all cases.

The best examples of halo nuclei, exotic systems with a diffuse nuclear cloud surrounding a tightly-bound core, are found in the light, neutron-rich region, where the halo neutrons experience only weak binding and a weak, or no, potential barrier. Modern direct reaction measurement techniques provide powerful probes of the structure of exotic nuclei. Despite more than four decades of these studies on the benchmark one-neutron halo nucleus Be-11, the spectroscopic factors for the two bound states remain poorly constrained. In the present work, the Be-10(d,p) reaction has been used in inverse kinematics at four beam energies to study the structure of Be-11. The spectroscopic factors extracted using the adiabatic model, were found to be consistent across the four measurements, and were largely insensitive to the optical potential used. The extracted spectroscopic factor for a neutron in a nlj = 2s1/2 state coupled to the ground state of Be-10 is 0.71(5). For the first excited state at 0.32 MeV, a spectroscopic factor of 0.62(4) is found for the halo neutron in a 1p1/2 state., Comment: 5 pages, 4 figures

The location of electron capture heat sources in the crust of accreting neutron stars depends on the masses of extremely neutron-rich nuclei. We present first results from a new implementation of the time-of-flight technique to measure nuclear masses of rare isotopes at the National Superconducting Cyclotron Laboratory. The masses of 16 neutron-rich nuclei in the scandium -- nickel range were determined simultaneously, improving the accuracy compared to previous data in 12 cases. The masses of $^{61}${V}, $^{63}${Cr}, $^{66}${Mn}, and $^{74}${Ni} were measured for the first time with mass excesses of $-30.510(890)$ MeV, $-35.280(650)$ MeV, $-36.900(790)$ MeV, and $-49.210(990)$ MeV, respectively. With the measurement of the $^{66}$Mn mass, the locations of the two dominant electron capture heat sources in the outer crust of accreting neutron stars that exhibit superbursts are now experimentally constrained. We find that the location of the $^{66}$Fe$\rightarrow^{66}$Mn electron capture transition occurs significantly closer to the surface than previously assumed because our new experimental Q-value is 2.1 MeV (2.6$\sigma$) smaller than predicted by the FRDM mass model., Comment: 4 pages, 3 figures

The (d,p) neutron transfer and (d,d) elastic scattering reactions were measured in inverse kinematics using a radioactive ion beam of 132Sn at 630 MeV. The elastic scattering data were taken in a region where Rutherford scattering dominated the reaction, and nuclear effects account for less than 8% of the cross section. The magnitude of the nuclear effects was found to be independent of the optical potential used, allowing the transfer data to be normalized in a reliable manner. The neutron-transfer reaction populated a previously unmeasured state at 1363 keV, which is most likely the single-particle 3p1/2 state expected above the N=82 shell closure. The data were analyzed using finite range adiabatic wave calculations and the results compared with the previous analysis using the distorted wave Born approximation. Angular distributions for the ground and first excited states are consistent with the previous tentative spin and parity assignments. Spectroscopic factors extracted from the differential cross sections are similar to those found for the one neutron states beyond the benchmark doubly-magic nucleus 208Pb., Comment: 22 pages, 7 figures

The first direct measurement of the proton separation energy, Sp, for the proton-unbound nucleus 69Br is reported. Of interest is the exponential dependence of the 2p-capture rate on Sp which can bypass the 68Se waiting-point in the astrophysical rp process. An analysis of the observed proton decay spectrum is given in terms of the 69Se mirror nucleus and the influence of Sp is explored within the context of a single-zone X-ray burst model., Comment: 6 pages, 6 figures, INPC 2010 conference proceedings

Atomic nuclei have a shell structure where nuclei with 'magic numbers' of neutrons and protons are analogous to the noble gases in atomic physics. Only ten nuclei with the standard magic numbers of both neutrons and protons have so far been observed. The nuclear shell model is founded on the precept that neutrons and protons can move as independent particles in orbitals with discrete quantum numbers, subject to a mean field generated by all the other nucleons. Knowledge of the properties of single-particle states outside nuclear shell closures in exotic nuclei is important for a fundamental understanding of nuclear structure and nucleosynthesis (for example the r-process, which is responsible for the production of about half of the heavy elements). However, as a result of their short lifetimes, there is a paucity of knowledge about the nature of single-particle states outside exotic doubly magic nuclei. Here we measure the single-particle character of the levels in 133Sn that lie outside the double shell closure present at the short-lived nucleus 132Sn. We use an inverse kinematics technique that involves the transfer of a single nucleon to the nucleus. The purity of the measured single-particle states clearly illustrates the magic nature of 132Sn., Comment: 19 pages, 5 figures and 4 tables

Spectroscopic factors have been extracted for proton rich 34Ar and neutron rich 46Ar using the (p,d) neutron transfer reaction. The experimental results show little reduction of the ground state neutron spectroscopic factor of the proton rich nucleus 34Ar compared to that of 46Ar. The results suggest that correlations, which generally reduce such spectroscopic factors, do not depend strongly on the neutron-proton asymmetry of the nucleus in this isotopic region as was reported in knockout reactions. The present results are consistent with results from systematic studies of transfer reactions but inconsistent with the trends observed in knockout reaction measurements., Comment: 11 pages, 2 figures. submitted to PRL

Cross sections, kinetic energy and angular distributions of fragments with charge 6$\le$Z$\le$28 emitted in 78,82Kr+40C at 5.5 MeV/A reactions were measured at the GANIL facility using the INDRA apparatus. This experiment aims to investigate the influence of the neutron enrichment on the decay mechanism of excited nuclei. Data are discussed in comparison with predictions of transition state and Hauser-Feshbach models., Comment: 8 pages, 1 figure, paper presented at the First Workshop on "State of the Art in Nuclear Cluster Physics" 13-16 May, 2008, at Strasbourg, France (SOTANCP2008) and accepted for publication at International Journal of Modern Physics E (Special Issue), Proceedings of SOTANCP2008 (to be published)

Atomic masses play a crucial role in many nuclear astrophysics calculations. The lack of experimental values for relevant exotic nuclides triggered a rapid development of new mass measurement devices around the world. The Time-of-Flight (TOF) mass measurements offer a complementary technique to the most precise one, Penning trap measurements, the latter being limited by the rate and half-lives of the ions of interest. The NSCL facility provides a well-suited infrastructure for TOF mass measurements of very exotic nuclei. At this facility, we have recently implemented a TOF-Brho technique and performed mass measurements of neutron-rich nuclides in the Fe region, important for r-process calculations and for calculations of processes occurring in the crust of accreting neutron stars., Comment: 8 pages, 4 figures, submitted to Journal of Physics G, proceedings of Nuclear Physics in Astrophysics III

Evaporation residue and fission cross sections of radioactive $^{132}$Sn on $^{64}$Ni were measured near the Coulomb barrier. A large sub-barrier fusion enhancement was observed. Coupled-channel calculations including inelastic excitation of the projectile and target, and neutron transfer are in good agreement with the measured fusion excitation function. When the change in nuclear size and shift in barrier height are accounted for, there is no extra fusion enhancement in $^{132}$Sn+$^{64}$Ni with respect to stable Sn+$^{64}$Ni. A systematic comparison of evaporation residue cross sections for the fusion of even $^{112-124}$Sn and $^{132}$Sn with $^{64}$Ni is presented., Comment: 9 pages, 11 figures

An algebraic analysis of Grover's quantum search algorithm is presented for the case in which the initial state is an arbitrary pure quantum state of n qubits. This approach reveals the geometrical structure of the quantum search process, which turns out to be confined to a four-dimensional subspace of the Hilbert space. This work unifies and generalizes earlier results on the time evolution of the amplitudes during the quantum search, the optimal number of iterations and the success probability. Furthermore, it enables a direct generalization to the case in which the initial state is a mixed state, providing an exact formula for the success probability., Comment: 13 pages

The degree to which the (p,gamma) and (p,alpha) reactions destroy 18F at temperatures 1-4x10^8 K is important for understanding the synthesis of nuclei in nova explosions and for using the long-lived radionuclide 18F, a target of gamma-ray astronomy, as a diagnostic of nova mechanisms. The reactions are dominated by low-lying proton resonances near the 18F+p threshold (E_x=6.411 MeV in 19Ne). To gain further information about these resonances, we have used a radioactive 18F beam from the Holifield Radioactive Ion Beam Facility to selectively populate corresponding mirror states in 19F via the inverse d(18F,p)19F neutron transfer reaction. Neutron spectroscopic factors were measured for states in 19F in the excitation energy range 0-9 MeV. Widths for corresponding proton resonances in 19Ne were calculated using a Woods-Saxon potential. The results imply significantly lower 18F(p,gamma)19Ne and 18F(p,alpha)15O reaction rates than reported previously, thereby increasing the prospect of observing the 511-keV annihilation radiation associated with the decay of 18F in the ashes ejected from novae., Comment: Error involving sum rule was corrected. Proton widths were recalculated using a Woods-Saxon potential. Both low-lying resonances (8- and 38-keV) are now included in the rate band. 12 pages, 4 figures, 1 table. Submitted to Phys. Rev. C

The effect of unitary noise on the performance of Grover's quantum search algorithm is studied. This type of noise may result from tiny fluctuations and drift in the parameters of the (quantum) components performing the computation. The resulting operations are still unitary, but not precisely those assumed in the design of the algorithm. Here we focus on the effect of such noise in the Hadamard gate $W$, which is an essential component in each iteration of the quantum search process. To this end $W$ is replaced by a noisy Hadamard gate $U$. The parameters of $U$ at each iteration are taken from an arbitrary probability distribution (e.g. Gaussian distribution) and are characterized by their statistical moments around the parameters of $W$. For simplicity we assume that the noise is unbiased and isotropic, namely all noise variables in the parametrization we use have zero average and the same standard deviation $\epsilon$. The noise terms at different calls to $U$ are assumed to be uncorrelated. For a search space of size $N=2^n$ (where $n$ is the number of qubits used to span this space) it is found that as long as $\epsilon < O(n^{-{1/2}} N^{-{1/4}})$, the algorithm maintains significant efficiency, while above this noise level its operation is hampered completely. It is also found that below this noise threshold, when the search fails, it is likely to provide a state that differs from the marked state by only a few bits. This feature can be used to search for the marked state by a classical post-processing, even if the quantum search has failed, thus improving the success rate of the search process.

Grover's quantum search algorithm is analyzed for the case in which the initial state is an arbitrary pure quantum state $|\phi>$ of $n$ qubits. It is shown that the optimal time to perform the measurement is independent of $| \phi>$, namely, it is identical to the optimal time in the original algorithm in which $| \phi > = | 0>$, with the same number of marked states, $r$. The probability of success $P_{\rm s}$ is obtained, in terms of the amplitudes of the state $| \phi>$, and is shown to be independent of $r$. A class of states, which includes fixed points and cycles of the Grover iteration operator is identified. The relevance of these results in the context of using the success probability as an entanglement measure is discussed. In particular, the Groverian entanglement measure, previously limited to a single marked state, is generalized to the case of several marked states.

Evaporation residue cross sections have been measured with neutron-rich radioactive $^{132}$Sn beams on $^{64}$Ni in the vicinity of the Coulomb barrier. The average beam intensity was $2\times 10^{4}$ particles per second and the smallest cross section measured was less than 5 mb. Large subbarrier fusion enhancement was observed. Coupled-channels calculations taking into account inelastic excitation and neutron transfer underpredict the measured cross sections below the barrier., Comment: 4 pages including 1 table and 3 figures

Angular distributions of oxygen produced in the breakup of $^{17}$F incident on a $^{208}$Pb target have been measured around the grazing angle at beam energies of 98 and 120 MeV. The data are dominated by the proton stripping mechanism and are well reproduced by dynamical calculations. The measured breakup cross section is approximately a factor of 3 less than that of fusion at 98 MeV. The influence of breakup on fusion is discussed., Comment: 7 pages, 8 figures

The rapid nuetron-capture process (r process) produces roughly half of the elements heavier than iron. The path and abundances produced are uncertain, however, because of the lack of nuclear strucure information on important neutron-rich nuclei. We are studying nuclei on or near the r-process path via single-nucleon transfer reactions on neutron-rich radioactive beams at ORNL's Holifield Radioactive Ion Beam Facility (HRIBF). Owing to the difficulties in studying these reactions in inverse kinematics, a variety of experimental approaches are being developed. We present the experimental methods and initial results., Comment: Proceedings of the Third International Conference on Fission and Properties of Neutron-Rich Nuclei

Angular distributions of fluorine and oxygen produced from 170 MeV 17^F incident on 208^Pb were measured. The elastic scattering data are in good agreement with optical model calculations using a double-folding potential and parameters similar to those obtained from 16^O+208^Pb. A large yield of oxygen was observed near \theta_lab=36 deg. It is reproduced fairly well by a calculation of the (17^F,16^O) breakup, which is dominated by one-proton stripping reactions. The discrepancy between our previous coincidence measurement and theoretical predictions was resolved by including core absorption in the present calculation., Comment: 9 pages, 5 figures