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One of the most important global properties of the atomic nucleus is its size. Experimentally determined nuclear charge radii carry unique information on the nuclear force and complex dynamics of protons and neutrons moving inside the nucleus. The intricate behaviour of charge radii along the chain of Ca isotopes, including the unexpectedly large charge radius of neutron-rich 52Ca, poses a daunting challenge for nuclear theory1. Here we present the measurements of the charge radii of proton-rich isotopes 36,37,38Ca, whose properties are impacted by the interplay between nuclear superfluidity and weak binding. Calculations carried out within nuclear density functional theory show that the combination of a novel interaction2 and a state-of-the-art theoretical method can successfully explain the behaviour of charge radii from the lightest to the heaviest Ca isotopes. Through this model, we show how the new data on 36,37,38Ca elucidate the nature of nucleonic pairing in weakly bound proton-rich isotopes. Spectral study on 36,37,38Ca isotopes and calculations based on density functional theory reveal the interplay between charge radii and nucleonic pairing correlations.

Author(s): Arrington, John; Brown, Ben; Beher, Steve; Cromaz, Mario; Dart, Eli; Edwards, Robert; Heyes, Graham; Jones, Clinton; Lauret, Jerome; Liddick, Sean; Mantica, Paul; Melo, Andrew; Monga, Inder; Morris, Brent; Pinkenburg, Chris; Porter, Jeff; Rai, Gulshan; Rockwell, Thomas; Rotman, Lauren; Simon, Richard; Stromsness, Rune; Wefel, Paul; Wiedlea, Andrew; Wilkinson, Sean; Winkler, Linda; Zurawski, Jason

A radiofrequency quadrupole (RFQ) ion beam cooler and buncher has been developed to deliver bunched beams with low transverse emittance, energy spread, and time spread to the BECOLA collinear laser spectroscopy system at the National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University. The beam cooler and buncher contains new features which enhance performance, especially for high count rate beams, as well as simplifying construction, maintenance, and operation. The transverse emittance, energy spread, and time spread of the bunched beam, as well as buncher efficiency are reported, showcasing the capabilities of the BECOLA facility to perform collinear laser spectroscopy measurements with bunched rare isotope beams at NSCL and at the future Facility for Rare Isotope Beams (FRIB).

The Facility for Rare Isotope Beams (FRIB) is a major new scientific user facility under construction in the United States for nuclear science research with beams of rare isotopes [1]. FRIB is funded by the United States Department of Energy Office of Science (DOE-SC) and is located on the campus of Michigan State University (MSU), in East Lansing, Michigan, USA. FRIB will support the mission of the Office of Nuclear Physics in the DOE-SC. The total FRIB project cost is $730 million and the facility is scheduled to be completed by 2022. The project has passed the 75% complete mark and is managed toward early completion in 2021. FRIB is estimated to provide access to 80% off all isotopes of elements lighter than uranium.

The population of low-energy metastable states in Ni I subsequent to charge-exchange reactions and cascade decay was studied for 29.85 keV Ni ions impinging on a sodium vapor. The charge-exchange cross sections were calculated using a semi-classic time-dependent perturbation theory and redistribution of the population via rapid spontaneous decays was simulated. Collinear laser spectroscopy experiments were performed on four emission lines in Ni I to determine populations in low-energy metastable states: 3d94s 3D3 (204.787 cm− 1), 3d94s 3D2 (879.816 cm− 1), 3d94s 3D1 (1713.087 cm− 1) and 3d94s 1D2 (3409.937 cm− 1). Fair agreement was obtained between the simulation and experimental results. The low-energy metastable states were preferentially populated after the cascade decay in contrast to weak populations in near-resonant high-energy states in Ni I.

The BEam COoler and LAser spectroscopy (BECOLA) facility at National Superconducting Cyclotron Laboratory at Michigan State University has been commissioned online to determine charge radii and electromagnetic moments of radioactive isotopes. A radioactive 37K beam was used for the commissioning tests and a 1.29 μs beam bunch-width from the buncher and a bunch repetition rate of 2.5 Hz led to a background suppression factor of 3.1 × 105 in resonant photon detection measurements. The hyperfine structure of 37K and its isotope shift relative to the stable 39K were determined. The obtained hyperfine coupling constants A(2 S 1/2) = 120.3 ± 1.4 MHz, A(2 P 1/2) = 15.2 ± 1.1 MHz, and A(2 P 3/2) = 1.4 ± 8 MHz, and the isotope shift δ ν 39,37 = −264 ± 3 MHz are consistent with the previously determined values, where available.

Spin polarization in fragmentation reactions has been studied in several nuclei showing mainly that large polarization is produced at the tail of the fragments momentum distribution. Such polarization features are not so well known in pickup reactions, especially in studies of unstable nuclei, such as 35K and 37K. The spin polarization of 35K and 37K produced in single-proton pickup reactions at intermediate energies at the Coupled Cyclotrons at NSCL and the results for the ground-state magnetic moment involving the /3-NMR technique are reported in the present work.

A collinear laser-spectroscopy (CLS) system in the BEam COoler and LAser spectroscopy (BECOLA) facility was constructed at National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University. The BECOLA facility will be used to advance measurements of nuclear properties of low-energy rare isotope beams generated via in-flight reactions and subsequent beam thermalization in a buffer gas. The CLS studies at BECOLA will complement laser spectroscopy studies of charge radii and nuclear moments mostly obtained so far at Isotope SeOn Line (ISOL) facilities. Commissioning tests of the CLS system have been performed using an offline ion source to produce stable-ion beams. The tests set the ground work for experiments at the future Facility for Rare Isotope Beams (FRIB) as well as experiments at the current Coupled Cyclotron Facility at NSCL.

Bunched-beam collinear laser spectroscopy is performed on neutron deficient $^{52,53}\mathrm{Fe}$ prepared through in-flight separation followed by a gas stopping. This novel scheme is a major step to reach nuclides far from the stability line in laser spectroscopy. Differential mean-square charge radii $\ensuremath{\delta}⟨{r}^{2}⟩$ of $^{52,53}\mathrm{Fe}$ are determined relative to stable $^{56}\mathrm{Fe}$ as $\ensuremath{\delta}⟨{r}^{2}{⟩}^{56,52}=\ensuremath{-}0.034(13)\text{ }\text{ }{\mathrm{fm}}^{2}$ and $\ensuremath{\delta}⟨{r}^{2}{⟩}^{56,53}=\ensuremath{-}0.218(13)\text{ }\text{ }{\mathrm{fm}}^{2}$, respectively, from the isotope shift of atomic hyperfine structures. The multiconfiguration Dirac-Fock method is used to calculate atomic factors to deduce $\ensuremath{\delta}⟨{r}^{2}⟩$. The values of $\ensuremath{\delta}⟨{r}^{2}⟩$ exhibit a minimum at the $N=28$ neutron shell closure. The nuclear density functional theory with Fayans and Skyrme energy density functionals is used to interpret the data. The trend of $\ensuremath{\delta}⟨{r}^{2}⟩$ along the Fe isotopic chain results from an interplay between single-particle shell structure, pairing, and polarization effects and provides important data for understanding the intricate trend in the $\ensuremath{\delta}⟨{r}^{2}⟩$ of closed-shell Ca isotopes.

Bunched-beam collinear laser spectroscopy is performed on neutron deficient ^{52,53}Fe prepared through in-flight separation followed by a gas stopping. This novel scheme is a major step to reach nuclides far from the stability line in laser spectroscopy. Differential mean-square charge radii δ⟨r^{2}⟩ of ^{52,53}Fe are determined relative to stable ^{56}Fe as δ⟨r^{2}⟩^{56,52}=-0.034(13) fm^{2} and δ⟨r^{2}⟩^{56,53}=-0.218(13) fm^{2}, respectively, from the isotope shift of atomic hyperfine structures. The multiconfiguration Dirac-Fock method is used to calculate atomic factors to deduce δ⟨r^{2}⟩. The values of δ⟨r^{2}⟩ exhibit a minimum at the N=28 neutron shell closure. The nuclear density functional theory with Fayans and Skyrme energy density functionals is used to interpret the data. The trend of δ⟨r^{2}⟩ along the Fe isotopic chain results from an interplay between single-particle shell structure, pairing, and polarization effects and provides important data for understanding the intricate trend in the δ⟨r^{2}⟩ of closed-shell Ca isotopes.

The performance characteristics of two charge-exchange cells (CECs) with horizontal and vertical cell configurations were determined by neutralizing a 10-keV rubidium ion beam in a potassium vapor. The neutralization efficiency and the fluorescence line shape of the 5s 2 S 1 / 2 ↔ 5 p 2P3/2 (D2) transition in neutral 85Rb were investigated as a function of the reservoir temperature used to control the potassium vapor density. The CECs exhibited similar neutralization performance and at neutralization efficiencies greater than 25–50% an asymmetric line shape of the rubidium D2 fluorescent signal was observed. The asymmetry was attributed to inelastic channels in the charge-exchange process. The rubidium D2 fluorescence line shape was fitted with a multiple Voigt function, which quantitatively models the inelastic channels. In the present Rb+ + K charge-exchange process, the optimum neutralization efficiency to accurately determine the centroid of the Rb D2 resonance line shape was obtained by fitting single Voigt functions to the resonance spectra obtained at neutralization efficiencies where inelastic processes were negligible.

A 624-channel Digital Data Acquisition System capable of instrumenting the Segmented Germanium Array at National Superconducting Cyclotron Laboratory has been implemented using Pixie-16 Digital Gamma Finder modules by XIA LLC. The system opens an opportunity for determination of the first interaction position of a γ ray in a SeGA detector from implementation of γ ‐ ray tracking. This will translate into a significantly improved determination of angle of emission, and in consequence much better Doppler corrections for experiments with fast beams. For stopped-beam experiments the system provides means for zero dead time measurements of rare decays, which occur on time scales of microseconds.

A new device has been designed and built at NSCL which provides additional filtering of radioactive beams produced via projectile fragmentation. The Radio Frequency Fragment Separator (RFFS) uses the time micro structure of the beams accelerated by the cyclotrons to deflect particles according to their time-of-flight, in effect producing a phase filtering. The transverse RF (Radio Frequency) electric field of the RFFS has superior filtering performance compared to other electrostatic devices, such as Wien filters. Such filtering is critical for radioactive beams produced on the neutron-deficient side of the valley of stability, where strong contamination occurs at intermediate energies from 50 to 200 MeV/u.

Observed neutron-capture elemental abundances in metal-poor stars, along with ongoing analysis of the extremely metal-poor Eu-enriched sub-class provide new guidance for astrophysical models aimed at finding the r-process sites. The present paper emphasizes the importance of nuclear physics parameters entering in these models, particularly β -decay properties of neutron-rich nuclei. In this context, several r-process motivated β -decay experiments performed at the National Superconducting Cyclotron Laboratory (NSCL) are presented, including a summary of results and impact on model calculations.

A beta-ray detecting nuclear quadrupole resonance system has been developed at NSCL/MSU to measure ground-state electric quadrupole moments of short-lived nuclei produced as fast rare isotope beams. This system enables quick and sequential application of multiple transition frequencies over a wide range. Fast switching between variable capacitors in resonance circuits ensures sufficient power delivery to the coil in the beta-ray detecting nuclear magnetic resonance technique. The fast switching technique enhances detection efficiency of resonance signals and is especially useful when the polarization and/or production rate of the nucleus of interest are small and when the nuclear spin is large.

New techniques to measure the g factors of picosecond-lived excited states of neutron-rich nuclei produced as radioactive beams are discussed along with their applications to study nuclear structure near 132 50Sn82 and in the region between N=20 and N=28.

Zhu, S. Janssens, R. V. F. Fornal, B. Freeman, S. J. Honma, M. Broda, R. Carpenter, M. P. Deacon, A. N. Kay, B. P. Kondev, F. G. Krolas, W. Kozemczak, J. Larabee, A. Lauritsen, T. Liddick, S. N. Lister, C. J. Mantica, P. F. Otsuka, T. Pawlat, T. Robinson, A. Seweryniak, D. Smith, J. F. Steppenbeck, D. Tomlin, B. E. Wrzesinski, J. Wang, X.

New high-spin levels have been identified in the nucleus 120Pd from analysis of the coincidence gamma rays observed in the 238U(α, f γ γ γ ) reaction. Recoiling fragments were detected with the Rochester heavy-ion detector array, CHICO, in coincidence with gamma-rays using Gammasphere. An A=110-130 mass-gated γ − γ − γ cube from this experiment was constructed and analyzed for coincident gamma-rays in several nuclei near A=120. New results for neutron-rich 118Pd and 120Pd have been obtained. The 120Pd level scheme was extended to spin of 10+ by building on the new low-energy gamma rays identified in decay studies of 120Rh. The details of the 118Pd level scheme derived from this work are compared to previous work. The systematics of the yrast levels in even-even Pd and Cd isotopes are presented and the symmetry of energy levels around N = 68 is discussed. A new 10+ level in 124Cd has been observed. The population intensity of even-even neutron-rich Cd isotopes is deduced, indicating that nuclides near 120Cd are preferentially populated following alpha-induced fission of 238U.

Background: The systematic trend in mean-square charge radii as a function of proton or neutron number exhibits a discontinuity at the nucleon-shell closures. While the established $N=28$ shell closure is evident in the charge radii of the isotopic chains of K through Mn, a similar signature of the $N=20$ shell closure is absent in the Ca region.Purpose: The isotope shift between neutron-deficient $^{36}\mathrm{K}$ and $^{37}\mathrm{K}$ was determined to investigate the change of the mean-square charge radii across $N=20$ in the K isotopic chain.Methods: The $D1$ atomic hyperfine spectra of $^{36}\mathrm{K}$ and $^{37}\mathrm{K}$ were measured using an optical pumping and subsequent $\ensuremath{\beta}$-decay asymmetry detection technique. Atomic rate equations were solved to fit the resonant line shape. The result was compared to Skyrme energy-density functional and shell-model calculations.Results: The isotope shift was obtained as $\ensuremath{\delta}{\ensuremath{\nu}}^{37,36}=\ensuremath{-}139(4)(3)$ MHz. Using a re-evaluated isotope shift, $\ensuremath{\delta}{\ensuremath{\nu}}^{39,37}=\ensuremath{-}264(2)(3)$ MHz, the isotope shift relative to $^{39}\mathrm{K}$ was determined to be $\ensuremath{\delta}{\ensuremath{\nu}}^{39,36}=\ensuremath{-}403(5)(4)$ MHz. The differential mean-square charge radius was then deduced as $\ensuremath{\delta}{\ensuremath{\langle}{r}^{2}\ensuremath{\rangle}}^{39,36}=\ensuremath{-}0.16(5)(8)\phantom{\rule{4pt}{0ex}}{\mathrm{fm}}^{2}$. The Skyrme energy-density functional and shell-model calculations overpredict the experimental values below $N=20$ and underpredict them above $N=20$, and their agreement is marginal.Conclusions: The absence of the shell-closure signature at $N=20$ in the K isotopic chain is understood as a balance between the monopole and the quadrupole proton-core polarizations below and above $N=20$, respectively.

β-delayed γ-ray measurements have been part of an experimental program at the NSCL to understand the role of the πf7/2 - νf5/2 proton-neutron monopole interaction in neutron-rich pf-shell nuclei above 48Ca. Central to this study has been an attempt to observe the development of new shell structure at N = 32, 34 through the systematic observation of E(2+1 ) as a function of neutron number. Additionally, the ground state spin and parity of odd-odd and odd-A nuclei were interpreted in an extreme single-particle model to follow the monopole migration of the νf5/2 as protons are removed from the πf7/2state.

The planned rare isotope accelerator facility RIA in the US would become the most powerful radioactive beam facility in the world. RIA's driver accelerator will be a device capable of providing beams from protons to uranium at energies of at least 400 MeV per nucleon, with beam power up to 400 kW. Radioactive beam production relies on both the in-flight separation of fast beam fragments and on the ISOL technique. In both cases the high beam power poses major challenges for target technology and handling and on the design of the beam production areas. This paper will give a brief overview of RIA and discuss aspects of ongoing conceptual design work for the RIA target areas.

The beta-decay properties of neutron-rich nuclides in the vicinity of doubly magic 48 Ca, 78 Ni, and 132 Sn have been measured recently using the beta counting system at the National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University. Some of the technical challenges faced by these experiments, including charge-state ambiguities, prompt gamma radiation flash, and presence of beta-decaying isomeric states, are discussed.

The results of measurements of β-delayed and isomeric γ rays from neutron-rich nuclides produced by projectile fragmentation at the NSCL are summarized.

The structure of magic neutron-rich nickel isotopes produced in the fragmentation of a 140 A MeV 86 Kr beam was investigated. For the first time four gamma transitions were assigned to the decay of the I π = 8 + , T 1 / 2 = 590 −110 +180 ns isomer, thus establishing the 0 + – 2 + – 4 + – 6 + – 8 + ground-state band in 76 Ni. The previously unknown 2 + and 4 + levels belonging to the ground-state band in 74 Ni were identified in the β decay of 74 Co ( T 1 / 2 = 30 ( 3 ) ms ) . The decay properties of 72 Co → 72 Ni were verified and confirmed on the basis of γ – γ coincidence data. The relevance of the measured level properties for the magicity of 78 Ni is analyzed with the help of advanced shell-model predictions.

New half-lives for neutron-rich ruthenium, rhodium and palladium isotopes close to the r-process path along the N=82 closed shell have been measured at the National Superconducting Cyclotron Laboratory at Michigan State University. The studied isotopes are close to the critical A=118-126 mass region in the astrophysical r-process, where incorrect nuclear structure development towards the shell closure may have the most pronounced effect on the abundances of elements produced. Neutron-rich nuclei were produced by fragmentation of a 120-MeV per nucleon 136 Xe beam on Be and were separated by the A1900 fragment separator. The nuclei of interest were implanted into a double-sided Si strip detector at the center of the MSU β counting system. Charge states and fully- stripped isotopes were distinguished by a measurement of the total kinetic energy of each implant. Our measured half-lives will provide first clues on whether our understanding of nuclear structure evolution in this mass region is correct.

Recent results of β-decay studies performed along the expected neutron shell closures at N = 20, 28, 50 and 82 are reviewed and discussed in view of the potentially dynamic nature of neutron single-particle states in nuclei having extreme neutron-to-proton ratios.

Despite a lot of experimental and theoretical progress the question of the r-process site and the origin of the heavy elements in nature remains one of the biggest open questions in nuclear astrophysics. We report first results from experiments with rare isotope beams of r-process nuclei at Michigan State University’s National Superconducting Cyclotron Laboratory. This includes a first measurement of the half-life of the doubly-magic waiting point nucleus 78Ni, which serves as a major bottle-neck for the synthesis of heavy elements in many r-process models

The regions around the doubly magic nuclei 100 Sn and 78 Ni are of great interest from a nuclear structure standpoint. These nuclei also play a key role in the astrophysical rp- and r-processes, respectively. Recently, nuclei in these regions were studied at the Coupled Cyclotron Facility at the National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University.

The β decay of neutron-rich nuclei in the π 1 f 7/2 and νpf shells has been studied at the NSCL to track the development of new proposed shell closures at N=32 and 34 due to the monopole migration of the ν 1 f 5/2 orbital. From a measurement of absolute γ -ray intensities following the β decay of odd-odd nuclei, the β -decay branching ratios to levels in the even-even daughters can be deduced. The branching ratios, especially to the 0 + daughter ground state, can be used to assign spins and parities to the parent ground states. The interpretation of these assignments for odd-odd nuclei in a single-particle limit can be used to track the migration of the ν 1 f 5/2 single-particle state.

A new beta counting system has been developed at the National Superconducting Cyclotron Laboratory to study the beta decay of exotic nuclei produced by fast fragmentation. This system uses a double-sided silicon strip detector to detect both fragment implants and their subsequent beta decays; these events are correlated on a pixel-by-pixel basis, providing a direct measurement of the decay time with specific particle identification information regarding the parent nucleus. The experimental capabilities of this system are described, and future plans discussed.

The level structure of 5422Ti32 has been explored for the first time by combining β-decay measurements from fragmentation products with prompt γ-ray spectroscopy following deep inelastic reactions. The latter technique was also instrumental in tracing 52Ti30 to higher spin. The data provide new tests of effective interactions for full pf-shell calculations in neutron-rich nuclei above 48Ca. The data indicate the presence of a significant subshell gap at N=32 and comparisons between theory and experiment suggest an additional shell closure at N=34 in Ca and Ti isotopes.

An 879.9(2) keV γ -ray transition has been identified following the β decay of 58 V and assigned as the 2 + 1 →0 + 1 transition in 58 Cr 34 . A peak in the energies of the first excited 2 + states for the even–even chromium isotopes is now evident at 56 Cr 32 , providing empirical evidence for a significant subshell gap at N =32. The appearance of this neutron subshell closure for neutron-rich nuclides may be attributed to the diminished π 1 f 7/2 – ν 1 f 5/2 monopole proton–neutron interaction as protons are removed from the 1 f 7/2 single-particle orbital.

The first fully on-line use of the angular distribution of beta emission in detection of NMR of nuclei oriented at low temperatures is reported. The magnetic moments of the single valence particle, intermediate mass, isotopes 67Ni(nup(-1)(1/2);1/2(-)) and 69Cu(pip(1)(3/2);3/2(-)) are measured to be +0.601(5) m(N) and +2.84(1) m(N), respectively, revealing only a small deviation from the neutron p(1/2) single-particle value in the former and a large deviation from the proton p(3/2) single-particle value in the latter. Quantitative interpretation is given in terms of core polarization and meson-exchange currents.

The existence of a subshell gap at neutron number N =40 is investigated in the odd-mass Ni and Cu isotopes using the Particle-Core Coupling Model (PCM). The calculated properties are in good agreement with the existing experimental data, suggesting that 68 Ni is a good core for the adjacent nuclei. Evaluation of the 2 + 1 excitation energies, two-neutron separation energies, and pairing properties of the Ni isotopes reveals discontinuities at neutron number N =40. These data support the existence of a significant neutron subshell gap at N =40, for Z =28. The systematic behavior of the proton single-particle energies extracted from the particle-core coupling calculations for the odd-mass Cu isotopes is discussed in terms of monopole shifts. The effect is consistent with the existence of a neutron subshell gap at N =40.

A new method has been developed for measuring the magnitude of nuclear spin polarization of a secondary, radioactive beam by making a pulsed magnetic field measurement that does not require advance knowledge of the nuclide's magnetic moment. Using a standard β NMR apparatus, a magnetic double ratio is determined from the counting rates in 0° and 180° β detectors for magnetic field on and off conditions. This ratio provides direct information on the induced spin polarization of a radioactive beam. A demonstration of the method was performed using spin polarized 12B nuclei produced by fragmentation of an 80 MeV/nucleon 18O beam in a Nb target.

A group of odd-mass neutron-rich nuclei with neutron excesses of 9--11 in the $13l~Zl~17$ region has been produced and studied by in-beam intermediate-energy Coulomb excitation using an array of NaI(Tl) detectors. The excitation energies of the observed states in ${}^{35}\mathrm{Al},$ ${}^{37}\mathrm{Si},$ ${}^{39}\mathrm{P},$ ${}^{41}\mathrm{S},$ ${}^{43}\mathrm{S},$ and ${}^{45}\mathrm{Cl}$ have been measured, and the $B(E2)$ values connecting these states to the ground states have been extracted. For the ${}^{41}\mathrm{S}$ and ${}^{43}\mathrm{S}$ cases the measurements have been compared to particle-rotor and particle-vibrator calculations. The measurements for ${}^{41}\mathrm{S}$ are consistent with an interpretation of the low-energy behavior of this nucleus as rotations of a deformed core, whereas for ${}^{43}\mathrm{S}$ no distinction can be made between the deformed (rotational) and spherical (vibrational) calculations.

The technique of nuclear magnetic resonance on beta-emitting nuclei (β-NMR) has been implemented at the NSCL to measure the ground state g factors of spin-polarized radioactive nuclei produced via projectile fragmentation. Recent developments to extend this technique to very exotic nuclear systems include the implementation of a multiple adiabatic fast passage technique for β-NMR and the use of thick production targets to improve the yield of spin-polarized secondary fragments.

The decays of ${}^{187}$Hg (2.2 min; ${J}^{\ensuremath{\pi}}$=13/2${}^{+})$ and ${}^{187}$Hg (2.4 min; ${J}^{\ensuremath{\pi}}$=3/2${}^{\ensuremath{-}})$ have been studied with mass-separated sources at the UNISOR facility. Multiscaled spectra of $\ensuremath{\gamma}$ rays, x rays, and conversion electrons, as well as $\ensuremath{\gamma}\ensuremath{\gamma}t$, $\ensuremath{\gamma}\mathrm{xt}$, $e\ensuremath{\gamma}t$, and $\mathrm{ext}$ coincidences were obtained. Decay schemes have been constructed incorporating $g$99$%$ of the decay intensities assigned to the high-spin and low-spin decays. The $\ensuremath{\gamma}$-ray gated conversion-electron spectra permitted determination of 367 conversion coefficients. A variety of coexisting band structures are established in ${}^{187}$Au. Some of these have near-identical analogs in the heavier odd-mass Au isotopes. The remaining bands reveal new degrees of freedom at low excitation energy in ${}^{187}$Au. Nine electric monopole $(E$0) transitions are observed to deexcite members of these new bands. The work represents an undertaking to achieve a benchmark in complete spectroscopy following radioactive decay. The most notable band structure in ${}^{187}$Au has two bands with identical spins, nearly identical relative energies, and electric monopole transitions connecting the favored members. These bands can be understood as $\ensuremath{\pi}{h}_{9/2}$ and $\ensuremath{\pi}{f}_{7/2}$ intruder configurations coupled to coexisting prolate or near-prolate $(\ensuremath{\gamma}$ $l~$ 20$\ifmmode^\circ\else\textdegree\fi{})$ cores having diabatic configurations that differ only in the number of protons occupying the $N$=5 intruder configurations.

The odd-odd N=Z nuclide, {sup 78}Y has been produced in the {sup 40}Ca({sup 40}Ca,pn) reaction at 125 MeV. Recoiling fragments separated by their A/Q values were implanted onto the tape of a moving tape collector and transported to a shielded position between two plastic {beta} detectors and two Ge {gamma}-ray detectors where {beta}-{gamma} coincidences were recorded as a function of time. {gamma} rays with energies of 279 (100{percent}), 504 (90{percent}), and 713 (40{percent}) keV, previously identified as yrast transitions in daughter {sup 78}Sr, were observed and found to decay with a half-life of 5.8 (6) s. From the relative intensities of the {gamma}-rays, a spin and parity of 5{sup +} and T=0 are assigned to the parent state in {sup 78}Y undergoing {beta} decay. A production cross section of 4{plus_minus}1{mu}b has been determined for {sup 78}Y by comparison of the counting rates with those of other reaction products with known cross sections. An upper limit of 500 keV can be set for the energy of this level relative to a possible highly deformed T=1 0{sup +} ground state. From this limit, it can be inferred that T=1 pn pairing is considerably quenched relative to such pairing in adjacent odd-odd N=Z more » {sup 74}Rb. Two-quasiparticle rotor model calculations have been used to account for the structure of {sup 78}Y and adjacent nuclides. {copyright} {ital 1998} {ital The American Physical Society} « less

We have measured the parallel momentum distributions of outcoming fragments in the one-neutron breakup of the odd-mass carbon isotopes ${}^{19,17,15}\mathrm{C}$ and ${}^{14}\mathrm{B}$. Owing to their low neutron separation energies, the study of these nuclei is of particular interest regarding the appearance of the halo phenomenon. Discrepancies between experiment and theory observed for ${}^{15}\mathrm{C}$ and ${}^{14}\mathrm{B}$ indicate that their halos are not as prominent as in ${}^{11}$Be or ${}^{11}$Li, and that core excitations start playing a role in the breakup mechanism. In ${}^{17}\mathrm{C}$ the halo appears hindered by a $d$-wave neutron ground state configuration. Finally, the data on ${}^{19}\mathrm{C}$ suggests an $s$-wave neutron around the 2${}^{+}$ excited state in ${}^{18}\mathrm{C}$.

The low energy level structure of neutron-rich 28,29Na has been investigated through β-delayed γ spectroscopy. The present work, which presents the first detailed spectroscopy of 29Na, clearly demonstrates that for Na isotopes between 28Na (N = 17) and 29Na (N = 18), intruder configurations start dominating the low lying excited states, suggestive of the small N = 20 shell gap.

Collinear laser spectroscopy was performed on an atomic beam of stable 55Mn. An ion beam of 55Mn+ was generated in an ion source, accelerated to 15 keV, and neutralized via charge-exchange reactions with a Na vapor. A long-lived metastable state of Mn I, near-resonantly populated in the charge-exchange process, was investigated via laser probing in addition to a laser excitation from the ground state in Mn I. The relative population of the Mn I metastable state compared to that of the ground state was found to be 0.7 ± 0.3. A theoretical calculation, which included feeding to the ground state and the metastable state from higher-energy excited electronic states populated in the charge-exchange process, agreed with the present result. The deduced A and B hyperfine coupling constants agreed with literature values, where available.

The ${\mathrm{\ensuremath{\beta}}}^{+}$ decay of 15.2 min $^{114}\mathrm{Te}$ into levels of $^{114}\mathrm{Sb}$ was studied through \ensuremath{\gamma}-ray and conversion electron spectroscopy at the UNISOR on-line isotope mass separator. Multiscaled \ensuremath{\gamma}-ray and electron singles provided for the determination of energies and intensities for 94 transitions (68 new) and of internal conversion coefficients for 12 of these transitions. The level scheme was deduced from \ensuremath{\gamma}-\ensuremath{\gamma} and \ensuremath{\gamma}-e coincidence measurements and extended to 2.1 MeV. The energy spectrum and electromagnetic properties of levels and associated transitions were calculated in the interacting boson-fermion-fermion model (IBFFM), with good agreement between experimental and theoretical results. \textcopyright{} 1996 The American Physical Society.

Beta-delayed proton emission from nuclides in the neighborhood of 100Sn was studied at the National Superconducting Cyclotron Laboratory. The nuclei were produced by fragmentation of a 120 MeV/nucleon 112Sn primary beam on a Be target. Beam purification was provided by the A1900 Fragment Separator and the Radio Frequency Fragment Separator. The fragments of interest were identified and their decay was studied with the NSCL Beta Counting System (BCS) in conjunction with the Segmented Germanium Array (SeGA). The nuclei 96Cd, 98Ing, 98Inm and 99In were identified as beta-delayed proton emitters, with branching ratios bp = 5.5(40)%, 5.5+3 -2%, 19(2)% and 0.9(4)%, respectively. The bp for 89Ru, 91,92Rh, 93Pd and 95Ag were deduced for the first time with bp = 3+1.9 -1.7%, 1.3(5)%, 1.9(1)%, 7.5(5)% and 2.5(3)%, respectively. The bp = 22(1)% for 101Sn was deduced with higher precision than previously reported. The impact of the newly measured bp values on the composition of the type-I X-ray burst ashes was studied.

Background: Nuclear structure plays a significant role on the rapid neutron capture process ($r$ process) since shapes evolve with the emergence of shells and subshells. There was some indication in neighboring nuclei that we might find examples of a new $N=56$ subshell, which may give rise to a doubly magic ${}_{34}^{90}$Se${}_{56}$ nucleus.Purpose: $\ensuremath{\beta}$-decay half-lives of nuclei around ${}^{90}$Se have been measured to determine if this nucleus has in fact a doubly magic character.Method: The fragmentation of a ${}^{136}$Xe beam at the National Superconducting Cyclotron Laboratory at Michigan State University was used to create a cocktail of nuclei in the $A=90$ region.Results: We have measured the half-lives of 22 nuclei near the $r$-process path in the $A=90$ region. The half-lives of ${}^{88}\phantom{\rule{-0.16em}{0ex}}$As and ${}^{90}$Se have been measured for the first time. The values were compared with theoretical predictions in the search for nuclear-deformation signatures of a $N=56$ subshell, and its possible role in the emergence of a potential doubly magic ${}^{90}$Se. The impact of such hypothesis on the synthesis of heavy nuclei, particularly in the production of Sr, Y, and Zr elements was investigated with a weak $r$-process network.Conclusions: The new half-lives agree with results obtained from a standard global QRPA model used in $r$-process calculations, indicating that ${}^{90}$Se has a quadrupole shape incompatible with a closed $N=56$ subshell in this region. The impact of the measured ${}^{90}$Se half-life in comparison with a former theoretical predication associated with a spherical half-life on the weak $r$ process is shown to be strong.