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The discrepancy between observations from γ-ray astronomy of the ^{60}Fe/^{26}Al γ-ray flux ratio and recent calculations is an unresolved puzzle in nuclear astrophysics. The stellar β-decay rate of ^{59}Fe is one of the major nuclear uncertainties impeding us from a precise prediction. The important Gamow-Teller strengths from the low-lying states in ^{59}Fe to the ^{59}Co ground state are measured for the first time using the exclusive measurement of the ^{59}Co(t,^{3}He+γ)^{59}Fe charge-exchange reaction. The new stellar decay rate of ^{59}Fe is a factor of 3.5±1.1 larger than the currently adopted rate at T=1.2 GK. Stellar evolution calculations show that the ^{60}Fe production yield of an 18 solar mass star is decreased significantly by 40% when using the new rate. Our result eliminates one of the major nuclear uncertainties in the predicted yield of ^{60}Fe and alleviates the existing discrepancy of the ^{60}Fe/^{26}Al ratio.

The $\ensuremath{\beta}$ decays of $^{103,104m}\mathrm{Nb}$ were studied with the Summing NaI(Tl) (SuN) detector at the National Superconducting Cyclotron Laboratory. The $\ensuremath{\beta}$-decay feeding intensity distribution ${I}_{\ensuremath{\beta}}(E)$ for each isotope was extracted by measuring $\ensuremath{\gamma}$ rays in coincidence with an emitted electron. The ${I}_{\ensuremath{\beta}}(E)$ was extracted via the total absorption spectroscopy technique. The ${I}_{\ensuremath{\beta}}(E)$ for each nucleus was compared to predictions made by the quasiparticle random-phase approximation (QRPA) model which is commonly used to calculate $\ensuremath{\beta}$-decay properties for astrophysical applications. The main goal was to provide experimental data for neutron-rich nuclei, relevant to the astrophysical $r$ process. In addition, the extracted $\ensuremath{\beta}$-decay feeding intensity distributions can lead to a better understanding of nuclear structure in a region of rapid structure changes around $A=100$. Finally, experimental data for $^{104m}\mathrm{Nb}$ are also of interest to antineutrino studies of nuclear reactors.

The $\ensuremath{\beta}$ decay of $^{101,102}\mathrm{Zr}$ and $^{109}\mathrm{Tc}$ was studied using the technique of total absorption spectroscopy. The experiment was performed at the National Superconducting Cyclotron Laboratory using the Summing NaI(Tl) (SuN) detector in the first-ever application of total absorption spectroscopy with a fast beam produced via projectile fragmentation. The $\ensuremath{\beta}$-decay feeding intensity and Gamow-Teller transition strength distributions were extracted for these three decays. The extracted distributions were compared to three different quasiparticle random-phase approximation (QRPA) models based on different mean-field potentials. A comparison with calculations from one of the QRPA models was performed to learn about the ground-state shape of the parent nucleus. For $^{101}\mathrm{Zr}$ and $^{102}\mathrm{Zr}$, calculations assuming a pure shape configuration (oblate or prolate) were not able to reproduce the extracted distributions. These results may indicate that some type of mixture between oblate and prolate shapes is necessary to reproduce the extracted distributions. For $^{109}\mathrm{Tc}$, a comparison of the extracted distributions with QRPA calculations suggests a dominant oblate configuration. The other two QRPA models are commonly used to provide $\ensuremath{\beta}$-decay properties in $r$-process network calculations. This work shows the importance of making comparisons between the experimental and theoretical $\ensuremath{\beta}$-decay distributions, rather than just half-lives and $\ensuremath{\beta}$-delayed neutron emission probabilities, as close to the $r$-process path as possible.

This paper reports on the first β-decay study of 74,75Cu isotopes using the technique of total absorption spectroscopy (TAS). The experiment was performed at the National Superconducting Cyclotron Laboratory at Michigan State University using the Summing NaI(Tl) (SuN) detector. The Cu isotopes are good candidates to probe the single-particle structure in the region because they have one proton outside the Z = 28 shell. Comparing the β-decay intensity distributions in the daughter Zn isotopes to the theoretical predictions provides a stringent test of the calculations. The nuclei in this region are also identified as playing an important role in the astrophysical r-process. The measured β-decay intensity distributions provide essential nuclear physics inputs required to better understand heavy element nucleosynthesis.

Excited states in ${}^{73,75}\mathrm{Ni}$ were investigated through the $\ensuremath{\beta}$ decay of ${}^{73,75}\mathrm{Co}$ in an experiment performed at the National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University (MSU). The experimental results extended the level scheme of $^{73}\mathrm{Ni}$ to 3.2-MeV excitation energy and provided the experimental information on excited states in $^{75}\mathrm{Ni}$. The $\ensuremath{\beta}$-delayed neutron branching ratio for $^{73}\mathrm{Co}$ was obtained. The experimental results are discussed in comparison with shell-model calculations.

Electron-capture reactions play important roles in the late evolution of core-collapse supernovae. The electron-capture rates used in astrophysical simulations rely on theoretical calculations which have to be tested against and guided by experimental data. We report on the measurement of the Gamow-Teller strength distribution of the odd-mass nucleus $^{93}\mathrm{Nb}$ via the ($t,^{3}\mathrm{He}$ + $\ensuremath{\gamma}$) charge-exchange reaction at a beam energy of 115 MeV/u. The Gamow-Teller strength distributions were extracted up to an excitation energy in $^{93}\mathrm{Zr}$ of 10 MeV. The results were compared with shell-model and quasiparticle random-phase approximation (QRPA) calculations. The theoretical calculations fail to describe the details of the strength distribution, but estimate reasonably well the integrated Gamow-Teller transition strength. Electron-capture rates derived from the measured and theoretical strength distributions match reasonably well, especially at the higher stellar densities of importance for deleptonization during the collapse of the stellar core, since the electron-capture $Q$ value is close to zero and the Fermi energy sufficiently high to ensure that the details of the strength distribution do not have a strong impact on the derived rates. At stellar densities in excess of ${10}^{9}$ g/${\mathrm{cm}}^{3}$, the electron-capture rate based on a single-state approximation used in astrophysical simulations is slightly higher than the rates based on the data and the shell-model and QRPA calculations, likely due to the fact that the approximation includes temperature-dependent effects, which increase the rates. However, the difference is much smaller than that observed in recent studies of nuclei with $Zl40$ near $N=50$, suggesting that the single-state approximation does not account for Pauli-blocking effects for nuclei with $Zl40$ that are much stronger than those for $^{93}\mathrm{Nb}$ with $Z=41$.

The Gamow-Teller strength distribution from ${}^{88}$Sr was extracted from a $(t,{}^{3}\text{He}+\gamma)$ experiment at 115 MeV/$u$ to constrain estimates for the electron-capture rates on nuclei around $N=50$, between and including $^{78}$Ni and $^{88}$Sr, which are important for the late evolution of core-collapse supernovae. The observed strength below an excitation energy of 8 MeV was consistent with zero and below 10 MeV amounted to $0.1\pm0.05$. Except for a very-weak transition that could come from the 2.231-MeV $1^{+}$ state, no $\gamma$ lines that could be associated with the decay of known $1^{+}$ states were identified. The derived electron-capture rate from the measured strength distribution is more than an order of magnitude smaller than rates based on the single-state approximation presently used in astrophysical simulations for most nuclei near $N=50$. Rates based on shell-model and quasiparticle random-phase approximation calculations that account for Pauli blocking and core-polarization effects provide better estimates than the single-state approximation, although a relatively strong transition to the first $1^{+}$ state in $^{88}$Rb is not observed in the data. Pauli unblocking effects due to high stellar temperatures could partially counter the low electron-capture rates. The new data serves as a zero-temperature benchmark for constraining models used to estimate such effects.

$\ensuremath{\beta}$-decay half-lives of neutron-rich nuclides in the $A=100--110$ mass region have been measured using an implantation station installed inside of the Summing NaI(Tl) (SuN) detector at the National Superconducting Cyclotron Laboratory. Accurate half-lives for these nuclides are important for nuclear astrophysics, nuclear structure, and nuclear technology. The half-lives from the present work are compared with previous measurements, showing overall good agreement.

Use of a beam-based approach is presented for establishing a rigidity calibration for the A1900 fragment separator located at the National Superconducting Cyclotron Laboratory. Also presented is why an alternative approach to the rigidity calibration – using detailed field maps of individual magnetic components – is not a feasible basis for deriving an accurate calibration. The level of accuracy achieved for the rigidity calibration is ±0.1%.

Previous experiments observed a 4^{+} state in the N=28 nucleus ^{44}S and suggested that this state may exhibit a hindered E2-decay rate, inconsistent with being a member of the collective ground state band. We populate this state via two-proton knockout from a beam of exotic ^{46}Ar projectiles and measure its lifetime using the recoil distance method with the GRETINA γ-ray spectrometer. The result, 76(14)_{stat}(20)_{syst} ps, implies a hindered transition of B(E2;4^{+}→2_{1}^{+})=0.61(19) single-particle or Weisskopf units strength and supports the interpretation of the 4^{+} state as a K=4 isomer, the first example of a high-K isomer in a nucleus of such low mass.

International audience; Note from publisher: This article contains the abstract only.

The \( N=28\) isotone 60Ge , \( T_{z}=-2\), was produced and selected among the products of the fragmentation reaction of a 78Kr beam at 150 MeV/nucleon and a Be target by means of the A1900 fragment separator at the National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University (MSU). Its decay was studied for the first time using the optical time projection chamber. The \( \beta\)-decay of 60Ge was found to be dominated by \( \beta\)-delayed proton emission, with a branching of \( \approx 100\)% and half-life \( T_{1/2}=20_{-5}^{+7}\) ms.

The longitudinal momentum distributions of 34 neutron-rich isotopes of elements produced by fragmentation of a 76Ge beam at 132 MeV/u with 13 ≤ Z ≤ 27 were scanned using a novel experimental approach where a variety of targets with different thicknesses was used with the fragment separator at constant magnetic rigidity. In comparison to models that describe the shape and centroid of fragment momentum distributions, a parametrization based on the measured data was derived. Details of the transmission calculations, the analysis of their uncertainties, as well as the general analysis of momentum distributions and deduced cross-sections obtained with this approach are presented.

The complete three-body correlation pictures are experimentally reconstructed for the two-proton decays of the Be-6 and (45)e ground states. We are able to see qualitative similarities and differences between these decays. They demonstrate very good agreement with the predictions of a theoretical three-body cluster model. Validity of the theoretical methods for treatment of the three-body Coulombic decays of this class is thus established by the broad range of lifetimes and nuclear masses spanned by these cases. Implementations for decay dynamics and nuclear structure of 2p emitters are discussed. (C) 2009 Elsevier B.V. All rights reserved.

Background: In neutron-rich nuclei around $N=40$ rapid changes in nuclear structure can be observed. While $^{68}\mathrm{Ni}$ exhibits signatures of a doubly magic nucleus, experimental data along the isotopic chains in even more exotic Fe and Cr isotopes---such as excitation energies and transition strengths---suggest a sudden rise in collectivity toward $N=40$.Purpose: Reduced quadrupole transition strengths for low-lying transitions in neutron-rich $^{58,60,62}\mathrm{Cr}$ are investigated. This gives quantitative new insights into the evolution of quadrupole collectivity in the neutron-rich region close to $N=40$.Method: The recoil distance Doppler-shift (RDDS) technique was applied to measure lifetimes of low-lying states in $^{58,60,62}\mathrm{Cr}$. The experiment was carried out at the National Superconducting Cyclotron Laboratory (NSCL) with the SeGA array in a plunger configuration coupled to the S800 magnetic spectrograph. The states of interest were populated by means of one-proton knockout reactions.Results: Data reveal a rapid increase in quadrupole collectivity for $^{58,60,62}\mathrm{Cr}$ toward $N=40$ and point to stronger quadrupole deformations compared to neighboring Fe isotopes. The experimental $B(E2$) values are reproduced well with state-of-the-art shell-model calculations using the LNPS effective interaction. A consideration of intrinsic quadrupole moments and ${\mathrm{B}}_{42}$ ratios suggest an evolution toward a rotational nature of the collective structures in $^{60,62}\mathrm{Cr}$. Compared to $^{58}\mathrm{Cr}$, experimental ${\mathrm{B}}_{42}$ and ${\mathrm{B}}_{62}$ values for $^{60}\mathrm{Cr}$ are in better agreement with the $E(5)$ limit.Conclusion: Our results indicate that collective excitations in neutron-rich Cr isotopes saturate at $N=38$, which is in agreement with theoretical predictions. More detailed experimental data of excited structures and interband transitions are needed for a comprehensive understanding of quadrupole collectivity close to $N=40$. This calls for additional measurements in neutron-rich Cr and neighboring Ti and Fe nuclei.

In an experiment at the A1900 spectrometer of the National Superconducting Cyclotron Laboratory at Michigan State University the new isotope $^{59}\mathrm{Ge}$ was identified and the cross section for production of the most neutron-deficient $^{59\ensuremath{-}62}\mathrm{Ge}$ isotopes in the fragmentation of a $^{78}\mathrm{Kr}$ beam at 150 MeV/nucleon measured. This experimental information is relevant for the hunt of new two-proton emitters in the region above the doubly-magic $^{48}\mathrm{Ni}$.

Properties of nuclei beyond the proton drip line are studied via measurement of the spontaneous emission of protons from these exotic nuclei. Different components of the wave function of the proton emitter can be accessed by observation of fine structure in the proton spectrum. Fine structure has been observed in the proton radioactivity of 145Tm, 146Tm and 141Ho in measurements at the Holifield Radioactive Ion Beam Facility. These nuclei range over an interesting transition region going from nearly spherical to rather deformed. Detailed analysis of the rich fine structure spectrum of 146Tm is presented in terms of the particle-core vibration coupling model. Other models are discussed briefly.

Rare isotope beams at the National Superconducting Cyclotron Laboratory are produced by projectile fragmentation of medium energy primary beams on beryllium targets. The fragments of interest are selected by the A1900 high-acceptance fragment separator. The A1900 consists of superconducting magnets: four 45° dipoles and eight quadrupole triplets with a maximum magnetic rigidity of 6 Tm. A momentum acceptance of Δ p / p = 5% with a solid angle acceptance of Δ Ω = 8 msr makes the A1900 one of the highest-acceptance separators in the world. Detector systems installed within the device allow tracking and unambiguous identification of individual isotopes. During the first three years of operation of the A1900, more than 200 different rare isotope beams approaching both the neutron and proton driplines have been delivered to experiments.

Two new neutron-deficient isotopes, 60Ge and 64Se, were observed for the first time. They were produced by projectile fragmentation of a 140-MeV/nucleon 78Kr primary beam on a beryllium target. The secondary fragments were unambiguously identified after separation in the A1900 fragment separator. The nucleus 60Ge corresponds to the heaviest isotone of the N = 28 neutron shell and it is most likely the last isotone to be observed. Lower limits of 110 and 180 ns were extracted for the half-life of 60Ge and 64Se, respectively. No events of 59Ga or 63As have been observed, providing very strong evidence that these nuclei are unbound with respect to proton emission. This non-observation implies an upper limit for the half-life of 43 ns for 59Ga and 63As.

New level schemes of odd-Z 99,101Y (Z = 39) and 101,105Nb (Z = 41) are established based on the measurement of prompt gamma rays from the fission of 252Cf at Gammasphere. Bands of π5/2+[422], π5/2−[303] and π3/2−[301] are observed and extended to provide spectroscopic information concerning nuclear shapes in this important odd-Z region. In combination with the level structure of the odd-Z Tc (Z = 43), Rh (Z = 45) and the neighbouring even-Z isotopes the level systematics, signature splittings and kinematic and dynamic moments of inertia of the bands in the Y and Nb isotopes are discussed in terms of shape transition and triaxiality. The pronounced difference observed in the signature splittings between Y and Tc, Rh isotopes is interpreted as evidence of the axially symmetric deformed shape in the Y isotopes, and, as previously reported, large and near maximum triaxiality in Tc–Rh isotopes. The likely lowering of crossing frequencies of the ground-state bands in Tc and Rh isotones in comparison with those in Y isotones also implies a shape transition from axially symmetric deformed shapes in Y nuclei to triaxiality in Tc and Rh isotones. Triaxial-rotor-plus-particle model calculations strongly support a pure axially symmetric shape with large quadrupole deformation in Y isotopes. The model calculations yielded γ values ranging from −19° to −13° for the 5/2+[422] ground-state bands of 101,103,105Nb and of −5° for the two negative-parity bands in 101Nb. The fact that Nb isotopes have intermediate values of signature splitting and band crossing frequencies between those of Y and Tc, Rh isotopes is interpreted as that the Nb isotopes are transitional nuclei with regard to triaxial deformation. A correlation of quadrupole deformations and of triaxiality is seen in the neutron-rich nuclei with Z = 39–45.

Very neutron-deficient fragments were produced by projectile fragmentation of a 140 MeV/ nucleon 78Kr primary beam on a beryllium target. The secondary fragments were unambiguously identified after separation in the A1900 fragment separator. Three events of 60Ge and four events of 64Se have been observed for the first time, making 60Ge the heaviest known isotone of the N = 28 neutron shell. No events of 59Ga and 63As have been observed providing very strong evidence that these nuclei are unbound with respect to proton emission.

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.

From prompt γ-γ-γ coincidence studies with a 252Cf source, the yrast levels were identified from 2+ to 16+ and 14+ in neutron-rich 162,164Gd, respectively. Transition energies between the same spin states are higher and moments of inertia lower at every level in N = 100 164Gd than in N = 98 162Gd. These observations are in contrast to the continuous decrease in the 2+ energy to a minimum at neutron midshell (N = 104) in Er, Yb, and Hf nuclei.

This talk will discuss the fragment yield measurements from the reaction of 58 Ni+ 9 Be and 58 Ni+ 181 Ta using the A1900 fragment separator at the National Superconducting Cyclotron Laboratory. In addition, the charge distributions for these two reactions as well as reactions using the 86 Kr beam will also be presented.

Neutron-rich nuclei populated in spontaneous fission are providing new tests for microscopic models. The neutron-rich 104–108 Mo, 108–114 Ru and 112–116 Pd exibit a variety of rapidly changing collective band structures to high spins. The first evidence for chiral band doublets in any even-even nucleus are reported.

In bombardments of 248Cm with 143.7-146.8 MeV 26Mg ions the nuclides 269Hs and presumably 270Hs were produced. After chemical isolation, Hs atoms were identified by observing genetically linked nuclear-decay chains. Three chains originating from 269Hs confirmed the decay properties observed previously in the decay of 277112. Two chains exhibited the characteristics expected for the new nuclide 270Hs, which was predicted to be a deformed ”doubly magic” nucleus. From the measured $E_\alpha =9.16^{+0.07}_{-0.03}$ MeV an $\alpha$ -decay half-life of 3.6+0.8 -1.4 s was estimated.

Recently, the first successful chemical investigation of element 108, hassium (Hs) has been reported [1]. Based on 7 detected atoms, Hs was shown to form a higly volatile oxide, most probably HsO4. Therefore it behaves similarly to Os, which is known to form highly volatile OsO4. The enthalpies of adsorption-ΔHa O(T) of the compounds on silicon nitride were evaluated as (46±2) kJ/mol for HsO4, compared to (39±1) kJ/mol for OsO4 under identical experimental conditions. Hs should therefore be considered as a member of group 8 of the periodic table.

The Cryo-Thermochromatographic Separator (CTS) was designed and constructed for rapid, continuous on-line separation and simultaneous detection of highly volatile compounds of short-lived α-decaying isotopes of osmium and hassium (Hs, Z =108). A flowing carrier gas containing the volatile species is passed through a channel formed by two facing rows of 32 α-particle detectors, cooled to form a temperature gradient extending from 247 K at the channel entrance down to 176 K at the exit. The volatile species adsorb onto the SiO 2 -coated detector surfaces at a characteristic deposition temperature and are identified by their observed α-decay energies. The CTS was tested on-line with OsO 4 prepared from 169–173 Os isotopes produced in 118,120 Sn( 56 Fe, 3,4,5n) reactions. An adsorption enthalpy for OsO 4 of −40.2±1.5 kJ/mol on SiO 2 was deduced by comparing the measured deposition distribution with Monte Carlo simulations. This successful result demonstrates that such a system provides a viable means for the first chemical study of hassium—which is expected to be a homologue of osmium—by determining whether it forms a similarly volatile oxide.

Particle radioactivity studies using the XIA DGF-4C digital signal processing units at the Recoil Mass Separator of Oak Ridge National Laboratory are presented. Proton emission signals were observed starting from 500 ns after recoil implantation. An energy threshold below 100 keV for particle detection was achieved. For the 145 Tm and 146 Tm decay, evidence for the fine structure in proton emission was obtained. An experiment to search for a new proton emitter 149 Lu is described as an example where the combination of a 56 Ni radioactive beam and digital signal processing is a major advantage.

New excited states built on the 11/2− isomeric states are identified in neutron-rich 121,123Cd, respectively, with gammasphere by measuring three-and higher-fold prompt coincidences following the spontaneous fission of 252Cf. The new data suggests that the 11/2− isomers in the odd-A Cd isotopes with N ≥ 75 have spherical shape similar to those in Te. It is suggested that the backbending observed at ω ≈ 0.445 MeV in 121Cd is because of the alignment of the h11/2 neutron pair.

Results of decay spectroscopy on nuclei in the vicinity of the doubly magic $^{48}\mathrm{Ni}$ are presented. The measurements were performed with a time projection chamber with optical readout which records tracks of ions and protons in the gaseous volume. Six decays of $^{48}\mathrm{Ni}$, including four events of two-proton ground-state radioactivity, were recorded. An advanced reconstruction procedure yielded the $2p$ decay energy for $^{48}\mathrm{Ni}$ of ${Q}_{2p}=1.29(4)$ MeV. In addition, the energy spectra of $\ensuremath{\beta}$-delayed protons emitted in the decays of $^{44}\mathrm{Cr}$ and $^{46}\mathrm{Fe}$, as well as half-lives and branching ratios, were determined. The results were found to be consistent with the previous measurements made with Si detectors. A new proton line in the decay of $^{44}\mathrm{Cr}$ corresponding to the decay energy of 760 keV is reported. The first evidence for the $\ensuremath{\beta}2p$ decay of $^{46}\mathrm{Fe}$, based on one clear event, is shown.

The spontaneous fission of 252Cf has been studied via γ-γ-γ coincidence and γ-γ light charged particle coincidence with Gammasphere. The binary fission yields of correlated Mo−Ba pairs with 0–10 neutron emission have been remeasured. The existence of “hot” fission mode with 8–10 neutron emission seen previously in the Mo−Ba split is confirmed but with lower intensities. By gating on the light charged particles detected in ΔE-E detectors and a γ ray in one partner, the relative yields of correlated pairs in alpha ternary SF with zero to 6n emission are observed for the first time with the distribution peaked at 2.5n. New correlated pairs are identified in 10Be ternary SF. We observed essentially only cold, On 10Be and little, if any, hot, xn 10Be. New γ-γ-γ data with 2.3 times the total events show weak non-Doppler broadened high energy peaks in coincidence with transitions in correlated pairs in 10Be SF shifted by the same 6,1 to 26 keV from the 2-0 energy in 10Be as seen earlier.

The recently commissioned Recoil Mass Spectrometer (RMS) at the Holifield Radioactive Ion Beam Facility (HRIBF) is described. Consisting of a momentum separator followed by an E-D-E Rochester-type mass spectrometer, the RMS is the centerpiece of the nuclear structure endstation at the HRIBF. Designed to transport ions with rigidities near K = 100, the RMS has acceptances of +/- 10% in energy and +/- 4.9% in mass-to-charge ratio. Recent experimental results are used to illustrate the detection capabilities of the RMS, which is compatible with many detectors and devices. (C) 2000 Elsevier Science B.V. All rights reserved.

X-ray bursts have been suggested as a possible site for the astrophysical $\mathrm{rp}$-process. The time scale for the process is governed by $\ensuremath{\beta}$-decay half-lives of several even-even $N\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}Z$ waiting point nuclei, in particular, $N\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}Z\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}40$ ${}^{80}\mathrm{Zr}$. A $4.1({+0.8}{\ensuremath{-}0.6})\ensuremath{-}\mathrm{s}$ ${\ensuremath{\beta}}^{+}/\mathrm{EC}$ half-life for ${}^{80}\mathrm{Zr}$ was determined by observing delayed 84-keV $\ensuremath{\gamma}$ rays depopulating a ${T}_{1/2}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}4\ensuremath{-}\ensuremath{\mu}\mathrm{s}$ isomer at 312 keV in the daughter ${}^{80}\mathrm{Y}$. As this half-life is lower than many previously predicted values, the calculated excessive production of $A\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}80$ nuclides in astrophysical x-ray burst scenarios is reduced, and less extreme conditions are necessary for the production of heavier nuclides.

An isomer of {sup 151}Lu was identified by its direct proton radioactivity. It was produced by bombardment of {sup 96}Ru with 266-MeV {sup 58}Ni from the Holifield Radioactive-Ion Beam Facility, mass separated with a recoil separator and implanted in a double-sided silicon strip detector, which provided signals to correlate each proton decay with a particular implant. The proton energy and half-life of {sup 151}Lu{sup m} were measured to be 1310(10) keV and 16(1) {mu}s, respectively. The half-life of the previously known h{sub 11/2} ground state was observed to be 80(2) ms in agreement with the previously adopted value of 88(10) ms. Comparison of the half-life of {sup 151}Lu{sup m} with WKB barrier-penetration calculations leads to the conclusion that the isomer is a d{sub 3/2} proton state. A two-potential approach predicts a half-life of 5.5{sub {minus}1.1}{sup +1.4} {mu}s which yields an experimental spectroscopic factor of 0.34{sub {minus}0.08}{sup +0.12}. {copyright} {ital 1999} {ital The American Physical Society}

Several new high-spin states are observed in the prompt $\ensuremath{\gamma}$-rays emitted from the neutron-rich, odd-$Z$ ${}^{145,147}\mathrm{La}$ fragments produced in the spontaneous fission of ${}^{252}\mathrm{Cf}.$ Alternating parity bands are extended up to spins 41/2 and 43/2 in ${}^{145,147}\mathrm{La},$ respectively. A new band completes the evidence for two sets of parity doublets expected for octupole correlations. Eight $B(E1)/B(E2)$ ratios (four new) between two bands in ${}^{145}\mathrm{La}$ are all essentially constant and somewhat larger than similar ratios in ${}^{143,144}\mathrm{Ba},$ where stable octupole deformation and/or correlations are reported. The new ratios out of the ${31/2}^{\ensuremath{-}}$ levels in both nuclei show a sharp spike compared to other states, presumably from a strong reduction in $E2$ strengths in this backbending region. In ${}^{145}\mathrm{La},$ collective bands show competition and coexistence between symmetric and asymmetric shapes. Band crossings occur in both nuclei around $\ensuremath{\Elzxh}\ensuremath{\omega}\ensuremath{\approx}0.26--0.30 \mathrm{MeV}.$ Their backbends are associated with the alignment of two ${i}_{13/2}$ neutrons according to cranked shell model calculations.

Several $\ensuremath{\gamma}$ transitions in ${}^{101--103}\mathrm{Nb},{ }^{98,100}\mathrm{Y},{ }^{146}\mathrm{La},$ and ${}^{149}\mathrm{Pr}$ are identified from spontaneous fission studies employing a ${}^{252}\mathrm{Cf}$ source and the Gammasphere detector array. In particular, the pairing-free bands in ${}^{100}\mathrm{Y}$ and ${}^{102}\mathrm{Nb}$ are extended, two rather highly deformed negative-parity bands feeding the 4.3-s $\ensuremath{\beta}$-decaying isomer of ${}^{102}\mathrm{Nb}$ are identified, and their possible bandhead configurations are discussed. In the case of ${}^{101,103}\mathrm{Nb},$ we have extended previously reported level schemes by over 20 transitions in each nuclide. For the fission partners ${}^{146}\mathrm{La}$ and ${}^{149}\mathrm{Pr}$ we have elucidated their previously unreported yrast level schemes.

Gamma rays decaying from the excited states of the proton-unbound ${}^{151}\mathrm{Lu}$ were observed for the first time in an experiment using the ${}^{96}\mathrm{Ru}{(}^{58}\mathrm{Ni},{p2n)}^{151}\mathrm{Lu}$ reaction. These \ensuremath{\gamma} rays were identified by correlating prompt \ensuremath{\gamma} radiations at the target position with ${}^{151}\mathrm{Lu}$ proton radioactivities at the focal plane of a recoil mass separator. Systematic data on $N=80$ isotones suggest a possible isomeric level at high spin in ${}^{151}\mathrm{Lu}.$ Our measurement was unable to observe such an isomer, but provided an upper limit on its half-life. The observed \ensuremath{\gamma} rays in ${}^{151}\mathrm{Lu}$ can be interpreted in terms of two possible level structures.

Gamma rays in the N=Z + 1 nucleus Y-79 were identified using the reaction Si-28(Fe-54, p2n)Y-79 at a 200 MeV beam energy and an experimental setup consisting of an array of Ge detectors and the Recoil Mass Spectrometer at Oak Ridge National Laboratory. With the help of additional gamma-gamma coincidence data obtained with Gammasphere, these gamma rays were found to form a strongly coupled rotational band with rigid-rotor-like behavior. Results of conventional Nilsson-Strutinsky cranked shell model calculations, which predict a deformation of beta(2)similar to 0.4, are in excellent agreement with the properties of this band. Similar calculations for the neighboring N=Z and N=Z + 1 nuclei are also in good agreement with experimental data. This suggests that the presence of the putative T=0 neutron-proton pairing does not significantly affect such simple observables as the moments of inertia of these bands at low spins. [S0556-2813(98)50612-7].

A fundamental question in nuclear physics is what combinations of neutrons and protons can make up a nucleus. Many hundreds of exotic neutron-rich isotopes have never been observed; the limit of how many neutrons a given number of protons can bind is unknown for all but the lightest elements, owing to the delicate interplay between single particle and collective quantum effects in the nucleus. This limit, known as the neutron drip line, provides a benchmark for models of the atomic nucleus. Here we report a significant advance in the determination of this limit: the discovery of two new neutron-rich isotopes--40Mg and 42Al--that are predicted to be drip-line nuclei. In the past, several attempts to observe 40Mg were unsuccessful; moreover, the observation of 42Al provides an experimental indication that the neutron drip line may be located further towards heavier isotopes in this mass region than is currently believed. In stable nuclei, attractive pairing forces enhance the stability of isotopes with even numbers of protons and neutrons. In contrast, the present work shows that nuclei at the drip line gain stability from an unpaired proton, which narrows the shell gaps and provides the opportunity to bind many more neutrons.