P. F. Bertone

First Results from the CARIBU Facility: Mass Measurements on the r-Process Path

The Canadian Penning Trap mass spectrometer has made mass measurements of 33 neutron-rich nuclides provided by the new Californium Rare Isotope Breeder Upgrade facility at Argonne National Laboratory. The studied region includes the 132Sn double shell closure and ranges in Z from In to Cs, with Sn isotopes measured out to A=135, and the typical measurement precision is at the 100 ppb level or better. The region encompasses a possible major waiting point of the astrophysical r process, and the impact of the masses on the r process is shown through a series of simulations. These first-ever simulations with direct mass information on this waiting point show significant increases in waiting time at Sn and Sb in comparison with commonly used mass models, demonstrating the inadequacy of existing models for accurate r-process calculations.

Decay and Fission Hindrance of Two- and Four-Quasiparticle K Isomers in (254)Rf

Two isomers decaying by electromagnetic transitions with half-lives of 4.7(1.1) and 247(73) μs have been discovered in the heavy ^{254}Rf nucleus. The observation of the shorter-lived isomer was made possible by a novel application of a digital data acquisition system. The isomers were interpreted as the K^{π}=8^{-}, ν^{2}(7/2^{+}[624],9/2^{-}[734]) two-quasineutron and the K^{π}=16^{+}, 8^{-}ν^{2}(7/2^{+}[624],9/2^{-}[734])⊗8^{-}π^{2}(7/2^{-}[514],9/2^{+}[624]) four-quasiparticle configurations, respectively. Surprisingly, the lifetime of the two-quasiparticle isomer is more than 4 orders of magnitude shorter than what has been observed for analogous isomers in the lighter N=150 isotones. The four-quasiparticle isomer is longer lived than the ^{254}Rf ground state that decays exclusively by spontaneous fission with a half-life of 23.2(1.1) μs. The absence of sizable fission branches from either of the isomers implies unprecedented fission hindrance relative to the ground state.

Fusion reactions with the one-neutron halo nucleus 15 C

The structure of (15)C, with an s(1/2) neutron weakly bound to a closed-neutron shell nucleus (14)C, makes it a prime candidate for a one-neutron halo nucleus. We have for the first time studied the cross section for the fusion-fission reaction (15)C+(232)Th at energies in the vicinity of the Coulomb barrier and compared it to the yield of the neighboring (14)C+(232)Th system measured in the same experiment. At sub-barrier energies, an enhancement of the fusion yield by factors of 2-5 was observed for (15)C, while the cross sections for (14)C match the trends measured for (12,13)C.

Direct Measurement of the

In our Letter [Phys. Rev. Lett. 112, 152701 (2014)] we reported the direct measurement of the 23Naðα; pÞ26Mg reaction cross section at energies relevant for the production of Galactic Al. Our results, which relied on the extracted absolute cross sections given in Table I, have been found to be in error, overestimating the reported cross sections by a factor of 100. In the experiment, protons from the reaction were measured in an annular silicon strip detector placed downstream from a cryogenic He gas target. The cross sections were normalized to the yield of scattered Na ions from a separate Au foil in an upstream monitor detector. The data acquisition system was triggered by a logic “OR” of the proton detector and the “downscaled” monitor detector. The monitor detector rate was downscaled in order to reduce dead time in the data acquisition system. The down-scale factor was n 1⁄4 100, while in the analysis, the factor was mistakenly taken as n 1⁄4 1. Therefore, the cross section numbers given in Table I should be divided by a factor of 100. The stellar rate reported in our Letter should also be down scaled by the same factor of 100, which makes it in agreement, within the experimental uncertainties, with the recommended rate. This problem came to light due to results from recent experiments where the same reaction was studied in regular and inverse kinematics [1,2]. Those studies obtained similar results and were in disagreement with our measurement. A subsequent experiment by our group was carried out with a different technique to verify the results. In this experiment, an active target and detector system measures both the heavy Mg recoils as well as the incoming Na beam, thus avoiding normalization errors [3]. The new results [3] are in agreement with the reported results [1,2] and also with the values in our Letter, within their experimental uncertainties, if the down-scale factor is correctly included.

^{23}

The 33S(p,\gamma)34Cl reaction is important for constraining predictions of certain isotopic abundances in oxygen-neon novae. Models currently predict as much as 150 times the solar abundance of 33S in oxygen-neon nova ejecta. This overproduction factor may, however, vary by orders of magnitude due to uncertainties in the 33S(p,\gamma)34Cl reaction rate at nova peak temperatures. Depending on this rate, 33S could potentially be used as a diagnostic tool for classifying certain types of presolar grains. Better knowledge of the 33S(p,\gamma)34Cl rate would also aid in interpreting nova observations over the S-Ca mass region and contribute to the firm establishment of the maximum endpoint of nova nucleosynthesis. Additionally, the total S elemental abundance which is affected by this reaction has been proposed as a thermometer to study the peak temperatures of novae. Previously, the 33S(p,\gamma)34Cl reaction rate had only been studied directly down to resonance energies of 432 keV. However, for nova peak temperatures of 0.2-0.4 GK there are 7 known states in 34Cl both below the 432 keV resonance and within the Gamow window that could play a dominant role. Direct measurements of the resonance strengths of these states were performed using the DRAGON recoil separator at TRIUMF. Additionally two new states within this energy region are reported. Several hydrodynamic simulations have been performed, using all available experimental information for the 33S(p,\gamma)34Cl rate, to explore the impact of the remaining uncertainty in this rate on nucleosynthesis in nova explosions. These calculations give a range of ~ 20-150 for the expected 33S overproduction factor, and a range of ~ 100-450 for the 32S/33S ratio expected in ONe novae.

Na(

Abstract A new decay-spectroscopy station has been commissioned for experiments with low-energy, fission-fragment radioactive beams from the CARIBU ion source. The new set-up consists of the ‘X-Array’, a highly-efficient array of HPGe clover detectors, and ‘SATURN’ (Scintillator And Tape Using Radioactive Nuclei), a plastic scintillator detector combined with a tape-transport system for detection of β particles and removal of long-lived isobaric decay products.

\alpha

Low-spin states in the neutron-rich, N=90 nuclide Ba146 were populated following β decay of Cs146, with the goal of clarifying the development of deformation in barium isotopes through delineation of their nonyrast structures. Fission fragments of Cs146 were extracted from a 1.7-Ci Cf252 source and mass selected using the CAlifornium Rare Ion Breeder Upgrade (CARIBU) facility. Low-energy ions were deposited at the center of a box of thin β detectors, surrounded by a highly efficient high-purity Ge array. The new Ba146 decay scheme now contains 31 excited levels extending up to ~2.5 MeV excitation energy, double what was previously known. These data are compared to predictions from the interacting boson approximation (IBA) model. It appears that the abrupt shape change found at N=90 in Sm and Gd is much more gradual in Ba and Ce, due to an enhanced role of the γ degree of freedom.

,p)

In our Letter [Phys. Rev. Lett. 112, 152701 (2014)] we reported the direct measurement of the 23Naðα; pÞ26Mg reaction cross section at energies relevant for the production of Galactic Al. Our results, which relied on the extracted absolute cross sections given in Table I, have been found to be in error, overestimating the reported cross sections by a factor of 100. In the experiment, protons from the reaction were measured in an annular silicon strip detector placed downstream from a cryogenic He gas target. The cross sections were normalized to the yield of scattered Na ions from a separate Au foil in an upstream monitor detector. The data acquisition system was triggered by a logic “OR” of the proton detector and the “downscaled” monitor detector. The monitor detector rate was downscaled in order to reduce dead time in the data acquisition system. The down-scale factor was n 1⁄4 100, while in the analysis, the factor was mistakenly taken as n 1⁄4 1. Therefore, the cross section numbers given in Table I should be divided by a factor of 100. The stellar rate reported in our Letter should also be down scaled by the same factor of 100, which makes it in agreement, within the experimental uncertainties, with the recommended rate. This problem came to light due to results from recent experiments where the same reaction was studied in regular and inverse kinematics [1,2]. Those studies obtained similar results and were in disagreement with our measurement. A subsequent experiment by our group was carried out with a different technique to verify the results. In this experiment, an active target and detector system measures both the heavy Mg recoils as well as the incoming Na beam, thus avoiding normalization errors [3]. The new results [3] are in agreement with the reported results [1,2] and also with the values in our Letter, within their experimental uncertainties, if the down-scale factor is correctly included.

^{26}

Using an array of high-purity Compton-suppressed germanium detectors, we performed an independent measurement of the

Mg Reaction Cross Section at Energies Relevant for the Production of Galactic

\beta