J. C. Lighthall

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.

Studying X-ray burst nucleosynthesis in the laboratory

Type I X-ray bursts are the most common explosions in the Galaxy; however, the nucleosynthesis that occurs during the thermonuclear runaway and explosion is poorly understood. In this proceedings we discuss current experimental efforts and techniques that are being used to study X-ray burst nucleosynthesis in the laboratory. Specifically, radioactive ion beam techniques that have recently been developed have allowed the study of some of the most important (α, p) reactions in X-ray bursts for the first time.

HELIOS - progress and possibilities

The helical orbit spectrometer, HELIOS, at Argonne National Laboratory has been developed to measure transfer reactions in inverse kinematics with good Q-value resolution. The technique is discussed alongside examples of measurements with medium-mass beams, the first exploration of reactions in the the forward hemisphere, and a future outlook.

Trends in the g7/2 and h11/2 neutron single-particle energies in N = 51 isotones

The energies of the g7/2 and h11/2 neutron orbitals in N = 51 isotones have been investigated. The single-neutron adding reactions (d,p) and (?, 3He) have been performed on 88Sr, 90Zr and 92Mo targets, at beam energies of 15 MeV and 50 MeV, respectively. These measurements were supplemented by studying the d(86Kr,p)87 Kr reaction at an energy of 10 MeV/u, in inverse kinematics. Absolute cross sections were measured, ? assignments made and spectroscopic factors extracted. The energy centroids of the single-particle strength have been deduced and the observed trends are discussed.

Study of valence neutrons in 136Xe with HELIOS

The single-neutron adding (d,p) reaction has been performed on 136Xe in inverse kinematics at 10 MeV/u. The position, time-of-flight, and energy of the outgoing protons were analyzed by the new helical orbit spectrometer, HELIOS, at Argonne National Laboratory. An excitation-energy resolution of 100 keV was obtained in the outgoing proton spectra. The experimental setup is described, along with a technique of extracting absolute cross sections. Data are shown which illustrate the performance of the device. This measurement clearly demonstrates the potential of HELIOS for future heavy radioactive-beam studies.

{sup 15}C(d,p){sup 16}C Reaction and Exotic Behavior in {sup 16}C

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

THE HELIOS SPECTROMETER AND THE RADIOACTIVE BEAM PROGRAM AT ARGONNE

The near-term radioactive beam capabilities of ATLAS include radioactive beams produced in flight in a gas cell, or starting in the fall of 2009, re-accelerated beams of 252Cf fission fragments provided by the new CARIBU injector. The availability of such exotic beams will allow for detailed studies of the single-particle aspects of nuclear structure in neutron-rich nuclei reaching out to the astrophysical r-process path by employing light-ion reactions in inverse kinematics. The HELIOS spectrometer is based on a new concept that is especially well suited for such studies. This concept was recently demonstrated using the reactions D(28Si,p)29Si with a (stable) 168 MeV 28Si beam. Since then D(12B,p)13B, D(17O,p)18O, and D(15C,p)16C have been studied successfully. The combination of neutron-rich beams from CARIBU and the HELIOS spectrometer opens a fertile research area of precision studies of the single particle strengths and collective excitations in exotic nuclei, and is likely to have applications in other reactions as well.

12B(n,γ)—the influence on r‐process nucleosynthesis of light elements

Because of interest in the 11,12B(n,γ) reaction in seeding r‐process nucleosynthesis through light neutron‐rich nuclei, we have measured the 12B(d,p) reaction for the first time using the ATLAS in‐flight facility at Argonne National Laboratory. We also measured the 11B(d,p) reaction in the same way for calibration. The spectroscopic factors of excited states and the branching ratio of the neutron‐unbound state in 12B are obtained from the current experiment and the reaction rate for 11B(n,γ) is discussed in comparison with the theoretical prediction.

The Structure of 7He

We have studied the properties of the unbound isotope of helium 7He, using the 2H(8Li,3He)7He reaction. When combined with prior measurements of the 2H(6He,p)7He reaction the data present a consistent picture for the low‐lying excited states of 7He, specifically the negative parity sequence 3/2− (ground state) 1/2− (first‐excited) and 5/2− (second‐excited). The shapes but not the absolute magnitudes of the angular distributions are reproduced by ab‐initio theory coupled with a DWBA reaction framework.