A high efficiency, low background detector for measuring pair-decay branches in nuclear decay

We describe a high efficiency detector for measuring electron-positron pair transitions in nuclei. The device was built to be insensitive to gamma rays and to accommodate high overall event rates. The design was optimized for total pair kinetic energies up to about 7 MeV.Comment: Accepted for publication by Nucl. Inst. & Meth. in Phys. Res. A (NIM A

Initial operation of the recoil mass spectrometer EMMA at the ISAC-II facility of TRIUMF

The Electromagnetic Mass Analyser (EMMA) is a new vacuum-mode recoil mass spectrometer currently undergoing the final stages of commissioning at the ISAC-II facility of TRIUMF. EMMA employs a symmetric configuration of electrostatic and magnetic deflectors to separate the products of nuclear reactions from the beam, focus them in both energy and angle, and disperse them in a focal plane according to their mass/charge (m∕q) ratios. The spectrometer was designed to accommodate the γ-ray detector array TIGRESS around the target position in order to provide spectroscopic information from electromagnetic transitions. EMMA is intended to be used in the measurement of fusion evaporation, radiative capture, and transfer reactions for the study of nuclear structure and astrophysics. Its complement of focal plane detectors facilitates the identification of recoiling nuclei and subsequent recoil decay spectroscopy. Here we describe the facility and report on commissioning efforts

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 (α, 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, ℓ assignments made and spectroscopic factors extracted. The energy centroids of the single-particle strength have been deduced and the observed trends are discussed. © Published under licence by IOP Publishing Ltd

Study of valence neutrons in \u3csup\u3e136\u3c/sup\u3eXe 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

Stretched states in B 12,13 with the (d,α) reaction

The (d,α) reaction is highly selective, favoring final states in which the removed neutron and proton are completely aligned in a J=2j configuration. We have studied the C14,15(d,α)B12,13 reactions in inverse kinematics using the Helical Orbit Spectrometer (HELIOS) at Argonne National Laboratory. In B12, the reaction strongly favors the population of a known 3+ state at 5.61 MeV, and for B13, we observe a possible unreported doublet of states at high excitation energy, probably corresponding to the B12(3+) state coupled to the 1s1/2 neutron from the C15 ground state. In contrast to single-nucleon transfer, deuteron-transfer reactions have not been widely studied with exotic nuclei

Recent results from HELIOS

Transfer and inelastic scattering reactions using light-ion beams and stable or long-lived targets have traditionally provided detailed information on the structure of nuclei near the line of beta stability. Such studies can now be extended to nuclei away from the line of β-stability as radioactive beams are becoming available at a number of facilities around the world, including the CARIBU facility at Argonne. These measurements must, however, be carried out in inverse kinematics, resulting in a loss of the effective experimental resolution when conventional detection schemes are employed. The HELIOS spectrometer is based on a new concept, that is especially well suited for such studies by reducing the resolution problem, providing simple particle identification, and giving high detection efficiency with moderate Si detector area. In this talk, the HELIOS concept and results from the first series of experiments will be presented

Structure of C 14 and B 14 from the C 14,15 (d, He 3) B 13,14 reactions

We have studied the C14,15(d,He3)B13,14 proton-removing reactions in inverse kinematics. The (d,He3) reaction probes the proton occupation of the target ground state, and also provides spectroscopic information about the final states in B13,14. The experiments were performed using C14,15 beams from the ATLAS accelerator at Argonne National Laboratory. The reaction products were analyzed with the HELIOS device. Angular distributions were obtained for transitions from both reactions. The C14-beam data reveal transitions to excited states in B13 that suggest configurations with protons outside the π(0p3/2) orbital, and some possibility of proton cross-shell 0p-1s0d excitations, in the C14 ground state. The C15-beam data confirm the existence of a broad 2- excited state in B14. The experimental data are compared to the results of shell-model calculations

Fusion reactions with the one-neutron halo nucleus C15

The structure of C15, with an s1/2 neutron weakly bound to a closed-neutron shell nucleus C14, 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 C15+Th232 at energies in the vicinity of the Coulomb barrier and compared it to the yield of the neighboring C14+Th232 system measured in the same experiment. At sub-barrier energies, an enhancement of the fusion yield by factors of 2-5 was observed for C15, while the cross sections for C14 match the trends measured for C12,13. © 2011 American Physical Society