ISAC-II Facility Research Articles

Investigating nuclei produced in 9Li +11B reaction

In this contribution, a preliminary analysis of the first part of the experiment S2012 conducted at the ISAC-II facility of Canada’s particle accelerator center TRIUMF in Vancouver will be presented. The experiment aims to study highly clustered structures of nuclei created in multi-nucleon transfer reactions of 9Li radioactive beam on natural boron target (11B and 10B). The main objective of the experiment is to study exotic structures created in neutron-rich 16C nucleus in the range of higher excitation energies. The analysis presented here probes the existence of exotic cluster configurations and the quality of detected results using the invariant mass techniques.

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 gamma-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.

TRIFIC: The TRIUMF Fast Ion Counter

Abstract The TRIUMF Fast Ion Counter (TRIFIC) is a tilted-grid gas counter detector for identifying the impurities in radioactive ion beams (RIBs). Placed at zero degrees downstream of a primary reaction target at the TRIUMF ISAC-II facility, TRIFIC provides accurate Z identification of beam species on an event-by-event basis at a rate of up to 5 × 1 0 5 particles per second and functions as a beam composition monitor at rates up to 5 × 1 0 7 particles per second. With this additional level of diagnostics, RIB experiments at TRIUMF are able to reach levels of sensitivity not previously possible. Details regarding the construction and instrumentation of TRIFIC as well as results from the commissioning of the detector are presented.

In-beam internal conversion electron spectroscopy with the SPICE detector

The SPectrometer for Internal Conversion Electrons (SPICE) has been commissioned for use in conjunction with the TIGRESS γ-ray spectrometer at TRIUMF’s ISAC-II facility. SPICE features a permanent rare-earth magnetic lens to collect and direct internal conversion electrons emitted from nuclear reactions to a thick, highly segmented, lithium-drifted silicon detector. This arrangement, combined with TIGRESS, enables in-beam γ-ray and internal conversion electron spectroscopy to be performed with stable and radioactive ion beams. Technical aspects of the device, capabilities, and initial performance are presented.

Recoil distance method lifetime measurements at TRIUMF-ISAC using the TIGRESS Integrated Plunger

The TIGRESS Integrated Plunger device (TIP) has been developed for recoil distance method (RDM) lifetime measurements using the TIGRESS array of HPGe γ-ray detectors at TRIUMF’s ISAC-II facility. A commissioning experiment was conducted utilizing a 250 MeV 84Kr beam at ≈2×108 particles per second. The 84Kr beam was Coulomb excited to the 21+ state on a movable 27Al target. A thin Cu foil fixed downstream from the target was used as a degrader. Excited nuclei emerged from the target and decayed by γ-ray emission at a distance determined by their velocity and the lifetime of the 21+ state. The ratio of decays which occur between the target and degrader to those occurring after traversing the degrader changes as a function of the target–degrader separation distance. Gamma-ray spectra at 13 target–degrader separation distances were measured and compared to simulated lineshapes to extract the lifetime. The result of τ=5.541±0.013(stat.)±0.063(sys.) ps is shorter than the literature value of 5.84±0.18 ps with a reduction in uncertainty by a factor of approximately two. The TIP plunger device, experimental technique, analysis tools, and result are discussed.

Recoil distance method lifetime measurement of the 21+ state in Sr94 and implications for the structure of neutron-rich Sr isotopes

A high precision lifetime measurement of the ${2}_{1}^{+}$ state in $^{94}\mathrm{Sr}$ was performed at TRIUMF's ISAC-II facility by coupling the recoil distance method implemented via the TIGRESS integrated plunger with unsafe Coulomb excitation in inverse kinematics. Due to limited statistics imposed by the use of a radioactive $^{94}\mathrm{Sr}$ beam, a likelihood ratio ${\ensuremath{\chi}}^{2}$ method was derived and used to compare experimental data to Geant4 simulations. The $B(E2;{2}_{1}^{+}\ensuremath{\rightarrow}{0}_{1}^{+})$ value extracted from the lifetime measurement of $7.{80}_{\ensuremath{-}0.40}^{+0.50}\text{(stat.)}\ifmmode\pm\else\textpm\fi{}0.07\text{(sys.)}$ ps is approximately 25% larger than previously reported while the relative error has been reduced by a factor of approximately 8. A baseline deformation has been established for Sr isotopes with $N\ensuremath{\le}58$ which is a necessary condition for the quantum phase transition interpretation of the onset of deformation in this region. A comparison to existing theoretical models is presented.

Preliminary study of the10Li nucleus via one-neutron transfer

The structure of the 10 Li unbound nucleus is a subject of large interest and its description is nowadays a matter of debate. We have investigated this system using the d( 9 Li,p) 10 Li one-neutron transfer reaction at 100 MeV in inverse kinematics. The experiment was performed at the ISACII facility at TRIUMF laboratory. The excitation energy spectrum has been reconstructed by measuring the emitted protons at backward angles and the 9 Li at forward angles.

Fabrication of calcium-gold and calcium-lead targets for ion beam experiments at TRIUMF ISAC-II

Multi-layer thin targets are vital in low-energy nuclear physics experiments employing the use of accelerated beams. In this paper, we describe the fabrication of encapsulated calcium targets produced by evaporation on gold and lead backings for radioactive beam experiments at TRIUMF’s ISAC-II facility.

The TIGRESS Integrated Plunger ancillary systems for electromagnetic transition rate studies at TRIUMF

The TIGRESS Integrated Plunger device is a new experimental tool for nuclear structure investigations via gamma-ray spectroscopy with post-accelerated beams from the ISAC-II facility at TRIUMF. Several ancillary detection systems integral to the device׳s capabilities for charged-particle tagging and light-ion identification following a variety of nuclear reaction mechanisms have been constructed and characterized. In particular, a silicon PIN diode wall, an annular silicon segmented detector, and a CsI(Tl) scintillator wall have together enabled particle-gamma correlations for reaction channel selectivity and precision kinematic reconstruction in recent measurements. We highlight the construction, characteristics, and implementation of the device׳s ancillary detectors as they enable a rich set of electromagnetic transition rate measurements via Doppler-shift lifetime techniques and low-energy Coulomb excitation.

Two-neutron transfer reaction mechanisms in12C(6He,4He)14C using a realistic three-body6He model

The reaction mechanisms of the two-neutron transfer reaction $^{12}$C($^6$He,$^4$He) have been studied at 30 MeV at the TRIUMF ISAC-II facility using the SHARC charged-particle detector array. Optical potential parameters have been extracted from the analysis of the elastic scattering angular distribution. The new potential has been applied to the study of the transfer angular distribution to the 2$^+_2$ 8.32 MeV state in $^{14}$C, using a realistic 3-body $^6$He model and advanced shell model calculations for the carbon structure, allowing to calculate the relative contributions of the simultaneous and sequential two-neutron transfer. The reaction model provides a good description of the 30 MeV data set and shows that the simultaneous process is the dominant transfer mechanism. Sensitivity tests of optical potential parameters show that the final results can be considerably affected by the choice of optical potentials. A reanalysis of data measured previously at 18 MeV however, is not as well described by the same reaction model, suggesting that one needs to include higher order effects in the reaction mechanism.

Towards 26Na via (d,p) with SHARC and TIGRESS and a novel zero-degree detector

Nucleon transfer experiments have in recent years begun to be exploited in the study of nuclei far from stability, using radioactive beams in inverse kinematics. New techniques are still being developed in order to perform these experiments. The present experiment is designed to study the odd-odd nucleus 26Na which has a high density of states and therefore requires gamma-ray detection to distinguish between them. The experiment employed an intense beam of up to 3×107 pps of 25Na at 5.0 MeV/nucleon from the ISAC-II facility at triumf. The new silicon array SHARC was used for the first time and was coupled to the segmented clover gamma-ray array TIGRESS. A novel thin plastic scintillator detector was employed at zero degrees to identify and reject reactions occurring on the carbon component of the (CD)2 target. The efficiency of the background rejection using this detector is described with respect to the proton and gamma-ray spectra from the (d,p) reaction.

Narrowing of the neutron [formula omitted] shell gap in 29Na

The wave-function composition for the low-lying states in 29Na was explored by measuring their electromagnetic properties using the Coulomb-excitation technique. A beam of 29Na ions, postaccelerated to 70 MeV, bombarded a 110Pd target with a rate of up to 600 particles per second at the recently commissioned ISAC-II facility at TRIUMF. Six segmented HPGe clover detectors of the TIGRESS γ-ray spectrometer were used to detect deexcitation γ rays in coincidence with scattered or recoiling charged particles in the segmented silicon detector, BAMBINO. The reduced transition matrix element |〈521+||E2||32gs+〉| in 29Na was derived to be 0.237(21) e b from the measured γ-ray yields for both projectile and target. This first-time measured value is consistent with the most recent Monte Carlo shell-model calculation, indicating a significant admixture of both sd and pf components in the wave function, and also providing evidence for the narrowing of the neutron sd–pf shell gap from ∼6 MeV for stable nuclei to ∼3 MeV for 29Na.

Radioactive beam experiments with large gamma-ray detector arrays

High-resolution γ-ray spectroscopy is one of the most powerful and versatile experimental techniques in low-energy nuclear physics research. With the continuing development of hyper-pure germanium (HPGe) detector technology, including multi-crystal detectors, contact segmentation, and digital signal processing techniques, large γ-ray detector arrays will continue to play a major role in the experimental programs at existing and future radioactive ion beam facilities. This paper provides an overview of recent progress in, and future plans for, the development of large γ-ray spectrometers at such facilities, including the recent commissioning of the 8π spectrometer at ISAC-I and the proposed TRIUMF-ISAC gamma-ray escape suppressed spectrometer array for the ISAC-II facility.