An overview of the decay spectroscopy experiments being carried out by scientists led by Professor Corina Andreoiu from the Department of Chemistry at Simon Fraser University.
Professor Corina Andreoiu works in the Nuclear Structure Group in the Department of Chemistry at Simon Fraser University (SFU), which is located in Burnaby, British Columbia, Canada. The group is currently undertaking decay spectroscopy experiments using stable and radioactive beams primarily at TRIUMF, Canada’s accelerator centre in Vancouver. The work presented here was carried out in collaboration with the GRIFFIN and FIPPS collaborations.
Conducting decay spectroscopy experiments using rare isotope beams
SFU and TRIUMF have a long collaborative history; in fact, SFU was one of the three founding members of TRIUMF, alongside the University of British Columbia and University of Victoria. This explains the origin of TRIUMF’s acronym — TRI-University Meson Factory; this has stuck with the institution since its inception despite its evolution to a consortium of currently 14 Canadian universities.
At TRIUMF, Andreoiu’s group is currently performing decay spectroscopy experiments using rare isotope beams (RIBs) produced by the ISOL technique in the ISAC facility of TRIUMF. RIBs are generated via the interaction of a 520-MeV proton beam on a heavy production target, (mostly used are Ta and UCx targets) coupled to various ion sources for extracting the beams according to their chemistry.
The ISAC facility is the descendent of the TISOL facility developed by an SFU Chemistry Professor, J. D’Auria, in collaboration with TRIUMF; TISOL produced the first radioactive beam at TRIUMF in 1985.
Analysing isotopes in tin
Andreoiu’s main interest is the study of nuclei close to the Z = 50 magic number of protons that confers extra stability to nuclei, similarly to the closed shells of noble gases. This enhanced stability manifests in the highest number (10) of stable isotopes for the element 50 on the periodic table, tin. This stability also results in a high excitation energy of the first excited state in the even-mass tin isotopes of about 1.2 MeV, as well as almost spherical shapes, which are attributes of closed-shell nuclei.
Researchers at SFU have investigated excited states in tin through the beta decay of their corresponding radioactive indium beams that are produced in both their ground state and one to two metastable states.
The indium beams are implanted on a Mylar tape in the middle of a high-efficiency Compton-suppressed array of up to 16 high-purity germanium clover detectors (HPGe) known as Gamma Ray Infrastructure For Fundamental Investigations of Nuclei (GRIFFIN) (Fig. 1). These are utilised for the detection of photons emitted from excited states in tin when they decay to the lowest energy, known as the ground state.
In addition, the geometry of GRIFFIN allows for angular correlation measurements for angular momentum determination of nuclear states. The beta particles emitted by In are detected by plastic scintillators placed downstream, while conversion electrons are detected by five Si (Li) detectors. Inorganic scintillator LaBr3(Ce) detectors coupled with GRIFFIN can be used for fast lifetime measurements of nuclear levels.
The Mylar tape moves in cycles behind a Pb wall according to the half-life of the RIB to maximise the activity of the source and create a fresh implantation spot. This allows for background measurements and determination of the half-life of the states in indium or tin.
Investigating different shape coexistence
At stability, 114,116,118Sn, with neutron number N = 64,66,68, are situated in the middle between the N = 50 and N = 82 closed shells. The research group studied these mid-shell to investigate deformed rotational structures built upon excited 0+ states which are based on two-particle two-hole excitations across the Z = 50 proton shell gap.
Andreoiu’s group was interested to learn how a nucleus that displays single-particle excitations at low energies can shift its nuclear potential and exhibit collective phenomena. This collectivity manifests through a phenomenon called shape coexistence in a narrow energy gap and has been experimentally observed for the three isotopes of interest.
However, the degree of collectivity from one isotope to the next in these intruder states seems to differ and has been the focus of the team’s recent experimental beta decay studies. Results on the two-particle two-hole states populated via beta decay of 116In and 118In have been published for 116Sn and 118Sn, respectively, and recent experimental data on the beta decay of 114Sb into 114Sn is currently being analysed.
Exploring regions of magicity
Additionally, to the work on the tin isotopes, other regions of magicity have been explored by the Andreoiu group. Notably, in the region around doubly magic 78Ni, the 80Ge nucleus is of interest, since it lies near what has been proposed as the fifth island of inversion.
Furthermore, a prior study of this nucleus had reported an excited 0+ state below the first excited 2+ state, indicating the presence of shape coexistence in this nucleus. This potentially indicates the boundaries of the island, as this phenomenon is known to occur at its shores.
However, a recent experimental study of the same nucleus using the GRIFFIN spectrometer, its Si(Li)s (PACES), beta-tagging (SCEPTAR) and fast-timing (LaBr3) arrays has been carried out to observe the beta-decay of 80Ga into excited states of 80Ge. Researchers were unable to observe the presence of this state, despite the use of a superior detector system and higher statics afforded to the TRIUMF experiment. Theoretical calculations undertaken in concert with this experiment were also unable to reproduce this low-lying state, placing it in the range of 2 MeV.
This experiment also uncovered the presence of a multitude of newly observed energy states and transitions. Subsequent studies of the same nucleus corroborated our findings, experimentally where no observation was made of the alleged excited 0+ state at 639 keV and theoretically where calculations placed it in the expected ~2 MeV range. The capabilities of the GRIFFIN spectrometer to uncover key nuclear structure information have been shown time and time again, proving that there is still much to learn about the nature of the structure of matter.
Studying pygmy dipole resonances and pygmy quadrupole resonances
A new research interest of the group is the study of pygmy dipole resonances (PDR) and pygmy quadrupole resonances (PQR). The PDR phenomenon describes the presence of additional electric dipole strength in the region of the neutron separation energy; this manifests as a resonance-like structure of 1– levels. The PDR is interpreted in a geometric picture as an out-of-phase of a neutron-skin against an N = Z core. This interpretation of a neutron-skin oscillation has led to theoretical approaches to connect the neutron skin to the symmetry term of the nuclear binding energy and the nuclear equation of state.
However, this interpretation is being debated and other theories have been proposed. The levels associated with the PDR are understood to impact neutron capture rates in nuclei and therefore influence astrophysical calculations for processes such as the r-process, which is responsible for the creation of around half of the elements heavier than iron.
To pursue this interesting phenomenon, the group recently performed an experiment to investigate levels associated with the PDR in 92Sr with GRIFFIN. This was achieved by exploiting the beta decays of the parent nuclei 92Rb. The large Q value of the decay combined with the low-spin of the ground-state of 92Rb are ideal conditions to populate the levels associated with the PDR in 92Sr. Utilising beta decay to study Pygmy Quadrupole Resonances is a rather new approach and is only applicable for a small number of nuclei where conditions are favourable and will reveal more information about this phenomenon than has been learned with standard experimental methods.
While the PDR has been investigated theoretically and experimentally through numerous studies, the PQR has only recently been identified. Experimental results have concluded PQR states in 124Sn as a grouping of 2+ levels in the 3 to5 MeV region which are supported by theoretical studies. Furthermore, studies on 112, 114Sn isotopes have also revealed a low-energy quadrupole mode in the 3 to5 MeV region, which is believed to be attributed to the PQR. With this being a new topic of interest for the TRIUMF group, the team is looking to identify the PQR in 116Sn and 118Sn via thermal neutron capture experiments. This is expected to further enhance the entire level scheme of these nuclei and the scientists’ understanding of their structure.
Experiments using the Fission Product Prompt gamma–ray Spectrometer
Experimental studies of 116,118Sn were performed recently at the Institut Laue-Langevin (ILL) in Grenoble, France. The research reactor of the ILL produces a high flux of neutrons via fission of the reactor fuel element, which are then cooled using neutron moderators. Excited states in 116,118Sn were populated following neutron capture of 115,117Sn stable targets.
These targets were placed in the centre of the Fission Product Prompt gamma–ray Spectrometer (FIPPS), an array of eight HPGe clover detectors, which could observe de-excitation of 116,118Sn via gamma-ray emission. The statistical decay of the capture state enables access to multiple levels in 116,118Sn ar which can be inaccessible with other experimental probes. Researchers hope that this access will help to reveal further information regarding the structure of these isotopes.
Unfortunately, it is not feasible to perform an experimental study of 114Sn using neutron capture due to the unstable nature of 113Sn. Therefore, researchers performed an experiment to study excited states in 114Sn by utilising beta+/Electron Capture decay of 114Sb with GRIFFIN. An intense beam of 114Sb at a rate of approximately 500,000 particles per second was delivered to the centre of the GRIFFIN spectrometer and a data set with significant statistics was collected. This data will provide precise information on beta-feeding, gamma-ray branching ratios, lifetime measurements via the fast-coincidence method and E0 measurements. This experiment further offers insight into the two-particle two-hole intruder states, as well as the possible population of PQR states which are expected to lie well below the beta-decay Q-value.
Studies far from stability
Far from stability around the 82-neutron shell gap, 129,131,132Sn have been studied via the beta decay of corresponding indium beams that are produced at TRIUMF in both their ground and isomeric states. Due to the high-efficiency of the experimental set-up, the team was able to improve beta-delayed neutron values for 131,132Sn and observe a new beta-decay branch in 129Sn. By expanding the knowledge and information available near two regions of magicity, the group provided crucial inputs to improve the nuclear shell model, especially in the case of 131Sn.
In addition to the research conducted, the group was heavily involved in the validation of the detectors used in the GRIFFIN array. The Compton suppression shields of HPGe and LaBr3(Ce) detectors were tested in house at SFU before being implemented.
This work was supported in part by the Natural Sciences and Engineering Research Council of Canada. The GRIFFIN infrastructure has been funded jointly by the Canada Foundation for Innovation, the University of Guelph, TRIUMF, the British Columbia Knowledge Development Fund, and the Ontario Ministry of Research and Innovation.
TRIUMF receives federal funding via a contribution agreement through the National Research Council Canada. We thank the GRIFFIN and FIPPS collaborations, and TRIUMF for their support in obtaining these results.
D Annen, I Djanto, F H Garcia, K Ortner, P Spagnoletti