The first paper from transfer reactions at TRIUMF has been published: The low energy excited 0+2,3 states in 96Sr are amongst the most prominent examples of shape coexistence across the nuclear landscape. In this work, the neutron (2s1/2)2 content of the 0+1,2,3 states in 96Sr was determined by means of the d(95Sr, p) transfer reaction at the TRIUMF-ISAC2 facility using the SHARC and TIGRESS arrays. Spectroscopic factors of 0.19(3) and 0.22(3) were extracted for the 96Sr ground and 1229 keV 0+ states, respectively, by fitting the experimental angular distributions to DWBA reaction model calculations. A detailed analysis of the γ-decay of the isomeric 0+3 state was used to determine a spectroscopic factor of 0.33(13). The experimental results are compared to shell model calculations, which predict negligible spectroscopic strength for the excited 0+ states in 96Sr. The strengths of the excited 0+2,3 states were also analyzed within a two-level mixing model and are consistent with a mixing strength of a2 = 0.40(14) and a difference in intrinsic deformations of |Δβ|=0.31(3). These results suggest coexistence of three different configurations in 96Sr and strong shape mixing of the two excited 0+ states. S. Cruz et al.,"Shape Coexistence and Mixing of Low-Lying 0+ States in 96Sr", Physics Letters B 786 (2018) 94
Spectroscopy of 70Kr has been published. Excited states in the nucleus 70Kr have been populated using inelastic scattering of a radioactive 70Kr beam as well as one- and two-neutron removal reactions from 71,72Kr at intermediate beam energies. The level scheme of 70Kr was constructed from the observed γ-ray transitions and coincidences. Tentative spin and parity assignments were made based on comparison with the mirror nucleus 70Se. A second 2+ state and a candidate for the corresponding 4+2 state suggest shape coexistence in 70Kr. K. Wimmer et al., "Shape coexistence and isospin symmetry in A = 70 nuclei: Spectroscopy of the Tz = -1 nucleus 70Kr", Physics Letters B 785 (2018) 441
An invited topical review paper on transfer reactions has been published in Journal of Physics G. K. Wimmer et al., "Nucleon transfer reactions with radioactive beams", Journal of Physics G 45 (2018) 033002
The second paper on two-neutron transfer reactions using radioactive beams and a radioactive target has been published. The IS499 experiment investigated the shell closure at neutron number 28 using the t(44Ar,p)46Ar at REX-ISOLDE (CERN). We could identify a new excited 0+ state and measure it's cross section. The existence of a low-lying 0+ state indicates shape coexistence at the N=28 shell closure. Our results are compare with state-of-the-art shell model calculations using various effective interactions. K. Nowak, K. Wimmer et al.,"Spectroscopy of 46Ar by the (t,p) two-neutron transfer reaction", Physical Review C 93 (2016) 044335
The first paper from NSCL experiment e12003 has been published. For the analysis of the excitation energy spectrum of 28Na we combined data from a β decay experiment performed at GANIL with our in-beam spectroscopy from NSCL. We have identified several new states, among them candidates for negative parity states. Together with theoretical calculations we can predict the evolution of negative parity states towards the neutron drip line. A. Lepailleur, K. Wimmer et al.,"Spectroscopy of 28Na: shell evolution toward the drip line", Physical Review C 92 (2015) 054309
Experiment at NSCL successfully completed
Our experiment to study the singe-particle structure at N=28 through nucleon knockout reactions has finished. We used the S800 spectrograph, the GRETINA γ-ray spectrometer, and the IsoTagger developed by our group to study 43,44S.
New experiment at RIBF approved
A proposal for a new experiment at RIBF has been approved by the Program Advisory Committee. The experiment "Magicity at N=32: Collective and single-particle properties of 52Ca" will investigate the new magic number N=32 by two complementary methods.
Our paper on measurements of the elastic breakup fraction of well bound nucleons has been published in Physical Review C. In this work we look in great detail at the one-proton removal reaction at intermediate beam energies. We deduce the contributions from the elastic and inelastic nucleon removal mechanisms and compare them with the eikonal reaction model predictions. The measured and calculated elastic breakup fractions are found to be in good agreement. K. Wimmer et al., "Elastic breakup cross sections of well-bound nucleons", Physical Review C 90 (2014) 064615
Opening for Postdocs at CNS
Opening of Postdoctoral Researchers at Center for Nuclear Study, University of Tokyo
CNS has several devices in RIBF in RIKEN Nishina Center and pursues the experimental nuclear physics such as nuclear astrophysics and nuclear structure/reaction studies with radioactive isotope beams. We also have branches of high-energy nuclear physics, nuclear theory and development of ion sources. APPLICATION MATERIALS: (1) CV (2) List of Publication/Talks/etc. (3) Research summary and future plan at CNS (4) Names and contact details of reference will be sent as a combined PDF file (1-4) to n.imai _at_ cns.s.u-tokyo.ac.jp ("_at_" replaced by "@") with a subject field "Application to PostDoc at CNS ". Interested candidates are encouraged to contact the relevant researchers at CNS.
Our paper describing a new experimental technique to tag the population of isomeric states in secondary fragmentation reactions has been published in Nuclear Instruments and Methods A. This new approach is crucial for the analysis of experiments done at the National Superconducting Cyclotron Laboratory (NSCL). K. Wimmer et al., "IsoTagger: Identification of isomeric nuclear states produced in fragmentation reactions with radioactive beams", Nuclear Instruments and Methods A 769 (2015) 654
Moved to Tokyo
I recently moved from Central Michigan University to The University of Tokyo. The new position is a Lecturer at the Department of Physics, with adjunct appointments at the Center for Nuclear Study and RIKEN RI Beam Factory.
Our paper titled "Isotopic 32S/33S ratio as a diagnostic of presolar grains from novae" has been published in Physics Letters B. A measurement of the isotopic abundances of sulfur isotopes in grains could help to discriminate between grains from novae and other stellar environments, like type-II supernova explosions. However, in order to use the 32S/33S isotopic ratio as a production site discriminant a much better understanding on the production of sulfur in novae is required. For a more precise determination of the 33S(p, γ)34Cl at temperatures encountered in novae explosions, we have investigated states in 34Cl through the 33S(3He, d)34Cl reaction at the MLL tandem accelerator in Munich. Experimentally extracted spectroscopic factors allow us to determine the 33S(p, γ) resonance strengths of the key low-energy resonances. Using these new results in hydrodynamic nova models we show that the present uncertainty in the 33S(p, γ)34Cl rate results in uncertainties of ≤20 % for nova yields of species between S and Ca. This would allow to use the 32S/33S isotopic ratio to distinguish between presolar grains of nova and supernova origin. A. Parikh, K. Wimmer et al., "Isotopic 32S/33S ratio as a diagnostic of presolar grains from novae" Physics Letters B 737 (2014) 314
Record intensities of radioactive beams achieved at TRIUMF
This week the S1389 experiment team achieved record beam intensities at the radioactive beam facility TRIUMF in Vancouver, Canada. The experiment, lead by scientist at TRIUMF and the CMU nuclear structure group, was devoted to study single-particle properties in the neutron-rich strontium, Sr, isotopes. In these nuclei the shape of the ground state rapidly changes from round, like a soccer ball, to strongly deformed, like an American football. Two CMU students, graduate student Ntiana Sachmpazidi and undergrad Nick Terpstra went to Vancouver to participate and help with the experiment. The experiment was extremely successful, record beam intensities of 10 million particles per second were achieved. The beam intensity was so high that within a few days it burnt a hole into the target foil (shown on the right).
|Ntiana Sachmpazidi in front of the experimental setup at TRIUMF||Nick Terpstra (right) discussing the experiment with collaborators|
New experiment at NSCL approved
A proposal for a new experiment at the National Superconducting Cyclotron Laboratory has been approved by the Program Advisory Committee (PAC 38). The experiment "Single-particle structure at N=28" will investigate the triple shape coexistence in 44S by neutron removal reactions.
First paper on results from the Gretina campaign at NSCL published
The first paper describing results from the 12 month campaign of the gamma-ray tracking array Gretina at the National Superconducting Cyclotron Laboratory has been published in Physical Review C as a rapid communication. The article was highlighted as an editor's suggestion. F. Recchia, et al., "Configuration mixing and relative transition rates between low-spin states in 68Ni" Physical Review C 88 (2013) 041302(R)
Undergraduate student conducts world-class nuclear physics research in Canada
Physics undergraduate student Caleb Bancroft recently spent eight days at TRIUMF laboratory in Vancouver, Canada conducting nuclear physics research. Caleb prepared and performed an experiment that investigated the shape of very exotic atomic nuclei.
TRIUMF is one of the world's leading subatomic physics laboratories. In addition to basic research in nuclear and particle physics, TRIUMF researchers use accelerators to produce medical isotopes which are, for example, used in the diagnostics and treatment of cancer.
The experiment that Caleb participated in involved an international team, led by CMU assistant professor of physics Kathrin Wimmer. Their goal was to explore the phenomenon of shape coexistence in heavy Strontium isotopes. While some nuclei are round like soccer balls, others exhibit a more deformed shape, along the lines of what an American football looks like. In the case of the Strontium isotopes, the transition between spherical and deformed nuclei is very abrupt - only two additional neutrons change the shape of the nucleus completely.
In order to investigate this sudden change, a beam of radioactive 94Sr, consisting of 38 protons and 56 neutrons, was guided to the experiment where it collided with a foil containing Deuterium, a special form of Hydrogen. In some cases, only one neutron was transferred in the collision onto the 94Sr nucleus, making it 95Sr with 57 neutrons. The properties of this newly formed 95Sr were then investigated in great detail to draw conclusions about the rapid shape change in the Strontium nuclei.
The success of the experiment was a major breakthrough for the TRIUMF facility. Before, the heaviest beam that was accelerated and used for experiments at TRIUMF had a mass number of 30. Now, it is possible to do experiments with nuclei that are over three times more massive. This allows researchers to do many more exciting experiments and gives CMU physics majors the opportunity to participate in cutting-edge science.
GrROOT version 2.0 with tracking has been released today. Download here (password protected)
Our paper on the 18F(p,α)15O reaction rate has been published in Physical Review Letters. Novae are nuclear explosions in white dwarf stars. They are caused by the accretion of hydrogen from a companion star in a binary system onto the surface of the white dwarf. The burning of hydrogen is thermally unstable leading to an explosion and large amounts of hydrogen are rapidly converted into heavier elements. The γ-ray emission from unstable nuclei could be measured experimentally. One of these isotopes is 18F, however the amount of 18F produced in a nova is influenced by the 18F(p,α)15O reaction which destroys 18F. The 18F(p,α)15O reaction proceeds through the compound nucleus 19Ne, and the reaction rate is strongly influenced by the structure of 19Ne. We have studied states in 19Ne using the 19F(3He,t)19Ne reaction and determined the spin and parity of around the proton threshold. The new experimental data strongly influences theoretical predictions for the amount of 18F produced in nova explosions. This, in turn, affects the maximum distance at which novae can be observed by detection of their γ-ray emission. A.M. Laird, et al., "Is γ-ray emission from novae affected by interference effects in the 18F(p,α)15O reaction?" Physical Review Letters 110 (2013) 032502
GrROOT version 1.2 has been released today. Apart from several bug fixes this new version supports now several new detection systems and provides a faster way to analyze data by splitting the tree into several sub-trees. Download here (password protected)
The new webpage has been launched