Nuclear Structure - Nuclear Astrophysics - Detector Development

Welcome

Nuclear physics is one of the key ingredients to understand how the elements, which we observe in our universe, are synthesized during the life cycle of stars. The heavier elements beyond Iron are formed during different nucleosynthesis processes in explosive stellar scenarios. Among these, the rapid neutron-capture process (r process) produces the most neutron-rich nuclei. Neutron-star mergers, like the recent multi-messenger event GW170817, are believed to be one of the main sites for this process. Another process, the p process, a huge network of different nuclear reactions on seed nuclei, produces proton-rich nuclei, which cannot be synthesized in neutron-capture reactions and the competing β-decays. For this process, Core-Collapse and Type-1A Supernovae are discussed as possible sites.

The research group of Dr. Spieker performs experiments to study the influence of nuclear-structure phenomena on reactions taking place in explosive stellar scenarios. The experiments are performed at the John D. Fox Laboratory at Florida State University. At the heart of the program stands the Super-Enge Split-Pole Spectrograph. A magnetic spectrometer which allows for detailed studies of excited states of the atomic nucleus due to its excellent energy resolution. γ-decay properties of excited states at energies relevant for nucleosynthesis processes can be selectively studied in particle-γ coincidence experiments. For these experiments, the group will build a highly-efficient CeBr3 scintillator array. The group also performs nuclear-structure experiments with GRETINA and the S800 at the National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University. Plans to continue the experiments at the upcoming Facility for Rare-Isotope Beams (FRIB) are in place.

Research Projects for Graduate Students

Research projects, which are listed below, are currently available. Please contact Dr. Spieker directly if you are interested in one of the topics or if you want to get more information.

The microscopic structure of the Pygmy Dipole Resonance

Neutron-star mergers (picture credit: NASA/Swift/Dana Berry) are discussed as one of the main sites for the rapid neutron-capture process. Different nuclear physics inputs shape the final isotopic abundance pattern. A low-lying dipole mode of the atomic nucleus, possibly corresponding to a dipole-type oscillation of the neutron skin (compare lower left corner), strongly influences neutron-capture rates.

Theoretical studies have shown that the excitation strengths to 1- states below and around the neutron-separation threshold critically influence neutron-capture rates in explosive stellar scenarios. Not including the low-lying electric dipole strength, also often denoted by Pygmy Dipole Resonance (PDR), can lead to variations of the capture rates by up to a factor of thousand. It is, however, not clear whether the strength, i.e. γ-ray strength function, will be the same in the excitation and decay channel. More experimental data are needed to study the γ-ray strength function in both channels.

A unique structure change was predicted in stable fp-shell nuclei, which is expected to influence the PDR strength. A sudden increase of the excitation strength has already been observed. If a connection to an underlying structure change can be experimentally proven, important and new information for the formation of this strength in neutron-rich nuclei far off stability would be obtained.

Using the Super-Enge Split-Pole Spectrograph in combination with highly-efficient γ-ray detectors, (d,pγ) experiments will be performed to study the single-particle character and γ-decay behavior of excited 1- states in the fp shell.

Alpha clustering and its connection to enhanced E1 strength - Possible implications for (γ,α) reactions on rare-earth nuclei

α-cluster states can be populated with the (6Li,d) α-transfer reaction. Their γ decay can be studied in coincidence with the residual deuterons. In combination with data from (γ,γ') experiments, information about the (γ,α) reaction, relevant for the p process, can be obtained.

As part of the p process, the rare-earth nuclei 144,146Sm and their neighbors are produced in photodisintegration reactions on seed nuclei, which were synthesized during the slow neutron-capture process (s process). (γ,α) reactions become increasingly important when approaching the N = 82 magic neutron shell closure because of the comparably low Qα values. However, direct measurements of this reaction on heavy nuclei are extremely challenging.

The goal of this project is to populate possible A+α structures in rare-earth nuclei via the (6Li,d) α-transfer reaction and to study their γ decay in coincidence with the residual deuterons, which will be detected with the Super-Enge Split-Pole Spectrograph. In a macroscopic picture, α-cluster states are expected to feature enhanced electric dipole (E1) strength due to their intrinsic mass asymmetry. If these states exist, their enhanced E1 strength will lead to a strong excitation with real photons and their pre-clustered structure to a larger α-decay width. Consequently, α-cluster states can strongly influence the outcome of (γ,α) reactions on rare-earth nuclei.

Octupole collectivity in the neutron-deficient Kr isotopes

The excitation energy of the first 3- state clearly minimizes around 72Kr (Z = 36, N = 36). Enhanced octupole correlations could be observed. Selective experiments are needed.

Atomic nuclei close to 72Kr are expected to feature enhanced octupole correlations since both proton and neutron single-particle levels are close to the Fermi surface, which differ by ΔJ = Δl = 3. Microscopic, theoretical calculations predicted only small octupole strength in this mass region, which is at odds with the experimental systematics gathered in the stable Kr isotopes. At the same time, these calculations underlined that the strength fragmentation is intimately connected to the type of quadrupole ground-state deformation. During the last two decades, the latter has been accessed in various experimental studies which revealed a delicate interplay between prolate and oblate configurations at low excitation energies, which is still a hot topic in nuclear-structure physics.

Inelastic proton scattering experiments in inverse kinematics will be performed at the National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University using GRETINA, the S800 spectrograph and the URSINUS College Liquid Hydrogen Target. The experimental approach will be used and the program continued at the Facility for Rare-Isotope Beams (FRIB), which is currently being built at Michigan State University.

Implementing particle-γ coincidences at the Super-Enge Split-Pole Spectrograph - A new CeBr3 γ-ray detection array

A particle-γ coincidence matrix recorded with the SONIC@HORUS setup at the University of Cologne. Particle-γ coincidences are a powerful tool in nuclear physics to selectively study the γ decays of excited states to different final states of the atomic nucleus of interest. This example shows how ground-state decays and decays to the first 2+ state of 112Sn can be distinguished.

In order to perform the particle-γ coincidence experiments outlined above, a new γ-ray detection array will be built at the John D. Fox Accelerator Laboratory at Florida State University. Low-background CeBr3 scintillation detectors will be tested for this array and the γ branch will be merged with the SPS digital data-acquisition system. In addition, a new target-chamber concept will need to be developed to minimize the distance of the γ-ray detectors to the target.

The moderate γ-ray energy resolution of the CeBr3 detectors is compensated for by the excellent particle-energy resolution of the SPS, which allows to select single excitations or groups of excited states. Additional selectivity to excited states will be gained by using different reactions, which probe different classes of excited states, and by measuring angular distributions with the Split-Pole. Ambiguities for Spin-Parity assignments will be resolved by measuring particle-γ angular correlations. The experimental methods for these measurements will be developed.

Dr. Mark Spieker

Assistant Professor


About

In 2017, Dr. Spieker received the title of Dr. rer. nat. in Experimental Physics (PhD equivalent) from the Faculty of Mathematics and Natural Sciences of the University of Cologne in Germany. As part of the group of Professor Andreas Zilges, he performed particle-gamma coincidence experiments to study different aspects of nuclear structure. From November 2017 to October 2019, he worked at the National Superconducting Cyclotron Laboratory at Michigan State University as NSCL fellow with Professor Alexandra Gade. Dr. Spieker joined Florida State University as Assistant Professor in October 2019. He is currently a member of the GRETINA-GRETA User Executive Committee.

email: mspieker at fsu.edu phone: (850)644-2066 office: Keen Bldg. - Room 217