Research Group of Dr. Mark-Christoph Spieker

Artistic depiction of nuclear fission (picture credit: Adobe Stock). Nuclear fission is one of the oldest problems in nuclear science. Because of advances in state-of-the-art theory, new questions have recently been asked about the fission dynamics and correlations between the fission fragments. State-of-the-art experiments coupling different detectors can possibly address these open questions.

U.S. citizens only (NNSA project)

Detector-development project

This NNSA-supported program to indirectly study the neutron-induced fission of ²⁴⁰U will use the triton beam at the FSU laboratory, which is currently being developed, in conjunction with the SE-SPS spectrograph. A compact solid-state detector array, serving as a fission detector, will be built within the scope of the project. A design similar to the SCARY array, which was used at the Wright Nuclear Structure Laboratory, or to the SORCERER solar-cell array, which is being used at the IFIN-HH, is envisioned.

The planned ²³⁸U(t,pf) experiment will establish a fission probability distribution as a function of excitation energy. Measuring angular distributions with the SE-SPS will provide access to a possible spin dependence of the fission probabilities. Particle-fission-fragment angular correlations can also be measured thanks to the segmentation of the planned fission detector. Problems of past (t,pf) experiments with Oxygen and Carbon contaminants can be effectively mitigated by performing measurements with the SE-SPS at different angles (should there be any problems), which will shift the contaminants’ position in the focal plane, and, thus, allow to determine the fission probabilities at various angles for consistency checks.

Tracking particles through the focal plane of the SE-SPS with a position sensitive focal plane detector. Currently, two position sensitive anode wires allow to reconstruct the track achieving a position resolution of about 2 mm.

Detector-development project

The current focal-plane detector consists of a position-sensitive proportional counter with two anode wires, separated by about 4.3 cm, to measure position, angle, and energy loss, and a large plastic scintillator to determine the rest energy of the residual particles passing through the detector. Under favorable conditions, the detector can be operated at rates as high as two kilocounts/s (kcps). Both anode wires are inductively coupled to pick-up pads that are read out at the left and right ends of delay lines. The measured time differences allow to track particles through the focal plane of the SE-SPS. As the resolution depends on the solid angle, target thickness and beam-spot size, it may vary from experiment to experiment. In standard operation and with a global kinematic correction, i.e., assuming a vertical shift of the real focal plane with respect to the two position-sensitive sections of the detector, a full width at half maximum of 30-70 keV has been routinely achieved (see red line in figure). This corresponds to a position resolution of about two millimeters. This resolution can be improved further with position-dependent offline corrections, e.g., assuming a linear dependence of the z shift on the focal-plane position (green, dashed line in figure).

To overcome current limitations, plans are in place to design a new focal-plane detector with increased position resolution (~ factor of 4) and higher count rate capabilities (~ factor of 10) based on the multi-layer thick gaseous electron multiplier (M-THGEM) technology and a highly segmented anode pad matched to the M-THGEM design. The M-THGEM-based design also allows us to detect heavier ions opening new possibilities for experimental studies with heavy-ion beams from the combined Tandem+LINAC accelerator system at the FSU John D. Fox Laboratory.

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 and to understand the microscopic origin of the PDR.

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 the CeBrA γ-ray detectors, (d,p) singles and (d,pγ) coincidence experiments will be performed to study the single-particle character and γ-decay behavior of excited 1^- states in the fp shell.

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 Q_α 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.

CeBr₃ detectors around the new Gamma-target chamber at the FSU Super-Enge Split-Pole Spectrograph (CeBrA@SE-SPS).

Characterization of CeBr₃ for γ-ray detection at the Fox Laboratory

γ-ray spectroscopy is an essential tool in Nuclear Physics and Nuclear Astrophysics. Our group has developed a new array, called CeBrA, for particle-γ coincidence experiments at the Super-Enge Split-Pole Spectrograph. This project will show you how γ rays interact with matter, how they can be detected, how to record data and how to analyze data coming from these detectors.

Once you have mastered these items, you will characterize the detectors thoroughly using different standard calibration sources and also learn how coincident detection of γ rays and particles works.

Data-analysis projects

Data-analysis projects are also available. The data were collected at the FSU Super-Enge Split-Pole Spectrograph as well as at the National Superconducting Cyclotron Laboratory of Michigan State University. If you want to know more about these possible projects, contact Dr. Spieker. Some programming skills are needed.