Figure 1: Artist's depiction of a recurrent nova RS Ophiuchi system (a) and a neutron star collision after inspiral (b). (Credit: (a) David A. Hardy/PPARC and (b) NASA/Swift/Dana Berry)
Nuclear Astrophysics is an interdisciplinary research field of nuclear physics and astrophysics, understanding properties of nuclei that help explain both the formation of elements in the universe and the evolution of stellar bodies and cosmic explosions (Figure 1). While we cannot travel to stars and observe what is happening inside them, many indirect astronomical observations have provided clear evidence that nuclear physics plays a major role in creating energy and elements in the stellar environment. An observed abundance pattern of elements as shown in Figure 2, for example, offers one of the most powerful clues to the history of nucleosynthesis processes occurring in the interior of stars. Now, we have a question left how to understand the patterns being made.
After Eddington's hypothesis for the energy production in a star in 1920, we understand that various elements heavier than helium in the Universe were created by a chain of nuclear reactions during the life cycle of stars. First, light elements up to iron are formed during the fusion process inside the stars. A few important reactions of these fusion processes are identified as the pp chain or the CNO cycle. Both processes effectively fuse four hydrogen nuclei into a helium nucleus. The former is the main reaction inside the Sun, for instance, and the latter is more dominant inside more massive stars with carbon nuclei being the catalyst.
In order to explain the observed abundance curve of the elements heavier than iron, neutron capture nucleosynthesis was suggested by Burbidge, Burbidge, Fowler, and Hoyle (B2FH) in 1957 and independently by Cameron in the same year. The synthesis proceeds by neutron capture reactions and beta decays, effectively creating higher atomic number nuclei. The neutron capture process can be either slow (called s-process, an order of 103 years) or rapid (called r-process, an order of 10−3 sec) relative to the lifetime of the unstable nuclei, depending on thermal condition and neutron density of the stellar environment. These processes involve various neutron-rich isotopes, some of which have never been produced or observed.
Figure 2: Solar system isotopic and elemental abundances, normalized to 106 28Si atoms, adapted from C. A. Barnes, D. D. Clayton, D. N. Schramm and M. A. Fowler, Essays in Nuclear Astrophysics, Cambridge University Press (1982).
Another astronomical observations of novae, X-ray burst or accreting binary systems can be explained by other processes including the rapid proton capture process (rp-process), the γ-ray capture process (γ-process), the neutrino capture process (ν-process), and (α,p) reaction process (αp-process). Figure 3 shows the relationship between relevant isotopes and production sites/mechanisms in different regions of the nuclear chart.
As seen in Figure 3, most of these stellar processes involve unstable isotopes far from stable nuclei, and their nuclear properties have large uncertainties due to experimental difficulties at the laboratory, resulting in huge uncertainties of the nucleosynthesis processes. It is very challenging to produce short lived isotopes and measure their spectroscopic properties before they decay to another one at the laboratory. However, nuclear physicists have pioneered rare isotope (RI) beam productions and detector devices to change the "impossible" to the "I'm possible". There are now many opportunities to address the challenges with advances in rare-isotope production around the world including Radioactive Isotope Beam Factory (RIBF), National Superconducting Cyclotron Laboratory (NSCL), Californium Rare Isotope Breeder Upgrade (CARIBU), CNS RI beam separator (CRIB), Momentum Achromat Recoil Separator (MARS) and new generation RI beam accelerator facilities such as Facility for Rare Isotope Beam (FRIB) in the US and Rare isotope Accelerator complex for ON-line experiments (RAON) in South Korea.
Nuclear astrophysics group at CENS is aiming 1) to study unknown nuclear properties of key exotic nuclei mainly influencing the uncertainty of nucleosynthesis processes, 2) to demonstrate astronomical observations with a good precision of nuclear physics and 3) to join the mainstream of frontiers on research collaborations in the world. The nuclei are also important research topics themselves in terms of nuclear structure and theory. In order to achieve the missions, we are currently working on experimental measurements of spectroscopic properties including nuclear masses, half-lives and reaction rates using beams at RI facilities in the world. Moreover, we develop many new detector systems providing high detection efficiency and good position/energy resolution of detected particles. Here are a few examples of our research activities.
Figure 3: The chart of nuclides showing various stellar processes and the believed sites where such processes occur. Black square boxes represent stable nuclei and others are unstable ones. Taken from H. Schatz, J. Phys. G: Nucl. Part. Phys. 43, 064001 (2016).
Novae and type I X-ray bursts (XRBs) are the most frequently observed explosive stellar events in our universe. Much work has been dedicated to study these events in the form of theoretical descriptions and computational modeling. Such work requires an experimental knowledge of the constituent reactions that drive these violent outbursts. For example, the resonance properties of 19Ne above the alpha threshold at Ex=3.529 MeV are relevant for the α-cluster structure and the 18F(p,α)15O reaction rate, which constrains the 18F production in novae. The reaction rate is dominated by resonances of 19Ne above the proton threshold at Ex=6.410 MeV, but there are some ambiguities concerning the spin-parity assignment and the missing states. To address the α-cluster states and near-proton-threshold states of 19Ne, an elastic resonance scattering experiment on alphas was performed at CRIB.
Masses of neutron-rich nuclei heavier than iron have a substantial impact on elemental abundance predictions and the path of r-process nucleosynthesis. Moreover, masses of nuclei around A=56 are important on a robustness of shallow Urca-type neutrino cooling processes in the crust of accreting neutron stars. The collaboration work between CENS and the Rare-RI Ring group at RIBF has been initiated to measure the masses of medium-heavy exotic neutron-rich nuclei using the Rare-RI Ring and develop an intelligent, fast digital-readout based data acquisition system for a large area position-sensitive micro-channel plate (MCP) detector for high-precision mass measurements.
Recent studies of r-process abundance pattern found charged-particle reactions, mainly (α,xn) reactions, are the main production mechanism of Z=38-47 abundances in neutron-rich neutrino driven winds during core-collapse supernovae scenario, so called the weak r-process. The uncertainty of the charged-particle reaction rates including 75Ga(α,xn) and 85Br(α,xn) reactions was critical to control the production of elements. The studies of (α,xn) compound reactions using 75Ga and 85Br beams were performed via reactions in inverse kinematics at the ReA3, NSCL.
For reaction studies with rare isotopes, high detection efficiency as well as high energy and position resolution of particles are critical due to relatively low beam intensities and unfavorable beam purities. One of suitable detector systems is an active target time projection chamber (AT-TPC) providing high efficiency and great resolutions of detected particle's track, energy and position. We are developing a new AT-TPC (called AToM-X: Active target TPC for Multiple nuclear astrophysics eXperiments) for future reaction studies including fusion, elastic resonance scattering, β-delayed charged particle, (α,p), (α,n) or (d,p) reactions. Another suitable detector system, especially for the study of neutron transfer reaction in inverse kinematics, is a large solid angle silicon detector array with position sensitive silicon strip detectors. Silicon Telescope Array for Reactions in inverse Kinematics (STARK) is under development at CENS to measure excitation energy, spin and parity of low-lying states of exotic nuclei. Details of the AToM-X and STARK are described in the Infrastructure page.
In addition to detector developments, we are also working on developments of Wien filter and cryogenic gas cell systems in collaborations with the Rare Isotope Science Project (RISP). The Wien filter will be installed at a beamline between F2 focal plane and F3 focal plane of KOrea Broad acceptance Recoil spectrometer and Apparatus (KoBRA) to separate different velocity beam ions. The cryogenic gas cell will be located at the F0 focal plane of the KoBRA beamline to produce RI beams by bombarding primary beams onto the gas cell. Both equipment are important ones for the RI beam quality at RAON as a Day-one science. Details of the Wien filter and cryogenic gas cell are also described in the Infrastructure page. Besides the RAON equipment developments, we will work closely with the RISP to develop an experimental program during the construction of RAON and perform nuclear astrophysics experiments upon its completion.
Machine Learning (ML) has been incredible progress over the last decade and applied to many research fields. While there is a relatively slow increase of nuclear research activities using the ML technique yet, we believe it's high reliability of pattern recognition and fast determination in a large parameter space can be successfully used to study nuclear research including empirical evidence of nuclear spectroscopic properties such as excitation energy and spin/parity. We are working on developing a new research tool with the ML. In addition, development of the ML based online/offline analysis software will trigger collaborators to take advantage of the unique technique.
Finally but most of all, we have built internal, domestic and international collaborations. There are very active close collaborations with nuclear structure group as well as nuclear theory group in the center. Many researchers from the groups are involved in research discussions and detector developments including AToM-X and STARK. In addition, students from schools including Sungkyunkwan University are actively involved and make great contributions to our projects. Furthermore, we established a memorandum of understanding (MOU) with many international collaborators including CNS/RIKEN, RIBF/RIKEN (in progress), ATOMKI, Texas A&M University (in progress). The collaborations will promote many benefits of our center including new research topics, new detector developments as well as experiment proposals to RI beam facilities in the world. The activities will also encourage and motivate young physicists for their future research.
Center for Exotic Nuclear Studies