Is nuclear fission occurring in the cosmos? (v1.0)
A key reference is IE (interesting engineering)
In nuclear fusion light elements like hydrogen combine to form heavier elements. In nuclear fission, heavy elements like uranium split to create lighter elements. Replicating nuclear fusion on Earth involves overcoming challenges such as creating and maintaining the extreme temperatures and pressures required for fusion reactions, achieving stable plasma confinement, and developing materials that can withstand harsh conditions within a fusion reactor. Instead, nuclear reactors on Earth run on nuclear fission, where a heavier atom splits into smaller atoms, releasing a large amount of energy.
Vast regions of space go through a series of life cycles of births and deaths and mergers of stars, and all this accumulates in terms of heavier elements created. A star goes through a single life cycle governed by its mass. The bigger the mass, the shorter the life cycle. It can transition from gas and dust in a nebula to the birth as a protostar, to a main sequence star, to a red giant, to a helium or heavier element burning star, to a white dwarf or a supernova or a neutron star or a black hole. Stars can die in many ways including by burning up all available fuel and becoming inert, push away most of its outer mass, explode in a supernova, convulse and collapse and reinflate again and again, or merge with another star. The cycle can start again if what is left behind is material for a new cycle.
All the energy of stars is from nuclear fusion. It is believed that elements before Iron in the periodic table are created through fusion in stars while elements above iron are created through other ways including fission. But where and how is this fission occurring? Scientists have suspected that nuclear fission is occurring in the cosmos but lacked evidence of how and where. Several decades ago, scientists theorized that about half of the elements heavier than iron are produced through a process called the Rapid Neutron Capture Process, or the R-process. The rest are thought to originate through slow neutron capture, or the S-process—a relatively well-understood sequence of reactions that occurs in long-lived, low-mass stars. Recently Scientists from Los Alamos National Laboratory and North Carolina State University have uncovered compelling evidence of nuclear fission occurring in the cosmos associated with the R-process, specifically during the merger of neutron stars.
The mergers of neutron stars are one of the most chaotic events in the cosmos. They are so strong that they send ripples throughout the fabric of space-time. The study focused on an important process called the R-process, a phenomenon occurring in neutron-rich environments such as neutron star mergers or certain types of supernovae. The r-process is thought to be one of the ways for creating elements heavier than iron. However, its intricate details remained elusive, as it cannot be studied directly in a laboratory setting.
The research team analyzed element abundances in 42 stars enriched by R-process materials and the distribution of elements such as ruthenium, rhodium, palladium, and silver. This analysis aimed to spot correlated excesses in element distribution, crucially signaling the processes that occurred during neutron star mergers. Correlated excesses, observed in the abundance of certain elements, provide valuable insights into the specific mechanisms and interactions involved in neutron star mergers, offering a unique window into the formation of heavy elements in the universe. The observed correlation between light precision metals and rare earth nuclei served as a pivotal clue. This correlation pattern not only validated the team's earlier predictions but also pointed towards the involvement of nuclear fission. "The only plausible way this can arise among different stars is if there is a consistent process operating during the formation of the heavy elements," said a researcher. After exploring various scenarios, the team found that only fission could accurately replicate the observed trend.
Atomic nuclei seize neutrons during the rapid-neutron capture process (r-process), creating heavier elements. Some may become excessively heavy, risking instability and prompting a split or fission, resulting in two atoms of lighter yet still substantial elements. Researchers also used fission models developed at Los Alamos to compare with measured data. They found excellent agreement between the two, lending credibility to their findings. The discovery not only validated long-standing suspicions but also hinted at the potential existence of elements with an atomic mass surpassing 260. These findings challenge existing models of heavy element formation and expanding our understanding of the upper limits of the periodic table.
A word about the S-process. The S-process or slow-neutron-capture-process is a nucleosynthesis process that occurs at relatively low neutron density and intermediate temperature conditions in stars. Under these conditions the rate of neutron capture by atomic nuclei is slow relative to the rate of radioactive beta-minus decay. A stable isotope captures another neutron; but a radioactive isotope decay to its stable daughter before the next neutron is captured. This process produces stable isotopes. The S-process produces approximately half of the isotopes of the elements heavier than iron, and therefore plays an important role in the galactic chemical evolution. The S-process differs from the more rapid R-process of neutron-capture by its slow rate of neutron captures.
Observational evidence that nutron star mergers generate elements heavier than iron is reported here.
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