Microscopic study of quasifission dynamics in hot fusion reactions for synthesizing superheavy nuclei with Z = 112-120
This study employs microscopic time-dependent Hartree-Fock theory to analyze quasifission dynamics in 18 hot fusion reactions for synthesizing superheavy nuclei (Z=112-120), revealing that shell effects (specifically spherical 208Pb and octupole-deformed Z=88/N=136) govern fragment formation while heavier projectiles accelerate quasifission, thereby explaining the reduced fusion probabilities observed in attempts to create elements Z=119 and Z=120.
Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
In the quest to understand the very limits of matter, physicists are constantly trying to build the heaviest possible atoms. These superheavy elements, which sit at the far end of the periodic table, do not exist naturally on Earth; they must be created artificially in laboratories. To make them, scientists smash two smaller atomic nuclei together at incredibly high speeds, hoping they will fuse into a single, heavier nucleus. However, this process is notoriously difficult. When the two nuclei collide, they often fail to stick together. Instead of merging, they touch for a fleeting moment and then immediately split apart again. This specific failure mode, where the nuclei interact briefly but separate before forming a new element, is known as quasifission. It is the primary obstacle preventing scientists from creating elements heavier than those currently known, such as the element with 118 protons. Understanding exactly why and how this split happens is crucial for the next step in the journey: creating elements 119 and 120.
A team of researchers in China has taken a deep, microscopic look at this process to see what is really happening inside the collision. Using powerful computer simulations based on the laws of quantum mechanics, they modeled eighteen different types of collisions designed to create superheavy nuclei. They focused on reactions where a smaller, fast-moving projectile nucleus hits a larger, stationary target nucleus. The researchers were particularly interested in how the internal structure of the nuclei influences the outcome. They examined collisions involving different types of projectiles, including calcium, scandium, titanium, vanadium, and chromium, all aimed at creating elements with between 112 and 120 protons.
The study revealed that the outcome of these collisions is not random; it is heavily guided by the "shells" inside the nucleus. Just as electrons in an atom arrange themselves in specific energy levels or shells, protons and neutrons inside the nucleus also prefer to arrange themselves in certain stable groups. The researchers found that in collisions using lighter projectiles like calcium, scandium, titanium, and vanadium, the system has a strong tendency to break apart in a way that leaves one of the fragments looking very much like lead-208. Lead-208 is a particularly stable nucleus because its protons and neutrons fill up complete, spherical shells. The simulations showed that the colliding nuclei seem to "aim" for this stable configuration, causing the heavy fragment to form with a specific number of protons and neutrons that match this stable lead-like structure. This behavior was consistent across all the reactions using these lighter projectiles, suggesting a universal rule at play.
However, the story changed when the researchers used a heavier projectile: chromium-54. In these collisions, the guiding force was different. Instead of forming a spherical, lead-like fragment, the system showed a strong tendency to produce a heavy fragment that resembles radium-224. This radium-like nucleus is not spherical; it is shaped more like a pear. This shape is caused by a different kind of internal stability, where the protons and neutrons settle into deformed, octupole-shaped shells. The simulations indicated that when chromium-54 is used, the collision dynamics are driven by this pear-shaped stability, leading to a distinct type of quasifission that differs fundamentally from the reactions using lighter projectiles.
Beyond the shape of the fragments, the researchers also investigated how long the two nuclei stay in contact before splitting. They found a clear relationship between the weight of the projectile and the speed of the breakup. As the projectile became heavier, moving from calcium to titanium and then to chromium, the time the nuclei spent touching each other became significantly shorter. This happens because heavier projectiles carry more electrical charge, creating a stronger repulsive force that pushes the nuclei apart more quickly. This rapid separation leaves less time for the nuclei to fully merge and settle into a new, stable element. The researchers suggest that this shortened contact time is a major reason why the success rate for creating new elements drops so dramatically when scientists switch from calcium projectiles to heavier ones like titanium or chromium.
The study also looked at how the balance of neutrons and protons changes during these collisions. They found that even though the nuclei split apart so quickly, they manage to share their neutrons and protons with each other very efficiently. By the time they separate, the difference in the ratio of neutrons to protons between the two fragments is very small and consistent across all the different reactions studied. This finding provides a precise, microscopic constraint that other scientists can use to build better models of how these reactions work.
Ultimately, this work provides a clearer picture of the invisible forces that dictate whether a superheavy element is born or fails to form. It confirms that the internal shell structure of the nuclei acts as a blueprint for the collision, determining whether the fragments will be spherical or pear-shaped. It also highlights the physical barrier created by electrical repulsion, which speeds up the breakup process for heavier projectiles. These insights are vital for the ongoing effort to synthesize elements 119 and 120. By understanding exactly which projectile and target combinations are likely to succeed and why others fail, scientists can make more informed choices in their experiments, potentially guiding the next successful creation of a new element on the periodic table.
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