Superheavy elements (SHEs) represent the limit of the periodic table, possessing atomic numbers of 104 or greater. These atoms are entirely synthetic and must be created in a laboratory setting. Due to the immense repulsive forces between their numerous protons, superheavy elements are profoundly unstable, with many decaying in fractions of a second. Scientists create these fleeting atoms by fusing the nuclei of two lighter elements. The methods used to synthesize these elements allow researchers to explore the fundamental forces governing the atomic nucleus.
Specialized Facilities for Creation
The creation of superheavy elements happens inside highly specialized research centers that house powerful particle accelerators. These accelerators function as sophisticated atomic cannons. Their purpose is to propel a beam of projectile ions to speeds approaching ten percent of the speed of light, ensuring the nuclei have enough energy to overcome their natural electrostatic repulsion.
The accelerated ions are directed toward a microscopic target made of a heavy element, such as Californium or Berkelium. This entire apparatus operates under an ultra-high vacuum to prevent the high-speed projectile from colliding with stray air molecules instead of the intended target nuclei. The successful synthesis of even a single atom of a new element is an extremely rare event, often requiring weeks or months of continuous bombardment. The precise control over the beam’s energy and the purity of the target material are required for a successful fusion reaction.
The Nuclear Fusion Process
Creating a superheavy element requires the successful fusion of the projectile nucleus with the target nucleus, a process known as nuclear fusion. The two primary techniques scientists use to achieve this atomic merging are categorized as hot fusion and cold fusion reactions. These methods differ significantly in the types of nuclei used, the energy of the collision, and the subsequent stability of the newly formed nucleus.
Hot Fusion
Hot fusion involves bombarding a heavy, radioactive actinide target, like Californium-249 or Americium-243, with a lighter projectile, such as Calcium-48 ions. Calcium-48 is often favored because it is a neutron-rich, relatively stable isotope that helps increase the neutron count in the resulting superheavy nucleus. The collision produces a highly excited compound nucleus with a considerable amount of internal energy. To achieve a more stable state, this “hot” nucleus must shed its excess energy, which it does primarily by evaporating three to five neutrons. This method has been successfully used to create the heaviest elements, including those up to Oganesson (element 118).
Cold Fusion
The alternative approach is cold fusion, which uses a medium-weight target, typically Lead-208 or Bismuth-209, combined with a medium-weight projectile. Since Lead-208 is a doubly stable nucleus with a closed shell structure, the fusion reaction occurs at lower collision energies. This lower energy results in a compound nucleus with much less internal excitation energy. Consequently, the nucleus stabilizes by evaporating only one or two neutrons, or sometimes none at all, resulting in a “colder” nucleus. This technique was effectively used to synthesize elements with atomic numbers from 107 to 112, such as Copernicium (element 112). While cold fusion produces isotopes with fewer neutrons, the reaction’s lower energy helps prevent the newly formed nucleus from immediately breaking apart through fission.
Tracking and Verification
The discovery of superheavy elements relies on sophisticated detection and verification methods. The newly formed superheavy nuclei must first be separated from the intense beam of unreacted projectile ions and other unwanted reaction products. This separation is accomplished using specialized devices like velocity filters or gas-filled separators, which employ magnetic and electric fields to isolate the single, heavy atoms.
Once isolated, the superheavy atom is implanted into a detector, where its existence is confirmed by tracking its decay signature. Superheavy elements typically decay through a sequence of alpha particle emissions, where the nucleus sheds two protons and two neutrons with each step. Scientists record the specific energy of each alpha particle and the time interval between each decay. This characteristic decay chain continues until the atom transforms into a known, lighter, and more stable nuclide. By tracing the sequence back to the first measured event, researchers can definitively prove the synthesis of a new element, even if only a few atoms were created.
The Theoretical Goal of Stability
The intense research into superheavy elements is driven by a theoretical prediction known as the Island of Stability. This concept suggests that while most superheavy elements are extremely short-lived, certain isotopes with specific numbers of protons and neutrons may possess unexpectedly long half-lives. The stability is thought to arise from the nuclear shell model, which predicts that a nucleus gains extra stability when its shells of protons and neutrons are completely filled, similar to the stability of noble gases in chemistry.
These specific nucleon counts are referred to as “magic numbers,” and theory predicts a region of enhanced stability centered around a proton number of 114 and a neutron number of 184. Isotopes within this predicted island could have half-lives ranging from seconds to possibly minutes or even longer, a significant increase over the milliseconds typical of their neighbors. Successfully reaching this region would allow scientists to conduct detailed chemical studies on these elements, advancing our understanding of nuclear physics.