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The synthesis of superheavy elements
A chemical element is a species of atoms having the same number of protons in its nucleus. The nucleus also contains neutrons, which carry no net electric charge. The number of protons in nucleus is characteristic of the element. However, the neutron numbers can vary. While nucleus of Hydrogen (the lightest element) has only one proton, Uranium (the heaviest naturally occurring element) has 92.
Researchers have identified three major cosmic processes namely, Big Bang nucleosynthesis, Stellar nucleosynthesis, and Supernova. Our own solar system is believed to have been formed 4.5 billion years ago from the remnants of a supernova. Elements from Hydrogen to Uranium, some of them are radioactive with half-lives longer than the age of Earth, are known as primordial elements - these are elements that are present from the time of Earth’s formation and are thus naturally occurring. The remaining 26 elements undergo radioactive decay with half-lives much shorter than the age of Earth.
So, if any nuclides of these elements were present in the primordial soup, they would not have survived. They have long since decayed. Hence, they do not occur in nature. These elements, called superheavy elements, have only been synthesised in nuclear reactors and accelerators.
How they were created
In theory, creating superheavy elements is quite simple. Smash together nuclei of two different elements, each with sufficient number of protons and neutrons and hope they will fuse to form a new element. However, in order to overcome the Coulomb repulsive force between the two positively charged colliding nuclei, ions of the lighter of the two elements are accelerated to almost a tenth of the speed of light in particle accelerators and beamed at a target made of the second element.
Even in such an arrangement, most of the times the colliding nuclei break apart, but on a few occasions they fuse to form a new element. Invariably, the new element will be highly unstable and undergo radioactive decay, characteristic of a new element. Detection of this decay chain confirms the synthesis of the new element. Since the probability of nucleosynthesis is very small, the fusion experiments have to run for months together to get even a few atoms of the new element.
The credit for synthesising the latest four new elements goes to the scientists from Japan, USA and Russia. Between 2000 and 2012, Japanese scientists at RIKEN Nishina Centre for Accelerator-Based Science bombarded a thin target of bismuth ions (83 protons and 126 neutrons) with high speed zinc ions (30 protons and 40 neutrons) accelerated to 30,000 km/s2 to create a new element of atomic number 113. The element was later named by them as Nihonium (Nh).
Syntheses of elements 115 and 117 were carried out in collaborative research
between the Joint Institute for Nuclear Research, Russia, and Lawrence Livermore National Laboratory, USA in 2003 and 2010 respectively. The researchers bombarded targets of Americium-243 and Berkelium-249 with high speed Calcium-48 ions in a cyclotron. They were named Moscovium (Mc - proton number 115) and Tennessine (Ts- proton number 117).
The last of the four new elements, element-118 was first synthesised in 2002 by a team of Russian and American scientists at the Joint Institute of Nuclear Research, Russia. It was named Oganesson (Og) in honour of the Russian physicist Yuri Oganessian, leader of the team. It was synthesised by projecting high speed calcium ions on to a target of californium. Oganesson has the highest number of protons and neutrons of all known elements.
As the elements become heavier with an increasing number of protons and neutrons, they become even more unstable. However, researchers suspect that there may be an 'island of stability’, where elements with proton numbers 114, 120 and 126 and with neutron number 184 may exist with half-life long enough to study their chemical and physical properties.
Since superheavy elements are produced at a high cost and decay so fast, one may ask what is their use? Researchers believe that a study of the decay of the massive elements may give further insight into the forces that hold the subatomic particles together in the nucleus, the nucleus’ structure and the processes involved in nucleosynthesis. These are fundamental questions in nuclear physics. And so the effort will go on.