Physics

Scientists discover promising way to create new superheavy elements

Chart of superheavy elements (SHE) plotted by atomic number (protons) and number of neutrons. Boxes are discovered SHEs with predicted half-lives. The yen is an island of stability. Credit: Wikipedia Commons

What is the heaviest element in the universe? Are there an infinite number of elements? Where and how are superheavy elements naturally produced?

The heaviest element known to exist is uranium, which has 92 protons (atomic number “Z”). However, scientists have succeeded in synthesizing superheavy elements up to oganeson, with a Z of 118. Immediately in front of it are livermorium, which has 116 protons, and tennessine, which has 117 protons.

All have short half-lives (the time it takes for half of an element’s collection of atoms to decay), typically less than a second, and some as short as microseconds. Creating and detecting such elements is not easy, requiring powerful particle accelerators and precise measurements.

However, common methods of manufacturing high-Z elements are reaching their limits. In response, a group of American and European scientists has devised a new way to produce superheavy elements that goes beyond existing dominant technologies. Their research, conducted at Lawrence Berkeley National Laboratory in California, was published in Physical Review Letters.

“Today, the concept of an ‘island of stability’ remains an interesting topic, and its precise location and extent on the Segre diagram continues to be actively pursued in both theoretical and experimental nuclear physics. ” said JM Gates of LBNL and colleagues. wrote in their paper.

Islands of stability are regions where superheavy elements and their isotopes (nuclei with the same number of protons but different numbers of neutrons) can have much longer half-lives than their nearby elements. It is expected to occur in isotopes around Z=112.

Although there are several techniques for discovering superheavy elements and creating their isotopes, one of the most effective techniques is to target actinide series elements with calcium atoms, specifically the calcium isotope 48-calcium ( 48Ca) beam. , there are 20 protons and 28 (48 minus 20) neutrons. Actinide elements have a proton count of 89 to 103, but 48Ca is special because it has a “magic number” of both protons and neutrons. This means that the number completely fills the available energy shell within the nucleus.

The magical number of protons and/or neutrons means that the nucleus is very stable. For example, Ca 48 has a half-life of approximately 60 billion (6 x 1019) years, which is much longer than the age of the universe. (49Ca, by contrast, has only one more neutron and decays by half in about 9 minutes.)

These reactions are called “thermal fusion” reactions. In another technique, called a “cold fusion” reaction, a beam of isotopes ranging from 50 titanium to 70 zinc was accelerated over a lead or bismuth target. These reactions led to the discovery of superheavy elements up to oganeson (Z=118).

However, the time required to generate new superheavy elements, quantified by the reaction cross section, which measures the probability of their occurrence, has become increasingly long, with run times of several weeks in some cases. Ta. Because we are so close to the predicted island of stability, scientists need techniques to go further than Oganesson. Einsteinium and fermium targets are themselves super heavy and cannot be produced in sufficient quantities to make suitable targets.

“New reactive approaches are needed,” Gates and his team wrote. And that’s what they found.

A nuclear theoretical model successfully predicted the production rate of superheavy elements below oganeson using actinide targets and beams of isotopes heavier than 48-calcium. These models also agree that the largest cross-section 50 titanium beam is optimal for manufacturing Z=119 and Z=120 elements.

However, not all of the necessary parameters, such as the required beam energy, have been specified by theorists, and some of the mass required by the model has not been measured by experimentalists. Accurate numbers are important because otherwise the production rate of superheavy elements can vary significantly.

Some experimental efforts have already been made to create atoms with 119 to 122 protons. None of them were satisfactory, and the limits determined on the cross section made it impossible to constrain the various theoretical nuclear models. Gates and his team investigated the production of the isotope of livermorium (Z=116) by irradiating a 244-Pu (plutonium) target with 50-titanium.

Using an 88-inch cyclotron accelerator at Lawrence Berkeley National Laboratory, the researchers generated a beam of titanium ions at an average rate of 6 trillion per second, which they shot out of the cyclotron. These impacted a plutonium target with a circular area of ​​12.2 cm over a period of 22 days. After numerous measurements, it was determined that 290-livermorium was produced via two different nuclear decay chains.

“This is the first reported production of SHE (superheavy element) near a predicted stable island by a beam other than 48-calcium,” the researchers concluded. Although the reaction cross section, and therefore the probability of interaction, did decrease, as expected with heavier beam isotopes, “the success of this measurement confirms that the discovery of new SHEs is certainly within the experimental range.” I proved it.”

This discovery shows for the first time that other superheavy atoms and isotopes (or both) can be produced by collisions of non-magical nuclei, and is expected to pave the way for future discoveries. Approximately 110 isotopes of superheavy elements are known to exist, and 50 more are expected to exist, waiting to be discovered through new techniques such as these.

Further information: JM Gates et al, Toward the Discovery of New Elements: Production of Livermorium ( Z=116 ) with Ti50, Physical Review Letters (2024). DOI: 10.1103/PhysRevLett.133.172502

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Citation: Scientists discover promising way to create new superheavy elements (October 27, 2024) from https://phys.org/news/2024-10-scientists-superheavy-elements.html October 2024 Retrieved on 27th

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