The path is open for the synthesis of new superheavy elements ⚛️

In an experiment conducted at the Berkeley National Laboratory (United States) with the participation of a team from the IPHC, scientists have for the first time produced livermorium-290 (Z=116), a superheavy atomic nucleus, using a beam of titanium-50 (Z=22).


This pathway was known to be very promising, but physicists invested several years of development to obtain beams intense enough for this purpose. With this success, a new pathway for the synthesis of superheavy nuclei has emerged. A pathway that should allow the production of new nuclei beyond oganesson-294 (Z=118), the heaviest nucleus ever studied by nuclear physicists. The next step is to synthesize element 120.

Although element 116 had been known and synthesized for about twenty years, the two isotopes of livermorium that briefly appeared at the cyclotron of the Berkeley National Laboratory on April 27 and June 16 last year have stirred the nuclear physics community. The reason is that the two isotopes of this superheavy element, absent in nature, resulted from an unprecedented union: that of plutonium-244 (Z=94) and titanium-50.


The use of titanium-50 in such laboratory reactions, known as fusion-evaporation, had indeed been a challenge for physicists for many years. But the effort was worth it: under the right conditions, the use of this isotope and its neighbor, chromium-54 (Z=24), could unlock the quest for nuclei with even more protons by propelling the fusion-evaporation technique into new realms.

This process, used in nuclear physics to synthesize artificial superheavy nuclei, seems at first glance as simple as it is brutal: take a heavy nucleus (here plutonium-244) and bombard it with lighter nuclei (here titanium-50). With a bit of luck, some of these projectiles will overcome the repulsion between the positive charges of the two nuclei to merge with the heavy nuclei of the target.

The practical implementation of the fusion-evaporation reaction has allowed scientists to produce many artificial elements beyond uranium in the laboratory, deepening our understanding of nuclear mechanisms and our knowledge of these quantum structures. But here's the catch: the calcium-48 beams (Z=20), which until now were the basis of this process, have reached their limit when bombarding californium targets, the heaviest that can be produced.

Indeed, it was the fusion of californium, with its 98 protons, and calcium-48 that allowed the production of oganesson, the heaviest element ever produced in the laboratory, with 118 protons. To surpass this limit, only one solution is currently conceivable: using new heavier metal beams than calcium-48, such as titanium-50 or chromium-54.

However, using heavier nuclei is a challenge. The more the number of protons increases, the more the electrostatic barrier opposing fusion intensifies, not to mention that the higher kinetic energy of these nuclei makes the synthesized nucleus more excited, and thus more unstable. The chances of survival for these nuclei are therefore very slim, and it is difficult to simultaneously have the energy and beam intensity required. Moreover, titanium is one of the most difficult beams to produce at high intensity continuously.


To overcome this problem and achieve the 2024 result, two methods were successively updated and then adopted by the IPHC team led by Benoît Gall in what would become a true scientific epic. The group first followed the so-called MIVOC path (for Metal Ion from Volatile Organic Compounds), where metal ion isotopes are isolated and then associated with volatile organic compounds to form a stable powder. The vapors from the sublimation of this powder then feed the ion source to produce the beams.

Using this method, Zouhair Asfari, a chemist at the IPHC, notably enabled the generation of a titanium-50 beam intense enough to produce over 2000 nuclei of rutherfordium-256 (Z=104) in 2011. The same method was applied several years later to chromium-54 to study the fission of element 120 in Dubna, Russia. "Under these experimental conditions," explains Benoît Gall, "it had little chance of survival. It fissioned almost immediately, but the manipulation allowed us to learn more about this process."

At higher intensity, the vapors associated with MIVOC compounds saturate the source. This is why the IPHC team subsequently turned to an alternative method, that of direct metal vaporization using induction micro-furnaces. This technique has the advantage of generating pure metal vapors, increasing the intensity produced by the sources and thus the number of fusion reactions on the target. But while 400°C is enough to vaporize calcium, it takes 1660°C to produce a titanium beam with this method, requiring the development of adapted and more powerful furnaces.


The Strasbourg scientists therefore invested in an induction micro-furnace project for the study of superheavy nuclei with the S³ spectrometer at GANIL as well as for their superheavy element synthesis program. They demonstrated the ability of their furnace to vaporize chromium and titanium in 2019 in Dubna, a project that has since been affected by the international context.


In 2020, the group joined forces with colleagues from Berkeley, who are also developing an induction furnace, and brought their expertise. It is within the framework of this fruitful collaboration that the synthesis of livermorium at the Berkeley cyclotron rewards the team's long-standing efforts.

"This experiment is an important step towards the synthesis of new elements as it not only provides proof of the feasibility of synthesizing element 120 with a titanium-50 beam but also an estimate of the time it will take to produce it!," rejoices Benoît Gall. The experiment can be started as soon as the experimental facility at Berkeley is prepared to accommodate the much more radioactive californium target than plutonium-244.

Thanks to heavy metal beams, the discovery of the next superheavy element could then be envisaged by 2026. A promising prospect for both experimentalists and theorists: synthesizing and then studying new elements beyond current limits sheds light on the structure of the nucleus - element 120 could, for example, reveal a hypothetical island of stability where the lifespan of nuclei would be much longer than that of the superheavy nuclei produced so far.