In an experiment carried out at Berkeley National Laboratory (United States) with the participation of a team from the IPHC, scientists produced for the first time livermorium-290 (Z=116), a superheavy atomic nucleus, from a beam of titanium-50 (Z=22).
We knew this path was very promising, but physicists invested several years of development to obtain beams sufficiently intense for this use. With this success, a new route for the synthesis of superheavy nuclei is emerging. A path which should allow in the future to produce new nuclei beyond oganesson-294 (Z=118), the heaviest nucleus ever studied by nuclear physicists. Next step, succeed in synthesizing element 120. Although element 116 had been known and synthesized for around twenty years, the two isotopes of livermorium appeared briefly near the cyclotron of the Berkeley National Laboratory last April 27 and June 16 put the community of the nuclear physics in excitement. This is because 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 the context of such laboratory reactions, called fusion-evaporation, has indeed been giving physicists a hard time for many years. But the game was worth it: under the right conditions, the use of this isotope and that of its neighbor, chromium-54 (Z=24), could unlock the quest for nuclei ever richer in protons by propelling the technique of fusion-evaporation into new spheres.
This process, used in nuclear physics to synthesize superheavy artificial 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). Hopefully, some of these projectiles will overcome the repulsion between the positive charges of the two nuclei to amalgamate with the heavy nuclei of the target.
Putting the fusion-evaporation reaction into practice has allowed scientists to produce numerous artificial elements beyond uranium in the laboratory, thereby deepening our understanding of nuclear mechanisms and our knowledge of these quantum structures. But here it is: the beams of calcium-48 (Z =20), on which this process has been based until now, have reached their limit by bombarding the californium targets, the heaviest that it is possible to produce.
It is in fact the fusion of californium, with its 98 protons, and calcium-48 which made it possible to produce oganesson, the heaviest element ever produced in the laboratory, with 118 protons. To overcome this limit, only one solution is currently possible: using new metal beams heavier than calcium-48, such as titanium-50 or chromium-54.
However, using heavier cores is a challenge. The more the number of protons increases, the more the electrostatic barrier which opposes fusion intensifies, not to mention that the kinetic energy of these nuclei being higher, it makes the synthesized nucleus more excited, and therefore more unstable. The chances of survival for these nuclei are therefore very slim and it is difficult to have at the same time theenergy and beam intensity required. What’s more, titanium is one of the most difficult beams to produce at high intensity continuously.
To get around 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 began by following the so-called MIVOC trail (for Metal Ion from Volatile Organic Compounds), where isotopes of metal ions are isolated and then combined with volatile organic compounds to form a stable powder. The vapors resulting from the sublimation of this powder then supply the ion source to produce the beams.
-Using this method, Zouhair Asfari, chemist at IPHC, notably made it possible to generate a beam of titanium-50 sufficiently intense to produce more than 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 conditionsexplains Benoît Gall, he was given little chance of survival. It fissioned almost immediately, but manipulation allowed us to learn more about this process“.
At higher intensity, vapors linked to MIVOC compounds saturate the source. This is why the IPHC team subsequently turned to an alternative method, that of direct vaporization of metals using induction micro-furnaces. This technique has the advantage of generating pure metal vapors, increasing the intensity produced by the sources and therefore the number of fusion reactions on the target. But if 400°C is enough to vaporize calcium, it is necessary to go up to 1660°C to produce a beam of titanium with this method, which requires the development of suitable and more powerful furnaces.
Strasbourg scientists have therefore invested in an induction micro-furnace project for the study of superheavy nuclei with the S spectrometer3 at GANIL as well as for their superheavy element synthesis program. They were able to demonstrate the ability of their furnace to vaporize chromium and titanium in 2019 in Dubna, a project which has since suffered the consequences of context international.
In 2020, the group joined forces with colleagues from Berkeley, who are also developing an induction oven, and provided them with 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 constitutes an important step towards the synthesis of new elements because it not only provides proof of the feasibility of the synthesis of element 120 with a beam of titanium-50, but also an estimate of the time it will take us to produce it!“, rejoices Benoît Gall. The experiment can be started as soon as the installation experimental in Berkeley will have been prepared to accommodate the californium target, which is much more radioactive than plutonium-244.
Thanks to heavy metal beams, the discovery of the next superheavy element would then be possible by 2026. A pleasing prospect for experimentalists and theorists alike: synthesizing and then studying new elements beyond current limits enlightens physicists on the structure of the core – element 120 could for example reveal a hypothetical island of stability where the lifespan of the cores would be much longer than that of the superheavy cores produced up to here.
