Can we make even heavier elements?2019.05.13 12:19 - Marek Pawłowski
Joint press release of FUW and NCBJ.
Researchers from Faculty of Physics UW (University of Warsaw) and National Centre for Nuclear Research point out, that within a short time there might be a possibility of creating two new superheavy elements, as well as a few isotopes of such elements, which have already been discovered. The calculations which led to this conclusions used theoretical model created in Warsaw and took into account previously omitted processes.
Vacant places in the periodic table of elements have recently been filled up and the new elements have already been given their names. The heaviest (with the number of protons Z=118) has been named oganesson in honour of an academician and discoverer Yuri Oganessian. However, scientists still wonder whether artificially made elements can get even heavier? If so, then which group of their periodic table will they belong to? Due to strong relativistic effects, which deform the distribution of electrons on atom shells, answer to this question is not so simple and obvious. Plus, the impact, which these relativistic deformations have on chemical properties is hard to predict.
Superheavy elements are created by bombarding heavy nuclear targets with much lighter, sped-up ions. The target, the „bullets”, as well as the energy pf collision need to be precisely selected. The probability of a desired nuclear reaction topped with creation of a nucleus with a novel composition is extremely small. Accelerators used for this type of experiments have already reached their limits, but new colliders are being built, such as SHE-Factory in the international institute in Dubna, Russia, which surely will enhance the „production potential” by as much as a hundred.
„In Warsaw we created a simple, but reliable model for estimating the probability of production of new elements in the new installations.” – explains Professor Krystyna Siwek-Wilczyńska from Faculty of Physics UW. „In the model, which we call >>Fusion by Diffusion<< (FBD), we can separate the process of production of new nuclei (nuclear synthesis) into three independent, subsequent stages. The first one describes the probability of overcoming the repulsive barrier caused by the large positive charge of nuclei in the initiated reaction. This stage is relatively easy to model.”
„The next stage is much more difficult to describe. It defines the probability of a specific reconfiguration of the two components, which results in a configuration strong enough to be considered a briefly existing, independent, standalone nuclear system” – further explains Professor Michał Kowal, head of the NCBJ Theoretical Physics Division, co-author of the paper. „The probability of such process taking place is incredibly small. If this process occurs, such nucleus is called a compound nucleus.”
„For calculations of the second stage we use Smoluchowski equations, describing the process of diffusion and that’s where our model’s name comes from” – describes Professor Wilczyńska. „However, the analogy to a regular diffusion is not so obvious. We can tell, in a very simplified way, that a nuclear system diffuses from the initial configuration to the configuration of a compound nucleus. The potential barrier separating these configurations is an obstacle for this process. The diffusion is possible because of thermal fluctuations of the system’s shape. The third stage is the decay of the compound nucleus. Our calculations include a few possible decay channels. The most important ones are neutron emission and fission. A new feature of our FBD model is inclusion of previously not taken into account emissions of protons or even alpha particles. Probability of charged particle emission is smaller than the probability of both of the competing processes – neutron emission and fission. However, we discovered, that calculated cross section values for these new decay channels point out, that they can be observed in the newly built colliders. The processes of proton or alpha particle emissions leads to production of new superheavy nuclei, which are relatively richer in neutrons and in turn are closer to the hypothetical island of stability.”
„We have already discovered earlier, that it is essential to properly include the relationship between the determined cross sections and the angular momentum of the system at the beginning of the process” – adds Tomasz Cap, PhD, from NCBJ Nuclear Physics Division, co-author of the paper. „This relationship was included in every stage of the reaction. It was also crucial to use a consistent set of input data, such as nuclear mases, fission barriers, shell corrections, nuclear deformations. Our team working in NCBJ specialises in such calculations for superheavy elements and the credibility of data acquired in NCBJ has been confirmed multiple times while comparing it with existing experimental data. Thus, we can assume with high dose of probability, that results of calculations, which are reliable in the area of known nuclei, can also be applied to new, undiscovered nuclei in order to estimate the probability of their production.”
The results obtained by the authors are intriguing and spectacular. They predict, that the chance of creating two new elements with Z=119 and Z=120 in the new experiments is not that small at all. „The most promising reaction is the one with target made of 249Bk (berkelium) and 50Ti (titanium) bullet” – tells Professor Kowal. „The probability of producing an element with Z=119 should be smaller only by one order of magnitude with the target of 248Cm (curium) with vanadium (51V) as a bullet. Such reaction is being tested right now in the RIKEN laboratory in Japan. There is also an interesting possibility of producing an element with Z-120 as a result of bombarding curium-248 with chromium-54 nuclei.”
„Apart from the prospect of creating new elements, the issue of discovering new isotopes of already known elements is also an optimistic vision.” – adds Tomasz Cap, PhD. „We predict, that there is a possibility of creating around twenty new superheavy nuclides! These are the isotopes of copernicum (Z=112), nihonium (Z=113), moscvium (Z=115), livermorium (Z=116), as well as tennessine (Z=117). This is a really exciting prospect.”
„Despite large dose of optimism considering the perspective of production of new elements and new isotopes, it is always good to be somewhat cautious, considering the complicated nature of this phenomenon” – stresses Professor Siwek-Wilczyńska. „For the first time in such calculations we managed to estimate the theoretical error with a simple, yet smart method. We showed, that the cross section and, consequently, probabilities of production of new superheavy nuclei, cannot be determined with precision greater than the order of magnitude.”
The paper titled „Exploring the production of new superheavy nuclei with proton and α-particle evaporation channels” has been published at the beginning of May this year in a leading journal of American Physical Society, Physical Review C. It can be found under following address: