Nuclear scientists at the Lawrence Berkeley National Laboratory (LBNL) in the US have produced and identified molecules containing nobelium for the first time. This element, which has an atomic number of 102, is the heaviest ever to be observed in a directly-identified molecule, and team leader Jennifer Pore says the knowledge gained from such work could lead to a shake-up at the bottom of the periodic table.

“We compared the chemical properties of nobelium side-by-side to simultaneously produced molecules containing actinium (element number 89),” says Pore, a research scientist at LBNL. “The success of these measurements demonstrates the possibility to further improve our understanding of heavy and superheavy-element chemistry and so ensure that these elements are placed correctly on the periodic table.”

The periodic table currently lists 118 elements. As well as vertical “groups” containing elements with similar properties and horizontal “periods” in which the number of protons (atomic number Z) in the nucleus increases from left to right, these elements are arranged in three blocks. The block that contains actinides such as actinium (Ac) and nobelium (No), as well as the slightly lighter lanthanide series, is often shown offset, below the bottom of the main table.

The end of a predictive periodic table?

Arranging the elements this way is helpful because it gives scientists an intuitive feel for the chemical properties of different elements. It has even made it possible to predict the properties of new elements as they are discovered in nature or, more recently, created in the laboratory.

The problem is that the traditional patterns we’ve come to know and love may start to break down for elements at the bottom of the table, putting an end to the predictive periodic table as we know it. The reason, Pore explains, is that these heavy nuclei have a very large number of protons. In the actinides (Z > 88), for example, the intense charge of these “extra” protons exerts such a strong pull on the inner electrons that relativistic effects come into play, potentially changing the elements’ chemical properties.

“As some of the electrons are sucked towards the centre of the atom, they shield some of the outer electrons from the pull,” Pore explains. “The effect is expected to be even stronger in the superheavy elements, and this is why they might potentially not be in the right place on the periodic table.”

Understanding the full impact of these relativistic effects is difficult because elements heavier than fermium (Z = 100) need to be produced and studied atom by atom. This means resorting to complex equipment such as accelerated ion beams and the FIONA (For the Identification Of Nuclide A) device at LBNL’s 88-Inch Cyclotron Facility.

Producing and directly identifying actinide molecules

The team chose to study Ac and No in part because they represent the extremes of the actinide series. As the first in the series, Ac has no electrons in its 5f shell and is so rare that the crystal structure of an actinium-containing molecule was only determined recently. The chemistry of No, which contains a full complement of 14 electrons in its 5f shell and is the heaviest of the actinides, is even less well known.

In the new work, which is described in Nature, Pore and colleagues produced and directly identified molecular species containing Ac and No ions. To do this, they first had to produce Ac and No. They achieved this by accelerating beams of ⁴⁸Ca with the 88-Inch Cyclotron and directing them onto targets of ¹⁶⁹Tm and ²⁰⁸Pb, respectively. They then used the Berkeley Gas-filled Separator to separate the resulting actinide ions from unreacted beam material and reaction by-products.

The next step was to inject the ions into a chamber in the FIONA spectrometer known as a gas catcher. This chamber was filled with high-purity helium, as well as trace amounts of H₂O and N₂, at a pressure of approximately 150 torr. After interactions with the helium gas reduced the actinide ions to their 2+ charge state, so-called “coordination compounds” were able to form between the 2+ actinide ions and the H₂O and N₂ impurities. This compound-formation step took place either in the gas buffer cell itself or as the gas-ion mixture exited the chamber via a 1.3-mm opening and entered a low-pressure (several torr) environment. This transition caused the gas to expand at supersonic speeds, cooling it rapidly and allowing the molecular species to stabilize.

Once the actinide molecules formed, the researchers transferred them to a radio-frequency quadrupole cooler-buncher ion trap. This trap confined the ions for up to 50 ms, during which time they continued to collide with the helium buffer gas, eventually reaching thermal equilibrium. After they had cooled, the molecules were reaccelerated using FIONA’s mass spectrometer and identified according to their mass-to-charge ratio.

A fast and sensitive instrument

FIONA is much faster than previous such instruments and more sensitive. Both properties are important when studying the chemistry of heavy and superheavy elements, which Pore notes are difficult to make, and which decay quickly. “Previous experiments measured the secondary particles made when a molecule with a superheavy element decayed, but they couldn’t identify the exact original chemical species,” she explains. “Most measurements reported a range of possible molecules and were based on assumptions from better-known elements. Our new approach is the first to directly identify the molecules by measuring their masses, removing the need for such assumptions.”

As well as improving our understanding of heavy and superheavy elements, Pore says the new work might also have applications in radioactive isotopes used in medical treatment. For example, the ²²⁵Ac isotope shows promise for treating certain metastatic cancers, but it is difficult to make and only available in small quantities, which limits access for clinical trials and treatment. “This means that researchers have had to forgo fundamental chemistry experiments to figure out how to get it into patients,” Pore notes. “But if we could understand such radioactive elements better, we might have an easier time producing the specific molecules needed.”

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The first precise mass measurements of an extremely short-lived and proton-rich nucleus, silicon-22, have revealed the “magic” – that is, unusually tightly bound – nature of nuclei containing 14 protons. As well as shedding light on nuclear structure, the discovery could improve our understanding of the strong nuclear force and the mechanisms by which elements form.

At the lighter end of the periodic table, stable nuclei tend to contain similar numbers of neutrons and protons. As the number of protons increases, additional neutrons are needed to balance out the mutual repulsion of the positively-charged protons. As a rule, therefore, an isotope of a given element will be unstable if it contains either too few neutrons or too many.

In 1949, Maria Goeppert Mayer and J Hans D Jensen proposed an explanation for this rule. According to their nuclear shell model, nuclei that contain certain “magic” numbers of nucleons (neutrons and/or protons) are more bound because they have just the right number of nucleons to fully fill their shells. Nuclei that contain magic numbers of both protons and neutrons are even more bound and are said to be “doubly magic”. Subsequent studies showed that for neutrons, these magic numbers are 2, 8, 20, 28, 50, 82 and 126.

While the magic numbers for stable and long-lived nuclei are now well-established, those for exotic, short-lived ones with unusual proton-neutron ratios are comparatively little understood. Do these highly unstable nuclei have the same magic numbers as their more stable counterparts? Or are they different?

In recent years, studies showing that neutron-rich nuclei have magic numbers of 14, 16, 32 and 34 have brought scientists closer to answering this question. But what about protons?

“The hunt for new magic numbers in proton-rich nuclei is just as exciting,” says Yuan-Ming Xing, a physicist at the Institute for Modern Physics (IMP) of the Chinese Academy of Sciences, who led the latest study on silicon-22. “This is because we know much less about the evolution of the shell structure of these nuclei, in which the valence protons are loosely bound.” Protons in these nuclei can even couple to states in the continuum, Xing adds, forming the open quantum systems that have become such a hot topic in quantum research.

Mirror nuclei

After measurements on oxygen-22 (14 neutrons, 8 protons) showed that 14 is a magic number of neutrons for this neutron-rich isotope, the hunt was on for a proton-rich counterpart. An important theory in nuclear physics known as isospin symmetry states that nuclei with interchanged numbers of protons and neutrons will have identical characteristics. The magic numbers for protons and neutrons for these “mirror” nuclei, as they are known, are therefore expected to be the same. “Of all the new neutron-rich doubly-magic nuclei discovered, only one loosely bound mirror nucleus for oxygen-22 exists,” says IMP team member Yuhu Zhang. “This is silicon-22.”

The problem is that silicon-22 (14 protons, 8 neutrons) has a short half-life and is hard to produce in quantities large enough to study. To overcome this, the researchers used an improved version of a technique known as Bρ-defined isochronous mass spectroscopy.

Working at the Cooler-Storage Ring of the Heavy Ion Research Facility in Lanzhou, China, Xing, Zhang and an international team of collaborators began by accelerating a primary beam of stable ³⁶Ar¹⁵⁺ ions to around two thirds the speed of light. They then directed this beam onto a 15-mm-thick beryllium target, causing some of the ³⁶Ar ions to fragment into silicon-22 nuclei. After injecting these nuclei into the storage ring, the researchers could measure their velocity and the time it took them to circle the ring. From this, they could determine their mass. This measurement confirmed that the proton number 14 is indeed magic in silicon-22.

A better understanding of nucleon interactions

“Our work offers an excellent opportunity to test the fundamental theories of nuclear physics for a better understanding of nucleon interactions, of how exotic nuclear structures evolve and of the limit of existence of extremely exotic nuclei,” says team member Giacomo de Angelis, a nuclear physicist affiliated with the National Laboratories of Legnaro in Italy as well as the IMP. “It could also help shed more light on the reaction rates for element formation in stars – something that could help astrophysicists to better model cosmic events and understand how our universe works.”

According to de Angelis, this first mass measurement of the silicon-22 nucleus and the discovery of the magic proton number 14 is “a strong invitation not only for us, but also for other nuclear physicists around the world to investigate further”. He notes that researchers at the Facility for Rare Isotope Beams (FRIB) at Michigan State University, US, recently measured the energy of the first excited state of the silicon-22 nucleus. The new High Intensity Heavy-Ion Accelerator Facility (HIAF) in Huizhou, China, which is due to come online soon, should enable even more detailed studies.

“HIAF will be a powerful accelerator, promising us ideal conditions to explore other loosely bound systems, thereby helping theorists to more deeply understand nucleon-nucleon interactions, quantum mechanics of open quantum systems and the origin of elements in the universe,” he says.

The present study is detailed in Physical Review Letters

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