FIONA measures the mass number of two super-heavy elements

A team led by nuclear physicists at the Lawrence Berkeley National Laboratory (Berkeley Lab) of the Department of Energy reported first direct measurements of the mass numbers of two super-heavy-core nuclei: moscovium, which is the most 39, element 115, and nihonium, element 113.

They obtained the results using FIONA, a new tool from the Berkeley lab designed to solve the nuclear and atomic properties of heavier elements. The results are detailed in the 28 November edition of the Letters of physical examination newspaper.

FIONA is an acronym which means: "For the identification of the nuclide A", with "A" representing the scientific symbol for the mass number of an element – the total number of protons and neutrons in the nucleus from an atom. Protons are positively charged and the number of protons is also called atomic number; neutrons have a neutral charge. The super-heavy elements are of human origin and have a higher atomic number than those found in natural elements.

The world race for mass numbers

Gathering and validating these first FIONA data was a top priority for the laboratory's 88-inch nuclear and cyclotron division since the completion of FIONA's commissioning in early 2018. Cyclotron staff worked with scientists visiting and internal to conduct the first experimental test of FIONA, which lasted five weeks.

"It's very exciting to see FIONA go online because it's extremely important to pin down the mass of extremely heavy elements," said Barbara Jacak, director of the Nuclear Science Division. "Until now, mass assignments have been made with indirect evidence rather than direct measurements."

Jackie Gates, a scientist in the Berkeley Lab Nuclear Science Division, who has been instrumental in the design, construction and testing of FIONA, and who is leading the efforts to determine the numbers. mass of FIONA, said: "The interest aroused by making an experimental measurement of super-heavy mass numbers".

Mr. Gates added that this effort to measure the mass number of super-heavy elements is of global interest. The Argonne National Laboratory and Japan Nuclear Research Program teams also perform mass measurements of super heavy elements using slightly different approaches or tools.

Guy Savard, principal investigator at the Argonne National Laboratory, designed, built and supplied several components for FIONA. He also helped with the commissioning of FIONA and its first scientific campaign.

Roderick Clark, a senior scientist in Berkeley Lab's Nuclear Science Division, said, "Everyone is coming together in this great race, and that can open up a whole range of physics from these heavy and super-heavy samples," as well as new ones. structural studies. and the chemistry of these exotic elements, and a deeper understanding of how they relate to other elements.

"If we can measure the mass of any of these super heavy elements, you can fix the entire region," Clark said.

A new chapter in heavy element research

The number of mass and atomic number (or "Z") – a measure of the total number of protons in the nucleus of an atom – very heavy elements rely on the accuracy of nuclear mass models. It is therefore important to have a reliable way to measure these numbers with the help of experiments in case of model problems, said Ken Gregorich, a recently retired scientist in the Nuclear Science Division of the United States. Berkeley Lab, who worked closely with Gates to build and control FIONA.

For example, super-heavy elements could potentially have unexpected nuclear shapes or densities of protons and neutrons that are not considered in the models, he said.

Berkeley Lab has made a significant contribution to the field of heavy element research: laboratory scientists have played a role in the discovery of 16 elements of the periodic table, dating back to the synthesis of neptunium in 1940, and have also provided hundreds of isotopic identifications. . Isotopes are different forms of elements that share the same number of protons but have a different number of neutrons in their nuclei.

FIONA (see related article) is a complement to the Berkeley gas separator (BGS). For decades, BGS has separated heavy elements from other types of charged particles that can be undesirable "noise" in experiments. FIONA is designed to trap and cool individual atoms, separate them according to their mass and charge properties and transmit them to a low-noise detection station on a time scale of 20 milliseconds, or 20 thousandths of a second.

"One atom a day"

"We can make an atom a day, because of," a super-heavy element desired, Gregorich noted. At the beginning of its operation, FIONA had the specific mission of trapping individual moscovium atoms. "We have about a 14% chance of trapping every atom," he added. The researchers had therefore hoped to capture a single measure of the number of Moscow mass per week.

Moscovium was discovered in 2015 in Russia by a US-Russian team including scientists from the national laboratory Lawrence Livermore. The discovery of nihonium was awarded to a team in Japan in 2004. The names of the elements were officially approved in 2016.

To produce moscovium, the 88 cm Cyclotron scientists bombarded a target made of americium, an isotope of an element discovered by Glenn T. Seaborg of Berkeley Lab in 1944, with a particle beam produced from the rare isotope Calcium 48. The required half-gram of calcium 48 was provided by the DOE isotope program.

There is a separate loop signature for each atom trapped and measured by FIONA – a bit like looking at a fixed point on a bike tire when the bike is moving forward. The trajectory of this loop behavior is related to the "mass-to-charge ratio" of the atom – the timing and position of the measured energy signal in the detector indicate to the scientists the mass number.

Ideally, the measurement involves several steps in the particle's decay chain: Moscovium has a half-life of about 160 milliseconds, which means that one atom has a 50% chance of disintegrating into another element called "daughter" element of the disintegration chain. milliseconds. Capturing its energetic signature at several stages of this decay chain can confirm which parent atom has started this cascade.

"We have been trying to establish the mass number and number of protons here for many years now," said Paul Fallon, senior scientist at Berkeley Lab's Nuclear Science Division, who leads the program's reduction program. division energy consumption. The sensitivity of the detectors has steadily improved, as has the ability to isolate individual atoms from other noises, he noted. "Now we have our first definitive measures."

Checking the mass numbers of elements 113 and 115

In the first scientific study of FIONA, researchers identified a moscovium atom and its related decay daughters, as well as an atom of nihonium and its decaying daughters. Measurements of atoms and decay chains confirm the predicted mass numbers for both elements.

While the researchers were only looking to create and measure the properties of a moscovium atom, they were also able to confirm a measure of nihonium after the decay of a moscovium atom into nihonium prior to To reach FIONA.

"The success of this first step is incredibly exciting," said Jennifer Pore, a postdoctoral fellow who participated in FIONA's command experiments. "FIONA's unique capabilities have unleashed a new renaissance in heavy-weight research with the 88-inch Cyclotron."

Gregorich thanked the efforts of the 88 cm cyclotron staff (including mechanical, electrical, operations, and control system experts) to optimize the duration of the FIONA experiment during its five weeks of research.

He highlighted the special contributions of other members of the BGS group and the FIONA group, including Greg Pang, a former project scientist involved in the construction and testing of FIONA; Jeff Kwarsick, a graduate student whose doctorate thesis is focused on FIONA outcomes; and Nick Esker, a former graduate student whose Ph.D. work focused on FIONA's built-in mass separator technique.

Plans for new measurements and the addition of SHEDevil & # 39;

Fallon said that another scientific operation is planned for FIONA in the next six months, during which nuclear physics researchers could pursue a new set of measurements for moscovium and nihonium, or for other elements super heavy.

It is also planned to install and test a new tool, dubbed "SHEDevil" (for very heavy element detectors for Extreme Ventures In Low), which will help scientists understand the shape of the nuclei of Very heavy atoms by detecting the gamma rays produced during their disintegration. These gamma rays will provide clues about the arrangement of neutrons and protons in nuclei.

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