The group, led by nuclear physicists at the Lawrence Berkeley National Laboratory (Berkeley Laboratory), reported on the first direct measurements of the mass numbers of the nuclei of two superheavy elements: Muscovy, which is element 115, and element 113 nikhoniya.
They obtained results using the FIONA, a new tool at Berkeley Lab, which is designed to resolve the nuclear and atomic properties of the heaviest elements. The results are described in detail in the November 28 issue. Physical review letters magazine.
FIONA is an abbreviation meaning: “Identification of 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 of an atom. Protons are positively charged, and the number of protons is also known as the atomic number; neutrons have a neutral charge. Super heavy elements are made by man and have a higher atomic number than those found in naturally occurring elements.
Global peak for mass numbers
The collection and validation of these first data from FIONA has been one of the top priorities for the 88-inch cyclotron and nuclear research unit of the laboratory since the commissioning of FIONA in early 2018. Cyclotron employees worked with visitors and their own scientists to conduct the first experimental launch of FIONA, which lasted five weeks.
“It is very interesting to see how FIONA went online, as it is extremely important to attach masses of super-heavy elements,” says Barbara Jakak, director of nuclear research. “So far, mass assignments have been made with indirect evidence, and not direct measurement.”
Jackie Gates, researcher at Berkeley Laboratory of Nuclear Research Laboratory, who played a leading role in the concept, construction and testing of FIONA, and who heads FIONA’s efforts to determine the number of mass numbers, said: “There was great interest in making an experimental measurement of super-heavy mass numbers.”
Gates added that this desire to measure the mass numbers of superheavy elements is of global interest: teams from the Argonne National Laboratory and the Japan Nuclear Research Program among them also carry out mass measurements of superheavy elements using slightly different approaches or tools.
Guy Saward, a senior research scientist at the Argonne National Laboratory, developed, built, and implemented several components for FIONA. He also assisted in the commissioning of FIONA and in his first scientific campaign.
Roderick Clark, a senior researcher at the Berkeley Laboratory of Nuclear Research, said: “Everyone gathers together in this great race, it can open up a whole series of physicists of these heavy and super-heavy samples,” as well as new studies of the structure and chemistry of these exotic elements, and more profound understanding how they are related to other elements.
“If we can measure the mass of one of these super-heavy elements, you can break the whole region,” said Clark.
New chapter on heavyweight research
The mass number and atomic number (or "Z") – a measure of the total number of protons in the nucleus of an atom – superheavy elements relied on the accuracy of nuclear mass models. Therefore, it is important to have a reliable way to measure these numbers with experiments if there is a problem with the models, said Ken Gregoryh, recently retired senior researcher at the nuclear science division of Berkeley Lab, who worked closely with Gates to create and commission FIONA.
For example, superheavy elements might have unexpected nuclear forms or proton and neutron densities that are not included in the models, he said.
Berkeley Lab made an enormous contribution to the field of heavy element research: laboratory scientists played a role in the discovery of 16 elements on the periodic table related to the synthesis of neptunium in 1940, and also provided hundreds of isotopic identifications. Isotopes represent different forms of elements that have the same number protons, but have a different number of neutrons in their nuclei.
FIONA (see related article) is an addition to the gas-filled Berkeley separator (BGS). For decades, BGS has separated heavy elements from other types of charged particles, which can act as unwanted “noise” in experiments. FIONA is designed to trap and cool individual atoms, their separation based on their mass and charge properties and bring them to a low-noise detector station on a time scale of 20 milliseconds or 20 thousandths of a second.
"One atom per day"
Gregorich noted that "we can make one atom per day, give or take," from the desired superheavy element. At the beginning of work, FIONA was specifically tasked with capturing individual atoms of Muscovy. “We have a 14 percent chance of capturing every atom,” he added. Therefore, the researchers hoped to fix one dimension of the mass number of Muscovites per week.
Moscovium was opened in 2015 in Russia by the joint US team, which included scientists from the Lawrence Livermore National Laboratory, and the discovery of the nihonium is attributed to a team in Japan in 2004. The names of the elements were officially approved in 2016.
To create the Muscovy, scientists at the 88-inch cyclotron bombarded a target consisting of americium, an isotope of the element discovered by Glenn T. Seaborg from the Berkeley Lab and others in 1944, with a particle beam derived from the rare calcium isotope-48. The required half gram of calcium-48 was provided by the DOE isotope program.
There is a clear signal loop for each atom captured and measured by FIONA – a bit like watching a fixed point on a bicycle tire when the bike is rolling forward. The trajectory of this cyclic behavior is related to the atomic “mass-to-charge ratio” —the time and position of the energy signal measured in the detector, the mass number is reported to scientists.
In the ideal case, the measurement includes several steps in the decay chain of a particle: Moscovium has a half-life of about 160 milliseconds, which means that the atom has a 50% chance of decomposition into another element, known as a "child" element in the decay chain, every 160 milliseconds. Capturing your energy signature at a few steps in this decay chain can confirm which parent atom started this cascade.
“We’ve been trying to establish a mass number and proton number for many years now,” said Paul Fallon, senior researcher at Berkeley Lab’s nuclear science department, who heads the low-energy program of the unit. He noted that the sensitivity of the detector is steadily improving, as is the ability to isolate individual atoms from other noises. "Now we have the first final measurements."
Confirmation of the mass numbers of element 113 and element 115
In the first scientific experience of FIONA, researchers identified one atom of Muscovy and the decay daughters associated with it, and one atom of nihonium and its dissolute daughters. Measurements of atoms and decay chains confirm the predicted mass numbers for both elements.
While the researchers were only trying to create and measure the properties of the Muscovy atom, they were also able to confirm the measurement of nihonium after the atom of Muscovy disintegrated into nichonium before reaching FIONA.
“The success of this first measurement is incredibly fascinating,” said Jennifer Pora, a postdoc who participated in the commissioning experiments of FIONA. “The unique capabilities of FIONA have sparked a new renaissance of research into super-heavy elements on the 88-inch cyclotron.”
Gregorich attributed the efforts of the staff of the 88-inch cyclotron, including experts in the field of mechanical, electrical, operating and control systems, to maximize the FIONA experimental time during its initial five-week scientific mileage.
He noted the special contributions of other members of the BGS and FIONA groups, including Greg Pang, a former project research scientist who participated in the construction and testing of the FIONA; Jeff Kvarsik, a graduate student whose Ph.D. thesis focuses on FIONA results; and Nick Esker, a former PhD student, Ph.D. The work focused on the mass separator method incorporated by FIONA.
Plans for new measurements and the addition of “SHEDevil”
Fallon said that over the next six months, FIONA is scheduled to have another scientific run, during which nuclear physics researchers may conduct a new round of measurements for Muscovy and Nihonium or for other superheavy elements.
It is also planned to install and test a new instrument, called SHEDevil (for superheavy element detectors for extreme enterprises with low statistics), which will help scientists find out the shape of the nuclei of superheavy atoms by detecting gamma rays produced during their decay. These gamma rays will be the key to the location of neutrons and protons in the nuclei.
Measurement of the mass quantity of human extra heavy elements