A team led by nuclear physicists at the Lawrence Berkeley National Laboratory Department (Berkeley Lab) has reported the first direct measurement of mass numbers for the core of two superheavy elements: moscovium, which is element 115, and nihonium, element 113.
They got results using FIONA, a new tool at Berkeley Lab designed to solve the nuclear and atomic properties of the heaviest elements. The results are detailed in the November 28 issue Physical Review Letter journal.
FIONA is an acronym which means: "For Identification of Nuclide A," with "A" representing the scientific symbol for the mass number of elements – the total number of protons and neutrons in the atomic nucleus. The proton is positively charged and the number of protons is also known as the atomic number; neutrons have a neutral charge. Superheavy elements are man-made and have a higher atomic number than those found in natural elements.
Global turmoil for mass numbers
Collecting and validating this first data from FIONA has been a top priority for Lab's 88-Inch Cyclotron and Nuclear Science Division since FIONA's commissioning was wrapped up in early 2018. Cyclotron staff work with visiting and in-house scientists to conduct the first experimental experiments FIONA, which spans five weeks.
"It is interesting to see FIONA coming online, because it is very important to determine the mass of the superheavy element," said Barbara Jacak, director of the Nuclear Science Division. "Until now mass assignments have been made with indirect evidence and not by direct measurement."
Jackie Gates, a staff scientist at Berkeley Lab's Nuclear Science Division who played a leading role in the conception, construction, and testing of FIONA, and who led FIONA's mass-numbering efforts, said, "There is a lot of interest in making experimental measurements of numbers superheavy mass. "
Gates added that efforts to measure the mass of superheavy elements were of global importance, with a team from Argonne National Laboratory and a Japanese nuclear research program among them also making mass measurements of superheavy elements using a slightly different approach or tool.
Guy Savard, a senior scientist at Argonne National Laboratory, designed, built and contributed several components to FIONA. He also helped in FION preparation and in his first scientific campaign.
Roderick Clark, a senior scientist at the Berkeley Lab Nuclear Science Division, said, "Everyone comes together in this big race. It can open up a variety of physics from these heavy and superheavy samples," as well as new studies of the structure and chemistry of elements these exotic elements, and a deeper understanding of how they are tied to other elements.
"If we can measure the mass of one of these superheavy elements, you can nail down the entire region," Clark said.
New chapter in heavy element research
The mass number and atomic number (or "Z") – the measure of the number of protons in the superheavy nucleus depends on the accuracy of the nuclear mass model. So it's important to have a reliable way to measure these numbers with experiments if there is a problem with the model, said Ken Gregorich, a senior scientist who recently retired at Berkeley Lab's Nuclear Science Division who worked closely with Gates to build and assign FIONA.
For example, superheavy elements might indicate the shape or unexpected nuclear density of protons and neutrons that are not taken into account in the model, he said.
Berkeley Lab has made a major contribution to the field of heavy element research: Lab scientists have played a role in the discovery of 16 elements in the periodic table, dating back to neptunium synthesis in 1940, and have also supplied hundreds of isotope identification. Isotopes are various forms of elements that have the same number of protons but have different numbers of neutrons in their nucleus.
FIONA (see related article) is an add-on to Berkeley Gas-filled Separator (BGS). For decades, BGS has separated heavy elements from other types of charged particles that can act as unwanted "noise" in experiments. FIONA is designed to trap and cool individual atoms, separate them based on the mass and charge of property, and deliver them to low noise detector stations on a time scale of 20 milliseconds, or 20 thousandths of a second.
& # 39; One atom a day & # 39;
"We can make one atom a day, give or receive," from the desired superheavy element, Gregorich noted. In its initial operation, FIONA was specifically assigned to trap individual moscovium atoms. "We have about 14 percent chance of trapping every atom," he added. So the researchers hope to capture a measurement of the amount of moscovium mass per week.
Moscovium was discovered in 2015 in Russia by a joint US-Russian team which included scientists from Lawrence Livermore National Laboratory, and the discovery of nihonium was credited to a team in Japan in 2004. The names of the elements were officially approved in 2016.
To produce moscovium, scientists in the 88-Inch Cyclotron bombarded targets consisting of americium, an isotope of an element discovered by Berkeley Lab Glenn T. Seaborg and others in 1944, with a bundle of particles produced from the isotope calcium-48 which rare. Half a gram of calcium-48 is provided by the DOE Isotope Program.
There are different looping marks for each atom that is trapped and measured by FIONA – a little like paying attention to fixed points on bicycle tires when the bike rolls forward. The trajectory of repetitive behavior is related to the ratio of "mass-to-charge" atoms – the time and position of the energy signal measured in the detector tells the scientists the amount of mass.
Ideally, measurements cover several steps in the particle decay chain: Moscovium has a half-life of around 160 milliseconds, which means that atoms have 50 percent chance of decaying into other elements known as "daughter" elements in the decay chain every 160 milliseconds. Capturing its energy sign at several steps in this decay chain can confirm which parent atom starts this cascade.
"We have tried to assign mass numbers and proton numbers here for years now," said Paul Fallon, a senior scientist at the Berkeley Lab Nuclear Science Division who heads the division's low energy program. The sensitivity of the detector has continued to increase, because it has the ability to isolate individual atoms from other noise, he said. "Now, we have our first definitive measurement."
Confirm the mass number of elements 113 and element 115
In the first FIONA scientific study, the researchers identified one atom of moscovium and related decay children, and one atom of nihonium and its decaying children. Atomic and decay chain measurements confirm the predicted mass for the two elements.
While researchers have searched only to create and measure the atomic properties of moscovium, they were also able to confirm measurements for nihonium after the moscovium atom decayed into nihonium before reaching FIONA.
"The success of this first measurement is very interesting," said Jennifer Pore, a postdoctoral colleague involved in the FIONA commissioning trial. "FIONA's unique capabilities have triggered a new revival of research on superheavy elements on the 88-Inch Cyclotron."
Gregorich credited the efforts of staff at the 88-Inch Cyclotron – including mechanical, electrical, operations, and control system experts – to maximize FIONA's experimental time during the initial five weeks of scientific work.
He recorded special contributions from other members of the BGS and FIONA groups, including Greg Pang, a former project scientist involved in the construction and testing of FIONA; Jeff Kwarsick, a graduate student who is a Ph.D. thesis is focused on FIONA results; and Nick Esker, a former graduate student who is a Ph.D. work focused on the technique of mass separations that are united by FIONA.
Plans for new measurements and additions & # 39; SHEDevil & # 39;
Fallon said that another scientific trip was planned for FIONA in the next six months, where nuclear physics researchers could pursue a new round of measurements for moscovium and nihonium, or for other superheavy elements.
There are also plans to install and test a new tool, dubbed "SHEDevil" (for Super Heavy Element Detectors for Extreme Ventures in Low statistics) that will help scientists study the shape of the superheavy atomic nucleus by detecting gamma rays produced in its decay. This gamma ray will give clues to the arrangement of neutrons and protons in the nuclei.
Measuring the number of superheavy masses, man-made elements