Physicists Measure Mass Numbers of Nihonium and Moscovium

Physicists using the FIONA (For the Identification Of Nuclide A) device at the Department of Energy’s Lawrence Berkeley National Laboratory’s 88-inch Cyclotron have performed the first direct measurements of the mass numbers for the nuclei of nihonium (Nh, element 113) and moscovium (Mc, element 115).

Left: average of known decay properties assigned to ²⁸⁸Mc and its daughters. Right: details of decay chains detected at the FIONA focal plane. Unobserved decays within each decay chain are indicated as ‘unobserved’ and are assumed to have been emitted out of the open end of the detector array. The x position of decays observed in the focal-plane detector is also given. Image credit: Gates et al, doi: 10.1103/PhysRevLett.121.222501.

“The mass number and atomic number (or ‘Z’) — a measure of the total number of protons in an atom’s nucleus — of superheavy elements have relied on the accuracy of nuclear mass models,” said Dr. Ken Gregorich, a recently retired senior scientist in Berkeley Lab’s Nuclear Science Division.

“So it’s important to have a reliable way to measure these numbers with experiments in case there is a problem with models.”

“For example, superheavy elements could possibly exhibit unexpected nuclear shapes or densities of protons and neutrons that aren’t accounted for in the models.”

To produce moscovium, the Berkeley Lab scientists at the 88-inch Cyclotron bombarded a target composed of americium with a particle beam produced from the rare isotope calcium-48.

There is a distinct looping signature for each atom trapped and measured by FIONA — a bit like watching a fixed point on a bicycle tire as the bicycle rolls forward. The trajectory of this looping behavior is related to the atomic ‘mass-to-charge ratio’ — the timing and position of the energy signal measured in the detector tells the physicists the mass number.

Ideally, the measurement includes several steps in the particle’s decay chain: moscovium has a half-life of about 160 milliseconds, meaning an atom has a 50% chance to decay to another element known as a ‘daughter’ element in the decay chain every 160 milliseconds.

Capturing its energy signature at several steps in this decay chain can confirm which parent atom began this cascade.

“We have been trying to establish the mass number and the proton number here for many years now,” said Dr. Paul Fallon, a senior scientist in Berkeley Lab’s Nuclear Science Division.

“Detector sensitivity has steadily improved, as has the ability to isolate individual atoms from other noise. Now, we have our first definitive measurements.”

In FIONA’s first scientific run, the researchers identified one moscovium atom and its related decay daughters, and one nihonium atom and its decay daughters.

The measurements of the atoms and the decay chains confirm the predicted mass numbers for both elements.

While the physicists had been seeking only to create and measure the properties of a moscovium atom, they were also able to confirm a measurement for nihonium after a moscovium atom decayed into nihonium before reaching FIONA.

“The success of this first measurement is incredibly exciting,” said Dr. Jennifer Pore, a postdoctoral fellow at Berkeley Lab.

“The unique capabilities of FIONA have sparked a new renaissance of superheavy element research at the 88-inch Cyclotron.”

The results were published in the journal Physical Review Letters.

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J. M. Gates et al. 2018. First Direct Measurements of Superheavy-Element Mass Numbers. Phys. Rev. Lett 121 (22); doi: 10.1103/PhysRevLett.121.222501