Sat. Apr 17th, 2021

Scientists discovered the lightest isotope of mendelevium thanks to the direct measurement of its mass number.

Nuclear scientists are extremely interested in how different combinations of protons and neutrons affect the structure of the nucleus. Newly discovered isotopes have unique combinations of protons and neutrons that scientists have never before observed. For the heaviest elements, neighboring isotopes often have very similar properties. This makes it challenging to disentangle the properties of a new isotope. However, scientists can differentiate these neighboring isotopes by their differing masses.

Isotopes of mendelevium took unique two-looped paths that were dependent on their mass through a device called For the Identification of Nuclide A (FIONA). Scientists measure the mass of the new isotope by seeing where the isotope landed on a detector. Image courtesy of J.L. Pore / LBL

Scientists use a device called FIONA (For the Identification of Nuclide A), an instrument at Lawrence Berkeley National Laboratory’s (LBNL) 88-Inch Cyclotron, to measure the masses of heavy-element isotopes. For the first time, scientists have used FIONA to discover a new heavy-element isotope, mendelevium-244.

The Impact

For decades, nuclear scientist faced ambiguity when assigning properties to isotopes of heavy elements. These are elements with atomic numbers higher than 92. This ambiguity results from the need to rely on previously published results and theoretical predictions. The new device, FIONA, directly identifies isotopes of heavy and super-heavy elements (atomic numbers higher than 103) by their mass numbers. FIONA allows for direct experimental confirmation that given properties correspond to a specific isotope. This removes all ambiguity. The result is a much clearer picture of the behavior of heavy and super-heavy nuclei. Researchers can use this technique to identify and study unknown isotopes.


A recent nuclear physics experiment discovered a new isotope, mendelevium-244. This is the lightest isotope of mendelevium researchers have ever observed. In total, the experiment produced ten mendelevium-244 atoms in the nuclear reaction where ions of argon-40 bombarded a target of bismuth-209 at the 88-Inch Cyclotron. The FIONA device at the Cyclotron was critical to the discovery of mendelevium-244. As isotopes travel through FIONA, they take unique two-looped flight paths before they land on a detector. The trajectories of these two-looped paths are dependent on that isotope’s mass. Researchers can therefore measure an isotope’s mass just from seeing where that isotope landed on the detector. This apparatus allowed nuclear scientists to directly measure the mass number (244), of the new mendelevium isotope. This gave unambiguous confirmation that the experiment had created a new isotope.

With the discovery of mendelevium-244, researchers have observed seventeen isotopes of mendelevium, including twelve discovered at LBNL. Studies of new isotopes allow nuclear scientists to understand the production mechanisms and decay properties of these rare nuclei and to test models at the limits of nuclear existence. Scientists are now conducting additional measurements to discover more isotopes.


Pore, J.L., et al., Identification of the new isotope 244Md. Physical Review Letters 124, 252502 (2020). [DOI: 10.1103/PhysRevLett.124.252502]

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