That headline may sound like the set-up for a geeky joke aimed at physicists, but in reality, it was the research question asked by a team of MSU scientists at the superconductor located within RIKEN’s Radioactive Isotope Beam Facility in Wako, Japan. The answer, found in the form of calcium-60, was far more than we expected.
That punchline came about when researchers from Michigan State University working at the facility spotted a calcium isotope with its usual 20 protons, but a whopping 40 neutrons, double what it would commonly have. It is not only more than the previous record holder for a calcium atom but also defies current models which predict that such heavy isotopes of calcium shouldn’t exist.
Particular elements always have the same number of protons in their atomic nucleus, which is made up of protons and neutrons held together by the strong nuclear force. The number of neutrons is free to vary, however. Thus the atomic mass of an element, which is given by the number of protons and neutrons, collectively known as nucleons, in the atomic nucleus, can also vary.
The more neutrons we add to an atomic nucleus, the heavier we describe that isotope as being. The heaviest isotope of a particular element tells us the upper limit of the amount of neutrons that element can hold.
We call these versions of an element with different atomic masses isotopes. They are generally identified by the number of nucleons, so common calcium would be calcium-40, whereas this newly observed isotope would be referred to as calcium-60.
Different isotopes of the same element can have different properties. Some are stable, they can potentially last forever, whilst heavier isotopes can last from between years and a matter of minutes. The heaviest known isotopes exist for only fractions of a second.
The recent experiments conducted at RIKEN created two heavy isotopes of calcium, calcium-59 and calcium-60. The most neutron packed nuclei known to physics This is 12 neutrons more than the closet record holder, calcium-48. Whereas calcium-48 is stable, calcium-60 lasted for only a few thousandths of a second.
The experiment is designed to allow researchers to probe the limits of the strong nuclear force, one of nature’s fundamental forces and one that scientists know relatively little about in comparison to gravity and electromagnetism.
“At the heart of an atom, protons and neutrons are held together by the nuclear force, forming the atomic nucleus,” said Oleg Tarasov, a staff physicist at MSU’s National Superconducting Cyclotron Laboratory. “Scientists continue to research what combinations of protons and neutrons can exist in nature even if it is only for fleeting fractions of a second.”
The experiment, which also produced sulfur and chlorine isotopes, sulfur-49 and chlorine-52, has prompted scientists to reassess models of just what is possible terms of nucleus sizes. “Some of these models that describe nuclei at the highest resolution scale predict that 20 protons and 40 neutrons will not hold together to form Ca-60,” Alexandra Gade, professor of physics at MSU and NSCL chief scientist said. “The discovery of calcium-60 will prompt theorists to identify missing ingredients in their models.”
To create these new heavy isotopes, researchers accelerated an intense beam of heavy zinc ions onto a block of beryllium. The heavy nuclei were created as ‘debris’ from the collision. The now hope that MSU’s Facility for Rare Isotope Beams, scheduled for completion in 2022 will enable scientists to create even heavier isotopes, with calcium-68 or even calcium-70 being the next possible ‘heaviest isotope’ contenders.
The findings were published in the latest edition of the journal Physical Review Letters.