Mysterious change of shape in the mercury atom

Jumps in the nuclear form detected in certain mercury isotopes

The atomic nuclei of mercury bounce between round and oval, depending on the number of neutrons. © CERN
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Unique Phenomenon: The atomic nucleus of mercury is a real shape-changer - it jumps back and forth between a round and an elliptical shape depending on the number of its neutrons, as an experiment reveals. Such behavior is unknown to any other element. Strange also: The change between round football and oval rugby ball shape happens only with mercury isotopes with between 181 and 185 core building blocks, as the researchers report in "Nature Physics".

Mercury is a real nerd. Because it is the only metal that is liquid at room temperature. The silvery liquid has such a high density that even a piece of iron floats in mercury. The reason for its low melting point is the heavy nucleus: In addition to the 80 protons, the mercury core contains between 95 and 128 neutrons, depending on the isotope. This mass leads to relativistic effects for the electrons of the atomic shell

Deformed atomic nuclei

But that's not the only thing that's special about mercury, as Bruce Marsh from the CERN research center and his team now report. Using the ISOLDE ion source at CERN, they systematically produced various mercury isotopes and examined their atomic nuclei more closely using special laser and mass spectrometers. The range ranged from the isotope 185 Hg with 105 neutrons to the extremely short-lived isotope 177 Hg, which has a half-life of only 27 milliseconds.

The reason: Earlier experiments had shown first indications that some radioactive mercury isotopes behave differently than expected. Normally, the atomic nuclei of heavy metals gradually become smaller with decreasing neutron number. With mercury, however, some isotopes seemed to be out of line: their nuclei were unusually large and deformed. What's up, Marsh and his team have now checked.

Mean square charge radius of mercury atomic nuclei depending on the mass number Frank Wienholtz

Abrupt change between "football" and "rugby"

And indeed, the measurements revealed that only the mercury isotopes with an even number of neutrons follow the rule. Their cores are round and are becoming smaller and smaller with decreasing mass number. However, the isotopes with the odd mass numbers are different: If the nucleus contains 101, 103 or 105 neutrons, an abrupt change takes place: The nucleus is much larger and no longer round, but oval like a rugby ball. display

"This change in shape manifests characteristic features of a quantum phase transition, " explain the researchers. "The two phases an almost round and a strongly deformed core do not mix. However, small changes in one parameter, in this case the neutron number, are enough to make the system jump between the two phases. "

ISOLDE experiment at CERN CERN

Only six times

Strangely enough, this change of nuclear forms does not occur with mercury in all isotopes with odd mass numbers. Instead, the researchers observed the phenomenon only at neutron numbers between 101 and 105. "In the isotope chain of mercury, this phase transition occurs only six times, " said Marsh and his colleagues. "This confirms that this dramatic change of form is a localized phenomenon and that mercury below 100 neutrons returns permanently to a round shape."

But why? In search of the cause of this unique phase change, scientists analyzed the quantum states of mercury isotopes using elaborate computer models. "The most striking differences between the isotopes Hg, Hg and Hg and the rest can be found in the nucleon occupation of two orbitals, "report Marsh and his team.

103 neutrons are the critical number

According to this, four protons and eight neutrons form an excited complex in these anomalous isotopes, leading to a special occupation of the nuclear orbitals. This in turn causes the change to an oval core shape, as the researchers explain. Because this configuration occurs only at a neutron number close to 103, this change of shape is due to the three mercury isotopes Hg, Hg and Hg limited.

The riddle of the strange change in the shape of mercury could finally be solved. (Nature Physics, 2018; doi: 10.1038 / s41567-018-0292-8)

(CERN, 08.10.2018 - NPO)