What is the charge of the mercury atoms

Charge radii of the mercury nuclei 207Hg and 208Hg were measured for the first time

Similar to the electrons in the atomic shells, the building blocks of the atomic nuclei, the protons and neutrons, also form shell structures. With a number of 8, 20, 28, 50, 82 or 126 - also known as magic numbers in nuclear physics - shell closures are achieved. These atomic nuclei are particularly stable. The chemical elements are defined by the number of protons in their atomic nuclei. Although only the protons are electrically charged, the nuclear charge radius also depends on the number of neutrons in the nucleus. Therefore the atomic radii of the different isotopes of an element differ. In general, the following applies: the more neutrons a nucleus has, the larger its radius. But there are some special features. The actual distribution of the protons in the nucleus is influenced by the distribution of the neutrons. This in turn depends crucially on the number of neutrons. Once the shell has been closed, further neutrons have to move to the next shell. Therefore, after the shell has been closed, the charge radius increases significantly more with the increasing number of neutrons than before. The shape of the cores can also change abruptly after the shell is closed. There are also even-odd effects. These are systematic changes in the atomic charge radii between nuclei with an even and an odd number of neutrons.

“The secrets of the size and shape changes in the atomic nucleus are investigated with the help of laser spectroscopic methods. With our measurements we have the charge radii of the neutron-rich mercury nuclei 207Hg and 208Hg examined. Both nuclei have the same number of protons Z = 80. In terms of the number of neutrons, they therefore surpass a magic number: 207With 127 neutrons, Hg has one more neutron and 208Hg with 128 neutrons even two more than the magic number 126. We were now able to determine the charge radii of these two isotopes experimentally for the first time and compare them with corresponding calculations. In addition to a change in slope after the shell has closed, we were also able to demonstrate the even-odd effect between neighboring isotopes in the investigated mercury nuclei, ”reports Dr. Frank Wienholtz, who was a doctoral student in the Greifswald working group for atomic and molecular physics at the time of the measurements.

With half-lives of 2.9 and 42 minutes apply 207Hg and 208Hg as unstable atomic nuclei. For the measurements, the radioactive mercury isotopes must first be produced in a complex manner. For this purpose, liquid lead (Z = 82) is bombarded with high-energy protons at the ISOLDE apparatus at CERN. In this way, the exotic mercury atoms can be produced, but only in very small quantities. In addition, when the isotopes of mercury are produced, they become stable 207Pb and 208Pb lead atoms released. You contaminate the sample.

To determine the nuclear charge radii of the mercury atoms, the atoms are ionized by means of multiple laser transitions. The desired core properties result from the resonance frequencies of the laser ionization. However, a large number of other particles are ionized in such experiments. The small amounts of mercury ions in which the researchers are interested must therefore be differentiated from the contamination ions of the almost equally heavy lead atoms. This task is performed by the highly sensitive and highly selective Greifswald multi-reflection time-of-flight mass spectrometer of the ISOLTRAP apparatus at CERN. In addition to the size of the charge radii, this Greifswald spectrometer was also used to precisely determine the mass of the atomic nuclei. The results of the mass measurements are currently still being evaluated, analogous to earlier experiments on exotic calcium atoms.

In addition to the Schweikhard working group of the Institute for Physics at the University of Greifswald, members of the CERN, the Max Planck Institute for Nuclear Physics in Heidelberg and the universities in Dresden, Manchester (Great Britain) and Paris-Sud (France) were involved in the ISOLTRAP collaboration. There were also several other experimental groups, including laser spectroscopists from the University of Mainz.

additional Information

ISOLTRAP apparatus at CERN
Atomic and Molecular Physics working group at the University of Greifswald
Institute for Nuclear Physics at the TU Darmstadt
Stored and cooled ions department at the Max Planck Institute for Nuclear Physics

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Laser spectroscopy of neutron-rich 207,208Hg isotopes: Illuminating the kink and odd-even staggering in charge radii across the N = 126 shell closure. T. Day Goodacre, A.V. Afanasjev, A.E. Barzakh, B.A. Marsh, S. Sels, P. Ring, H. Nakada, A.N. Andreyev, P. Van Duppen, N.A. Althubiti, B. Andel, D. Atanasov, J. Billowes, K. Blaum, T.E. Cocolios, J.G. Cubiss, G.J. Farooq-Smith, D.V. Fedorov, V.N. Fedosseev, K.T. Flanagan, L.P. Gaffney, L. Ghys, M. Huyse, S. Kreim, D. Lunney, K.M. Lynch, V. Manea, Y. Martinez Palenzuela, P.L. Molkanov, M. Rosenbusch, R.E. Rossel, S. Rothe, L. Schweikhard, M.D. Seliverstov, P. Spagnoletti, C. Van Beveren, M. Veinhard, E. Verstraelen, A. Welker, K. Wendt, F. Wienholtz, R.N. Wolf, A. Zadvornaya, K. Zuber.Physical Review Letters. 2021: 126, 032502. DOI: https://doi.org/10.1103/PhysRevLett.126.032502

The graph shows the increase in the mean square charge radius (r²) of neutron-rich mercury atomic nuclei as a function of the number of neutrons. The mean square charge radius (r²) includes not only the volume of the nuclei, but also their shape. On the one hand, the figure illustrates the different nuclear shapes depending on the neutron number N. It also becomes clear that mercury atoms with a neutron number between 100 and 105 have a noticeable even-odd effect (zigzag behavior) between neighboring isotopes. The new measurements concern the steeper rise in the charge radii of the mercury isopes 206Hg, 207Hg and 208Hg (marked in red). This change in gradient could now be determined experimentally for the first time and verified through new calculations. © Thomas-Day-Goodacre

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Contact Person

Prof. Dr. Lutz Schweikhard
Head of the Atomic and Molecular Physics working group
Institute for Physics at the University of Greifswald
Felix-Hausdorff-Strasse 6, 17489 Greifswald
Telephone 03834 420 4700

Dr. Frank Wienholtz
Institute for Nuclear Physics
Schlossgartenstrasse 9, 64289 Darmstadt
Telephone 06151 1623537

Prof. Dr. Klaus Blaum
Spokesman for the ISOLTRAP collaboration
Max Planck Institute for Nuclear Physics
Saupfercheckweg 1, 69117 Heidelberg
Telephone 06221 516850