Anomalous enhancement of the nuclear size in Ca isotopes
An international group of scientists from universities and research institutes, including Osaka University, has measured nuclear radii for 42–51Ca, finding a significant enhancement of the nuclear size beyond Calcium-48 (48Ca).
Inside the atomic nucleus, protons and neutrons are organized in shell structures similar to the electron shell around an atom. When proton or neutron shells are filled with 2, 8, 20, 28, 50, 82, or 126 nucleons (nucleon is the collective term for protons and neutrons), these numbers are called “magic numbers.” Atomic nuclei with a magic number of protons or neutrons are more stable than other nuclei.
Isotopes of an element have different mass numbers but the same number of protons. Nuclei that have both a neutron number and a proton number equal to one of the magic numbers are called "doubly magic" and are especially stable against decay.
Electrons can interact with protons because they are both electrically charged. In order to measure the nuclear radius, it is convenient to bombard electrons to the nucleus and observe their reaction. The nuclear charge (proton) distribution can be measured by electron scattering. The nuclear radius, or the size of nucleus, can be deduced from proton distribution in a nucleus.
The nuclear charge radius, also called rms (root mean square) charge radius, is a measure of the size of an atomic nucleus, particularly the proton distribution, and gives complemental information on the size of the nucleus. Scientists study the size of unstable nuclei near magic numbers by measuring nuclear charge radii.
Recently, the nuclear size of the doubly magic Calcium-48 (48Ca) core in Ca isotopes beyond N = 28 has received considerable attention. 48Ca is doubly magic: it has 20 protons and 28 neutrons.
Measurements of charge radii for 42–52Ca revealed a swelling of the bare 48Ca core in Ca isotopes beyond N = 28. In order to clarify the mechanisms behind this swelling, measurements of the interaction cross section, which is sensitive to the rms radius of nucleon density distribution, or the matter radius, are important.
This international joint group produced calcium-42-51 isotopes, 42-51 Ca (20 protons, 22~31 neutrons), with a 345 MeV/nucleon 238U primary beam bombarding a rotating beryllium production target installed at the focal plane of the BigRIPS superconducting fragment separator at the RIKEN Radioactive Isotope Beam Factory (RIBF). They calculated the nuclear radii of these isotopes by measuring the interaction cross section.
This group observed a sudden growth in matter radii in neutron-rich Ca isotopes beyond N= 28. This enhancement of matter radii was much larger than that of the previously measured proton distribution (charge radii), indicating a sudden increase in the surface diffuseness of the neutron density distribution.
A dramatic enhancement of matter radii beyond N = 28 was similar to the growth in the charge radii of Ca isotopes but was much greater in magnitude. The neutron skin thickness of Ca isotopes exhibited a striking increase beyond N = 28.
Measurements of matter radii revealed that neutrons played a crucial role in anomalous enhancement of charge radii near the magic number 48Ca in Ca isotopes beyond N = 28, which had been pointed out in previous studies, suggesting that measurements of matter radii near magic numbers, or determining the surface diffuseness of the neutron density distribution, is important. For deeper understanding of these findings, further studies of nuclear charge radii in a wide mass region and theoretical research discussions will be necessary.
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The article, “Swelling of Doubly Magic 48Ca Core in Ca Isotopes beyond N = 28,” was published in Physical Review Letters at DOI: https://doi.org/10.1103/PhysRevLett.124.102501
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