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New Experiment Quantifies Neutron Skin in Calcium

Heavier types of calcium nuclei are comparatively thin-skinned. This was disclosed by a new measurement of the neutron skin in calcium.

New Experiment Quantifies Neutron Skin in Calcium.
A precision measurement of the neutron skin in a calcium nucleus surprisingly reveals a thinner skin than expected from a similar measurement in lead. Image Credit: Jefferson Lab.

The latest measurement, performed by the 48Ca Radius EXperiment (CREX) collaboration at DOE’s Thomas Jefferson National Accelerator Facility, was presented at the 2021 Fall Meeting of the APS Division of Nuclear Physics of the American Physical Society. It is known to be the first highly strong electroweak measurement of the neutron skin in a medium-weight nucleus, and it features an accuracy of around 0.025 millionths of a nanometer.

The measurement originates from a whole range of experiments targeted at teasing out the way in which neutrons and protons are circulated in dense nuclei. Earlier in 2021, the 208 Pb Radius EXperiment (PREX) verified that lead nuclei have comparatively thick skin. The lead-208 nucleus consists of 82 protons and a high number of 126 neutrons.

In lead, a comparatively equal number of its neutrons and protons tend to assemble toward the core. Additional neutrons in the lead nuclei get pushed out to the fringe to make the neutron-rich “skin” near the nucleus.

This new outcome from CREX is obtained from measurements composed of the neutron-rich nuclei of calcium-48, having 28 neutrons but just 20 protons. The scientists quantified the weak form factor of the calcium-48 nucleus to a precision of 4.3%.

Our new, extremely precise CREX measurement on calcium-48 indicates a thin neutron skin around its nucleus. In contrast, our PREX measurement on lead-208 revealed a somewhat thick skin around that nucleus.

Caryn Palatchi, Research Associate, University of Virginia

Palatchi presented the study findings on behalf of the CREX collaboration.

This variance seems to be an exciting one for nuclear physics. It offers the chance for additional exploration to identify why there is such a huge difference between the high-density lead nucleus and the medium-density calcium nucleus.

The smaller calcium nucleus has fewer protons and neutrons, and its just within the reach of certain types of calculations that come from first principles, or ab initio calculations. The CREX result is consistent with the thin neutron skin predictions from coupled cluster calculations.

Caryn Palatchi, Research Associate, University of Virginia

Robert Michaels, a staff scientist at Jefferson Lab and lead spokesperson for this experiment, describes that such theoretical calculations apply well to calcium-48.

It is gratifying that the theory seems to work for calcium-48.

Robert Michaels, Staff Scientist, Jefferson Lab

This comparison with theory displays that the experimental method has been well-calibrated for nuclei that could be evaluated dependably. Also, this indicates that the lead measurement, performed with the same calibrated method, is tested to work, enhancing confidence in that outcome.

The scientists say the next level is to go nearer the final publication of the new result. The last reported result will feature data on calcium-48’s form factor, its neutron radius and neutron skin. Also, they aim to perform painstaking comparisons of the CREX and PREX experiments with theory to identify what extra data can be gathered from the experiments.

Journal Reference:

Hagen, G., et al. (2021) Coupled-cluster computations of atomic nuclei. Reports on Progress in Physics.


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