Some atomic nuclei are thin-skinned — they’re surrounded by a slim shell of neutrons.
Physicists now know how thick that neutron skin is for one particular type of nucleus. The skin of lead-208 — a variety of lead with 126 neutrons in addition to its 82 protons — is about 0.28 trillionths of a millimeter thick, researchers report online April 27 in Physical Review Letters.
Lead-208’s nucleus is approximately spherical, a ball of protons embedded within a slightly bigger ball of neutrons. Measuring the difference between the sizes of the spheres reveals the thickness of lead’s sleek neutron skin.
Gauging the size of the proton sphere is relatively straightforward:
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