How thick is the neutron skin of lead-208?

• A. 0.28 trillionths of a millimeter
• B. 0.28 millimeters
• C. 2.8 millimeters
• D. 28 millimeters

Answer: (A)

Neutron skin thickness describes how much farther the neutron distribution extends beyond the protons inside a heavy nucleus like lead-208. The value given, 0.28 trillionths of a millimeter, is an incredibly small length—so small that it sits at the scale of fractions of a femtometer (fm), where 1 fm = 1e-15 meters.

Convert it to familiar units: 0.28 × 10^-12 mm equals 0.28 × 10^-15 meters, which is 0.28 fm. That means the extra neutron layer around the nucleus is about a few tenths of a femtometer thick. This is perfectly reasonable since nuclear dimensions are on the order of several femtometers (the nucleus itself is roughly 7 fm across for lead-208).

So the thickness corresponds to about 0.28 femtometers, i.e., 0.28 × 10^-12 millimeters.