Are Nuclei Thick Skinned?Category: Science & Technology
Posted: March 9, 2012 05:25PM
Most people know an atom consists of electrons orbiting a nucleus, and that a nucleus contains protons and neutrons, but not everyone knows the layout of the protons and neutrons within the nucleus. At the core of the nucleus is the protons and some neutrons, but outside of the core is a skin of neutrons, and researchers want to know how thick that skin is.
Finding the proton radius of a nucleus is not terribly challenging; researchers just look at the deflection of electrons as they pass by the protons. The neutron radius though is going to be much trickier because neutrons do not so readily interact with other particles. (The difference of the neutron and proton radii is the thickness of the neutron skin.) Instead of using the electromagnetic force, the researchers had to play with the weak nuclear force, which works quite differently.
When electrons speed by the neutrons, they are affected by the weak nuclear force, but how they are affected depends on the mechanical spin of the electrons (the electrons are literally spinning in the experiment). If the electrons are spinning in one direction, the weak nuclear force reacts one way, and if they spin in the opposite direction, the reaction is different. This asymmetry is what the researchers wanted to examine to find the neutron radius.
The results came back to say that the skin of a lead-208 nucleus is 0.33 millionths of a nanometer thick, +/- 50%. Obviously that margin of error is not particularly good, but the real trouble is that this measurement differs from current theory. As the experiment is repeated and improved upon it may become necessary for rethink the current models of the nucleus, or the current measurement could be proven wrong. Either way, the results will have impacts from nuclear physics and quantum mechanics to astrophysics and cosmology, as the size and interactions of neutron stars are affected by the skin of a nucleus.