How do you know how strong interaction forces are?
1 Answer
The strength of the strong interaction can be measured by binding energy.
Explanation:
The strong interaction is responsible for binding protons and neutrons in an atomic nucleus. It has to overcome the electromagnetic force which is causing the positively charged protons to repel each other.
Free protons and neutrons has known masses. The mass of an atomic nucleus is less that the sum of the masses of the equivalent number of free protons and neutrons. This difference is called the mass deficit or binding energy. Dividing this mass difference by the total number of nucleons gives the binding energy per nucleon which is a measure of the strength of the strong interaction.
Atomic masses are measured in Atomic Mass Units or amu. One amu is defined to be 1/12 of the mass of a Carbon-12 nucleus. It is about
A proton has a mass of 1.007825 amu. A neutron has a mass of 1.008665 amu.
A Helium-4 nucleus has a mass of 4.002602 amu. This gives a binding energy per nucleon of 0.0075945 amu. Helium-4 is particularly stable and hence quite abundant.
The element with the highest binding energy per nucleon is Nickel-62. It has a mass of 61.928345 amu and a binding energy per nucleon of 0.00944137.
The strong interaction is short ranged and the electromagnetic force is long ranged. For elements heavier than nickel, every proton in the nucleus is repelling every other one but the strong interaction only binds nucleons to their neighbours. Heavier elements require more neutrons to make them stable. Compare Nickel-62 with 28 protons and 34 neutrons with Uranium-238 with 92 protons and 146 neutrons. Uranium-238 is the most common Uranium isotope and it is not stable and is radioactive. Uranium-238 has a binding energy per nucleon of 0.0081269.
