= Nuclear physics
{wiki}
<Nuclear physics> studies <atomic nuclei>, their internal structure and the processes that change them. A <nucleus> contains <protons> and <neutrons>, whose short-range <nuclear forces> bind them while the <Coulomb force> repels the <protons>. The resulting <nuclear energy levels> and <nuclear binding energies> connect this subject to the low-energy consequences of <quantum chromodynamics>. <Weak interactions> additionally permit changes of nuclear composition through <beta decay>.
The <mass defect> makes the connection between structure and released <energy> quantitative. For a <nucleus> with $Z$ <protons>, $N$ <neutrons> and nuclear <mass> $M_{\rm nuc}$, its <nuclear binding energy> is
$$
B=[Zm_p+Nm_n-M_{\rm nuc}]c^2.
$$
A bound <nucleus> has less rest <energy> than its separated <nucleons>. In a <nuclear reaction>, the difference between the sums of initial and final rest <energies> supplies the <Q value>. The sign of that difference and the <Coulomb barrier> answer different questions: a reaction can release <energy> overall while still requiring a way to cross an entrance barrier.
Scattering experiments supplied an early structural clue. Rare large deflections of <alpha particles> by thin matter could be explained by close encounters with a highly concentrated central charge. <Ernest Rutherford>'s 1911 analysis connected such angular measurements to a compact atomic centre, establishing a route from measured <scattering cross-sections> to internal structure.<The Scattering of Alpha and Beta Particles by Matter and the Structure of the Atom by Ernest Rutherford>[Sections 1–2 and 7, large single deflections and the concentrated central charge]{ref}
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