![]() ![]() ![]() However, the strong nuclear force is an extremely short range force, and this negative potential energy essentially disappears at the radius of the nucleus as illustrated below. Within the nuclear interior, the strong force attraction between nucleons produces a deep well of potential energy, even though it must overcome the strong electromagnetic repulsion of the protons to do so. By examining the coupling constants for the fundamental forces, it is clear that the strong force would be expected to dominate for phenomena within the nucleus. The nuclear strong force and the electromagnetic force influence radioactivity in the form of alpha decay. Strong and Electromagnetic Forces in Alpha Decay Calculating the ratio of the wavefunction outside the barrier and inside and squaring that ratio gives the probability of alpha emission. Inside the barrier, the solution to the Schrodinger equation becomes a decaying exponential. To evaluate this probability, the alpha particle inside the nucleus is represented by a free-particle wavefunction subject to the nuclear potential. Quantum mechanical tunneling gives a small probability that the alpha can penetrate the barrier. The Coulomb barrier faced by an alpha particle with this energy is about 26 MeV, so by classical physics it cannot escape at all. The illustration represents an attempt to model the alpha decay characteristics of polonium-212, which emits an 8.78 MeV alpha particle with a half-life of 0.3 microseconds. ![]() This extraordinary dependence upon kinetic energy suggests an exponential process, and is modeled by quantum mechanical tunneling through the Coulomb barrier. This half-life range depends strongly on the observed alpha kinetic energy which varies only about a factor of two from about 4 to 9 MeV. The half-lives of heavy elements which emit alpha particles varies over 20 orders of magnitude, from about a tenth of a microsecond to 10 billion years. Alpha Particle Tunneling Alpha Halflife vs Kinetic Energy ![]()
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