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Fusion at the barrier
Two nuclei fuse when they get over, or through, the barrier made by their Coulomb repulsion and the short-range nuclear pull. Near and below its top the measured rate is far above what one barrier allows: the colliding nuclei are not rigid balls. A deformed target offers a lower barrier along its tips, a soft one couples to its vibrations; the single barrier becomes a distribution, and the second derivative of Eσ shows it.