Ternary fission

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Ternary fission is a rare type of nuclear fission in which three charged fragments are produced instead of two. Like other fission processes, it also releases uncharged particles such as neutrons and gamma rays. It can happen during neutron-induced fission or in spontaneous fission. Spontaneous fission produces about 25% more ternary events than the same fission after a thermal neutron is captured, showing small but real differences between the two processes. A rarer cousin, quaternary fission, produces four charged products and occurs in about 1 in 10 million fissions.

In regular fission (binary fission), two large charged fragments are formed. Ternary fission happens in roughly 2 to 4 out of 1000 fissions in a nuclear reactor. The smallest of the three charged fragments can be as small as a proton (Z = 1) or as large as an argon nucleus (Z ≈ 18). The vast majority of small fragments are helium-4 nuclei (about 90%), followed by tritons (about 7%) and helium-6 (about 0.8%). The remaining small pieces are protons and other light fragments.

The two larger fragments from ternary fission are similar to those in binary fission. The third, smaller fragment typically carries 10–20 MeV of energy, and alpha particles (helium-4) usually have around 16 MeV, making them “long-range” alphas. The other two larger fragments share the rest of the fission energy, about 170 MeV total, so each is a bit less energetic than in binary fission.

Ternary fission can contribute to helium-4 and tritium buildup in reactor fuel. It was first observed in 1957 near Savannah River. A very rare form called true ternary fission, where three nearly equal-sized fragments (Z ≈ 30) split the energy evenly, occurs in about 1 in 100 million fissions and has been seen only with heavy, high-energy ion bombardment.