Beryllium-8 | properties

Properties

Triple-alpha process

Beryllium-8 is unbound with respect to alpha emission by 92 keV; it is a resonance having a width of 6 eV.[4] The nucleus of helium-4 is particularly stable, having a doubly magic configuration and larger binding energy per nucleon than 8Be. As the total energy of 8Be is greater than that of two alpha particles, the decay into two alpha particles is energetically favorable,[5] and the synthesis of 8Be from two 4He nuclei is endothermic. The decay of 8Be is facilitated by the structure of the 8Be nucleus; it is highly deformed, and is believed to be a molecule-like cluster of two alpha particles that are very easily separated.[6][7] Furthermore, while other alpha nuclides have similar short-lived resonances, 8Be is exceptionally already in the ground state. The unbound system of two α-particles has a low energy of the Coulomb barrier, which enables its existence for any significant length of time.[8] Namely, 8Be decays with a half-life of 8.19×10−17 seconds.[9]

8Be also has several excited states. These are also short-lived resonances, having widths up to several MeV and varying isospins, that quickly decay to the ground state or into two alpha particles.[10]

Decay anomaly and possible fifth force

A 2015 experiment by Attila Krasznahorkay et al. at the Hungarian Academy of Sciences's Institute for Nuclear Research found anomalous decays in the 17.64 and 18.15 MeV excited states of 8Be, populated by proton irradiation of 7Li. An excess of decays creating electron-positron pairs at a 140° angle with a combined energy of 17 MeV was observed. Jonathan Feng et al. attribute this 6.8-σ anomaly to a 17 MeV protophobic X-boson dubbed the X17 particle. This boson would mediate a fifth fundamental force acting over a short range (12 fm) and perhaps explain the decay of these 8Be excited states.[10] A 2018 rerun of this experiment found the same anomalous particle scattering, and set a narrower mass range of the proposed fifth boson, 17.01±0.16 MeV/c2.[11] While further experiments are needed to corroborate these observations, the influence of a fifth boson has been proposed as "the most straightforward possibility".[12]

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