Tetraneutron

A tetraneutron is a hypothesised stable cluster of four neutrons. This cluster of particles is not supported by current models of nuclear forces. However, there is some empirical evidence which suggests this particle does exist, based on an experiment by Francisco-Miguel Marqués and co-workers at the Ganil accelerator in Caen using a novel detection method in observations of the disintegration of beryllium and lithium nuclei.^[1] Subsequent attempts to replicate this observation have failed.

Confirmation of the existence of a tetraneutron would be a significant discovery because current nuclear theory suggests that these clusters should not be stable, and thus should not exist. If it does, then it has been suggested that the substance be considered an "element", and be placed on the Periodic Table of the Elements, with an atomic number of 0 (zero).

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Marqués' experiment

As with many particle accelerator experiments, Marques' team fired atomic nuclei at one another and observed the 'spray' of particles from the resulting collisions. In this case the experiment involved firing beryllium-14, beryllium-15 and lithium-11 nuclei at a small carbon target, the most successful being beryllium-14. This isotope of beryllium has a nuclear halo that consists of four clustered neutrons; this allows it to be easily separated intact in the high speed collision with the carbon target. Their approach to the production and detection of bound neutron clusters was new and novel.^[1] Current nuclear models suggest that four separate neutrons should result when beryllium-10 is produced, but the signal detected in the production of beryllium-10 suggested a multineutron cluster in the breakup products; most likely a beryllium-10 nucleus and four neutrons fused together into a tetraneutron.

Since Marqués' experiment

A later analysis of the detection method used in the Marques' experiment suggested that at least part of the original analysis was flawed^[2], and attempts to reproduce these observations with different methods have not successfully detected any neutron clusters.^[3] If, however, the existence of stable tetraneutrons was ever independently confirmed, considerable adjustments would have to be made to current nuclear models. Bertulani and Zelevinsky^[4] proposed that, if it existed, the tetraneutron could be formed by a bound state of two dineutron molecules. However, attempts to model interactions which might give rise to multineutron clusters have failed,^[5][6][7] and it:

"does not seem possible to change modern nuclear Hamiltonians to bind a tetraneutron without destroying many other successful predictions of those Hamiltonians. This means that, should a recent experimental claim of a bound tetraneutron be confirmed, our understanding of nuclear forces will have to be significantly changed."^[8]

See also

• Neutron
• Free neutron
• Dineutron
• Neutronium

References

1. ^ ^{a b} F. M. Marqués et al., Detection of neutron clusters, Phys. Rev. C 65, 044006 (2002)
2. ^ B. M. Sherrill and C. A. Bertulani, Proton-tetraneutron elastic scattering, Phys. Rev. C 69, 027601 (2004)
3. ^ D. V. Aleksandrov, et al. Search for Resonances in the Three- and Four-Neutron Systems in the 7Li (7Li, 11C) 3n and 7Li (7Li, 10C) 4n Reactions, JETP Letters, 81, 43 (2005)
4. ^ C.A. Bertulani and V.G. Zelevinsky, Tetraneutron as a dineutron-dineutron molecule, J. Phys. G 29 (2003) 2431
5. ^ Rimantas Lazauskas, and Jaume Carbonell, Three-neutron resonance trajectories for realistic interaction models, Phys. Rev. C 71, 044004 (2005)
6. ^ Koji Arai, Resonance states of 5H and 5Be in a microscopic three-cluster model, Phys. Rev. C 68, 034303 (2003)
7. ^ A. Hemmdan, W. Glöckle, and H. Kamada, Indications for the nonexistence of three-neutron resonances near the physical region, Phys. Rev. C 66, 054001 (2002)
8. ^ Steven C. Pieper, Can Modern Nuclear Hamiltonians Tolerate a Bound Tetraneutron?, Phys. Rev. Lett. 90, 252501 (2003)