In English

Experimental and theoretical refutation of Collinear Cluster Tri-partition

Patric Holmvall
Göteborg : Chalmers tekniska högskola, 2014. 79 s.
[Examensarbete på avancerad nivå]

A new mode of nuclear fission has been reported by the FOBOS collaboration, called Collinear Cluster Tri-partition (CCT). The claim is based on indirect observation via missing-energy events, measuring binary coincidences in thermal neutron-induced fission of 235U, and spontaneous fission of 252Cf. The events were interpreted as perfectly collinear emission of three heavy fragments. The proposed CCT seems to be an astonishing new aspect of nuclear fission, theoretically difficult to reconcile with traditional fission models, and experimentally surprising since the relatively high yield of 0.5 % of such events should have shown up in previous experiments. These claims call for an independent verification with a different experimental technique.

This thesis reports on direct searches for CCT events in thermal neutron-induced fission of 235U, which should manifest as a considerable excess yield around nuclear masses A ≈ 68–70 and A ≈ 34–36, compared to known binary fisson events. The experiments were performed with the fission fragment spectrometer Lohengrin at the high flux reactor of Institut Laue- Langevin. This spectrometer provides excellent mass and energy resolution and allows for clean measurements down to relative fission yields of 10−10. The known 70Ni yield in binary fission was confirmed, and a new upper limit of the 68Ni yield in binary fission was estimated. No indication of CCT events was found. The result of the experiments sets an upper limit on the order of 10-9 per fission for CCT events with A ≈ 68–70 and A ≈ 34–36, which is more than 5 orders of magnitude below the yields claimed by the FOBOS collaboration.

In addition, theoretical calculations and simulations have been performed to make sure that the Lohengrin experiments cover a sufficient and representative range of kinetic energies of possible CCT fragments. The calculations also show that CCT is an improbable event in low-energy fission.

In conclusion, the experimental results presented in this thesis clearly rule out the existence of CCT far below the level previously claimed, and the theoretical examination demonstrates that current models cannot explain the experimental interpretations as CCT by the FOBOS collaboration.

Nyckelord: CCT, Ternary Fission, Nuclear Fission

Publikationen registrerades 2015-01-05. Den ändrades senast 2015-01-15

CPL ID: 209567

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