Master thesis defense by Konstanteen Mantulnikov – Niels Bohr Institute - University of Copenhagen

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Niels Bohr Institute > Calendar > NBI Calendar 2015 > Master thesis defense ...

Master thesis defense by Konstanteen Mantulnikov

Neutron guide shielding for the BIFROST spectrometer at ESS

In recent years a great deal of effort has been put into designing more efficient guides to transport as many neutrons to the sample position as possible. Nonetheless, there are problems that have been given less attention, for example the design of the shielding for instruments. In this project the effect of background radiation induced by the interaction of cold/thermal neutrons from the source with the material of BIFROST spectrometer neutron guide is studied and a reasonable fast neutron shielding solution for the current guide geometry is being proposed using McStas and MCNPX. The essence of the project is a rather “new” approach to the guide model when the guide, shielding and the backgrounds are thought as a whole, rather than being designed or cared about in separate stages. It is also one of the first uses of the McStas-MCNPX coupling. It is shown that the steel shielding around the guide is not enough to drop the background at the sample position lower than the cosmic neutron rate at sea level. To decrease the background from the secondary emitted neutrons a shielding donut concept is being introduced and it’s ability to stop fast neutrons is tested to some extent. A conventional shielding method of adding a 20 cm layer of borated polyethylene around the guide is used to further lower the background level, now mostly in the cold/thermal neutron energy range. It finally proves to be sufficient to use both – a shielding donut and a layer of borated polyethylene to lower the background below the cosmic neutron rate. Additional two B4C layers of shielding for cold/thermal neutrons have been tested and proved to give very little difference.

Master thesis defense by Konstanteen Mantulnikov