Speaker
Description
To increase the ultracold neutron yield at the UCN source at PSI, a new design of the solid deuterium moderator vessel was developed within the frame of a major refurbishment project called ‘UCN EZE’. In addition to the improvements of the cold moderator, the project includes the replacement of the entire central insertion unit, which consists of several components, such as a central shutter, a new highly polished vertical neutron guide, and a large cryopump. This presentation will focus on the new moderator vessel, with particular emphasis on the newly developed aluminium lid.
The primary objective of the new vessel design was to increase the transmission of ultracold neutrons through a 500 mm diameter lid by reducing the aluminium thickness, while still ensuring structural integrity under two critical failure pressure conditions. Specifically, the new design must withstand either an internal vessel pressure of 3 bar or an external pressure of 1 bar at cryogenic temperatures. Furthermore, the vessel must meet a stringent maximum leakage rate of <10⁻⁸ mbar·l·s⁻¹, even after exposure to a failure pressure event.
The final design consists of a grid-supported aluminium foil with a thickness of only 0.254 mm. To verify compliance with the mechanical and vacuum requirements, a combined approach of finite element (FE) analysis and extensive experimental testing was employed. Several commercially available sheets of different aluminium alloys and of various thicknesses were evaluated to identify a suitable aluminium sheet metal. Preliminary testing included tensile tests on thin aluminium foil samples, pressure tests with different geometries, and sealing tests using indium foil as the sealing material.
To validate the structural integrity of the design, multiple pressure tests were performed on a prototype, both at room temperature and at 77 K in liquid nitrogen. A final burst test was conducted to determine the maximum pressure that the new lid design could withstand. The combined results from simulations and experimental testing demonstrate that the new design meets all specified requirements.