The measurement cells look deceptively simple; a plastic box, ~ 7x10x40 cm3. But they have some very challenging requirements:
They must be constructed from materials that minimize background events resulting from long-live radioactive contaminants produced by neutron activation.
The entrance and exit windows must be transparent to 8.9 Å neutrons.
They must store the UCN, produced by the interactions of the 8.9 Å neutrons with the liquid helium inside the cells, for hundreds of seconds.
They must convert the EUV light, produced in n+helium-3 capture events, to visible wavelengths and allow for efficient collection of resulting photons.
To reduce neutron activation backgrounds measurement cells must be fabricated from elements having atomic number less than 9. Combined with the optical requirement this leads to the choice of a clear plastic (polymethyl methacrylate, or PMMA).
A layer containing a high concentration of Lithium-6 is incorporated into one of the cell walls to absorb neutrons scattered out of the beam.
Protons strongly interact with neutrons, so the entrance and exit windows and the entire inner surface layer of the cell must be made from deuterated materials.
A thin polymer mirror (Vikuiti VM2000) is incorporated into five of the six cell walls to allow collection of the optical signal by an array of plastic fibers mounted on the sixth side (behind the ground electrode).
