Leak Checking on deuterated-PMMA films, using a helium-mass-spectrometer-based spray-mode leak detection apparatus.

At Oak Ridge National Laboratory the next series of experiments to measure the electric dipole moment (nEDM) at the neutron spallation source is currently being prepared. This is done by performing high-precision nuclear magnetic resonance experiments on nuclear-polarized ultracold neutrons and helium-3 atoms stored inside a 3-liter-sized measurement cell filled with superfluid helium-4 cooled to 400mK. This measuring cell is being developed at Montclair State University, among others, and must withstand the requirements of the project, such as extreme cold and vacuum, but also achieve the best possible physical effect by keeping the windows extremely thin (~0.5 mm) to reduce scattering and absorb the cold neutron beam, which causes unwanted background signals. The cell should also keep the neutrons in itself for as long as possible. The goal is to store the ultra-cold neutrons for an average time of 1000s. To achieve this storage time, the container must be sufficiently tight and that is where my task begins. I help set up the leak detecting apparatus and do the leak checking on the measuring cell. The spraying method is used for this. In this mode, He4 gas is sprayed around the measurement cell which is under ultra-high vacuum and connected to a mass spectrometer, which measures the helium atoms entering the cell from the outside and thus indicates the leak rate.