Study of the Behavior of Ions on Plastic Surfaces in Cryogenic Liquids.

In the nEDM@SNS experiment, a cryogenic apparatus will be used to improve the present limit on the neutron electric dipole moment(nEDM) by about two orders of magnitude with an ultimate sensitivity of ∼3x10^-28e-cm. In this apparatus, two PMMA cells will be filled with superfluid He-4 which can support strong electric fields. High voltage electrodes are placed outside of the measurement cells to generate a strong and stable electric field (∼75 kV/cm). During the experiment, however, ambient ionization radiation can ionize helium. These ionized charges (He ions and electrons) are bound on the cell’s walls. As a result, an opposing static electric field is generated, which will impact the stability of the electric field. This change in E-fields directly influences the desired sensitivity measurement in the measurement of the nEDM. Therefore, a compact test setup has been devised to study the behavior of ions inside cryogenic liquids using a scaled-down version of the nEDM cell and the electrodes. In our setup, ions are generated by ionizing the helium(nitrogen) with a ¹³⁷Cs source and the electric field is monitored via the electro-optical Kerr effect. We plan to develop a new method to measure the minimum energy required to remove the ions from the surface of the measuring cells and study the effect of fringe fields. Dependent on the results, we might neutralize the charge preferably by reversing the electric field direction to guarantee the required stability of the E-fields. This talk will present the experimental design and first results of measuring the binding energy of nitrogen ions on the PMMA surface inside the liquid nitrogen.