Preliminary measurements of the binding energy of ions on a plastic surface in cryogenic liquids.*

The nEDM@SNS experiment will use a cryogenic technique to improve the present limit of the neutron electric dipole moment (nEDM) by about two orders of magnitude with an ultimate sensitivity of ∼3x10^-28e-cm. The central part in this apparatus consists of two coated PMMA cells sandwiched between grounded and high voltage electrodes. Polarized ultracold neutrons are stored in a superfluid helium bath in a strong, stable electric field. During the experiment, several sources of ambient ionizing radiation generate charged particles in the cryogenic liquid. These ionized charges are adsorbed on the cell walls. As a result, an opposing static electric field is generated, which will impact the stability of the electric field. This instability of the E-field needs to be kept below 1% over a measurement cycle to achieve the required sensitivity of the nEDM measurement. 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, ion-electron pairs are generated by ionizing the nitrogen (helium) with a ¹³⁷Cs source and the electric field is monitored via the electro-optical Kerr effect in the cryogenic fluids. We plan to develop a new method to measure the binding energy of the ions bonded on PMMA. The determination of the binding energy can potentially reduce the data taking time for the nEDM@SNS experiment if partial field reversal is sufficient. Preliminary measurements of the binding energy will be presented.