Measurements of the binding energies of ions on plastic surfaces in cryogenic liquid.

To improve the present limit of the neutron electric dipole moment (nEDM) from 1.8 × 10^-26 e. cm to ∼ 3 × 10^-28 e. cm, the nEDM@SNS experiment plans to use a cryogenic technique. In this experiment, the central part of the apparatus consists of two deuterated tetraphenyl butadiene (dTPB) coated PMMA cells, which are sandwiched between grounded and high-voltage electrodes. To achieve such precision, the externally applied electric field has to be stable at the 1% level over a measurement cycle. During this experiment, several sources of ambient ionizing radiation generate charged particles in the cryogenic liquid. These ions are adsorbed on the cell walls. Consequently, an opposing static electric field is generated which will impact the stability of the electric field. At the University of Kentucky, we have devised a compact test setup 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 γ-source, and the electro-optic Kerr effect is utilized to understand the cell charging effects in the dummy measurement cell. We are in the process of developing a new method to measure the binding energy of the ions bonded on insulating surfaces. Determining the binding energy can potentially reduce the data-taking time of the nEDM@SNS experiment if partial field reversal is sufficient. I will present the binding energy measurements of ions on coated and uncoated PMMA surfaces in the cryogenic liquid.

This research is supported by DOE grants: DE-FG02-99ER41101, DE-AC05-00OR22725, DE-SC0014622.