Xenon plant development and light calibrations for nEXO

E. Brown, Rensselaer Polytechnic Institute (Principle Investigator)

This work will characterize the dynamics of radon (Rn) doped xenon (Xe) in order to develop a calibration scheme for nEXO experiment usiing dissolved radioactive sources in liquid Xe (LXe). This work is crucial for the success of the experiment, and builds on a combination of PI Brown’s prior contributions to nEXO and his extensive experience with LXe detectors and calibrations. nEXO is a neutrinoless double beta (0νββ) decay experiment at the forefront of its field. The experiment will reach a sensitivity to the 0νββ decay half life beyond 10²⁸ years, more than an order of magnitude beyond current half life limits, and in line with the global efforts for the next generation of 0νββ decay experiments. One major aspect of the experimental design is the safe handling and processing of 5 tons of enriched Xe. A major component of this work will be the conceptual and preliminary development of major components of this plant, including modeling of the plant performance. Precision calibration of the detector’s light response is required, and dissolved radioactive sources have been identified as the most promising way to do this. Understanding the transport properties of dissolved sources in LXe is the key missing ingredient to developing a calibration scheme for nEXO. In order to realize a dissolved source calibration in nEXO, three questions must be investigated and answered:

• How is Rn and its progeny transported in LXe: this will be characterized by laboratory measurements to develop a predictive model.
• What is the impact of the Xe recirculation system on Rn transport: This will be investigated with Xe process simulations and coupled with the conceptual and preliminary designs of the nEXO xenon processing plant.
• How scalable is the model to a large-scale Xe system: this will be answered by operating and measuring a full-scale recirculation system to complement simulations.

Measurements of transport properties will use 220Rn and 222Rn in a test stand at RPI to develop a data driven model that includes liquefaction of Rn doped Xe gas and surface plateout of Rn progeny. Process plant simulations will be conducted in the process engineering software, Aspen, to provide predictive scaling from the test stand to nEXO.

The final deliverable of this work is a working model of Rn transport in a gaseous and liquid xenon system, including all critical Xe process components needed in nEXO. This will be used to generate calibration schemes for nEXO in order to design realistic light map calibration procedures for the experiment. This research is integrated with the critical path for nEXO on multiple fronts, as evident in the alignment with priorities outlined in the pre-CDR. Both the science of dissolved source dynamics and the project development of the calibration and Xe handling systems that are proposed here are key components to the success of nEXO, and are thus critical to making the next major step in the search for 0νββ decay.