Design and simulation of cryogenic systems and calibrations techniques for the nexo neutrinoless double beta decay experiment
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Authors
Tidball, Adam
Issue Date
2024-08
Type
Electronic thesis
Thesis
Thesis
Language
en_US
Keywords
Physics
Alternative Title
Abstract
This dissertation presents a study of the dynamics of cryogenic plants and the implementation of radon injection as a calibration strategy for the nEXO neutrinoless double beta decay experiment. The research focused on three main areas: the design and optimization of cryogenic systems, the simulation of \textsuperscript{220}Rn progeny propagation in a xenon flow, and the development of a laboratory test stand to validate these simulations. Simulations of the EXO-200 xenon plant were developed using the Aspen engineering software suite, validating the modeling strategy for application to the nEXO refrigerant plants. Performance evaluations, conducted with Aspen Plus, included steady-state simulations and dynamic modeling of dual-stage heater and condenser systems based on those in EXO-200. These simulations assessed their ability to regulate pressure and temperature during and after transient upset conditions. Ramped pump speed changes were simulated to reflect typical operational variations, with both heater and condenser models utilizing PID controllers for temperature and pressure regulation. The dual-stage heater mitigated temperature excursions to 0.2 K while ensuring full xenon evaporation, achieving a temperature differential of just 0.001 K. The condenser model limited pressure excursions to $\Delta P = 6.21 \times 10^{-4}$ bar, well within the 0.35 bar TPC wall limit. Power consumption for the simulated condenser and heater closely matched theoretical values, confirming the reliability of this simulation strategy. Additionally, data from the LXTS slow control system was used to validate the modeling strategy, further demonstrating its applicability to simulating heat exchanger dynamics in xenon recirculation plants. Models were then created of the nEXO Novec-7000 refrigerant system to explore the viability of different operating conditions and plant configurations. These analyses revealed that using 1-inch piping for the entire Novec-7000 plant was inadequate for meeting the circulation needs of the nEXO system. Moreover, the ``top deck'' configuration, where the xenon circulation pump is placed above the cryostat, was found to be unsuitable due to cavitation issues in the pump caused by insufficient inlet pressure. The effect of incorporating valves with varying flow coefficients was analyzed, ruling out the use of valves with flow coefficient $C_V=13.2$, while indicating that valves with $C_V=26.5$ work under a variety of configurations. Results of these simulations allowed estimates to be placed on the Novec-7000 pump inlet pressure under these different system conditions, specifying which configurations ensure recirculation of refrigerant and which do not. For the unsuitable ``top deck'' configuration, required Novec-7000 vessel operating pressures to prevent recirculation pump cavitation are instead presented. A radon injection simulation model was developed in this study to leverage the entire decay chain of $^{220}$Rn down to stable $^{208}$Pb to understand the flow of calibration radioisotopes through the nEXO xenon plant. Statistical analyses, including z-tests, p-tests, and Kolmogorov-Smirnov tests, demonstrated the infeasibility of using late-chain isotopes for determining velocity and diffusion coefficients in a small-scale test stand. Fitting procedures applied to the model using krypton tracer data yielded best-fit velocity and diffusion parameters, which were $v = 0.635 \pm 0.005 \, \text{m/s}$ and $D = (2.0 \pm 1.9) \times 10^{-2} \, \text{m}^2/\text{s}$, respectively. These parameters were then used to estimate radon arrival times in the nEXO radon injection design, with an estimated arrival time to the TPC of $128 \pm 7_{\text{stat}} \pm 16_{\text{geom}}$ s in agreement with the calculated upper limit of 145 s. The developed simulation code will be made available to the collaboration, ensuring its utility in ongoing and future research. A concentration vs. time trend was determined for an updated TPC model with four outlet lines and a tangentially oriented inlet line using SolidWorks flow simulation. These results were coupled with outputs from the radon injection simulation code to generate a plot of activity vs. time in the TPC. From this plot, it is determined that activity in the TPC reaches a local maximum at 0.08 days, after which point total activity drops to 10\% after 1.41 days and to 1\% after 2.67 days. The updated TPC model is shown to promote effective mixing similar to a previously studied model with tangential inlet and outlet lines called Orientation 6. In contrast, Orientation 2, with radially positioned supply and return lines, demonstrates less effective mixing. Activity trends were determined for each of these configurations, with those encouraging better mixing correlating to higher activites in the TPC. These trends confirm that calibration with $^{220}$Rn is feasible using a standard 20 kBq source as 44\% of the $^{220}$Rn isotopes emanated from the calibration source reach the TPC. A laboratory test stand was constructed to experimentally validate the radon injection simulation model in a dual-phase system. This test stand was designed to be sensitive to xenon scintillation produced by alphas emitted along the $^{220}$Rn decay chain. The characterization of the QDrive and PT-100 components was detailed, ensuring their reliability for experimental use. Helium leak testing was performed to guarantee the integrity of the system, with a global leak rate of $8.37 \times 10^{-10}$ mbar L/s placed on the test stand, with no detection of localized leakage. The leak rate is below the approximate threshold of $\sim 10^{-7}$ mbar L/s which indicates that xenon leakage is dominated by molecular rather than bulk fluid flow, confirming the system's integrity and suitability for commissioning with xenon \cite{pfeiffer2013leak}. Cooling of the xenon condenser was initially achieved with a copper braid in the original design, which was upgraded to a custom thermal link with enhanced cooling capacity calculated to be 67 W for 40-Kelvin temperature differentials across the link. Custom data acquisition software was developed to support the robust operation of the test stand, facilitating efficient data collection and analysis. Argon was studied as a potential inexpensive proxy to xenon for research and development purposes, which necessitated exploring strategies to shift its short-wavelength scintillation light to wavelengths detectable by the photodiode in the test stand. Tetraphenyl butadiene (TPB) spectroscopy studies were conducted to explore the wavelength-shifting efficiency of TPB dissolved in ethanol and toluene for this application. The studies indicated that TPB dissolved in toluene showed markedly higher degrees of wavelength shifting compared to TPB dissolved in ethanol, with TPB coating thickness found to correlate to wavelength shifting efficiency. The TPB-coated slides exhibited flaking over time, making them unsuitable for incorporation in a high purity experimental appartus, indicating that future work must be done to refine the deposition strategy before this material can be used in the test stand.
Description
August2024
School of Science
School of Science
Full Citation
Publisher
Rensselaer Polytechnic Institute, Troy, NY