Abstract

1. A Radon in Water Sampling System for LUX Sashank Karri, Thomas Shutt Department of Physics, Case Western Reserve University Abstract Introduction The Separation Process Efficiency of Transfer Acknowledgements Hydrophobic Membrane Contactor We purchased a Liqui-Cel® hydrophobic membrane contactor for the purpose of extracting radon from water. The physical principle behind this transfer is diffusion. The contactor consists of tightly packed hydrophobic hollow fibers. A combination of a low flow sweep gas and a vacuum pump are applied to opposite ends of the hollow fibers. Water with some radon concentration flows counter-current to this gas flow outside the fibers. This creates a concentration gradient across the hydrophobic membrane, allowing radon to diffuse out of water into the sweep gas (nitrogen). The efficiency of transfer for all gases is dependent on the depth of vacuum and inversely dependent on the flow rate of water. The efficiency for individual gases is characteristic of the gases’ diffusion constants in water.. By measuring the efficiency of transfer of various atmospheric gases from water, one can get an estimate of the efficiency for radon. The following equations illustrate the dependence of the efficiency on the flow rate and diffusion constant. [3] (1) (2) E is the efficiency. RCo , RCi, dF, fP, fXare all characteristics of the contactor. ρL, μL are the density and viscosity of water. DL is the diffusion constant of the dissolved gas in water. QL is the flow rate of water. a, b, and c are parameters to be fit to the data. Provided one is using the same contactor at the same flow rates, the efficiency of transfer of any gas can be related by their diffusion constants: (3) If the theory is correct, one can fit the data for c by monitoring multiple atmospheric gases . Once this has been found, the efficiency of the contactor for radon transfer can be estimated. I would like to thank Adam Bradley and the rest of the LUX team for their assistance and support. The Large Underground Xenon (LUX) dark matter detector will use a water shield to reduce background events in the detector. However, a high radon concentration in the shield can increase the number of background events. Thus it is essential to monitor the radon concentrations in the water, particularly to confirm low levels on the order of 1 mBq/m3. The first step in any system that is able to measures these low concentrations is to separate the radon from water. One method is to use a hydrophobic membrane contactor which allows radon to diffuse across a membrane into a separate sweep gas. A hydrophobic membrane contactor is characterized by its efficiency of this transfer. This allows a prediction of the efficiency for radon removal. Knowing the efficiency of the process allows one to calculate the sample size necessary for a measurement. A gas system was built to test the efficiency of this membrane contactor in the first step with atmospheric gases. Motivation: Virtually all purified sources of water contain some amount of radon. Radon emits gamma rays in its decays. These gamma rays can Compton scatter inside the detector losing such energies that the signal that this scatter makes is indistinguishable from the signal that a weakly interacting massive particle (WIMP) would make. For this reason, water must be purified to low concentrations of radon, and a checking system must be constructed to measure these low concentrations. Radon concentrations of 1 mBq/m3 are too low for traditional methods used to measure gaseous concentrations in water. So a different approach (described below) must be taken. Four Step System to Measuring Low Concentrations of Radon Separate Radon from Water The first step is to extract radon from some sufficiently large volume of water. This process is characterized by some efficiency of transfer. (2) Dry Radon extract of any water residue Next this extract must be dried of any residual water vapor. (3) Purify and Concentrate the Radon After any residual water has been extracted, the extract must be further purified and also concentrated because of the limited volume of the detector in the final step. Measure radon with an alpha counter The final step is to sweep this purified radon extract into a detector that counts the alphas decayed , revealing the amount of radon that has been extracted. The apparatus for the first step in the process has been built and will tested for its efficiency in gas transfer. Fig. 1a: A P & ID of the apparatus for the extraction of radon and subsequent drying adapted from generic diagrams. [2] References Fig. 2: A diagram illustrating the application of the Liqui-Cel hydrophobic membrane. [3] Fig. 1b: The apparatus for the extraction of radon. [1] Gabelman, A. and Hwang, S. Hollow fiber membrane contactors. J. Membrane Sci. 159 (1999) 61-106. [2] Membrana. Design & Operating Procedures for Membrane Contactors. (2009) [3] Sengupta, A. et al. Separation and Purification Technology 14 (1998) 189–200