A Radon in Water Sampling System for LUX
Sashank Karri, Thomas Shutt
Department of Physics, Case Western Reserve University
Abstract
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.
Introduction
Efficiency of Transfer
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.
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
(1) 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.
(4) 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.
Fig. 1a: A P & ID of the apparatus for the extraction of radon
and subsequent drying adapted from generic diagrams. [2]
The following equations illustrate the dependence of the
efficiency on the flow rate and diffusion constant. [3]
(1)
The Separation Process
(2)
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).
E is the efficiency. RCo , RCi, dF, fP, fX are 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.
Acknowledgements
Fig. 2: A diagram illustrating
the application of the Liqui-Cel
hydrophobic membrane. [3]
The apparatus for the first step in the process has been
built and will tested for its efficiency in gas transfer.
I would like to thank Adam Bradley and the rest of the
LUX team for their assistance and support.
References
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