scale up of the krypton system. A new Storage and Recovery Vessel. (SRV) will
be included in line with the current condenser to func(on as the main condensing
...
Chromatographic Separa/on of Krypton 85 From Xenon Megan S. Van Vleet Case Western Reserve University, Department of Physics, NSF
Introduc/on
The Large Underground Xenon (LUX) Detector is a system designed for the direct detecCon of WIMPs as the leading Dark MaHer parCcles. This detector, as its name suggests will use a large amount of xenon (~350kg) in order to detect these elusive parCcles.
Results
In previous runs, the system funcConed in three disCnct phases of the purificaCon process, described in figure three:
Commercially available research grade xenon contains trace radioacCve contaminants, including krypton 85, which account for too much background radiaCon for LUX to clearly detect any signal of interacCng WIMPs.
Fig. 3: EluCon curve of the krypton system taken by a gas analyzer
A krypton removal system was devised, which uClizes a method of adsorpCon chromatography to separate the krypton from the xenon. The system is currently being scaled up to deal with conCnuous cycling of larger amounts of xenon per day.
The first phase, xenon feed, pictured in the green secCon, is where the impure xenon is introduced into the system. The blue line denotes the presence of xenon, while the red line denotes the presence of krypton.
Methods
Helium is pumped through the system as a carrier gas for the contaminated xenon throughout the circulaCon system. The gases are then introduced into a charcoal adsorber column. The properCes of the adsorber induce an electric dipole in the gases, which are then aHracted by a Van der Waals force to the porous surface of the adsorber. Each gas species has a specific adsorpCon constant (k), which governs the level of aHracCon between these gases and the adsorber. The krypton and xenon therefore propagate through the charcoal column at different rates, because their adsorpCon constants differ. In this case, the krypton has a smaller k, which results in it fully exiCng the gas column before the xenon beings to emerge. This difference in adsorpCon constants, balanced with the mass flow rate of the gases, is essenCally what allows for the complete separaCon of the two noble gases from each other, and therefore the ultra pure xenon desired for use in the LUX detector.
The current charcoal column is also too small to deal with the proposed mass flow rate of the system in order to purify the full 350 kg of xenon for use in the LUX detector. A new charcoal column has been designed and fabricated, and currently remains to be fiHed with its internal filtraCon system, as well as instrumented for use in line with the system.
[1] J.Mock 4/2008
The second phase is krypton purge, pictured in yellow. This is the period of Cme in which the krypton exits the adsorber column. It can clearly be seen that the krypton fully exits the column during this phase well before the xenon begins to emerge. This clear separaCon between the peaks illustrates the effecCveness of this chromatographic separaCon. The third phase is xenon recovery, which is several Cmes longer than the other phases, due to the need for a low mass flow rate in order to increase the efficiency of xenon recovery. Fig. 6: Current, exisCng charcoal column with smaller volume Fig. 4 (right): A measure of volume flow rate vs mass flow rate in the adsorber column. This data suggests that the pressure throughout the column is not constant, which implies a need for either a larger vacuum pump to keep the flows constant, or a changed geometry of the adsorber column, to allow for a more uniform pressure differenCal.
Fig. 7: New, larger volume charcoal adsorber column
Literature Cited
[1] J.Mock 4/2008
[1]Bolozdynya, A. I., P. P. Brusov, T. ShuH, C. E. Dahl, and J. Kwong. "A chromatographic system for removal of radioacCve 85Kr from Xenon." (2007). Print. [2]Mock, Jeremy. "Vacuum Recovery Phase in a Chromatographic System for Removal of 85 Kr from Xenon." Thesis. Case Western Reserve Univeristy, 2008. Print.
Fig. 1: (leH) Main panel of the krypton system
Fig. 2: (right) SchemaCc of the krypton system
Conclusions
Much more work remains to be done in order to complete the full scale up of the krypton system. A new Storage and Recovery Vessel (SRV) will be included in line with the current condenser to funcCon as the main condensing unit, given its higher efficiency and larger capacity.
[3]ShuH, T. KR purifica+on system at Case: First Results. Rep. 2006. Print. Fig. 5 (leH): Xenon recovery cycle for various mass flow rates. This data suggests that a mass flow rate of 20 slpm reduces the propagaCon Cme by one fourth. The higher the mass flow rate, the shorter the propagaCon Cme. [1] J.Mock 4/2008
Acknowledgements
I would like to thank the following people and organizaCons for their contribuCons to and assistance in this project: D. Akerib: Dept. Chair, Dept of Phyiscs, CWRU T. ShuH: Prof of Physics, P.I. LUX, Dept of Physics, CWRU A. Bolozdynya: Senior ScienCst, Dept of Physics, CWRU NSF REU grant DMR‐0850037
