J. A. Mock a , A. I. Bolozdynya a , C. E. Dahl b , T. Shutt a

1. Vacuum Recovery Phase in a Chromatographic System for Removal of 85Kr from Xenon J. A. Mocka, A. I. Bolozdynyaa, C. E. Dahlb, T. Shutta aDepartment of Physics, Case Western Reserve University, Cleveland, OH 44106 USA b Department of Physics, Princeton University, Princeton, NJ 08540 USA don’t Introduction The Large Underground Xenon (LUX) detector is a ~350 kg dual phase xenon detector searching for dark matter. Trace amounts of radioactive impurities such as 85Kr are present in the xenon and are responsible for significant backgrounds in the signal. 85Kr is a 0.687 MeV beta emitter with half life 10.71 years, and it is present in air at an abundance of 1.12 ppm, mostly from nuclear fuel reprocessing. Commercially available xenon has trace amounts of Kr at the level of ~5-10 ppb. In order to achieve the background goal of the experiment, this must be reduced to < 5 ppt. The xenon is purified using heated getter technology developed for the semiconductor industry, and it removes all contaminants except noble gases. There is currently no commercially available way to remove krypton from the xenon, so a dedicated krypton removal system was developed. The current system can purify approximately 2 kg of xenon per day, and this is too slow a rate for the amount of xenon to be used in this and future, larger generations of LUX. Thus, certain properties of the system will be studied to scale up the system to process more mass. The goal of the scale up is to purify 20 kg of xenon per day. Don’t’ delet Scale Up The current system needs to be scaled up to handle the large amount of xenon that must be purified. First, the column mass will be increased by a factor of ten. The geometry of the column will also be changed. This will allow xenon to be fed into the column at a faster rate. The xenon condenser will also be redesigned. It was discovered running in the current regime that the condenser has limited condensing power as it fills. Xenon was not condensing in the condenser but was instead returning to the column. This produced a loss of xenon. The condenser must also be able to hold the entire mass of xenon which the current condenser cannot do. The LUX gas storage system will be used to condense the purified xenon. Finally, the recovery cycle will be operated at low pressure. This will increase the volume flow rate and decrease the propagation time of xenon through the column. The mass scale up and condenser redesign will be explored at a later time because the vacuum recovery was studied first. Results Operating the recovery phase at low pressure will increase the volume flow rate of the carrier gas in the adsorber. Figure 3 shows that the volume flow rate is also related to the mass flow of the carrier gas. Higher mass flows lead to lower volume flows because the vacuum pump keeping the pressure in the column low cannot maintain low pressure at high mass flow. Fig 3. The volume flow rate as a function of mass flow. At higher mass flows the vacuum pump was unable to maintain low pressure in the column, so the volume flow rate was affected. Fig 2. The schematic of the CWRU krypton purification system. The lines in red are new additions for recovery at low pressure. The flow path for purification is shown as a heavy blue, and direction of flow is counterclockwise. Operating the recovery phase at low pressure does act to reduce τ. As Figure 4 shows, a factor of two reduction in pressure decreases τ by that same factor. CWRUPurificationSystem Krypton contaminated xenon is mixed with helium carrier gas and forced through a charcoal adsorber column. The propagation time of each species of gas through the column is determined by τ = kM/φ where k is the adsorption constant of a species, M is the mass of the adsorbant, and φis the volume flow rate of the carrier gas. Fig 4. Mass spectrometer measurement of the output of the charcoal column during the recovery phase at atmospheric pressure and at half an atmosphere. The recovery time at half the pressure takes half as long. Don’t delete Vacuum Recovery Inserting a vacuum pump immediately after the column allows the recovery cycle to operate at lower pressure. The lower pressure serves to increase the volume flow rate of the carrier gas while maintaining the mass flow rate. Decreasing the pressure by a factor of ten will increase the volume flow rate by the same factor and effectively decrease the propagation time by ten as well. The volume flow rate is related to the mass flow rate by the pressure, but in this system the relationship is not linear. As the mass flow rate is increased, the vacuum pump cannot maintain the desired pressure. The volume flow rate is maximized at lower mass flow rates. Figure 3 shows that the volume flow rate is higher for slower mass flow rates in the system. This suggests that the column should be operated with mass flow between 10 and 20 slpm. The system has three phases: the feed phase when krypton/xenon is injected into the system, the purge phase when krypton emerges from the column and is discarded, and the recovery phase when xenon emerges and is recovered. Fdfdsa Conclusions It has been demonstrated that the xenon purification system can operate at 10 torr during the recovery phase. This will increase the volume flow rate and drastically reduce τ. The optimal pressure and mass flow regime has yet to be determined experimentally. Increasing the volume flow rate by a significant factor is the first step in scaling up the system. Next, the column will be investigated, and its mass and geometry will change. Then the condenser will be redesigned to accommodate the increased purification rate. Finally, the system will be fully automated. Fig 1. The three cycles of the system. The green background is feed, yellow background is purge, and the white is recovery.