Outline

1. UHV and Surface Science Perspectives on the Purification of Liquid Argon for Large TPC DetectorsA. L. JohnsonDepartment of ChemistryUniversity of Nevada, Las Vegas

2. Outline • Introduction to UHV surface science technique • Analogy between LAr and UHV • Pumping contaminates • Detecting contaminates • Possible system layouts

3. UHV – 10-9 torr and better • In UHV we have an environment where electron based methods can be used – MFP of the electrons can be kilometers. • Surfaces can be extraordinarily reactive. • Ions and other energetic species can be stable enough to be studied.

4. Liquid Argon – Vacuum with a lot of stuff in it • Electrons have a large mean free path to capture. • The environment is rather unreactive. • Exotic species can be stable. • Can we use insights and techniques from UHV to clean and characterize LAr?

5. Calculations – the pressure equivalent of 1ppb of O2 in LAr • Some calculations of LAr - • Density of LAr is 1.4 g/cc. Thus one mole (40g) of LAr is 28.6 cc. If the concentration of the oxygen in LAr is 1 ppb by weight, then 28.5 cc contains 4x10-8 g O2 = 1.25x10-9 mole O2 (2.6x1013 molecules/cc). This corresponds to, using PV=nRT, 3.13x10-7 atm or 2.4x10-4 torr or 3.13x10-4mbar. • Now, the product of MFP and pressure is 6.5x10-5 m*mbar, and so the MFP (molecular) in O2 at this concentration is 20.8 cm. For electrons, the MFP is 4(2)1/2 times the molecular MFP which is 1.17 m. • From Physical Chemistry by Berry, Rice and Ross the excess thermodynamic quantities for O2 in LAr at 84K are: free energy 37 J/mol, enthalpy 60 J/mole, volume 0.14 cc/mole. • These would give a Boltzmann factor of 1.05 – Henrys Law Constant?

6. Titanium Sublimation Pumps (TSPs) • A clean layer of titanium is prepared, which reacts with atmospheric gases (even at LN2 temps) to provide vacuums as low as 10-9 torr. • TSPs do not pump (react with) argon

7. Electron assisted TSPs – “Ion Pumps” • A problem seems to be that the contamination in LAr moves very slowly (Beninni et. al. A study of the factors affecting the electron lifetime in ultra-pure liquid argon, Nuclear Instruments and Methods in Physics Research A305 (1991) 177-186). • Thus, to process the required quantities of LAr to maintain purity would be very difficult. • One way of addressing this problem may be to use electric fields to move the electron attaching contaminates towards the reactive surface. • The enabling technology would be efficient electron guns for LAr use. e-gun e-gun Cooling Jacket Cooling Jacket Ti depositor + + -

8. Electron guns for LAr • There are three easy candidates for LAr electron guns: • 1. photocathodes (expensive and low current) • 2. field emitters (can be high current {milliamps} – but also prone to contaminate) • 3. discharges (can be very high current, but deposit energy into the LAr and must deal with the positive ions)

9. Negative ions in UHV • We have studied negative ion formation from materials on surfaces using ESDIAD (angle resolved electron stimulated desorption) (A. L. Johnson, S. A. Joyce, and T. E. Madey, "Electron Stimulated Desorption Ion Angular Distributions of Negative Ions," Phys. Rev. Lett., 61, 2578, 1988 ). • The difficulty in observing negative ions is separating them from the electron flux. We used TOF techniques to both distinguish the electrons from the ions and to identify the ions. • In general, electronegative atoms form negative ions, as expected. • Further, in many cases the negative ions form without energy barrier, even in some cases where the atom source must fragment to create the ion (dissociative attachment). • Can a similar technique be used to study negative ion formation in LAr?

10. Ion TOF > Ion TOD (ion chromatography) Ion TOF • The ESDIAD apparatus uses a pulsed electron gun and gating hemispherical grids to do TOF of the charged species. • The ion chromatograph uses electrons/ions trapped around a grid, gated into a drift region, to inject a well defined “pulse” of ions towards a collection anode. detector e- Gating grid Ion TOD e-gun a + a b trapping Gating grid +++ analysis detector b ++++

11. Ar to Purification Ar to Purification Contaminate Transport LAr from Purification • We wish to process the LAr often enough to assure high purity. The large quantities of LAr being contemplated necessitate large throughput through the purification train. One way to address this problem is to do purification either in or adjacent to the detector, in the main LAr dewar. • One way to remove contamination from the main vessel is by distillation, if the contaminates are volatile. Oxygen should concentrate by at least a factor of three in gases distilled from the LAr. • This could be done using either heaters or He bubblers in an antechamber to the main chamber. • Proper flow regimes could use the low migration rate of contamination in LAr to isolate possible sources of trouble Detector Volume

12. Summary • Insights and techniques from UHV practice may be useful in working with LAr. • We calculate some comparative numbers and discussed some of their implications. • We suggest some purification techniques. • We suggest some analytical techniques • Some system ideas were presented

13. Acknowledgements • Dr. A Para for an introduction to the problem and useful discussions • Prof. T. Madey for the opportunity to work in the ESDIAD field.