Outgassing Test and the Getter Specification

1. Outgassing Test and the Getter Specification Dean Walters Nuclear Engineering Division Argonne National Laboratory

2. Gettering and Outgassing • System Requirements • Gettering Technology • Design layout • Test setup • Status • Future Work

3. Situation • What are the requirements for the vacuum • Pressure for MCP • Pressure for Photocathode CsI and CsTe • Pressure limits • For cesiated GaN cathodes degradation occurs above 10-7 torr • Pressures at 10-9 torr show no degradation of QE • CsI is hygroscopic • Bialkali is sensitive to hydrogen and oxygen • How long does it need to be functional • Current goal is ten years

4. Gettering • How does it work? • Chemisorption of active gases • O2, H2, CO2, H2O, and CO are active gases • Ar, He, & other inert gases cannot be sorbed • Two main types of getters • Flash getters - Ba, Ca, Sr, Ti • Non-evaporable getters – Zr, Th, Ti • Flash getters are heated and evaporated onto the walls of the vacuum vessel. • These have a long history in the CRT and picture tube industries. • Non-evaporable getters • These come in a variety of shapes but the most prominent are pills and strips • In both cases the getter material is milled to small particles to maximize their surface area. • The particles are either sintered into pills or pressed into a metal strip • Gettering takes place on the clean surface where active chemical sites are available to form a strong bond with the gas within the photo-detector From Saes Getter Literature

5. Gettering • Operational considerations • Need to heat it to activate the getter material • Activation in an atmosphere of vacuum or inert gas • Heated to 350 to 450 degree C for 10 minutes • During activation weakly sorbed gases are exhausted into the vacuum system. • While getters are more efficient at elevated temperatures they are used in numerous room temperature applications From Saes Getter Literature

6. Gettering • Pumping • Hydrogen • Pumped on the surface of the getter • Active gases: CO, O2, CO2, H2O • Pumped by diffusion into the bulk • Inert gases • Not pumped at all 10 m From Saes Getter Literature

7. Gettering • Capacity • Capacity will be based on the pressure, gas load of the active gases, exposure time, and the total functional area of the getter • Hydrogen presents a different situation, since it is pumped on the surface and by diffusion into the bulk it has a limitation where the getter become embrittled and ceases to pump. From Saes Getter Literature From Saes Getter Literature

8. Gettering • Materials • Example: Alloy of ST707 is made up of: 70% Zr, 25% V, and 5% Fe From Saes Getter Literature

9. Materials From Saes Getter Literature

10. Design Layout- Mechanical Design and Space Allocations • Plan is to attach the getter strips to the side walls of the housing • Getter will be activated during baking of the house assembly • Once the seal in made the getter will provide the pumping for the photo-detector • The getter will only be able to be reactivated if it is re-processed through the exhausting station and the active layer of the photo-cathode may need to be re-coated. Getter attached to side wall

11. Outgassing • Sources of gas within a vacuum system • Gas interaction with solids • Diffusion • Permeation • Vaporization • Desorption • Thermal • Electron stimulated desorption • Ion stimulated desorption • Photon stimulated desorption • Leaks • Real • Virtual or internal • Backstreaming • Related to diffusion and mechanical pumps “A User’s Guide to Vacuum Technology, Second Edition” J. O’Hanlon, 1989, pg. 57

12. Outgassing • In addition to the various sources of outgassing there is a different time factor associated with these sources. “A User’s Guide to Vacuum Technology, Second Edition” J. O’Hanlon, 1989, pg. 131

13. Outgassing System Cross Calibrated gauges • Schematic 100 AMU QMA with EM QSample=C(PA-PB)-QSystem The orifice has been calibrated to a flow meter using a single species of gas

14. Test Setup • Pictures Vacuum Pump VacuumGauge

15. System Under Bake Residual Gas Analyzer Vacuum Gauge Vacuum Pump

16. Status • Status • First bakeout was halted after 5 days due to limitations of temperature. • Max Temperature 275 C • After cool down the base pressure was high, 1 x 10-7 Torr. • Orientation of components with insufficient heat lead to cold spots that limited base pressure. • Small leak found in system. • Heating issues resolved and the system is able to reach 400 C • Pressures are improving with the reorientation of the valves and the heater tapes. • Both helium leak detection and the RGA have shown the system to be free of leaks. • Plan • Outgassing system continues bake

17. Future Activities • Going Forward • By measuring the outgassing rate and identifying the species of the residual gas evolving from the parts will aid in the estimation of the gas load that the getters will pump. • Will be able to experiment with different temperatures to determine the best bake out temperature. • Will be able to perform trials on some of the proposed processes. • Like Indium sealing and examples of glass frit joints. • A note of appreciation to Bob Garcia from Saes Getters for his assistance on this talk.