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
From Saes Getter Literature
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

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. “A User’s Guide to Vacuum Technology, Second Edition”
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. “A User’s Guide to Vacuum Technology, Second Edition”
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. QSample=C(PA-PB)-QSystem
Outgassing System
Schematic
Cross Calibrated gauges
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
Vacuum Gauge

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.