1. Helium Contamination of Rubidium Clocks and its Effect on Gravity Meters
Micro-g LaCoste, Inc.
Derek van Westrum
Rubidium Vapor Line Source

2. Outline Introduction, Clocks in Gravity Meters
Rubidium Clocks, Principle of Operation
Clock Drift Mechanisms, Effect of Helium
Typical Drifts and Controlled Large Exposures
Clock Steering Possibilities and Limits
Solutions and Mitigation
Conclusions and Recommendations

3. Clocks in Freefall Gravity Meters
A photodetector is used to observe the output signal of an interferometer.
Optical fringes are formed as the test mass falls (and accelerates).
The time of each “zero crossing” is recorded by counting the pulses (nominally 10MHz) output by a Rubidium atomic clock.
...

4. Rb Clocks, Principle of Operation*
lamp
85Rb
buffer
87Rb
buffer
Photodiode
Microwave Cavity
Optical Path Hz
Frequency
Synth
Quartz
Oscillator
Servo
feedback ÷
10 MHz
1PPS
A 87Rb lamp at ~110°C emits two spectral lines, one of which is absorbed by the 85Rb buffer gas. The second line enters the 87Rb in a shielded cavity. When the Frequency Synthesizer emits microwaves at this precise transition frequency, induced absorption causes a decrease in the signal observed by the Photodiode. Feedback minimizes this signal, driving the oscillator and the synthesizer.
*After NIST,

5. Clock Drift Mechanisms, Effect of He
Rubidium Cell is pure at time of manufacture
Atmospheric Helium (4 milliTorr) infiltrates glass cell
Collisions with Rubidium atoms interfere with atomic transition (Stark Effect), broadening and shifting the line frequency.
An exponential effect with a time constant ~100s of days (time depends on cell manufacturing characteristics)
Helium’s effect is to increase the clock frequency
Typically about +1milliHz in 5 years
Clocks are calibrated via Cesium-based GPS signal
Clocks are not forced back to an exact 10MHz when calibrated. The actual (drifted) frequency is used by software to calculate gravity.
A 1 mHz increase in clock frequency lowers gravity by 0.2 μGal

6. Typical Drift Examples
Typical Drift Data for four FG5s with well-documented histories (and no clock replacements)

7. Accidental Exposure to Large Concentrations of Helium
Two clocks from FG5s were recently accidentally exposed to large doses of Helium when in close proximity to Superconducting Gravimeters during refills
Quantities of Helium were unknown, but offsets ~6 weeks later were still 50mHz (10 μGal) and 160mHz (320 μGal), respectively!
After Helium source was removed, clocks exponentially drift back to nominal with time constants of 90 days and 104 days, respectively (while powered on).

8. Accidental, Large Exposures
Two clocks, τ = 90 and 104 days

9. Controlled, Large Exposures I
Purposely dosed with 100% Helium for 15 hours.
Max offset: mHz
Increase in f
after dosing
“Expected” exponential decay

10. Controlled, Large Exposures II
“Expected” exponential decay
Increase in f
after dosing
Attempt to lock. Probably insufficient time for PPL to steer clock
Locks after GPS “steering”
100% Helium for 10 minutes. Max offset: mHz
Similar to actual accidental exposures

11. Clock Steering Possibilities & Limits
Vacuum
Chamber
Freefalling
Upper Mirror
Interferometer
GPS 1PPS steering
PLL limits
Interference
Detector
Stationary Lower
Mirror

12. Possible Solutions & Mitigation Schemes
Time, Distance, and Shielding
Remove clocks during Helium “events”
Power clocks off during Helium “events”
Calibrate clocks at least once per year
Calibrate clocks before and after Helium “events”
“Steer” clocks with external (GPS/Cesium) sources

13. Conclusions & Recommendations
Calibrate at least once per year
Calibrate before and after every He exposure

14. Questions Why a “new” phenomenon in clocks lately?
Why doesn’t He disturb Iodine cells in WEO lasers?