Helium Contamination of Rubidium Clocks and its Effect on Gravity Meters

Micro-g LaCoste, Inc.

Derek van Westrum

www.microglacoste.com

derek@microglacoste.com

Rubidium Vapor Line Source

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

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Clocks in Freefall Gravity MetersA 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.

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Rb Clocks, Principle of Operation*

Optical Path

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.

Photodiode87Rblamp

85Rbbuffer

87Rbbuffer

QuartzOscillator

Servofeedback

FrequencySynth

÷ 1PPS10 MHz

6 834 682 608 Hz

Microwave Cavity

*After NIST, http://tf.nist.gov/general/enc-re.htm#rubidiumoscillator

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

Typical Drift Examples

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

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).

Accidental, Large Exposures

Two clocks, τ = 90 and 104 days

Controlled, Large Exposures IPurposely dosed with 100% Helium for 15 hours. Max offset: 255.7 mHz

Increase in fafter dosing

“Expected” exponential decay

Controlled, Large Exposures II

100% Helium for 10 minutes. Max offset: 55.9mHzSimilar to actual accidental exposures

Increase in fafter dosing

“Expected” exponential decay

Locks after GPS “steering”

Attempt to lock. Probably insufficient time for PPL to steer clock

Clock Steering Possibilities & LimitsVacuum Chamber

Interferometer

Stationary LowerMirror

Upper Mirror

Detector

GPS 1PPS steeringPLL limits

Freefalling

Interference

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

Conclusions & Recommendations

Calibrate at least once per year Calibrate before and after every He exposure

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

lasers?