Measuring buffer-gas pressure in sealed glass cells

In alkali rf-discharge lamps used for optical pumping in atomic clocks and magnetometers, a buffer-gas (Kr or Xe) allows electrons to extract energy from an rf-field, and these energized electrons eventually produce alkali resonant light. Contrary to naïve intuition, rf-discharge lamps can lose their noble-gas buffer over time. Recently, we began a long-term experimental program to better understand the mechanism of noble-gas loss in rf-discharge lamps, and needed a non-destructive means of measuring buffer-gas pressure in sealed glass cells. For this purpose, we employ the Kazantsev, Smirnova, and Khutorshchikov (KSK) technique, which is based on inferring buffer-gas pressure from the collision shift of an alkali ground-state hyperfine transition frequency ν_hfs. Here, we discuss the basic the KSK technique and two modifications that we have implemented for its improvement: use of a diode laser for optical pumping, and extrapolation of ν_hfs to zero magnetic field. Testing our system´s long-term performance with a very low pressure reference cell (i.e., 3.3 torr Xe), we find a reproducibility of 0.2% and an absolute accuracy of 5%. Further, our systematic drift is less than one mtorr/month.