Alkali metal consumption by discharge lamps fabricated from GE-180 aluminosilicate glass

Alkali rf-discharge lamps provide the light for optical pumping in vapor-cell atomic clocks and magnetometers. Traditionally, the discharge lamp´s envelope has been fabricated from alkali-resistant Corning 1720 aluminosilicate glass, and such lamps have demonstrated decade-long continuous operation. Specifically, the diffusion of alkali atoms into this glass during lamp operation has been shown to be sufficiently slow that lifetimes in excess of ten years can be obtained using only moderate initial alkali fill levels (<; 400 μg). However, Corning 1720 glass is no longer being manufactured. It is therefore important to identify alternative glass types that offer comparable alkali resistance, and that are readily available. Although Schott 8436 aluminosilicate glass has been shown to be a suitable substitute, it would be advantageous to identify an alternate glass type produced in higher volume. One alternative, which is manufactured in very high volume for use in automotive lamps, is GE-180 glass. Although this glass also offers high alkali resistivity due to its aluminosilicate composition, the rate of diffusion of alkali atoms into this material when employed as the glass envelope of an rf-discharge lamp has not been measured. We present here our initial results, obtained by Differential Scanning Calorimetry (DSC), of the rate of Rb consumption by discharge lamps manufactured with GE-180 glass. Our results suggest that GE-180 is an excellent substitute for Corning 1720 glass in rf-discharge lamps. We also discuss some unusual issues that have been observed during our DSC measurements, and we outline some new methods for attaining accurate alkali lamp fills using the DSC technique.