System and Handling Details

Abstract

Prior to operation, the system is baked for 1 hour at a temperature of ≈ 100°C and pumped to a pressure of ≈ 10−5 mbar. Cooling of cold trap CT1 with liquid nitrogen reduces the pressure to about 2 10−6 mbar. The sample is transferred from the copper tube to the glass bulb by removal of the lower pinchoff clamp, pressing of the copper seal and heating of the copper tube to ensure that all water is transferred into the expansion bulb. During expansion of the water into vacuum it is degassed to about 80%. After completion of the water transfer the ampoule is cooled by liquid nitrogen to establish a strong water vapor flow from the head space of the bulb over the capillary into the ampoule. The bulb is shaken by a motor driven excenter to increase the transfer velocity of dissolved gases into the head space. Water vapour is frozen out on the glass walls of the ampoule while permanent gases are adsorbed on the activated charcoal. Gases which can not be adsorbed on the charcoal quantitatively (helium and neon) are trapped inside the ampoule by the strong water vapour flow through the glass capillary (3He would have to diffuse against a high counter advection of about 50 m/s under given conditions). This process continues for about 30 minutes allowing all the helium in dead volumes of the extraction unit (e.g. the copper tube) to diffuse into the water vapour flow as well as to flush the bulb’s head space at a sufficient rate. The sample is sealed off by flame sealing of the ampoule. For optional tritium analysis, the water in the storage bulb is further pumped for a few minutes to ensure complete degassing. The bulb is sealed off and stored in a freezer for 3He ingrowth.