Some techniques in the assay of tritium.

insulator, and this in turn carries a 1-, in. brass rod, Q, fixed and sealed in position by a screw cap, R, and 0-ring gasket. The tungsten-anode wire, S (0-1 mm. diameter), is sheathed by a copper tube, T (diameter i16in.) to which it is soldered at its upper end. This is held in the rod, Q, by a small grubscrew. By this means the whole anode assembly is easily taken apart for cleaning and changing. The anode is terminated at its lower end, t in. above the planchet holder, by a glass bead 0-02 in. in diameter. The helium gas, after passing through dry ethanol, as described above, is admitted to the counting chamber via a channel within its wall (at U) and the insulator carrying the mounted anode. The gas escapes around the planchet throughtwin orifices, V, inthe baseoftheplanchetholderand thence via a series of drill holes, W, in the plates, as shown in the diagram of the section between stations. Theopening into this channel is sealed by the planchet holder in the lowered position duringthe transfer of samples. Thegas channels lead the gas issuing from the counting chamber over and around the second planchet holder (the preflushing position) and finallyoutthroughthe bubbler. This bubblerserves the triple purpose ofmaintaining a positive seal on the outlet ofthegas system, maintaining a pressure slightly above atmospheric within the system and enabling the rate of flow of gas to be observed. OPERATION