2 5 8 and 11 after these vials are lowered the rack advances one step and the cycle is repeated lifting and counting vials 3 6 9 and 12 thus completing all vials in the 12 vial rack the rack is then advanced a pre programed number of steps until it clears the area for the next rack to advance into engagement with rail 13 during actuation of motor 42 a motor 26 drives eject rods 25 to engage and move a rack which has just cleared the test station away from back rail 13 and into a post test storage area 27 of shelf 6 the entire process is repeated for the next rack of vials according to the rack size to obtain reliable and accurate counting results the photomultiplier tubes 24 and the sample under test must be effectively shielded from outside sources of light and radiation during the counting period test cell 19 consists of two diametrically opposed photomultiplier tubes 24 shielded all around with removable radiation shields 28 to minimize the effects of background radiation a capped hollow guide tube 29 is located directly above each test cell and contains a sleeve bearing 30 within which a weight rod 20 is telescopic during lifting and lowering of the vials a sealed cap 31 on the top of each guide tube 29 has a transverse vent hole 32 to prevent pressure buildup in the guide tube 29 during retraction of weight rod 20 vent hole 32 is baffled to prevent light from entering guide tube 29 by the installation of a porous sintered metal plug 33 in the bottom of cap 31.