SECREF UNCLASSIFIED #1175 -AEC -30-53 26 « The large sample counters have been built after a design by R. Wolfgang and W. F. Libby (Rev. Sci. Inst. 24, 511 (1953)). Sugihara, Thebe counters have lucite end pieces connected by three thin brass rods at their] periphery. Thin plastic film 2.69 mg/cm, with a conducting gold-costed surf for the counter wall. Miniature flow counters have been construc is used with field adjusting rings at the ends and with external end supports (to avaiid internal rods and their field distortion). These counters have been cove with DuPont Mylar fiim coated on the inner side with aluminum 0.975 mg/em. Mylar, a poly ester, is very durable, being resistant to moderate stress anc common organic sclvents. The background for the large The Pphysical Fkounters (1-1/2" x 6” to 10") runs about 6 to 10 counts per minute within mentioned above. The miniature counters (1.4 x 3.0 cm) have a 0.4 counts per minute. (The counters with aluminum-Mylar walls age quite photosensitive, but are ordinarily operated in the dark.) Samples are mounted on plastic half cylinders, two of which surround the sample counter. The geametry is as good as is permi}ted by the necessity for placing absorbers between the sample surface and For the large counters the geometry is 38% and for the small ones corresponding factors used in conversion of cpm to dpm are 2.7 These geometry determinations were made using potassium’© as a 6e and using the figure 26 dps/gm K. Most powdered samples are moun aid of dilute agar in alcohol and water, which provides a nearly binder after drying. The samples have usually been counted to a standard error of activity 1s sufficient. 1045327 If such precision would require an extra@rdinarily long SESS UNCLASSIFIED