Standardization: The efTIciency factor for converting net counts per minute into d/m is obtained by measuring the counting rate of an NBS standard of RaD and Rak plated on a palladium coated silver disc. The disc diameter is 3.8 cm amd the active area has a diameter of 204 cm. A standard having a disintegration rate of 5000 to 50,000 d/m is recommended, and the count is taken for 64,000 total counts. The factor should be about 0.12 ¢c/d. Counter Backgrounds The background counting rate should be measured before and after a day's run of samples. With the arrangement de=scribed, more frequent tests are unnecessary, unless a spill occurs. Semple Coumtings In order to maintain a constant precision of counting over most of the range to be covered, the following procedure should be used to set the minimum counting time. The maximum counting time for any sample is 30 minutes, as a practical limit, and this may be scaled down to 1 minute according to the values in the table below. The data is given for counters having background count- ing rates of 10, 15 and 20 c/me Gross Counting Rete Required for Various Cowting Times Minimum Counting Time 30 min. 20 LO 5 2 l 10 ¢/m 15 or less 17 22 30 50 85 or more Background 15 c/m 2l or less 23 30 35 60 95 or more 20 c/m 27 or less 30 35 45 65 100 or more These values are meant to act as guides to minimum counting times, and not as rigid rules for counting. For the purposes of this work, no sample should be counted less than 48 hours after the end of the collecting period, This will allow time for partial decay of thoron daughter products, Before any tests occur, each laboratory will have several days' samples to determine the background activity level at the collecting sites. The level of significant activity has been arbitrarily set at five times this background level. (The backgroud and sample activity levels are compared after subtraction of counter background). When the sample is significantly higher than background, it is set aside and recounted the next day. These two values will allow cor- rection for decay of the total activity (F.P. plus thoron daughters) and separation of the two activities. Since the data sheets were prepared before the second comt was proposed, it will be necessary to use & second data sheet to record this data, If the two sheets are fastened together, it should not be necessary to record sampling data on both.