2 “fhe radioactivity associated with the fall-out being studied decays pidty so that -eneed in the handling of samples is essential. For all. shipments, air mail is recommended where one day's transit time will be saved. In order to correlate the radioactivity measurements with the fallout, the date and hour of sampling and of measurement must be known for each sample. For uniformity, the use of the 24-hour time scale, rather than A.M. and P.M. notations is recommended. The time zone must be in‘eluded-for both collecting stations and laboratories. The data sheets, of which a sample is attached, should be filled out completelywiththe pertinent information, using a separate sheet for each sample. Collecting stations will record the data on the upper half of the sheet, while the laboratories will fill in the counting data. This will aid in the collation of data from the large number of samples to be handled. Four types of samples are to be used in the study of radioactive fallout in this program, but not every station will collect each type. 1. Precipitation samples taken by collecting the rain or snow-fall 2. Settled dust samples taken by washing down the surface of the collecting tray used for precipitation samples when no rain or during 24 hours on the specific area of a collecting tray. The insoluble particulate matter is filtered off for measurement. snow-fall occurs. The insoluble particulate matter is filtered off from the washings for measurement. ~ - 3 Settled dust samples collected on gummed paper mounted on a metal frame. The paper is destroyed by ashing and the ash used for measurement. 4. Airborne dust samples collected by drawing knowm volumes of air through a filter with a high-volume air sampler. The filter is destroyed by ashing -and the ash used for measurement. The procedure for taking and handling each of these samples will be given in detail so that comparable results may be obtained at all collecting stations and laboratoriss.