The sample collection procedure is described in a separate report, a copy of which is attached. This report describes the treatment of samples by the cooperating laboratories, ASHING PROCEDURE All samples are ashed before counting. The Whatman filter paper samples are ashed with the cellophane bag, while the gummed paper and air= dust filter samples are ashed without the bag, Platinum or silica dishes may be used, but platinum is preferred. After transferring the papsr to the dish, it is charred at a low temperature and then ignited for one-half hour at about 900° C in a muffle furnace or over a burner, The ash is brushed out into a plastic dish for counting. supplied is 12" in diametor with a 3/16" retaining wall. The dish COUNTING PROCEDURE In order to compare results from various laboratories, it is necessary to standardize the counting techniques to be used. While the conditions described may not be the optimum, it is believed that they can be reproduced in all of the cooperating laboratories. Ae Sample Dishs The container recommended is a ; diameter thin plastic dish with 35/16" retaining walls, It will hold the ash from all of the fall-out samples normally encountered. These dishes will be furnished by NYO, and are reusable if cleaned carefully. B. C. Geiger Tubes A thin window (3.5 mg/cm*) halogen-filled GM tube such as the Amperex 200N will be furnished by NYO. These tubes are very stable, and in an arrangement as described show a background of 1012 counts per minute. Mountings The tube is mounted on a standard lucite holder (such as AEC #AK-3A) in a vertical lead castle (such as AEC #AL-144). The dish is placed on a 3 mm thick aluminum shelf, spaced so that the top of the shelf is about 1.2 cm from the tube face. D. Scalers Any standard scaler having a scale of 64 or greater may be used. No modification is necessary, other than the insertion of a lO megohm resistor in series with the high voltage lead to the Gif tube. This may be done at the tube itself. To eliminate the need for plateau runs, a corona discharge regulator may be inserted in the high voltage supply. A circuit diagram for this modification will be included with each tube, as well as the corona voltage regulator.