Once the conversion factor relating the net photopeak counts, Np, . ; oa . to the activity per unit volume at the surface, sy is obtained for a source distribution, Eqs. 5 and 6 can be used to arrive at the corre- sponding conversion factor for the total activity per unit area S,, A? and for the average activity per unit volume in the top Z centimeters, sé, By dividing the 54 by the soil density, in ¢ per cm’, the results can be expressed in units of activity per unit mass. Table ] shows the conversion factors for 137¢¢ obtained Enewetak system for several different depth distributions. for the Also shown in the last column are the corresponding conversion factors for total external exposure rate, in R/h, at the 1 meter level. These results were obtained directly from the total activity per unit area conversion factors using data given by Beck, et al.394 Various assumptions must be made to derive these conversion factors.4:3 distribution. The most Significant assumption is made for the depth In general, it is very desirable to perform field meas- urements to establish the source distribution with depth, and thus, also allow for a direct measurement of the soil density. In @ situation where the depth distribution varies significantly from point to point within 2 given area, as on many islands at Enewetak, obtain, or assume, an.average depth distribution. islands at Enewetak, it is necessary to For the northern previous data (1) indicate that the average depth . distribution for !3’cs has a relaxation length on the order of 10 to 15 cm. In using the data given in Table 1, a reasonable first approach would be to take the average of the values given for a 10 cm and for a 15 cm relaxation length. More precise data can be obtained for any Ziven area if the depth distribution is better known. - 8- ME So bed