1.0 0.9 WITHOUT COLLIMATOR RELATIVE RESPONSE 0.8 0.7 0.6 0.5 WITH COLLIMATOR 0.4 03 0.2 0.1 0 10 20 30 40 50 60 70 80 90 ANGLE 6 (DEGREES) FIGURE 3-7. RELATIVE RESPONSE OF THE HPGe DETECTOR FOR 662 keV GAMMA RAYS AS A FUNCTION OF INCIDENT ANGLE (ZERO DEGREES BEING DIRECTLY BELOW THE DETECTOR). Conversion factors are given in Table 3-4 for 137Cs as a function of source depth distribution. Also shown in Table 3-4 are conversion factors relating external exposure rate (in,R/h at 1 meter) to photopeak count rate. The exposure rate conversion factors were obtained from data (Beck, et al 1968, 1972) which relate exposure rate at 1 meter to source distribution in the ground for a variety of radionuclides. It can be seen that, although a knowledge of the source depth distribution can be very critical in determining concentration values, it is not nearly so critical for determining exposure rate values. For the post-cleanup dose assessment, external exposure rate values for 137Cs were obtained using a conversion factor of 3.6 .R/h per eps. Conversion factors used for 59Co were 20.5 uR/n per eps for the 1173 keV peak and 22.3 yp R/h per cps for the 1333 keV peak. In principle, either peak could be used to determine the total external exposure rate due to 60Co. Both should lead to the same result. In practice, however, some measurements showed a slight difference in the two results. in these cases the average value was used. Table 3-5 shows the post-cleanup island average values for !37Cs and 69Co exposure rate from the IMP data. Also shown for comparison are the values obtained in November 1972 from the aerial survey (see Section 3-1). For comparison, the aerial data have been corrected for radioactive decay to November 1978. The two sets of data agree fairly well except in the obvious cases where cleanup activities have reduced the levels. It should be noted that the island average values for the aerial survey data were estimated from exposure rate contours while those for the IMP were obtained by numerically averaging discrete data points. The island average values for 187Cg exposure rate given in Table 3-5 can be converted to island ayerage concentration values using the data given in Table 3-4. Although the depth distribution for Cs ean vary significantly from point to point, the profile data obtained in 1972 showed that a reasonable compromise for all the northern islands would be to take an average of the values given for a 10 cm and a 15 em relaxation length. Table 3-6 gives the results for the 0-15 em average concentration based on a conversion factor of 5.4 pCi/g per cps. Shown for comparison are the results obtained from the 1979 Fission Product Data Base sampling program. The results, in general, agree quite well. 10]