Several of these detectors showed a significant change in efficiency after the original measurement. One detector suffered a 15 percent decrease in efficiency over a single weekend. The daily calibration measurements made in the field were monitored closely in order to detect any sudden change in efficiency. In addition, starting in July 1978 a remeasurement of detector efficiency (using an NBS cross-calibrated 24] Ar source) was made every three to four weeks. A new correction factor was applied whenever the efficiency changed by 5 percent or more from the efficiency at the time the last correction factor was determined. Detector Height AS can be seen from Table 3-2, variations in detector height do not significantly affect the 24lam conversion factor. This is primarily due to the assumption made in the derivation that the activity is distributed uniformly in the horizontal plane (see Section 3.2.5). (It is because of this assumption that an in situ measurement provides a direct method for obtaining an area-averaged value for the activity over the field-of-view of the detector.) As the detector height increases, the 1/r2 decrease in the gamma-ray flux at the detector due to a given source element is compensated for by the r increase in area, or source elements, within the detector field of view. The rather minor variations observed are due to slight additional attenuation for gamma rays ineident at a given angle due to an increased path length through the soil and air. For the Enewetak “4!Am conversion factor, a variation in the normal detector height (7.4 m) of +0.5 m leads to a 1 percent change in the conversion factor. For some areas, measurements were taken on a 12.5 m grid pattern with the detector at a height of 4.6m. For this height there is a 7 percent change in the conversion factor. Corrections were made to account for this difference on all measurements taken at 4.6 m (see Tech Note 12). 3.2.7 Other Sourees of Error Shielding by the IMP A portion of the ground area which is within the detector's field-of-view is shielded from the detector by oripinal the IMP. This reduces the flux arriving at the detector by approximately 4 percent. The Am conversion factor used during the cleanup did not correct for this effect. All final am data, however, were corrected to account for this 4 percent shielding factor (see Tech Note 23). For measurements taken at a detector height of 4.6 m, the IMP shielding factor is approximately 13 percent. Ail data obtained at the 4.6 m detector height were corrected for this factor throughout the cleanup {see Tech Note 12). Contributions Due to 155Eu One of the residual fission products found at Enewetak, 155 pu, emits a 60 keV gamma ray which interferes with the 59.5 keV gamma ray from 24!Am. Itis possible to correct for this interference by monitoring one of the two other gamma rays emitted by 25Eu: one at 86.5 keV and one at 105.3 keV. The ratio of 86.5 keV to 60.0 keV gamma rays from !55Eu is 24.3 to 1. For an in situ measurement, the ratio of these two gamma rays at the detector is somewhat dependent on the depth distribution of the europium; this is due to differences in soil attenuation at 60 keV (u/p = 0.333 em 4/g) and at 86.5 keV (u/p = 0.22 em “/g). A reasonable compromise for field measurements is to assume a ratio at the detector of 30:1. As discussed in Section 3.2.3, the field program processed the spectral data for the 86.5 keV photopeak. The contribution of 153Ey to the 60 keV photopeak was obtained by dividing the net counts at 86.5 keV by 30 and subtracting this from the net counts at 60 keV. This correction factor was never more than 3 percent (at a few locations on Pearl) and generally ran between 1 percent and 2 percent. For this reason, although the 199Ey was always monitored, no significant correction was required for the 241 am data. 96