Effects of Detector Distortion The typical symptom of detector degradation (due to icing, vacuum leak, lowered bias, ete.) was reduced resolution, i.e., wider photopeaks. The simple analysis program used in the field could not accommodate such an effect. Photopeak counts would be spread into the background windows resulting in an erroneously low value for net counts and, therefore, soil concentration. Window limits in the program could have been changed in the field if one cared to analyze peak shapes for a detector that was degraded but stable. The philosophy at Enewetak, however, was to correct the problem rather than attempt to correct the data. Brush Correction Factor Most of the islands surveyed were covered with a dense growth of Messerschmidia and Scaevola scrub vegetation, ranging in height from 1 to approximately 4 meters. A series of measurements were performed in October - November 1977 on Pearl to determine the effect of this vegetation on the 60 keV gamma ray from 24lam. Ten representative areas with brush covering 70-80 percent of the IMP field-of-view were measured. (The access road cut through the brush accounted for most of the open area.) Brush in each area was then earefully cleared by hand to prevent any soil disturbance and the measurements repeated. The results of this experiment gave a brush correction factor of 15 pereent for 100 percent brush cover (see Tech Notes 1.0 and 1.1 for details). No correlation was observed between the brush height and the brush attenuation factor. This was attributed to the fact that the vegetation normally encountered on the northern islands typically grew in the form of a canopy rather than solid cover. At each measurement location, an estimate of the percent brush cover within the detector field-of-view was made by the operator. This value was then used to provide a correction factor for brush attenuation. The estimate of brush cover was somewhat subjective and could have been in error by as much as 20 percent for some locations. Even a 20 percent error in the brush cover estimate, however, would only introduce a 3 percent error in the 41am concentration value. Thus, although some uncertainty was inherent in the method used to determine a brush attenuation correction factor, the uncertainty was less than would result from neglecting brush attenuation effects completely. Measurement Reproducibility A repeatability experiment was conducted on Pearl at location 3-N-0.5 in May 1979 to determineif any systematic variation could be observed in the IMP measurements over the course of a typical day. A total of 17 measurements were made, each for the standard 900-sec measurement time, with the detector fixed in position. The sample standard deviation for the series of measurements was 5 percent of the mean value. For the same set of measurements, the average one sigma error due to counting statistics was 6.7 percent. No systematic variation was observed between the early morning measurements, made when the ground was damp due to an early morning rain, and the afternoon measurements made during the hottest part of the day. One location on Janet was remeasured five times over a two-month period in the fall of 1977. The standard deviation for this set of measurements was 7.8 percent of the mean value. During the same period of time, two locations on Pearl were remeasured three times over a period of one month. The standard deviation was 4.4 percent of the mean for one location and 6.6 percent of the mean for the other location. These data indicate that the primary source of error in measurement reproducibility was associated with counting statistics, which generally ran from 5-7 percent. Additional details on measurement reproducibility can be found in Tech Note 21. 3.2.8 Detector Field-of-View The detector field-of-view is of some practical concern for an in situ measurement. However, as shown in Figure 3-3, even with a collimator the detector response does not drop abruptly to zero. Thus the "field-of-view" has an edge which is somewhat fuzzy. 97 The field-of-view ean only be