out that any bump in the spectral region assigned to a photopeak was analyzed and printed out. The
net/sigma value and a spectral plot were used to determine if the result was significant.
Below are numbers used for the 59.5 keV (24lAm) and 86.5 keV (19°Ey) analyses.
windows were used for higher energy peaks.
Sliding interval filter:
Region examined for americium-241:
Region examined for europium-155:
Slightly wider
rectangular, 3 channels wide
eh 155 to 162 (58.1 keV to 60.75 keV)
ch 227 to 234 (85.1 keV to 87.75 keV)
Low energy background window:
Photopeak:
High energy background window:
peak -8 to peak -5 channels
peak -4 to peak +3 channels
peak +4 to peak +7 channels
Analyzer gain:
0.3875 keV/channel
Laboratory Processing. Several correction factors had to be applied to the 241 Am data prior to its
use in determining the area-averaged total transuranic surface concentration values. These were all
made in the laboratory. The conversion factor used in the field program was the same for all
systems. This conversion factor assumed a detector height of 740 em and a detector efficiency of
19.0 cps per (y/em2 * sec). It did not include the possibility of any additional attenuating material
between the detector and the ground. Corrections had to be made if any of these assumptions were
not valid. Correction factors were routinely applied to correct for attenuation due to vegetation (a
maximum 15 pereent correction} and to correct for the different efficiencies of the various
detectors used at Enewetak (see Table 3-1). (The derivation of the brush attenuation correction
factor is deseribed in Technical Notes 1.0 and 1.1.)
TABLE 3-1. INITIAL DETECTOR EFFICIENCY CALIBRATION RESULTS FOR 24lAm
Detector
Serial
Number
386
393
483
496
513
635
Operating
Voltage
(kV)
-2.0
-2.0
Detector Efficiency
eps/( Y/em? see)
19.1
19.3
-2.5
18.7
-3.0
-3.0
17.2
18.1
~2.0
17.2
3.2.4 Operational Procedures
Prior to making any measurements, the detector system was calibrated to 0.375 keV per channel
(approximately 1500 keV full seale) using a combination ©9Co, 13%Cs, and 24! Am calibration source.
The calibration was checked periodically and any gain shift was corrected. (Maintaining power to
the preamplifier and amplifier on a 24-hour-a-day basis minimized gain shift problems.) The IMP
was moved from location to location with the boom fully retracted and the detector securely
fastened. At a measurement point the boom was extended to its full length and then inclined at an
angle of 20° away from the IMP. After completing the measurement(a typical acquisition time was
900 seconds), the boom was retracted and the detector secured for movement to the next
measurement location. The total time required for each measurement sequence was typically 20 to
minutes.
A five minute calibration run was made every morning, noon, and afternoon when a system was in
the field. This data was processed in the same way that a typical measurement was processed and
was also stored on magnetic tape for permanent retention,
Although the sources used were not
calibrated, the relative response as a function of time provided a means of monitoring for any
changes in the detector efficiency.
87