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.
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