Extraction of Point Source Gamma Signals from Aerial Survey Data Taken over a Las Vegas Nevada Residential Area Page: 2 of 17
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Detection of point-source gamma signals from aerial
measurements is complicated by widely varying terrestrial gamma
backgrounds, since these variations frequently resemble signals from
point-sources. Spectral stripping techniques have been very useful in
separating man-made and natural radiation contributions which exist
oi Energy Research and Development Administration (ERDA) plant sites
and other like facilities. However, these facilities are generally situated
in desert areas or otherwise flat terrain with few man-made structures to
disturb the natural background. It is of great interest to determine if the
stripping technique can be successfully applied in populated areas where
numerous man-made disturbances (houses, streets, yards, vehicles, etc. )
To determine the applicability of spectral stripping for
identifying low-energy point-sources in a populated area, a helicopter
overflight was made in the Las Vegas metropolitan area on 12 March
1975. The standard SANDS data acquisition system utilizing forty 5-inch
by 2-inch NaI gamma detectors was utilized. Nominal altitude was 150 ft;
nominal velocity was 100 ft/sec.
At one point in the survey, a Barium source was placed at
ground level in a two-story frame house. One pass was made near the_
house at ~-400 ft altitude, and three passes were made at -150 ft.
DETERMINATION OF STRIPPING PARAMETERS
To determine appropriate spectral windows for stripping,
data were examined to find a part of the survey which showed large count
rate extremes. The area chosen was a stretch of Eastern Avenue between
Russell and Natalie, fig. 1. The average count rate was 7363 1114 cps.
(If only counting statistics were involved, an approximate deviation of
+ 7 or + 86 cps would have been observed). Spectral data were extracted
from 'lo' and 'hi' gross count regions of this area (see fig. 2 & 3). The 'hi'
spectrum was divided channel by channel by the 'lo' spectrum (see fig. 4).
With the low-energy Barium source, virtually all source counts will occur
below 400 keV. Thus, the window 60 keV through 390 keV (channels 6 thru 39)
was selected as the 'source' window. It is observed from fig. 4 that the window
from 400 keV to 730 keV shows the same approximate ratio as does the source
window. Since virtually no Barium counts will fall in this window, it may be
used as a background window to indicate background counts present in the
source window. The constant which relates the counts in the source and
background window is
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Hendricks, Thane J. Extraction of Point Source Gamma Signals from Aerial Survey Data Taken over a Las Vegas Nevada Residential Area, report, May 1, 2007; United States. (https://digital.library.unt.edu/ark:/67531/metadc901182/m1/2/: accessed April 21, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.