APPENDIX
TECHNICAL CONSIDERA TIONS
Spatial Resolution
Spatial resolution is controlled by altitude, flight speed, data accumulation
time and gamma-ray energy.
The detectors exhibit a cosine angular
response for low energy gamma-rays such as the 60 keVline from
Am 241.
Inherent spatial resolution is approximately 125 ft at 100 ft
altitude for this energy.
Inherent spatial resolution is degraded for
higher energy gamma-rays due to the decrease in air attenuation and the
transmission of gamma-rays by the shielding around the sides of the
crystals.
The assumption of isotopic angular response and no air
attenuation for high energy gamma-rays gives a spatial resolution of
200 ft at an altitude of 100 ft.
Data accumulation time can be adjusted to be consistent with inherent
spatial resolution and flight speed.
Sensitivity
Sensitivity is controlled by survey rate, spatial resolution and background count rates under the photopeaks of interest.
Sensitivities can
be estimated by assuming background spectra in the region of interest
to be identical to that taken over typical U. S. Soil with an array of
4- by 4-inch crystals.
Amplitude of the spectra are scaled according
to crystal surface area and radiation levels three feet above the ground.
The following sensitivities are estimated with these assumptions and the
additional assumptions of 100 ft/sec flight speed, spatial resolution of
150 ft, survey speed 15000 ft? /sec, and cosine angular response for
the detector system