DETERMINATION OF TRANSURANIC CONCENTRATIONS
IN SURFACE SOIL AT ENEWETAK
The concentrations of transuranic radionuclides (Plutonium-238, -239, -240 and Americium-241) in surface
soils are determined using a sequence of procedures
involving instrumental surveys, radiochemical analyses
of soil samples, and statistical analysis of the data
to estimate the average concentrations of transuranics
in the soil.
An in-situ radiometric survey of the area under investigation is performed using a unique, self-propelled
instrument system called an "IMP"
(named for the small
tracked vehicle that carries the system).
Gamma radiation
from the ground is detected by a planar intrinsic
germanium detector suspended from the end of a retractable
boom on the IMP.
Gamma spectra from the detector are
analyzed and recorded.
From the 60 keV gamma, the average
concentration of Am-241 in the top 3 cm of soil within the
detector's field of view
determined...
(a 2l-meter diameter circle)
is
Soil samples are taken and radiochemically analyzed in the
Enewetak Radiation Laboratory.
The concentrations of
Plutonium and Americium are determined.
Conversion factors
are derived from these data which allow estimates of the
total transuranic concentrations in soil to be calculated
froma the Am-241 measurements of the IMP.
To survey a large area, the IMP travels from point-to-point
along a surveyed grid, making a measurement at each grid
intersection.
Data from the entire survey field are
statistically analyzed and isopleths are drawn of the
estimated average concentrations of total transuranics in
the surface soil.
The isopleths are based on the 70% upver
bound, i.e. the probability is at least 0.7 that the true
average concentration is no greater than the upper bound.
If soil is removed, this process is repeated to ascertain
the concentration values of newly exposed surfaces.