PREDICTIVE RADIOLOGICAL BACKGROUND DISTRIBUTIONS FROM GEOCHEMICAL DATA
Abstract:Gamma ray surveys are an important tool for both national security interests as well as industry in determining
locations of both anthropogenic radiological sources and natural occurrences of radiologic material. The purpose of
this project is to predict the radiologic exposure rate of geologic materials by creating a model using published
geochemical data, geologic data, GIS software, and freely available remote sensing data sets. If K, U, and Th
abundance values are known for a given geologic unit, the expected radiation exposure rate can be calculated. One
of the primary challenges surrounding this project is that alluvial units are classified by age rather than rock type. It
is therefore important to determine sediment sources and estimate their relative contribution to alluvial units.
ASTER data from the Terra satellite can differentiate between surface mineralogies and can aid us in calculating the
relative percentage of sediment from each source and by extension the geochemical concentrations of challenging
surfaces such as alluvium. An additional problem is that U and Th do not directly contribute to the measured
radiation exposure rate. Instead, daughter isotopes of these radioelements emit detectable gamma rays and may not
have reached equilibrium in younger surfaces. U can take up to 1.5 Ma to come to equilibrium with its daughter
isotopes while Th takes only about 40 years. Further modeling with software such as Monte Carlo N-Particle
Transport from Los Alamos National Laboratory, will help us correct for this disequilibrium in our models.
Once the predicted exposure rate is calculated for a geologic unit, it can then be assigned to a geographic area based
on geologic and geomorphic trends. This prediction will be subtracted from data collected through aerial surveys,
effectively ignoring geology, and allowing areas of interest to be narrowed down considerably. The study areas
include the alluvium on the west shore of Lake Mohave and Government Wash north of Lake Mead. In both of these
areas, strategically located soil and rock samples have been collected and are awaiting geochemical analysis.
This work was done by National Security Technologies, LLC, under Contract No. DE-AC52-06NA25946 with the
U.S. Department of Energy.