| Summary: | Accurate quantification of radionuclides detected during a scanning survey
relies on an appropriately determined scan efficiency calibration factor (SECF).
Traditionally, instrument efficiency was determined from a fixed instrument to
source geometry. However, as is often the case, the instrument is used in a
scanning mode where the source to instrument geometry is dynamic during the
observation interval. Three separate procedures were developed to determine the
SECF for a 10 cm x 10 cm source passing under the centerline of a 12.7 cm x 7.62
cm NaI(T1) detector. The procedures were first tested for determining the SECF
from a series of static point source measurements generated by the Monte Carlo N-Particle
(MCNP) code. These static efficiency values were then used to predict the
SECF for scan speeds ranging from 10 cm s����� to 80 cm s����� with a one second
observation interval. The investigator then used MCNP to directly determine the
SECF by simulating a scan of a 10 cm x 10 cm area source for scan speeds ranging
from 10 cm s����� to 80 cm s�����. Comparison of the MCNP static simulation with the
scan simulation showed the accuracy of the SECF prediction procedures to be
within ��5%. Experimental results further show the three procedures developed to
predict the actual SECF for a 10 cm x 10 cm source to be accurate to within ��10%.
Besides the obvious application to determine an SECF for a given scan speed, this
method can be used to determine the maximum detector or source velocity for a
desired SECF. These procedures are effective and can likely be extended to
determine an instrument specific SECF for a range of source sizes, scan speeds, and
instrument observation intervals. === Graduation date: 2002
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