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