Secondary Logo

Journal Logo

Institutional members access full text with Ovid®

PREDICTING INSTRUMENT DETECTION EFFICIENCY WHEN SCANNING POINT AND SMALL AREA RADIATION SOURCES

Hart, Kevin; Duffy, William; Higley, Kathryn; Marianno, Craig; Moss, Craig

PAPER
Buy

Abstract— Accurate quantification of radionuclides detected during a scanning survey relies on an appropriately determined scan efficiency calibration factor (SECF). Traditionally, instrument efficiency is determined with a stationary instrument and a fixed 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. Procedures were developed to determine the SECF for a point source (“hot particle”) and a 10 × 10 cm source passing under the centerline of a 12.7 × 7.62 cm NaI(Tl) detector. The procedures were first tested to determine the SECF from a series of static point source measurements using Monte Carlo N-Particle code. These point static efficiency values were then used to predict the SECF for scan speeds ranging from 10 cm s−1 to 80 cm s−1 with a simulated instrument set to collect integrated counts for 1 s. The Monte Carlo N-Particle code was then used to directly determine the SECF by simulating a scan of a point source and 10 × 10 cm area source for scan speeds ranging from 10 cm s−1 to 80 cm s−1. Comparison with Monte Carlo N-Particle scan simulation showed the accuracy of the SECF prediction procedures to be within ±5% for both point and area sources. Experimental results further showed the procedures developed to predict the actual SECF for a point and 10 × 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 minimum detectable activity. 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.

*Oregon State University, 130 Radiation Center, Corvallis, OR 97331.

Manuscript received 10 July 2002;

revised manuscript received 5 November 2002, accepted 16 January 2003

For correspondence or reprints contact: K. Hart at the above address, or email at kevin.hart@us.army.mil.

© 2003 by the Health Physics Society