Predicting instrument detection efficiency when scanning point and small area radiation sources.
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 x 10 cm source passing under the centerline of a 12.7 x 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 x 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 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 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.
Hart
Kevin
K
Oregon State University, 130 Radiation Center, Corvallis, OR 97331, USA. kevin.hart@us.army.mil
Duffy
William
W
Higley
Kathryn
K
Marianno
Craig
C
Moss
Craig
C
D002138:Calibration; D004784:Environmental Monitoring / Q000295:instrumentation / Q000379:methods; D019544:Equipment Failure Analysis / Q000295:instrumentation / Q000379:methods* / Q000592:standards; D015233:Models, Statistical; D009010:Monte Carlo Method; D011829:Radiation Dosage; D011868:Radioisotopes / Q000032:analysis; D011874:Radiometry / Q000295:instrumentation* / Q000379:methods* / Q000592:standards; D015203:Reproducibility of Results; D012588:Scintillation Counting / Q000295:instrumentation / Q000379:methods; D012680:Sensitivity and Specificity
D011868:Radioisotopes