A Lower limit of detection (LLD) is the smallest amount of sample activity which will yield a net count for which there is confidence at a predetermined level that activity is present . For a five percent probability of concluding falsely that activity is present, the LLD is approximately equal to 4.65 times the standard deviation of the background counts (assuming large numbers of counts where Gaussian statistics can be used [ANSI 1989, Pasternack and Harley 1971, U.S. DOE 1990]).