The detection limit is the answer to the question - what is the minimum number of counts we can be confident of detecting in a given time?

Suppose measure a weakly active sample, obtaining a count C. A measurement of background gives B. So the count due to the sample itself, S = C - B.

At the detection limit the average value of S: <S> = LD

The plot shows the distribution P(S) of repeated measurements. They are distributed about LD with a standard deviation sD

If the critical limit is LC, the detection limit LD = Lc + ks= k2 + 2LC.

 LD depends on background B and your choice of k. The fractional area of P(S) above LC gives the probability that any measurement of this source will be significantly above background, i.e. not consistent with zero. The area gives us our confidence level, which depends on our choice of k.

© 1999, 2000 The University of Liverpool, Department of Physics

 Project funded by The Engineering and Physical Sciences Research Council Website developed and maintained by the MATTER Project