SAM III Series | Why are some isotopes harder to identify at low dose rates/count rates?

The example given here is for a typical background of 7 urem/hr (70 nSv/hr) which corresponds to about 250 gamma counts per second (cps).    When a Ra-226 source is detected there will be an alarm at background levels but a firm ID will not occur until the cps reaches about twice the background level (500 – 600 cps for this example).  This higher cps level for ID is expected since Ra-226 has many peaks and many are very low in abundance (sometimes referred to as Branching Ratio, Branching intensity or intensity of peaks.) 

In addition Ba-133 has similar peaks with much higher branching intensities (about 20 times higher) which add to the complexity of identifying Ra-226 especially at low cps.  The library is carefully designed to take this into account but moving closer to the source or waiting a little longer for a solid ID is always important (Ba-133 ID may be seen momentarily but will cease as statistics improve).  Identifying two or more isotopes each with low abundant lines (peaks) may also require moving closer to the source and possibly waiting longer to obtain good performance. 

U-238 is another example of a source with low abundant lines and may take up to 30 seconds or more to identify at minimum count rate (<500 cps).  Users must realize that the basic specifications which are given for the Cs-137 standard is the result of a very high branching intensity (>85%) - whereas the branching intensity of U-238 lines are less than 1% of Cs-137.  It is therefore expected that acquisitions on some sources will take a little longer but it is common practice to use acquisitions of at least 1 minute and to move closer to the source if possible.