Choosing the LBGR is part of an iterative process used to determine the number of data points, N, needed. You can set the LBGR at the median residual radioactivity concentration believed to be remaining in the survey unit. If the true, but unknown, concentration in the survey unit is in the grey region, you will have difficulty in determining if the survey unit concentration is less than the DCGLW. You should take into account the following when choosing the LBGR:
- variation of the concentrations in the survey unit (determined from prior surveys of the survey unit)
- variation in the measurements due to instrumentation at the candidate values for the LBGR
- possibility a survey unit could fail even though its average concentration is, in fact, less that the DCGLW
- costs of measurements at the sensitivities needed to measure the candidate values for the LBGR
You can optimize the trade-offs between increased instrument sensitivity, costs, and the number of data points needed by setting the LBGR so that you get a relative shift greater than one and less than three.
When no other information is available, MARSSIM suggests a default value for the LBGR equal to ½ the DCGLW. Then you can begin the iterative process described in "How do I choose the LBGR?"
See, Chapter 5: Survey Planning and Design and Chapter D: The Planning Phase of the Data Life Cycle on the MARSSIM Manual download page.