Measurements required for the efficiency analysis

For full-energy detector efficiency determination, you need one or more multi-line measurements. The measurements should have the following properties:

taken with the same measurement setup (same detector, ADC, energy range),
taken roughly in the same time period (preferably within the same week),
their gamma lines of isotopes have exact energy and intensityvalues in HyperLab's database,
the lines of the isotopes spread across your entire, regularly used energy range,
the used decay gamma cascade does not show significant true-coincidence summing effect (either due to the structure of the nuclear transitions, or due to the appropriately large detector-source distance),
every relative measurement has at least 2 proper gamma lines, and every absolute calibrated measurement has at least one proper gamma line.

These are not stringent requirements, so the majority of usual calibration measurements – or even measurements acquired for material analysis – may be used for efficiency determination. You can even use isotopes with a few problematic lines, as you will be able to individually remove the disturbing or overlapping lines from the efficiency curve.