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Relative efficiency
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The relative efficiency is the ratio of the absolute efficiency of a detector to the efficiency of a sodium iodide detector.
  • Acquire a point source of 60Co of known activity and place precisely 250 mm from the centre of the top of the endcap of the detector.
  • Collect a spectrum for ~20,000 counts to be accumulated in the 1332 keV peak.
  • Determine the net peak area and divide by the live time to give counts per second.
  • Now divide this by the activity of 60Co, allowing for radioactive decay using the recommended half-life.
  • The result is now a measure of the full energy peak efficiency and can be compared to the efficiency of the NaI detector (1.2 10-3), expressed as a percentage.
Relative efficiency = 

Absolute efficiency

1.2 10-3

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