1. Radioactivity Check Source—a radioactive source, not necessarily calibrated but preferred to contain long-lived isotopes with both low and high-energy gamma rays (e.g., Eu-152), which should be used to confirm the continuing satisfactory operation of the counting instrument.

  2. Radioactivity Source Strength—a radioactive source selected as either a check source, or a calibration standard, should contain radioactivity high enough to produce a statistically reliable count rate in the full-energy gamma-ray peak of interest. However, it should be cautioned that when a source is counted at any distance from the detector, or in any geometry, the count rates should be low enough to reduce the effect of random summing of gamma rays to a level where it may be neglected. A better criterion may be the indication of counting system dead time when the source is placed at the counting position. Any source with ≥5% dead time should not be used in calibration at that position.

  3. Physical Form of Standard Source—the radioactive source can be in any physical form or size. However, the result of the calibration is applied to the same physical form and size of the standard used only (more discussion on source geometry in Section C.4.3).

C.2.2 Calibration Procedure

  1. Energy Calibration—Determine the energy calibration (channel number versus gamma-ray energy) of the detector system at a fixed gain by determining the full energy peak channel numbers from gamma rays emitted from one or more known energy standard sources. A linear correlation is normally obtained.

  2. Efficiency Calibration –

    1. Accumulate an energy spectrum using calibrated radioactivity standards at a desired and reproducible source-to-detector distance. At least 20,000 net counts should be accumulated in each full-energy gamma-ray peak of interest using Certified Radioactivity Standard Sources.

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