The equivalent volume method for gamma spectrometry calibration: A new method bridging the gap between distributed and point sources.

The high cost and limited availability of standard reference materials often restrict customized efficiency calibration in gamma spectrometry. This study introduces the Equivalent Volume Method (EQVM), a calibration approach that reduces dependence on such standard materials by using point sources together with a physically defined "Equivalent Volume" to generate efficiency curves directly within Genie 2000 software. Systematic experiments were conducted using various environmental matrices with densities ranging from 0.44 to 1.6 g/cm3 and multiple cylindrical geometries. A consistent linear correlation was observed between the spiked sample volume and the equivalent volume, defined by a constant scaling factor (F≈0.35±0.02), independent of gamma energy, density, and matrix composition. A similar proportionality was observed between self-absorption and linear attenuation coefficients, supporting the physical basis of the method. Four theoretical models were derived, supporting the experimental findings and introducing a critical point value as a physics criterion used for validating efficiency correction models. Validation against spiked samples and IAEA-certified reference material showed agreement within 1.5% relative deviation. The EQVM provides a precise, flexible, and economically superior alternative to the gamma spectrometry calibration, enabling customized calibration and accurate activity determination without requiring matrix-specific corrections over the studied range.