Φ = neutron fluence rate incident on sample of mass m; andPγ = emission probability of the gamma ray with the given energy, which is thefraction of the photons emitted per capture.From Eqn (2.8), it follows that the recorded count rate depends on the product of theinteraction cross-section, the isotopic abundance of the target nuclei, and the gamma-rayyield associated with the reaction. Higher values of the product of these quantities resultsin higher sensitivity for the particular isotope and hence a lower detection limit.The counting efficiency of an HPGe , i.e., the ratio of the number of pulsesrecorded to the number of gamma rays emitted by the source, depends on samplegeometry and the gamma-ray energy. The absolute efficiency includes the effect of thesolid angle subtended by the detector measured to the surface of the crystal, not thedetector housing. For PGNAA experimental conditions, gamma-ray emission from thesample can be considered isotropic. Neglecting attenuation effects between the sourceand detector, the number of photons in the photopeak N, over a period of time T is
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