where is total macroscopic cross-section (attenuation coefficient) ofthe emitted gamma-rays, and the term accounts for photon divergence,see section 3.6.2.A number of time-dependent factors also affect the magnitude of therecorded intensity. While the sample is being irradiated, the producedgamma-emitting nuclide decays. If the irradiation time is sufficientlylong, an equilibrium between the production rate by activation and theremoval rate by decay would be established, and the intensity of radiationwould be reduced by a factor of where is theactivation (irradiation) time, and is the decay-constant of the radioactiveisotope. If the object (or sample) is removed from the irradiationsite to the laboratory to measure the activity, the gamma intensity wouldbe further reduced by the decay factor where is the timeelapsed between irradiation and measurement of activity. During thecounting process itself, the sample would continue to decay, reducingthe activity by a factor of where is the counting period.Some isotopes decay by emitting several gamma energies with variousprobabilities, thus the measured activity needs to be corrected by theprobability of photon emission, at the monitored energy (also knownas the branching factor, as in branching to various photon energies).The measured intensity is also reduced by the detector’s efficiency,at the monitored energy. Since different isotopes of the same element
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