Emission probabilities of the main γ-rays following the decay of 222Rn and Daughters

There has been increasing interest over recent years in methods for measuring the activity of 222Rn (T1/2=3.8 d). Among these methods, γ-ray spectrometry would appear to be advantageous because the radionuclide activity can be measured directly without special preparation. Although 222Rn is not a γ-ray emitter, the activity of this radionuclide can be derived from measurements of the daughters in equilibrium. To obtain accurate results by γ-ray spectrometry, more precise nuclear data are needed. The photon emission probabilities of the main γ-rays have been measured using two calibrated germanium detectors and 4 bulbs filled with a 222Rn standard gas. The activity of the gas was accurately determined by an absolute measurement method based on counting in a defined solid angle the alpha particles emitted by radon condensed on a cold point. Efficiency responses were determined from a calibration using several standard point sources and two bulbs filled with 133Ba and 152Eu standard solutions; corrections factors were applied for the differences in geometry. An extensive number of γ-ray emission probabilities were determined as given in detail in the main text.