The method of atomic-resonance fluorescence has been applied to the detection of very small concentrations of radioactive <sup>20</sup>Na atoms (< 3 × 10<sup>9</sup> m<sup>-3</sup>). The <sup>20</sup>Na isotope is produced by the reaction <sup>20</sup>Ne(<i>p</i>,<i>n</i>)<sup>20</sup>Na by passing a 20 MeV proton beam through a neon gas target of density 3.5 × 10<sup>24</sup> atoms/m<sup>3</sup>. A beam of a continuous-wave dye laser tuned to the D2 line of sodium is transmitted through the production region and the fluorescence light is detected by means of photon counting. A digital synchronous-detection technique has been applied to measure the time-dependent behavior of the <sup>20</sup>Na atom density shortly after a proton irradiation. This behavior is determined both by radioactive decay and by diffusion of the atoms out of the production region. The absolute <sup>20</sup>Na atom density has been estimated, using Rayleigh scattering of the laser beam on the neon gas for calibration of the optical system. The density of neutral <sup>20</sup>Na atoms appeared to be an order of magnitude lower than the density produced. The detection method has also been used to measure the temperature dependence of saturated sodium-vapor density down to 3 × 10<sup>11</sup> atoms/m<sup>3</sup> (292 K).
F. C. M. Coolen and H. L. Hagedoorn, "Detection of 20Na atoms and measurement of sodium-vapor densities by means of atomic-resonance fluorescence," J. Opt. Soc. Am. 65, 952-955 (1975)