The method of atomic-resonance fluorescence has been applied to the detection of very small concentrations of radioactive 20Na atoms (< 3 × 109 m-3). The 20Na isotope is produced by the reaction 20Ne(p,n)20Na by passing a 20 MeV proton beam through a neon gas target of density 3.5 × 1024 atoms/m3. 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 20Na 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 20Na 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 20Na 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 × 1011 atoms/m3 (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)