FAST, LOW-LEVEL DETECTION OF SR-90 AND SR-89 IN ENVIRONMENTAL-SAMPLES BY COLLINEAR RESONANCE IONIZATION SPECTROSCOPY

Environmental assessment in the wake of a nuclear accident requires the rapid determination of the radiotoxic isotopes Sr-89 and Sr-90. Useful measurements must be able to detect 10(8) atoms in the presence of about 10(18) atoms of the stable, naturally occurring isotopes. This paper describes a new approach to this problem using resonance ionization spectroscopy in collinear geometry, combined with classical mass separation. After collection and chemical separation, the strontium from a sample is surface-ionized and the ions are accelerated to an energy of about 30 keV. Initially, a magnetic mass separator provides an isotopic selectivity of about 10(6). The ions are then neutralized by charge exchange and the resulting fast strontium atoms are selectively excited into high-lying atomic Rydberg states by narrow-band cw laser light in collinear geometry. The Rydberg atoms are then field-ionized and detected. Thus far, a total isotopic selectivity of S > 10(10) and an overall efficiency of xi = 5 x 10(-6) have been achieved. The desired detection limit of 10(8) atoms Sr-90 has been demonstrated with synthetic samples.