Abstract

The technique of resonance ionization spectroscopy (RIS) was extended to develop a means for counting individual atoms of a selected isotope of a noble gas. The experimental schematic is shown in Fig. 1. The concept for counting noble gas atoms with isotopic selectivity utilizes a laser for ionizing atoms of a selected atomic number (Z) and a mass spectrometer for atomic mass (A) selection. After atoms have been selectively ionized and mass selected, the ions are accelerated to 10 kV onto a Be-Cu target where they are implanted. The target will emit a pulse of several electrons which are detected by an electron multiplier. A four-wave mixing scheme was used to generate a 1165 Å beam with energy of 500 nJ/pulse. Thus, krypton atoms could be efficiently excited by resonant radiation and subsequently ionized. The use of a one-photon excitation step rather than a two-photon step also reduces the off-resonant multiphoton ionization background. An atom buncher was developed to enhance the probability that krypton atoms will be in the ionization laser beam at the time of the laser pulse, enabling a sample of ⁸¹Kr atoms to be counted in about 1 hr. Experimental data for counting 1000 atoms of ⁸¹Kr are shown in Fig. 2. The detection limit is believed to be less than 300 atoms of ⁸¹Kr.

© 1984 Optical Society of America