Selective counting of krypton atoms using resonance ionization spectroscopy

Resonance ionization spectroscopy (RIS) was used to count 1000 individual 81Kr atoms. In the method, lasers for RIS are used to obtain atomic species (Z) selectivity and a small quadrupole mass spectrometer provides isotopic (A) selectivity. After Z and A selection the krypton ions were counted as they were implanted into a target. Basically, the method works for any isotope whether stable or unstable. However, the detection limit may depend on the isotope because of outgassing of stable atoms from the enclosure walls. For 81Kr, we show that fewer than 1000 atoms can be counted with good accuracy.