  We measure the transverse velocity distribution in a thermal Sr atomic beam
precisely by velocity-selective saturated fluorescence spectroscopy. The use of
an ultrastable laser system and the narrow intercombination transition line of
Sr atoms mean that the resolution of the measured velocity can reach 0.13 m/s,
corresponding to 90$\mu K$ in energy units. The experimental results are in
very good agreement with the results of theoretical calculations. Based on the
spectroscopic techniques used here, the absolute frequency of the
intercombination transition of $^{88}$Sr is measured using an optical-frequency
comb generator referenced to the SI second through an H maser, and is given as
434 829 121 318(10) kHz.
