  We have constructed a collinear laser spectroscopy (CLS) system installed at
the Beijing Radioactive Ion-beam Facility (BRIF), aiming to investigate the
nuclear properties of unstable nuclei. The first on-line commissioning
experiment of this system was performed using the continuous stable ($^{39}$K)
and unstable ($^{38}$K) ion beams produced by impinging a 100-MeV proton beam
on a CaO target. Hyperfine structure spectra of these two isotopes are
reasonably reproduced, and the extracted magnetic dipole hyperfine parameters
and isotope shift agree with the literature values. The on-line experiment
demonstrates the overall functioning of this CLS system, opening new
opportunities for laser spectroscopy measurement of unstable isotopes at BRIF
and other radioactive ion beam facilities in China.
