  We present a new method for the measurements of photonuclear reaction
flux-weighted average cross sections and isomeric ratios using a laser-driven
bremsstrahlung $\gamma$-ray source. An ultra-bright ultra-fast
60$\,\thicksim\,$250 MeV bremsstrahlung $\gamma$-ray source was established
using the 200 TW laser facility in the Compact Laser Plasma Accelerator
Laboratory, Peking University, which could cover the energy range from knocking
out neutrons to producing pions. Stable quasi-monoenergetic electron beams were
generated via laser wakefield acceleration with a charge of
300$\,\thicksim\,$600 pC per shot. The averaged $\gamma$-ray intensities
($\geqslant$8 MeV) were higher than 10$^{8}$ per shot and the instantaneous
intensities can reach above 10$^{19}$ s$^{-1}$ with a duration time about 6.7
ps. $^{65}$Cu($\gamma,\,n$)$^{64}$Cu and $^{27}$Al($\gamma,\,x$)$^{24}$Na
reactions were used as $\gamma$-ray flux monitors in the experiments. The
flux-weighted average cross sections and isomeric ratios of
$^{197}$Au($\gamma,\,xn;\,x\,=\,1\thicksim9$) reactions were analyzed through
activation measurements. The results showed good agreement with previous works
and proved this method to be accurate. The
$^{197}$Au($\gamma,\,xn;\,x\,=\,7\thicksim\,9$) reaction cross sections were
first achieved with the highest threshold energy of 71.410 MeV. Theoretical
cross sections of TALYS 1.9 were calculated to compare with experiment results.
This method offered a unique way of gaining insight into photonuclear reaction
research, especially for short-lived isomers which extremely lack experimental
data.
