A measurement of the time profile of scintillation induced by low energy
  gamma-rays in liquid xenon with the XMASS-I detector
  We report the measurement of the emission time profile of scintillation from
gamma-ray induced events in the XMASS-I 832 kg liquid xenon scintillation
detector. Decay time constant was derived from a comparison of scintillation
photon timing distributions between the observed data and simulated samples in
order to take into account optical processes such as absorption and scattering
in liquid xenon. Calibration data of radioactive sources, $^{55}$Fe,
$^{241}$Am, and $^{57}$Co were used to obtain the decay time constant. Assuming
two decay components, $\tau_1$ and $\tau_2$, the decay time constant $\tau_2$
increased from 27.9 ns to 37.0 ns as the gamma-ray energy increased from 5.9
keV to 122 keV. The accuracy of the measurement was better than 1.5 ns at all
energy levels. A fast decay component with $\tau_1 \sim 2$ ns was necessary to
reproduce data. Energy dependencies of $\tau_2$ and the fraction of the fast
decay component were studied as a function of the kinetic energy of electrons
induced by gamma-rays. The obtained data almost reproduced previously reported
results and extended them to the lower energy region relevant to direct dark
matter searches.
