  We report an in-situ measurement of the nuclear recoil (NR) scintillation
decay time constant in liquid xenon (LXe) using the XMASS-I detector at the
Kamioka underground laboratory in Japan. XMASS-I is a large single-phase LXe
scintillation detector whose purpose is the direct detection of dark matter via
NR which can be induced by collisions between Weakly Interacting Massive
Particles (WIMPs) and a xenon nucleus. The inner detector volume contains 832
kg of LXe.
  $^{252}$Cf was used as an external neutron source for irradiating the
detector. The scintillation decay time constant of the resulting neutron
induced NR was evaluated by comparing the observed photon detection times with
Monte Carlo simulations. Fits to the decay time prefer two decay time
components, one for each of the Xe$_{2}^{*}$ singlet and triplet states, with
$\tau_{S}$ = 4.3$\pm$0.6 ns taken from prior research, $\tau_{T}$ was measured
to be 26.9$^{+0.7}_{-1.1}$ ns with a singlet state fraction F$_{S}$ of
0.252$^{+0.027}_{-0.019}$.We also evaluated the performance of pulse shape
discrimination between NR and electron recoil (ER) with the aim of reducing the
electromagnetic background in WIMP searches. For a 50\% NR acceptance, the ER
acceptance was 13.7${\pm}$1.0\% and 4.1${\pm}$0.7\% in the energy ranges of
5--10 keV$_{\rm ee}$ and 10--15 keV$_{\rm ee}$, respectively.
