  Cutoff energy in a synchrotron radiation spectrum of a supernova remnant
(SNR) contains a key parameter of ongoing particle acceleration. We
systematically analyze 11 young SNRs, including all historical SNRs, to measure
the cutoff energy, thus shedding light on the nature of particle acceleration
at the early stage of SNR evolution. The nonthermal (synchrotron) dominated
spectra in filament-like outer rims are selectively extracted and used for
spectral fitting because our model assumes that accelerated electrons are
concentrated in the vicinity of the shock front due to synchrotron cooling. The
cutoff energy parameter ($\varepsilon_0$) and shock speed ($v_{\rm sh}$) are
related as $ \varepsilon_0 \propto v_{\rm sh}^2 \eta^{-1}$ with a Bohm factor
of $\eta$. Five SNRs provide us with spatially resolved $\varepsilon_0$-$v_{\rm
sh}$ plots across the remnants, indicating a variety of particle acceleration.
With all SNRs considered together, the systematic tendency of $\eta$ clarifies
a correlation between $\eta$ and an age of $t$ (or an expansion parameter of
$m$) as $\eta \propto t^{-0.4}$ ($\eta \propto m^{4}$). This might be
interpreted as the magnetic field becomes more turbulent and self-generated, as
particles are accelerated at a greater rate with time. The maximum energy
achieved in SNRs can be higher if we consider the newly observed time
dependence on $\eta$.
