  Solar type III radio bursts are excited by electron beams propagating outward
from the Sun. The flux of type III radio burst has a time profile of rising and
decay phase at a given frequency, which has been actively studied since 1970s.
Several factors that may influence the duration of a type III radio burst has
been proposed. However, the major cause of the duration is still an open
question. In this work, to study the dominant cause of the duration, we
investigate the source positions of the front edge, the peak, and the tail edge
in the dynamic spectrum of a single and clear type III radio burst. The
duration of this type III burst at a given frequency is about 3 second for
decameter wave. The beam-formed observations by the LOw-Frequency ARray (LOFAR)
are used, which can provide the radio source positions and the dynamic spectra
at the same time. We find that, for this burst, the source positions of the
front edge, the peak, and the tail edge split with each other spatially. The
radial speed of the front edge, the peak, and the tail edge is 0.42 c, 0.25 c,
and 0.16 c, respectively. We estimate the influences of the corona density
fluctuation and the electron-velocity dispersion on the duration, and the
scattering effect by comparison with a few short-duration bursts from the same
region. The analysis yields that, in the frequency range of 30 - 41 MHz, the
electron-velocity dispersion is the dominant factor that determines the time
duration of type III radio bursts with long duration, while scattering may play
important role in the duration of short bursts.
