  Context. Observations of solar type II radio bursts provide a unique
opportunity to analyze the non-thermal electrons accelerated by coronal shocks
and also to diagnose the plasma density distribution in the corona. However,
there are very rare high-frequency resolution interferometric observations for
type II radio bursts that are capable of tracking these electrons.
  Aims. Recently, more spatially resolved high-resolution observations of type
II radio bursts have been recorded with the Low-Frequency Array (LOFAR). Using
these observations, we aim to track the location of a type II radio burst that
experiences a sudden spectral bump.
  Methods. Here, we present the first radio imaging observations for a type II
burst with a spectral bump. We measure the variation in source location and
frequency drift of the type II burst, and deduct the density distribution along
its propagation direction.
  Results. We identified a type II burst that experiences a sudden spectral
bump in its frequency-time profile. The overall frequency drift rate is 0.06
MHz/s and it corresponds to an estimated speed of 295 km/s. The projected speed
of the radio source obtained from imaging is 380 km/s towards the east
direction. At the spectral bump, a deviation in the source locations of the
type II split bands is observed. The band separation increases significantly in
the north-south direction.
  Conclusions. The spectral bump shows an 8 MHz deviation at 60 MHz which
corresponds to a 25% decrease in the plasma density. The estimated crossing
distance during the spectrum bump in type II is 29 Mm suggesting that this
density variation occurs in a confined area. This indicates that the shock most
likely encounters the upper extent of a coronal hole.
