  We examine the possible acceleration mechanisms of the relativistic particles
responsible for the extended radio emission in Abell 520. We used new LOFAR 145
MHz, archival GMRT 323 MHz and VLA 1.5 GHz data to study the morphological and
spectral properties of extended cluster emission. The observational properties
are discussed in the framework of particle acceleration models associated with
cluster merger turbulence and shocks. In Abell 520, we confirm the presence of
extended synchrotron radio emission that has been classified as a radio halo.
The comparison between the radio and X-ray brightness suggests that the halo
might originate in a cocoon rather than from the central X-ray bright regions
of the cluster. The halo spectrum is roughly uniform on the scale of 66 kpc.
There is a hint of spectral steepening from the SW edge towards the cluster
centre. Assuming DSA, the radio data are suggestive of a shock of
$\mathcal{M}_{SW}=2.6_{-0.2}^{+0.3}$ that is consistent with the X-ray derived
estimates. This is in line with the scenario in which relativistic electrons in
the SW radio edge gain their energies at the shock front via acceleration of
either thermal or fossil electrons. We do not detect extended radio emission
ahead of the SW shock that is predicted if the emission is the result of
adiabatic compression. An X-ray surface brightness discontinuity is detected
towards the NE region that may be a counter shock of
$\mathcal{M}_{NE}^{X}=1.52\pm0.05$. This is lower than the value predicted from
the radio emission ($\mathcal{M}_{NE}=2.1\pm0.2$). Our observations indicate
that the SW radio emission in Abell 520 is likely effected by the prominent
X-ray detected shock in which radio emitting particles are (re-)accelerated
through the Fermi-I mechanism. The NE X-ray discontinuity that is approximately
collocated with an edge in the radio emission hints at the presence of a
counter shock.
