LOFAR observations of galaxy clusters in HETDEX
  Diffuse cluster radio sources, in the form of radio halos and relics, reveal
the presence of cosmic rays and magnetic fields in the intracluster medium
(ICM). These cosmic rays are thought to be (re-)accelerated through ICM
turbulence and shock waves generated by cluster merger events. Here we
characterize the presence of diffuse radio emission in known galaxy clusters in
the HETDEX Spring Field, covering 424 deg$^2$. For this, we developed a method
to extract individual targets from LOFAR observations processed with the LoTSS
DDF-pipeline. This procedure enables improved calibration and joint imaging and
deconvolution of multiple pointings of selected targets. The calibration
strategy can also be used for LOFAR Low-Band Antenna (LBA) and
international-baseline observations.
  The fraction of Planck PSZ2 clusters with any diffuse radio emission
apparently associated with the ICM is $73\pm17\%$. We detect a total of 10
radio halos and 12 candidate halos in the HETDEX Spring Field. Five clusters
host radio relics. The fraction of radio halos in Planck PSZ2 clusters is
$31\pm11\%$, and $62\pm15\%$ when including the candidate radio halos. Based on
these numbers, we expect that there will be at least $183 \pm 65$ radio halos
found in the LoTSS survey in PSZ2 clusters, in agreement with predictions. The
integrated flux densities for the radio halos were computed by fitting
exponential models to the radio images. From these flux densities, we determine
the cluster mass (M$_{500}$) and Compton Y parameter (Y$_{500}$) 150 MHz radio
power (P$_{\rm{150 MHz}}$) scaling relations for Planck PSZ2-detected radio
halos. We find that the slopes of these relations are steeper than those
determined from the 1.4 GHz radio powers. However, considering the
uncertainties this is not a statistically significant result.
