  In order to understand the performance of the PARIS (Photon Array for the
studies with Radioactive Ion and Stable beams) detector, detailed
characterization of two individual phoswich (LaBr$_3$(Ce)-NaI(Tl)) elements has
been carried out. The detector response is investigated over a wide range of
$E_{\gamma}$ = 0.6 to 22.6 MeV using radioactive sources and employing
$^{11}B(p,\gamma)$ reaction at $E_p$ = 163 keV and $E_p$ = 7.2 MeV. The
linearity of energy response of the LaBr$_3$(Ce) detector is tested upto 22.6
MeV using three different voltage dividers. The data acquisition system using
CAEN digitizers is set up and optimized to get the best energy and time
resolution. The energy resolution of $\sim$ 2.1% at $E_\gamma$ = 22.6~MeV is
measured for the configuration giving best linearity upto high energy. Time
resolution of the phoswich detector is measured with a $^{60}$Co source after
implementing CFD algorithm for the digitized pulses and is found to be
excellent (FWHM $\sim$ 315~ps). In order to study the effect of count rate on
detectors, the centroid position and width of the $E_{\gamma}$ = 835~keV peak
were measured upto 220 kHz count rate. The measured efficiency data with
radioactive sources are in good agreement with GEANT4 based simulations. The
total energy spectrum after the add-back of energy signals in phoswich
components is also presented.
