  A large acceptance scintillator detector with wavelength shifting optical
fibre readout has been designed and built to detect the decay particles of
$\eta$-nucleus bound system (the so-called $\eta$-mesic nuclei), namely,
protons and pions. The detector, named as ENSTAR detector, consists of 122
pieces of plastic scintillator of various shapes and sizes, which are arranged
in a cylindrical geometry to provide particle identification, energy loss and
coarse position information for these particles. A solid angle coverage of
$\sim$95% of total 4$\pi$ is obtained in the present design of the detector.
Monte Carlo phase space calculations performed to simulate the formation and
decay of $\eta$-mesic nuclei suggest that its decay particles, the protons and
pions are emitted with an opening angle of 150$^\circ \pm 20^\circ$, and with
energies in the range of 25 to 300 MeV and 225 to 450 MeV respectively. The
detailed GEANT simulations show that $\sim$ 80 % of the decay particles
(protons and pions) can be detected within ENSTAR. Several test measurements
using alpha source, cosmic-ray muons etc. have been carried out to study the
response of ENSTAR scintillator pieces. The in-beam tests of fully assembled
detector with proton beam of momentum 870 MeV/c from the Cooler synchrotron
COSY have been performed. The test results show that the scintillator fiber
design chosen for the detector has performed satisfactorily well. The present
article describes the detector design, simulation studies, construction details
and test results.
