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.
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