The optimum collimator design in terms of aperture and thickness for accomplishing improved performance for wide energy radiation field has been investigated in the present paper. A 2"x2" NaI(Tl) detector was used as sensor in proposed Hot Spot Identification system. The spectral response of the detector with various collimator apertures and thickness was developed using Monte Carlo methods for optimizing the collimator design. The optimum size of collimator aperture is found to be 2 mm and thickness of 5 cm. Studies were also carried out to optimize lead shield thickness around the detector.