Optimum target for bremsstrah lunge mission is defined as the thickness of the target material, which produces maximum bremsstrah lung yield, on interaction of electron with the target. The bremsstrah lung dose rate per unit electron beam power at a distance of 1m from the target material gives the optimum target source term. In the present work, simulations were performed for three different electron energies, 450, 1000 and 2500 MeV using EGSnrc Monte-Carlo code to determine the optimum thickness. An empirical  relation for optimum target as a function of electron energy and atomic number of the target materials is found out from results. Using the simulated optimum target thickness, experiments are conducted to determine the optimum target source term. For the experimental determination, two available electron energies, 450MeV and 550MeV from booster synchrotron of Indus facility is used. The optimum target source term for these two energies are also simulated. The experimental and simulated source term are found to be in very good agreement within ±3%. Based on the agreement of the simulated source term with the experiment also urcetermat 450MeV and 550MeV, the same simulation methodology is used to simulate optimum target source term up to 2500MeV. The paper describes the simulations and experiments carried out onoptimum target brems strahlung source term and the results obtained.