Tell me Is it possible that a nucleus has negative mass defect   

If the nucleus has a mass defect it is likely that the strong force and the weak force have sustained a main decrease in equilibrium. This can cause the positive and negative charges to reverse and alter energy levels. Such a phenomenon has been described by Einstein in his paper on the speed of light and time deduction. You can check this with the use of an electron microscope to verify is the color spectrum had changed drastically. If so, then you may have a problem.