72 Hf is another reasonably stable nuclear isomer. It possesses a half-life of 31 years and the highest excitation energy of any comparably long-lived isomer. One gram of pure Hf contains approximately 1.33 gigajoules of energy, the equivalent of exploding about 315 kg (700 lb) of TNT. In the natural decay of Hf, the energy is released as gamma rays with a total energy of 2.45 MeV. As with Ta, there are disputed reports that Hf can be stimulated into releasing its energy. Due to this, the substance is being studied as a possible source for gamma-ray lasers. These reports indicate that the energy is released very quickly, so that Hf can produce extremely high powers (on the order of exawatts). Other isomers have also been investigated as possible media for gamma-ray stimulated emission.