Nuclear resonance spectroscopy of the 31-yr isomer of Hf-178

Induced release of the high energy densities stored in isomeric nuclear states may be important in the development of ultrashort wavelength lasers. Such a release could compensate the spontaneous power density radiated from the laser medium at threshold. The most promising candidate for such a role seems to be the 31-yr isomeric nucleus of Hf-178 that stores 1.3 GJ/g in the electromagnetic excitation of its constituent protons and neutrons. Successful studies of the induced release of energies from such isomeric states have required an extension of techniques for nuclear resonance spectroscopy using synchrotron radiation (SR) that had previously been applied only to ground state materials. In 2004, monochromatic X-rays from the SPring-8 SR source were used to identify one of the excited nuclear states that mediates the induced decay of the 31-yr isomer of Hf-178. That trigger level was found to lie at 2457.20(22) keV. It was excited when an isomeric nucleus absorbed an incident X-ray photon. We found that one branch of its subsequent decay consisted of a strong electromagnetic transition to the ground state of the nucleus. The energy of the gamma-photon emitted was equal to the energy of the trigger level. Proximity in energy of that level to the energy of 2446.06 keV stored by the isomer makes it easy to induce a release of the stored energy and encourages prospects for the development of a gamma ray laser.