Nuclear transitions induced by atomic excitations

In the two-step pumping scheme for a gamma-ray laser, an essential step is that of exciting the nucleus from a long-lived storage isomer to a nearby short-lived state that then decays to the upper lasing level. An experiment is in progress to induce this transfer by first exciting the atomic electrons with uv photons. The incident photons couple well to the electrons, which then couple via a virtual photon to the nucleus. As a test case, excitation of the 235U nucleus is being sought, using a high-brightness uv laser. The excited nuclear state, having a 26-minute half-life, decays by internal converison, resulting in emission of an atomic electron. A pulsed infrared laser produces an atomic beam of 235U which is then bombarded by the uv laser beam. Ions are collected, and conversion electrons are detected by a channel elctron multiplier. In preliminary experiments, an upper limit of 7 x 10-5 has been obtained for the probability of exciting a 235U atom in the uv beam for one picosecond at an intensity of about 1015W/cm2. Experiments with higher sensitivities and at higher uv beam intensities are underway. 6 refs., 1 fig