The nuclear lighthouse effect has been applied to samarium-149

The nuclear lighthouse effect has been applied to samarium-149. The NLE technique was developed last year by researchers from the University of Rostock in Germany. It allows physicists to get very accurate lifetime measurements of certain short-lived nuclear resonances. In their recent work, the Rostock scientists mounted a thin sheet of ¹⁴⁹Sm₂O₃ on the inside wall of a small cylinder. They then placed the cylinder in an x-ray beam at the Advanced Photon Source at Argonne National Laboratory and spun it at 15 kHz with jets of pressurized air. The nonresonant x rays went straight through the rotor, while those that were absorbed by the nuclei were reemitted after some slight delay. That delay provided enough time for the cylinder to rotate a few milliradians, and the forward-scattered resonant x rays were thus deflected into a detector. The group detected a resonance energy of 22.496 keV with a natural lifetime of 10.3 ns. Samarium is an important material for new permanent magnets but, like some other rare earths, is difficult to study with conventional methods (such as Mössbauer spectroscopy) for a variety of reasons. The physicists say that NLE is capable of resolving subpicosecond lifetimes, which are currently beyond the limits of x-ray detection. (R. Röhlsberger et al., Phys. Rev. Lett. 87 , 047601, 2001 http://dx.doi.org/10.1103/PhysRevLett.87.047601 .)