Coherent nuclear resonant scattering by ${}^{61}\mathrm{Ni}$ using the nuclear lighthouse effect

We have observed coherent nuclear resonant scattering of synchrotron radiation from the $67.41\text{−}\mathrm{keV}$ level of ${}^{61}\mathrm{Ni}$. The time evolution of the forward scattering signal was recorded by employing the nuclear lighthouse effect. This method is used to investigate Mössbauer isotopes in a coherent scattering process with synchrotron radiation at high transition energies. The decay of the excited ensemble of nuclei in Ni metal shows quantum beats that allowed the determination of the magnetic hyperfine field at the ${}^{61}\mathrm{Ni}$ nucleus. Moreover, we determined the lifetime of the $67.41\text{−}\mathrm{keV}$ level of ${}^{61}\mathrm{Ni}$ to be $7.4\left(1\right)\mathrm{ns}$.

Figure 1

(Color online) Experimental setup. The undulator radiation $U$ is monochromatized coarsely $(\delta E=9\mathrm{eV})$ by two Si (3 3 3) reflections $M$ and shaped by a slit system $S$. The rotor $R$ containing ${}^{61}\mathrm{Ni}$ is spinning at $\nu =8\mathrm{kHz}$. The noninteracting prompt direct beam (intensity $P$) hits the beam stop $B$. The prompt small-angle x-ray scattering SAXS is in competition with the delayed nuclear lighthouse effect signal NLE, thus demanding fast detection with an array of avalanche photo diodes $D$ and time-discriminating electronics $E$. The detector scans over a range of angles. Only for the nuclear delayed signal, angles are resolved proportional to the interaction time.

Figure 2

(Color online) The nuclear lighthouse effect with ${}^{61}\mathrm{Ni}$ at $180\mathrm{K}$. The detector signal was gated off during the first $7\mathrm{ns}$ to discriminate the prompt SAXS background $(\sim 10\mathrm{kHz})$. The SAXS is seen at early times when removing the gate temporarily for a short instant. The NLE data could be fitted by the theory of nuclear forward scattering using the program MOTIF (Ref. 33). The beat structure originates from the interference of the 12 transitions between the hyperfine-split ground and the $67.41\text{−}\mathrm{keV}$ excited states in the presence of a magnetic hyperfine field $B_{hf}$, as illustrated on the top right. Without hyperfine interactions, the magnetic sublevels would be degenerate.

Figure 3

(Color online)Time evolution of the fluorescence radiation of the $67.41\text{−}\mathrm{keV}$ ${}^{61}\mathrm{Ni}$ level. The setup to measure the nuclear fluorescence consists of two Si monochromator crystals $M$ on the (3 3 3) reflection, an absorber $A$ to reduce photon intensity transmitted via the Si (1 1 1) reflection, the ${}^{61}\mathrm{Ni}$ foil, and a fast detector $D$, with time-discriminating electronics $E$. An electronic gate blocks data aquisition during the first $7\mathrm{ns}$, i.e., especially the prompt photons.