Nuclear resonant scattering experiment with fast time response: Photonuclear excitation of ${}^{201}\mathrm{Hg}$

Nuclear resonant excitation and detection of its decay signal for the 26.27-keV level of ${}^{201}\mathrm{Hg}$ is demonstrated with high-brilliance synchrotron radiation (SR) and a fast x-ray detector system. This SR-based photonuclear excitation scheme, known as nuclear resonant scattering (NRS) in the field of materials science, is also useful for investigating nuclear properties, such as the half-lives and radiative widths of excited nuclear levels. To date, because of the limited time response of the x-ray detector, the nuclear levels to which this method could be applied have been limited to the one whose half-lives are longer than $\sim 1$ ns. The faster time response of the NRS measurement makes possible NRS experiments on nuclear levels with much shorter half-lives. We have fabricated an x-ray detector system that has a time resolution of 56 ps and a shorter tail function than that reported previously. With the implemented detector system, the NRS signal of the 26.27-keV state of ${}^{201}\mathrm{Hg}$ could be clearly discriminated from the electronic scattering signal at an elapsed time of 1 ns after the SR pulse. The half-life of the state was determined as 629 $\pm$ 18 ps, which has better precision by a factor of three compared with that reported to date obtained from nuclear decay spectroscopy.

Figure 1

Lower energy-level structure of ${}^{201}\mathrm{Hg}$ and the decay scheme from the excited state. The relative decay intensity $I_d$ is normalized for ${10}^2$ decays from the excited state.

Figure 2

Experimental setup for NRS of ${}^{201}\mathrm{Hg}$ nucleus.

Figure 3

Fluorescence energy spectrum from HgS target with 26.27-keV x-ray irradiation. The small peaks around 8 keV are characteristic x rays of Cu and Zn scattered from the brass around the target.

Figure 4

(a) Typical time spectrum of fluorescence signal from HgS target with incident x-ray energy tuned to nuclear excitation of ${}^{201}\mathrm{Hg}$. (b) Time spectrum with incident x-ray energy detuned from resonance. Integration time was 2000 s for each spectrum.

Figure 5

NRS count as a function of incident x-ray energy controlled by scanning the Si monochromators. The width was found to be $148\pm 22$ meV (FWHM).