Femtosecond Pumping of Nuclear Isomeric States by the Coulomb Collision of Ions with Quivering Electrons

Efficient production of nuclear isomers is critical for pioneering applications, like nuclear clocks, nuclear batteries, clean nuclear energy, and nuclear $\gamma$-ray lasers. However, due to small production cross sections and quick decays, it is extremely difficult to acquire a significant amount of isomers with short lifetimes via traditional accelerators or reactors because of low beam intensity. Here, for the first time, we experimentally present femtosecond pumping of nuclear isomeric states by the Coulomb excitation of ions with the quivering electrons induced by laser fields. Nuclei populated on the third excited state of ${}^{83}\mathrm{Kr}$ are generated with a peak efficiency of $2.34\times {10}^{15}\text{particles}/\mathrm{s}$ from a tabletop hundred-TW laser system. It can be explained by the Coulomb excitation of ions with the quivering electrons during the interaction between laser pulses and clusters at nearly solid densities. This efficient and universal production method can be widely used for pumping isotopes with excited state lifetimes down to picoseconds, and could be a benefit for fields like nuclear transition mechanisms and nuclear $\gamma$-ray lasers.

Figure 1

The schematic experimental setup. The femtosecond laser pulses are focused on the Kr nanoparticles ejected from a high-pressure Kr gas nozzle [27, 28]. A Faraday cup (FC) is used to measure the ion spectrum. A probe laser pulse is aligned through the gas-plasma during the interaction to provide interferograms [29]. After the interaction between the laser and clusters, the gas is pumped and frozen on surfaces of the liquid-nitrogen cold trap. The possible radioactive decays are recorded by a NaI detector under the trap. The right inset shows the decay scheme of ${}^{83}\mathrm{Kr}$. The $\mathrm{J}\pi$, energies, and half lifetimes of the ground state (0), the first (9.4 keV), second (41.6 keV), and third (562.5 keV) excited states are listed.

Figure 2

Experimental electron beam results. (a) Electron beam angular distribution. The laser pulses with $p$ polarization propagate at the 0° direction. Electron beams are recorded on an image plate which is covered by a $15\mu \mathrm{m}$ aluminum foil. (b) Electron spectrum measured at 90° direction for twenty cumulative shots and recorded on an image plate.

Figure 3

Detected ${}^{83}\mathrm{Kr}$ isomeric state and decay characteristics. (a) A typical energy spectrum measured by the NaI detector. The blue square markers represent the data measured while the red cross the background. The red solid line is the fit with sum of the decay (9.4 keV, $\mathrm{BR}=5.5\%$, black dash line), $K_{\alpha }$ (12.6 keV, $\mathrm{BR}=13.8\%$, black dot line), and $K_{\beta }$ (14.1 keV, $\mathrm{BR}=2.1\%$, black dot-dashed line), according to the NNDC database [39]. (b) The decay time spectrum. Blue squares represent the average values of three sets of experimental data, and the error bar denotes the maximum and minimum values. The red solid line represents a fit with an exponential function. The fitted half-life is $T_{1/2}^{\exp }=(1.80\pm 0.05)\mathrm{h}$.

Figure 4

Particle-in-cell simulation of nuclear Coulomb excitation in interaction of laser pulses with cluster. (a),(d) Laser driven electrons from a cluster at four different times ($t=6.75\mathrm{fs}$, 8 fs, and $t=30\mathrm{fs}$, 31.25 fs correspond to the process of linear resonance [35, 36] and nonlinear resonance [37, 38], respectively), where the arrow represents electron’s motion direction and the color shows electron’s energy. (e) The average density evolutions of electron and krypton ions in cluster region, and the insets are the density distributions at $t=60\mathrm{fs}$, the $n_c$ is critical density equaling to $1.74\times {10}^{21}{\mathrm{cm}}^{-3}$, the red dashed cycles in the insets represent the original cluster region. (f) The energy evolution of electrons in cluster region. (g) The excitation number evolution for three different energy states, and the inset is total excitation number in the laser irradiation region. The light red shaded areas in (e)–(g) represent the temporal window of laser-cluster interaction.