Novel Internal Target for Electron Scattering off Unstable Nuclei

A novel internal target has been developed, which will make electron scattering off short-lived radioactive nuclei possible in an electron storage ring. An “ion trapping” phenomenon in the electron storage ring was successfully utilized for the first time to form the target for electron scattering. Approximately $7\times {10}^6$ stable ${}^{133}\mathrm{Cs}$ ions were trapped along the electron beam axis for 85 ms at an electron beam current of 80 mA. The collision luminosity between the stored electrons and trapped Cs ions was determined to be $2.4(8)\times {10}^{25}{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}$ by measuring elastically scattered electrons.

Figure 1

Prototype of the SCRIT and the experimental setup. Typical form of the mirror potential is shown schematically.

Figure 2

Typical spectra measured by the analyzer for trapping times of 10, 50, and 100 ms at 75 mA. They are separated into three parts: the spectra (a) with Cs-ion injection, (b) without Cs ions, and (c) the difference (a)–(b). Residual gas ions are expected as: $\mathrm{CO}_2{}^{+}$ at channeltron No. 4, ${\mathrm{CO}}^{+}$ at No. 5, and ${\mathrm{H}}_2{\mathrm{O}}^{+}$, ${\mathrm{OH}}^{+}$, $\mathrm{CH}_4{}^{+}$, ${\mathrm{O}}^{+}$, ${\mathrm{C}}^{+}$ at No. 6.

Figure 3

(a) Time evolution of the number of trapped Cs ions together with fitted decay curves and (b) current dependencies of the fitting parameters $N_i$ and ${\tau }_{\mathrm{life}}$.

Figure 4

Calculations reproducing the time evolution of trapped ${\mathrm{Cs}}^{1+}$ (dotted curve), ${\mathrm{C}}^{2+}+{\mathrm{Cs}}^{3+}$ (dash-dotted curve) ions, and the estimated luminosity (solid curve) at 75 mA.

Figure 5

(a) Vertex distribution of scattered electrons and (b) the energy spectrum. Solid (dotted) line in both figures indicates the data with (without) Cs-ion trapping. In (b), closed circles show subtracted spectrum and the dash-dotted line is the response of the calorimeter to 120 MeV electrons reproduced by a GEANT simulation.