First Observation of Electron Scattering from Online-Produced Radioactive Target

We successfully performed electron scattering off unstable nuclei which were produced online from the photofission of uranium. The target ${}^{137}\mathrm{Cs}$ ions were trapped with a new target-forming technique that makes a high-density stationary target from a small number of ions by confining them in an electron storage ring. After developments of target generation and transportation systems and the beam stacking method to increase the ion beam intensity up to approximately $2\times {10}^7$ ions per pulse beam, an average luminosity of $0.9\times {10}^{26}{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}$ was achieved for ${}^{137}\mathrm{Cs}$. The obtained angular distribution of elastically scattered electrons is consistent with a calculation. This success marks the realization of the anticipated femtoscope which clarifies the structures of exotic and short-lived unstable nuclei.

Figure 1

Floor plan of the SCRIT electron scattering facility. The facility consists of a 150-MeV microtron, an electron storage ring equipped with the SCRIT system, an RI generator with ISOL and transportation system, a scattered electron spectrometer, and a luminosity monitor. (a) shows the conceptual design of the SCRIT method.

Figure 2

Achieved extraction rates of the cesium and barium isotopes. ALTO data [21] are also drawn as a reference. The mass number dependence among cesium isotopes was measured using a Faraday cup, and the absolute value was determined by measuring the gamma ray from ${}^{140}\mathrm{Cs}$. The ALTO data are normalized to the amount of ${}^{140}\mathrm{Cs}$ at the SCRIT. The amount of ${}^{137}\mathrm{Ba}$ contamination in the ${}^{137}\mathrm{Cs}$ beam was calculated to be less than 0.5% from the amount of ${}^{139}\mathrm{Ba}$, which is low, and only the upper end of the error bar is shown.

Figure 3

Reconstructed reaction vertex and momentum distributions. (a) and (c) show the vertex distributions along the beam and vertical directions with and without ${}^{137}\mathrm{Cs}$ ions (red and blue histograms, respectively). (b) shows the two-dimensional vertex distribution with ${}^{137}\mathrm{Cs}$ ions. (d) shows the momentum distribution after background subtraction. The black histogram is a Monte Carlo simulation based on the geant4 toolkit [22].

Figure 4

Obtained angular distribution for ${}^{137}\mathrm{Cs}$ and residual gas. Different slopes of the angular dependences for ${}^{137}\mathrm{Cs}$ and residual gas imply the radius difference of the target nuclei. Two lines are derived from calculations using the drepha code [23], assuming the two-parameter Fermi distribution for the nuclear charge density distributions.