Charge stripping of ${}^{238}\mathrm{U}$ ion beam by helium gas stripper

Development of a nondestructive, efficient electric-charge-stripping method is a key requirement for next-generation high-intensity heavy-ion accelerators such as the RIKEN Radioactive-Isotope Beam Factory. A charge stripper employing a low-$Z$ gas is an important candidate applicable to high-intensity uranium beams for replacing carbon-foil strippers. In this study, a high-beam-transmission charge-stripping system employing helium gas for ${}^{238}\mathrm{U}$ beams injected at $10.8\mathrm{MeV}/\mathrm{u}$ was developed and demonstrated for the first time. The charge-state evolution measured using helium in a thickness range of $0.24–1.83\mathrm{mg}/{\mathrm{cm}}^2$ is compared with theoretical predictions. Energy attenuation and energy spread due to the helium stripper are also investigated.

Figure 1

Uranium beam acceleration scheme at the RIBF. ${\mathrm{U}}^{35+}$ beams extracted from the 28-GHz ECRIS are accelerated to $0.68\mathrm{MeV}/\mathrm{u}$ using RILAC2 and are then fed into the RRC. The beams are accelerated to $10.75\mathrm{MeV}/\mathrm{u}$ in the RRC and then go through the first carbon-foil stripper ($0.3\mathrm{mg}/{\mathrm{cm}}^2$). A stripping efficiency of approximately 17% can be reached by selecting the charge state ${\mathrm{U}}^{71+}$. After the acceleration to $50\mathrm{MeV}/\mathrm{u}$ using the fRC, the charge state of the beams is converted using a second thick carbon-foil stripper ($17\mathrm{mg}/{\mathrm{cm}}^2$). The selected charge state, ${\mathrm{U}}^{86+}$, is obtained with an efficiency of approximately 27%. The ${\mathrm{U}}^{86+}$ beams are subsequently accelerated to an energy of $345\mathrm{MeV}/\mathrm{u}$ using the IRC and SRC. Finally, the beams are delivered to the in-flight RI beam generator, BigRIPS [10].

Figure 2

Cross-sectional view of the 0.5-m charge-stripping system. The system used here consists of two three-stage differential-pumping systems. Flow-disturbing plates are furnished to decouple the gas flows between stages 1 and 2. Beam current detectors with four segmental baffles, BF1–BF2, are attached at the beam entrance of the apertures to monitor the beam loss. The gas inlet line is connected to the gas-handling system. In addition, the target chamber is equipped with a foil changer for the carbon-foil stripper.

Figure 3

Required flow rate as a function of the helium thickness for both 0.5- and 8-m target systems.

Figure 4

Schematic view of the entire experimental setup. The RRC accelerates ${\mathrm{U}}^{35+}$ beams coming from RILAC2 to an energy of $10.8\mathrm{MeV}/\mathrm{u}$. The emitted beams of around 150 enA are fed into the He gas stripper. The charge-state distribution of the passing beams is analyzed using a dipole magnet, DMM1, and a Faraday cup, FC-M11. Plastic scintillators (SC-H12, SC-H16, and SC-K51) are used to measure the beam’s kinetic energy and longitudinal distribution. Many quadrupole magnets (QS*: singlet; QD*: doublet; QT*: triplet) were used in the beam transport system to optimize the beam trajectory.

Figure 5

Transmission efficiency of a selected charge state as a function of the helium thickness. The most probable charge state for each thickness was selected and transported for the measurements.

Figure 6

Results of Gaussian fitting to the charge-state distribution for He.

Figure 7

Measured and calculated charge-state evolution for He, Ne, and Ar [20]. The maximum mean charge state for He of around 65+ was obtained in both the 0.5-m and 8-m target systems. The calculation was performed using the charge-changing cross sections ${\sigma }_{\mathrm{BEM}}$ [30] and ${\sigma }_{\mathrm{Sch}}$ [31] for ${}^{238}\mathrm{U}$, while accounting for energy attenuation in the gas stripper. In addition, the calculation with scaled cross sections is shown.

Figure 8

Fitting results for the charge-state distribution in He gas strippers of various thicknesses. Solid lines indicate calculated probabilities, $F_{\xi }^{\mathrm{cal}}(q_i)$.

Figure 9

Dependence of the charge-state distribution width $w_q$ on the target thickness.

Figure 10

Obtained energy spread for He and carbon-foil strippers as a function of energy deposit. Systematic errors due to ${\tau }_w^{K51}(\xi =0)$ ($\sim 5\%$) are excluded from the plots. Solid curves are based on calculations using the Monte Carlo method and the scaled charge-changing cross sections and energy-loss cross sections from the CasP code. The calculated energy spread for the point charges (dashed lines) and the calculation including only collisional energy spread (dotted lines) are also shown.

Figure 11

Calculated dependence of the mean charge on the initial energy of ${}^{238}\mathrm{U}^{35+}$ for several gas thicknesses. If we could prepare a $2\mathrm{\text{−}}\mathrm{mg}/{\mathrm{cm}}^2$ He gas stripper, the optimum injection energy of the uranium beams, which would reach the highest charge state, would be around $20\mathrm{MeV}/\mathrm{u}$. This would yield a mean charge of approximately 75+.