Measurements of charge state distributions of 0.74 and $1.4\mathrm{MeV}/\mathrm{u}$ heavy ions passing through dilute gases

In many modern heavy-ion accelerator facilities, gas strippers are used to increase the projectile charge state for improving the acceleration efficiency of ion beams to higher energies. For this application, the knowledge on the behavior of charge state distributions of heavy-ions after passing through dilute gases is of special interest. Charge state distributions of uranium (${}^{238}\mathrm{U}$), bismuth (${}^{209}\mathrm{Bi}$), titanium (${}^{50}\mathrm{Ti}$), and argon (${}^{40}\mathrm{Ar}$) ion beams with energies of $0.74\mathrm{MeV}/\mathrm{u}$ and $1.4\mathrm{MeV}/\mathrm{u}$ after passing through hydrogen (${\mathrm{H}}_2$), helium (He), carbon dioxide (${\mathrm{CO}}_2$), nitrogen (${\mathrm{N}}_2$), oxygen (${\mathrm{O}}_2$), neon (Ne), and argon (Ar) gases were measured. Gas stripper target thicknesses up to $100\mu \mathrm{g}/{\mathrm{cm}}^2$ were applied. The observed behavior of the charge state distributions, including their width and mean charge state, are discussed. The measurements show the highest equilibrium charge state at $1.4\mathrm{MeV}/\mathrm{u}$ for ${}^{238}\mathrm{U}$ on ${\mathrm{H}}_2$ gas of $29.2\pm 1.2$. Narrow charge state distributions are observed for ${}^{238}\mathrm{U}$ and ${}^{209}\mathrm{Bi}$ on ${\mathrm{H}}_2$ and He gas, which are highly beneficial, e.g., for the production of beams of high intensities in accelerators.

Figure 1

Schematic of the gas stripper section at the GSI UNILAC: The charge state separation is achieved using a system of three dipole magnets. The ion beams are horizontally focused through the stripper by two quadrupole doublets onto the second analysing slit (Slit 2) behind the first dipole magnet. The beam current can be measured by several beam transformers along the beam line. The beam energy is measured with time-of-flight measurements using phase probes.

Figure 2

Schematic cross-sectional views of the main gas-stripper section: (a) vertical and (b) horizontal. The gas cell for the stripping process is fed by two pulsed gas valves (red) positioned on top and on the bottom of the beam line in the main stripper chamber, which are supplied through the top flange. A four-stage differential pumping system is used, comprising a roots vacuum pump (RVP) below the main stripper chamber and four turbomolecular pumps (TPU) on the sides of the adjacent chambers. A piezoelectric pressure gauge is attached to the side of the gas cell to enable pressure measurements.

Figure 3

High-current grid measurements of an ${}^{238}\mathrm{U}$ beam before (top) and after (bottom) passing Slit 2 (see Fig. 1) in horizontal (left) and vertical (right) plane. The desired charge peak is selected by adjusting the width of the slit, as indicated in panel (a) [16].

Figure 4

(a) Energy-loss measurements for ${}^{50}\mathrm{Ti}$ on ${\mathrm{H}}_2$ gas at $1.4\mathrm{MeV}/\mathrm{u}$ as a function of the back-pressure, using both gas valves. The red line represents a linear function fitted to the measurement data. (b) Estimated thickness of the ${\mathrm{H}}_2$ gas target at $1.4\mathrm{MeV}/\mathrm{u}$ depending on the back-pressure on the gas valves. The different lines represent linear functions fitted to the measurement data with corresponding equal color. (c) Pressure measurement using the piezoelectric pressure gauge near the interaction zone of the stripper with ${\mathrm{H}}_2$ gas. The pressure increases linearly with increasing back-pressure on the valves. The red line represents a linear function fitted to the measurement data.

Figure 5

Charge state distributions of ${}^{238}\mathrm{U}$, ${}^{209}\mathrm{Bi}$, ${}^{50}\mathrm{Ti}$, and ${}^{40}\mathrm{Ar}$ ion beams at $1.4\mathrm{MeV}/\mathrm{u}$ after passing through ${\mathrm{H}}_2$ and He gas for various target thicknesses. The lines are added to guide the eye.

Figure 6

Measured charge state fractions for ${}^{238}\mathrm{U}$, ${}^{50}\mathrm{Ti}$, and ${}^{40}\mathrm{Ar}$ at $1.4\mathrm{MeV}/\mathrm{u}$ on ${\mathrm{H}}_2$ and He gas for increasing target thicknesses, corresponding to the charge state distributions shown in Fig. 5.

Figure 7

Charge state distributions for ${}^{238}\mathrm{U}$ on ${\mathrm{O}}_2$ (a), ${}^{209}\mathrm{Bi}$ on He (b), and Ar gas (c) as well as ${}^{50}\mathrm{Ti}$ on ${\mathrm{N}}_2$ gas (d) at $1.4\mathrm{MeV}/\mathrm{u}$ for different back-pressures on the valve. For (a) and (c) the fit is shown, which was used to obtain missing charge fractions, as described in the text. For (b) and (d) lines were added to guide the eye.

Figure 8

Saturated charge state distributions for ${}^{238}\mathrm{U}$ on ${\mathrm{N}}_2$, ${\mathrm{CO}}_2$, Ne, and Ar gas at $1.4\mathrm{MeV}/\mathrm{u}$. The saturated charge state distributions for ${}^{238}\mathrm{U}$ on ${\mathrm{H}}_2$ and He (see Fig. 5) are shown for comparison.

Figure 9

Measured charge state fractions for ${}^{238}\mathrm{U}$ on ${\mathrm{O}}_2$ gas at $1.4\mathrm{MeV}/\mathrm{u}$, corresponding to the charge state distributions shown in Fig. 7.

Figure 10

Saturated charge state distributions for ${}^{238}\mathrm{U}$ on ${\mathrm{H}}_2$, He, ${\mathrm{N}}_2$, Ne, and Ar gas (a) and ${}^{50}\mathrm{Ti}$ on ${\mathrm{H}}_2$, He, and ${\mathrm{N}}_2$ gas (b) at $0.74\mathrm{MeV}/\mathrm{u}$ beam energy.

Figure 11

Mean charge states $\overline{q}$ for ${}^{238}\mathrm{U}$ (a), ${}^{50}\mathrm{Ti}$ (b), and ${}^{40}\mathrm{Ar}$ (c) ion beams at $1.4\mathrm{MeV}/\mathrm{u}$. Exponential fits are added to the data points to guide the eye. Saturated mean charge states, measured for ion-gas combinations without thickness estimation, are added as solid lines.

Figure 12

Width $d$ of the charge state distributions of ${}^{238}\mathrm{U}$ (a), ${}^{50}\mathrm{Ti}$ (b), and ${}^{40}\mathrm{Ar}$ (c) ion beams on ${\mathrm{H}}_2$ and He gas at $1.4\mathrm{MeV}/\mathrm{u}$.

Figure 13

Measured data plotted as ratio of the mean charge state to the atomic charge of the projectile ion against the scaling factor $x$, as described in the text, and compared to estimations by the Schiwietz formula [37].