Stopping Power in Insulators and Metals without Charge Exchange

The slowing-down process of pointlike charged particles in matter has been investigated by measuring the stopping power for antiprotons in materials of qualitatively very different nature. Whereas the velocity-proportional stopping power observed for metal-like targets such as aluminum over a wide energy range of 1–50 keV is in agreement with expectations, it is surprising that the same velocity dependence is seen for a large band-gap insulator such as LiF. The validity of these observations is supported by several measurements with protons and several checks of the target properties. The observations call for both a qualitative explanation and a quantitative theoretical model.

Figure 1

Schematic diagram of the stopping power experiment. The target foil is insulated and can be raised to a bias voltage in order to vary the projectile impact energy.

Figure 2

Voltage scan of the second deflector showing the energy-loss peaks from C and $\mathrm{C}+\mathrm{L}\mathrm{i}\mathrm{F}$ and the curve shows a fit of the data with two Gaussians. The collision energy is 33 keV due to a bias applied to the foil. The scatter of the points is mainly due to pulse-to-pulse variations of the AD.

Figure 3

Measured antiproton (▲, ▼, both $400Å$ foils; ● $200Å$) and proton (△ $200Å$; ▽ $400Å$) stopping powers of LiF. The squares are the proton and deuteron measurements from 14 and ◆ are from 25. See the text for an explanation of the curves.

Figure 4

Measured stopping powers for antiprotons in LiF (▲, ▼, ●) and Al (■) and for protons in LiF (△) as a function of velocity relative to the Bohr velocity. The lines represent fits to the data.

Figure 5

Measured antiproton (■) and proton (△, □) stopping powers of Al (this work). The data ◆ are from 6. See the text for an explanation of the curves.