7. Electron Emission from Foils: Application to the Beam Profile Monitor and Fast Detector

Yukio Sato and Daisuke Ohsawa

Keywords: secondary electron, foil, heavy ion, effec Live charge, stopping power

We precisely measured the secondary electronemission yield () from thin (1m thick) Al-, Ag and Au-foils by exposure to fully stripped 6 MeV/n heavy ions (H, He, C, N, O, Ne, Si and Ar), which were accelerated by the HIMAC injector linac. The dependence of the forward (_F) and backward (_B) yields on the projectile nuclear charge (z) showed a proportionality to the square of the effec Live charge (z_eff²) and an oscillatory behavior with atomic number z; the yields were comparatively low for exposures to He²⁺ and Ne¹⁰⁺ beams. The forward enhancement was significant for AT-foil (light metal) , depending on z; in contrast, it was small for Ag-and Au-foils (heavy metals). The accuracy of the 7 -values was evaluated by the determination of z_eff²(5%) and the surface reproducibility of the foil (2-3%). Thus, the overall error was about 6%

Figure 4 shows _F and _B from Al-foil scaled by z_eff² in order to consider their correlation to the stopping power (the well known A ) and to compare our data with that of other experiments. The z_eff^- values were calculated using Ziegler's empirical formula, in which the accuracy is 5% for z-numbers of 6-92. For proton (z=1) and helium (z=2), this kind of accuracy could be much better. The best-fit results were _Fz_eff^1.92 and _Bz_eff^1.78 The SE yield from foils was, to a first-order approximation, proportional to the stopping power; however the char acterustucs obvtously had an oscillation. The production mechanism of -electrons or the behav ior of two-center effects seemed to be related to this z-oscillation; the screening effects by target electrons in a continuum state may play an important role.

The ratio of _F/_B showed a large forward enhancement and its dependence on z, which suggested that many electrons initially ejected in the backward direction were pulled by the strong Coulomb field of a highly-charged projectile. For heavy metal (Ag and Au) foils, _F and _B were almost identical and the target dependence was small. These results showed that a sufficient relaxation of high-energy electrons occurred within dense materials, resulting in isotropic emission from the surface of heavy metal foils. Such secondary electrons (SE) are also applicable for a beam profile monitor and a fast detector, as briefly discussed in the following

Regarding sensitivity, the SE type monitor is compared with a conventional wire type. As can be seen from Fig.4, the forward SE yield from an Al-foil was around O.6z_eff²(6/E) per ion, where E is the projectile energy (MeV per nucleon) and z is the projectile charge. Further, the induced charge on a wire was basically z per ion, if ions were stopped within the wire. In the energy region around 6 MeV/n, the SE type was more sensitive than the wire by a factor of O.6z (3.6 for z=6), when z was roughly equal to z_eff. This meant that the SE type was particularly attractive for heavy ion beams. In addition, the energy loss within the foil was on the order of a few percent at this energy region; hence, this monitor can be used as a nearly non-destructive type.

Using this SE emission from foils, we observed a micro-bunch structure of the HIMAC linac beams with a rise time of Ins, which were accelerated at 100 MHz. The SE energy was generally lower than 20 eV and their traveling time between foils was of the order of 100 ps, which is comparable with that of the Pestov chamber. This fast-timing characteristic was also tested with a short-pulse obtained at the electron accelerator of the Nuclear Engineering Laboratory, University of Tokyo.

Fig.4. F and B per Zeff2 on Al vs. z with exposure to 6 MeV/n heavy ions (Fzeff1.92 and Bzeff1.78).Some other data (H1+, Li3+,C6+) are also plotted, and z-oscillation can be clearly seen (Borovsky et al., Nucl. Instrum. &Meth.,B36,377,1989 and B30,191,1988;Mironov et al., Sov. Ohys. JETP5, 188, 1957.)

Publications:
1)Sato, Y., et al.: Phys. Rev. A., 61, 0529011 0529016 2000.
2)I-Iigashi, A., et al.: Proc. 12thSympo. onAccel. and tech., 90-92, 1999.
3)Ohsawa, D., et al.: Proc. BEAMS98 91-96 1998.
4)Fujita, Y., et al.: Proc. EPAC98, 1503-1505, 1998.