1. INTRODUCTION
The interaction laser beams or of multi-charged heavy ions with solid targets has drawn considerable interest, not only from fundamental physics, but also from many applications such as material modification, X-ray source devices, ion-wall interaction in magnet confinement fusion, heavy ion driven plasma, fusion energy research, and other areas as well (Bock et al., Reference Bock, Hoffmann, Hofmann and Logan2005; Hoffmann et al., Reference Hoffmann, Blazevic, Ni, Rosmej, Roth, Tahir, Tauschwitz, Udrea, Varentsov, Weyrich and Maron2005, Reference Hoffmann, Blazevic, Korostiy, Ni, Pikuz, Rosmej, Roth, Tahir, Udrea, Varentsov, Weyrich, Sharkov and Maron2007; Tahir et al., Reference Tahir, Deutsch, Fortov, Gryaznov, Hoffmann, Kulish, Lomonosov, Mintsev, Ni, Nikolaev, Piriz, Shilkin, Spiller, Shutov, Temporal, Ternovoi, Udrea and Varentsov2005; Piriz et al., Reference Piriz, Cela, Serena Moreno, Tahir and Hoffmann2006; Logan et al., Reference Logan, Bieniosek, Celata, Henestroza, Kwan, Lee, Leitner, Prost, Roy, Seidl, Eylon, Vay, Waldron, Yu, Barnard, Callahan, Cohen, Friedman, Grote, Covo, Meier, Molvik, Lund, Davidson, Efthimion, Gilson, Grisham, Kaganovich, Qin, Startsev, Rose, Welch, Olson, Kishek, O'Shea and Haber2005; Sharkov, Reference Sharkov2001; Golubev, Reference Golubev, Turtikov, Fertman, Roudskoy, Sharkov, Geissel, Neuner, Roth, Tauschwitz, Wahl, Hoffmann, Funk, Suss and Jacoby2001; Sigmund, 1969; Pikuz et al., Reference Pikuz, Chefonov, Gasilov, Komarov, Ovchinnikov, Skobelev, Ashitkov, Agranat, Zigler and Faenov2010; Xin et al., Reference Xin, Zhu and Lei2010; Zavestovskaya, Reference Zavestovskaya2010). Accelerator physics and technology has made remarkable progress during recent years. Laser accelerated particle beams as well as conventional laser plasmas that are used as powerful ion sources have contributed to this development (Adonin et al., Reference Adonin, Turtikov, Ulrich, Jacoby, Hoffmann and Wieser2009; Renk et al., Reference Renk, Mann and Torres2008; Ter-Avetisyan et al., Reference Ter-Avetisyan, Schnurer, Polster, Nickles and Sandner2008). However, there still remains the necessity to investigate the details of the charge state effect on ion beam energy deposition in targets. Charge exchange effects in slow collisions of highly charged ions interacting with a surface still need to be explored in more detail.
Highly charged heavy ions are efficient carriers of energy, due to their kinetic energy and potential energy due to the high ionization state as well. When such a highly charged ion approaches a metallic surface at low speed (close to the Bohr velocity, 2.9 × 106 m/s), there is a high probability for electron capture processes, due to the high dynamic and static electric field of the ion that interacts with a surface constituent atom. Capture processes are most probable to populate high Rydberg states at a large distance to the surface. This is followed by a complex cascade of processes including recombination, Coulomb ionization, photon emission, Auger electron, and fast electron emission, when the ion crosses into the surface. At the same time most of the huge potential energy will be deposited into a small volume (of nanometer scale in diameter) close to the surface (Winter et al., Reference Winter, Eder and Aumayr1999; Schenkel et al., Reference Schenkel, Hamza, Barnes, Schneider, Banks and Doyle1998; Burgdörfer et al., Reference Burgdorfer, Lerner and Meyer1991). The physics mechanism is still unclear and the X-ray spectroscopy can provide important information to understand the dynamic of these processes. For example, Briand et al. (1990, 1996) used X-ray spectroscopy with high energy resolution to investigate the decay of the very slow and highly charged argon ions approaching a surface, and Rosmej (Reference Rosmej, Blazevic, Korostiy, Bock, Hoffmann, Pikuz, Efremov, Fortov, Fertman, Mutin, Pikuz and & Faenov2005a, Reference Rosmej, Pikuz, Korostiy, Blazevic, Brambrink, Fertman, Mutin, Efremov, Pikuz, Faenov, Loboda, Golubev and Hoffmann2005b) and colleagues (Rzadkiewicz et al., Reference Rzadkiewicz, Gojska, Rosmej, Polasik and Slabkowska2010) recently measured the X-ray emission with spatial resolution to analyze the stopping and ionization processes, and charge state evolution of swift heavy ion in dense matter.
In this paper, we present some recent experimental results on X-ray measurements originating from 129+Xe30+ ions beams impacting on an Au surface, the projectile initial kinetic energy ranged from 350 keV to 600 keV. Primary emphasis will be given to the investigation of projectile X-ray emission. The initial kinetic energy dependence of the X-ray yield will be discussed.
2. MAIN PERFORMANCE OF THE ECR ION SOURCES AT HIRFL
As shown in Figure 1, the Heavy Ion Research Facility in Lanzhou (HIRFL) is an accelerator complex consisting of heavy ion cyclotrons and cooling storage rings. The research activities at HIRFL cover a wide-range including nuclear physics, astrophysics, atomic physics, plasma physics, hadron physics and the applications of heavy ion beams in material science, heavy ion cancer therapy, and so on (Xu, Reference Xu2009, Reference Xu, Zheng, Xiao, Zhan, Zhou, Zhang, Sun, Wang, Gan, Huang and Ma2010; Zhao et al., Reference Zhao, Xiao, Xu, Zhao, Xia, Jin, Ma, Liu, Yang, Zhang, Wang, Li, Zhao, Zhan, Xu, Zhao, Li and Chen2009).
Fig. 1. The layout of the HIRFL complex. SFC: Sector Focusing Cyclotron (K = 69, ion beams with energy up to 10 MeV/u); SSC: Separated Sector Cyclotron (K = 450, ion beams with energy up to 100 MeV/u); CSRm: the main ring of CSR (Bρ < 12.1 T × m); CSRe: the experimental ring of CSR (Bρ < 9.4 T ×m).
In order to enhance HIRFL performance of beam intensity and energy, two electron cyclotron resonance (ECR) ion sources, LECR3 and SECRAL, were built during the past decade (Zhao et al., Reference Zhao, Zhang, He, Zhang, Guo, Cao, Yuan, Sun, Ma, Song, Zhan, Wei and Xie2004; Sun et al., Reference Sun, Zhao, Lu, Zhang, Feng, Li, Cao, Guo, Ma, Zhao, Shang, Ma, Wang, Li, Jin and Xie2010). LECR3 is the third ECR ion source at Lanzhou, which was commissioned in 2003. Very highly charged ion beams such as Ar18+ and Xe30+ were produced by this ion source. However, the intensity of such highly charged ion beam was quite limited, for instance, the beam current for Ar17+ and Xe30+ beam was much less than 1 µA.
After the completion of the SECRAL (a fully superconducting compact ECR ion source at Lanzhou), the beam intensity was increased by a factor of more than 30 as compared to the intensity of the same beam from LECR3. Figure 2 shows the spectrum of the highly charged Xe beams from SECRAL (Sun et al., Reference Sun, Zhao, Lu, Zhang, Feng, Li, Cao, Guo, Ma, Zhao, Shang, Ma, Wang, Li, Jin and Xie2010).
Fig. 2. (Color online) Optimized spectrum of highly charged xenon ion beams from SECRAL (the numbers upon the peaks are standing for the charge state of the xenon beams).
3. EXPERIMENTAL SETUP
The experiment was performed at LECR3. Highly charged Xe ion beams impacted onto a clean and pure (99.99%) Au target with an incident angle of 45° to the surface. The target area was about 19 × 24 mm that is much larger than the beam spot, which is 5 × 5 mm. The vacuum in the chamber was on the order of 10−8 mbar.
In this work, the X-rays were measured by a Si(Li) detector, which observed the target at an angle of 90 . Between the detector and the target was a beryllium vacuum window with thickness of 0.05 mm. The Si(Li) detector used in this experiment had an effective energy range of 1–60 keV and the energy resolution was 190 eV at 5.9 keV. It was calibrated with standard radioactive sources 241Am and 55Fe, for details please see our formal publication (Zhao et al., 2007). The geometrical solid angle was about 0.013 ± 0.001 sr (~ 0.1% of 4π). The spectrometer efficiency and the ion beam current were determined so that absolute values for the X-ray yield could be measured. Assuming that the X-ray emission is isotropic and taking the solid angle into account, the X-ray yield per ion can be given as
where C is the total X-ray yield, N is the total projectile ion number, q is the projectile charge state, I beam is the beam current (unit: nA), ω [keV] is the full width at half maximum of the spectral line, A is the peak value.
4. RESULTS AND DISCUSSION
The spectra shown in Figures 3 and 4 were taken during the experimental run 129Xe30+ impacting on the Au surface. The initial kinetic energy was 350 keV and 600 keV, respectively. Two lines can be identified in the spectra. The line at 1.65 keV has been identified as the characteristic X-ray M-shell radiation. It is observed with other targets in the same condition as well. The line at 2.15 keV has unambiguously been identified as Au Mα radiation (McMaster et al., 1969).
Fig. 3. (Color online) X-ray spectra for Xe30+ ions with kinetic energy 350 keV impacting on Au target.
Fig. 4. (Color online) X-ray spectra for Xe30+ ions with kinetic energy 600 keV impacting on Au target.
However, according to the same publication (McMaster et al., 1969), the energy of the Xe Mα X-rays should not extend beyond 1 keV, and should therefore be out of the sensitive range of our detector. We proposed earlier, that this line may possibly be induced by two-electron-one-photon (TEOP) transition of Xe ions. One may get details from our former publication [Zhao et al., 2007]. The probability for such TEOP transition is very low (Salem et al., 1984; Zou et al., 2003). But here we observed a very high X-ray yield of about 10−7 per injected ion. Therefore, we propose a different mechanism where radiation originates from the high Rydberg state of the Xe ions.
It is well-known that, a highly charged ion can capture many electrons from the metallic target into high Rydberg states at a very large distance. According to the classical over-barrier model, this critical distance R c from the ion to the surface can be given as
where q is the projectile charge state and W is the work function of the metal target; and the captured electrons will be located in the state with primary quantum number
Therefore in the present experiments, the Xe ions will capture the electrons into the level of n near to 10, which has a very low binding energy, about 10–100 eV. If we take into account that the inner most vacancy of the Xe30+ has a binding energy of about 1.71 keV, the X-rays with energy about 1.65 keV can be expected, if the transition from such Rydberg state takes place.
Furthermore, as shown in Figure 5, the yield of such X-rays is decreasing with the increasing of the initial projectile energy. It can be explained with this mode as well: Since the critical capture distance is the same, the ions with lower approaching velocity will have more time to decay through such Rydberg transition than the ions with higher velocity.
Fig. 5. (Color online) The X-ray yields per ion vs. the projectile initial kinetic energy.
5. CONCLUSION AND REMARKS
Recent experimental results on X-ray measurement during the 129+Xe30+ ions beams impacting on an Au surface are reported. The characteristic X-rays of the Xe ions with energy about 1.65 keV were observed. Based on the classical over-barrier model, the characteristic X-rays were proposed to be induced by the decay of very high Rydberg state of the Xe ions. The decreasing of this characteristic X-ray yield with the increasing of the projectile initial kinetic energy matches the model as well.
ACKNOWLEDGMENTS
We are in debt to the workers in ECRIS, Mr. Jinyu Li, Dr. Zimin Zhang, and Dr. Liangting Sun. The authors like to acknowledge the support from various funding agencies: The work was supported by the NSFC project in China (Grant Nos.11075135, 11075192, 11075125), the Major State Basic Research Development Program of China (Grant No. 2010CB832902), the Natural Science foundation of Shaanxi Province (Grant No. 2010JM1012), and partially supported by the WTZ project (Grant No. 1390).
