1 Introduction

Pelletron accelerators are used in today’s materials science research, e.g. ion beam analysis, ion implantation, materials modification, radiation testing etc. [1, 2]. In a pelletron accelerator a negative ion beam is produced and extracted with some initial energy in keV range. This negative beam is then attracted by central terminal of pelletron tank, which is kept at a positive potential of several MV. When the beam reaches at the center of high voltage terminal, some electrons of the negative ions are stripped off to make them positive, and this positive beam is repelled by high positive potential. Stripping can either be done by foil stripper or by the introduction of a kind of gas at low pressure. Due to degradation of the foil stripper with time, the transmission through it changes constantly. On the other hand, gas stripper remains the same all the time under the same experimental conditions. The ion beam, which leaves the stripper medium, consists of different charge states depending on the number of electrons stripped off. The energy of these ions are calculated as eV(q+1), where V is terminal potential, e is electron charge and q is the charge state acquired. The charge state distribution depends on terminal potential and stripper gas pressure. Previous studies on charge state distribution have been performed with energy variation effects on charge states for Be, Cl, Al, Ti, Ni and C ions on various accelerators around the world [3-8].

In this paper, we measure the charge state distribution of B, C, Ni, Si, Cu and Au beams on 5UDH-2 pelletron accelerator installed at the National Centre for Physics[9], Islamabad, Pakistan, so as to qualify our system for measuring the charge state distribution of different beams using Ar as target stripper gas. The knowledge of charge sate distribution is important since beam with higher charge state of an element can have greater energy than lower charge state at the same terminal potential of the machine. The charge state distribution with varying terminal potential (0.3 – 3.0 MV) is measured while making the stripper gas pressure at constant value. The advantage of choosing terminal potential is that we can cover a good range of energy especially with higher charge state at 3.0 MV potential. A range in atomic mass of elements in the periodic table is covered while studying the behavior of higher charge states, i.e. B, C and Si are low mass elements, Ni and Cu lie in middle range and Au is higher mass element. The stripper gas pressure effect on charge state distribution is studied, too, while keeping terminal potential constant. The results depict that the higher charge states are stable for B, C, Si, Ni and Cu, but some fluctuation are observed in the higher charge states of Au. In the case of C beam, with increasing stripper gas quantity and constant terminal potential, it is also observed that the higher charge states are more preferable but overall transmission of the beam is decreased.

2 Experimental setup

The accelerator has two beam lines at 15° and 30°. The 15° beam line is being used for ion beam analyses such as Rutherford backscattering spectroscopy (RBS), elastic recoil detection analysis (ERDA), nuclear resonance analysis (NRA), and particle-induced X-ray emission (PIXE), as well as ion beam irradiation for material modification. Whereas, the 30° beam line is dedicated for nuclear cross-section measurements of various materials [10].

Schematics of the accelerator setup with 15° beam line is shown in Fig. 1. The negative ion beams are produced by SNICS source and is extracted with initial energy of 61 keV. The beams are then bent to 45° using analyzing magnet (Fig. 1) and injected into pelletron accelerator. The initial current of negative beam is measured at faraday cup FC2. The negative beam enters stripper housing after passing through accelerating tube.

Fig.1:Full-screen View

The schematic diagram of the pelletron tandem accelerator 5UHD-2. The total beam current is measured at FC 3 and the individual beam current is measured after switching magnet at FC 4.

In stripper housing Ar gas is allowed to enter into stripper canal at low pressure by opening the gas stripper valve. The gas stripper valve is operated remotely in the control room. The pressure of Ar gas in stripper line, sized at Φ11.5 mm×966.6 mm, is measured by a thermocouple gauge. Two turbo molecular pumps (navigator TV301) are installed diametrically opposite at stripper housing, used for recirculation of the Ar gas and create a pressure at the center of stripper canal. Equilibrium between the incoming and recirculation gas is maintained after some time. Opening the stripper valve and switching on the turbo molecular pumps creates a small thickness in μg/cm² of Ar gas at center of stripper canal. The pressure in accelerator tube is measured by the ion gauges at both ends of the accelerator tank. The stripper gas pressure can be estimated indirectly by measuring the increase in pressure at the two ion gauges with increasing stripper gas quantity [8].

The positive beam consisting of all charge states leave the accelerator tank from high-energy side, and the current is measured at FC 3. Switching magnet bends the accelerated ion beams to the 15° beam line and the current is measured at FC 4.

3 Results and Discussion

3.2. Charge state distribution

NCP accelerators are utilized for different types of scientific experiments including ion irradiation / implantation and ion beam analysis. Therefore, it is of utmost importance to have a detailed knowledge of charge state distribution of different available ion beams is of significant importance. In this perspective, the charge state distribution of ¹¹B,¹²C, ²⁸Si, ⁵⁹Ni, ⁶³Cu and¹⁹⁷Au ion beams are measured as a function of terminal potential while stripper gas thickness is maintained at approximately 0.6 μg/cm².

Fig.2 shows the charge state distribution for ¹¹B with Ar as stripper gas at equilibrium pressure. The +1 charge state decreases with increasing terminal potential and in terminal potential range 0.3–1.2 MV +2 charge state is dominating while for 1.3–1.8 MV range +3 is dominating charge state. We have also observed good current in 1.5–1.8 MV range for +4 charge state.

Fig.2:Full-screen View

Charge states distribution of ¹¹B beam as a function of terminal potential. The stripper gas is at equilibrium with thickness of ~0.6 μg/cm².

Figure 3 shows the charge state distribution of ¹²C and ²⁸Si as a function of terminal potential with Ar as stripper gas at equilibrium pressure. Here again +1 charge state decreases with increasing terminal potential, +2 is dominating charge state in 0.3–1.2 MV range, and +3 is dominating charge state in 1.3–3.0 MV range. The +4 charge state has shown good current in 2.5–3.0 MV range, and +5 charge state shows reasonable current in 1.5–3.0 MV range for Si beam and in 2.5–3.0 MV for C beam.

Fig.3:Full-screen View

Charge states distribution of ¹²C (a) and ²⁸Si (b) beams as a function of terminal potential. The stripper gas is ~0.6 μg/cm² at equilibrium.

Fig.4 shows the charge state distribution for ⁵⁹Ni and ⁶³Cu. In both cases +1 charge state decreases with increasing potential, +2 charge state is dominating in 0.7–2.0 MV range, +3 charge state dominates in 2.0–3.0 MV range, and +4 and +5 charge states also have good currents in 1.5–3.0 MV range. A reasonable current is observed for higher charge states as well.

Fig.4:Full-screen View

Charge states distribution of ⁵⁹Ni (a) and ⁶³Cu (b) beam as a function of terminal potential. The stripper gas is ~0.6 μg/cm² at equilibrium.

Fig.5 shows the charge state distribution for ¹⁹⁷Au. The maximum limit of switching magnet in our accelerator is 1.4 Tesla. So the calculated fields for +1 charge state from above 0.7 MV terminal potential and in case of +2 charge state from above 2.0 MV are beyond these limits. The +3 and +4 are dominating charge states in 1.5–2.5 MV range. The +5, +6 and +7 are showing good currents in 1.0–2.5 MV range. For higher charge states, some fluctuations are observed for Au ions due to unknown reasons.

Fig.5:Full-screen View

Charge states distribution of ¹⁹⁷Au beam as a function of terminal potential. The stripper gas is ~0.6 μg/cm² at equilibrium.

It seems that there are systematic trends: The beam transmission losses are largest at the lowest and at highest energies. It means, at a given terminal potential, the higher the charge state, the greater will be the energy for a pelletron machine; so for the higher charge states the energy will be higher and for lower charge state the energy will be lower. At low energies the deviation could be assigned to scattering losses or caused by neutral particles. At the highest energies, the losses might be related to the ion optics. The small angle scattering in the stripper canal can lead to significant beam losses especially at low energies. The transmission through a tandem accelerator also depends on the optics. The losses due to the ion beam transport are related to the focusing properties of the acceleration tubes and following optical system (e.g. magnet and lenses) and depend therefore on the selected charge state.

Fig.6 shows the charge states distribution of ¹²C, ²⁸Si, ⁵⁹Ni and ⁶³Cu at 3.0 MeV. The +2 is dominating charge state for all beams while the +3 is also dominating for C and Si. In Fig.7 the charge state distribution for C is observed with varying stripper gas pressure while the terminal potential is fixed at 1.7 MV. The stripper gas thickness varies from 0.4–1.4 μg/cm². The best results are obtained at 0.6 μg/cm². The +3 charge state is in good agreement with the measured data previously [5].

Fig.6:Full-screen View

Fractional distribution of charge states measured at 3.0 MeV beam energy and stripper gas thickness 0.6 μg/cm².

Fig.7:Full-screen View

Effects on fraction of charge with increasing stripper gas pressure while making the terminal potential fix at 1.7 MV. HE pressure shows the reading on high-energy vacuum gauge. The stripper thickness varies from 0.4 μg/cm² to 1.4 μg/cm².

The error calculated between different measurements of same beam is approximately within 5%. In case of Au at higher charge states the error is increased to 10%. This error includes statistical error and error due to slight variations in stripper thickness. The error has also been developed because of different vacuum and operating conditions.

4 Conclusion

In this report, we have qualified our system for charge state distribution of different beams using Ar as target stripper gas. The previous results for charge states distribution are presented with varying energies. A good range of energies is covered by choosing terminal potential variation instead of energy variation in our experiments. An increase in number of charge states are observed with increasing terminal potential. For C beam, charge distribution behavior is also observed by making terminal potential stable and by varying the stripper gas pressure. With increasing stripper gas pressure the higher charge states are more preferable but it affects the overall transmission of the beam by increasing gas pressure in accelerator tubes. The results of C beam are in agreement with the pervious measurements performed by Kiisk et.al. [5] and Sarkar et.al. [6]. We have presented the results at higher charge states for B, Si, Ni, Cu and Au and at higher energies. In case of B, Si, Ni and Cu beam current stability at higher charge states are good but for Au fluctuation are observed above +8 charge states.