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The ion implanter

The vacuum chamber is interfaced with an ion implanter (200 kV; Danfysik) from which ions with energy up to 200 keV (400 keV for double ionizations) can be obtained. The ion beam produces on the target a spot larger than the area sampled by the IR beam and current densities are in the range of 10 nA cm$^{-2}$ to a few $\mu$A cm$^{-2}$ in order to avoid a macroscopic heating of the sample. The amount of energy released to the icy samples (dose) is expressed in units of eV/16 amu and is calculated from the knowledge of the ion fluence (ions cm$^{-2}$), the stopping power (eV cm$^{2}$ molecule$^{-1}$) of the chosen projectile, and its penetration depth or range in the target (molecules cm$^{-2}$). The fluence is obtained from a current integrator on the path of the ion beam, that measures the charge which reaches the sample during irradiation; the other two parameters are well known and can be provided by software such as SRIM. The penetration depth of 200 keV ions in the icy mixtures studied is about 0.2-2 $\mu$m. In order to have thicker irradiated samples, and thus spectra with a better signal-to-noise ratio, we have at times irradiated the icy mixtures during deposition. In this case the dose is estimated from the knowledge of the deposition rate (molecules cm$^{-2}$ s$^{-1}$) previously calibrated, the ion flux (ions cm$^{-2}$ s$^{-1}$) and the energy of impinging ions (eV). When irradiated samples are thicker than the penetration depth of impinging ions, the doses are given in units of ions cm$^{-2}$. A schematic depiction of three different types of performed experiments is reported in Figure 1.27.

Figure 1.27: Schematic description (not to scale) of three different types of experiments performed. The sketch on the left hand side refers to ion irradiation of icy films thinner than the penetration depth of impinging ions. In this case ions pass through the sample. The middle sketch refers to an icy sample irradiated during deposition. The process stops when the sample is as thick as required. The sketch on the right hand side refers to ion irradiation of a sample thicker than the penetration depth of impinging ions. In this case, only the uppermost layers of the sample are irradiated. The ice underneath remains unprocessed and ions remain implanted in the sample.
\includegraphics[width=140mm]{lasp/tretipi1.eps}

Figure 1.28: The setup of the ion implanter of the Laboratory of Experimental Astrophysics at Catania Astrophysical Observatory. The ion implanter is shielded with a conductive wall. All instruments and control systems are placed out of the restricted area where the beam is accelerated.
\includegraphics[width=150mm]{lasp/DSC00001.eps}


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Next: The UV lamp Up: Experimental facilities Previous: The vacuum chamber   Contents   Index
Innocenza Busa' 2005-11-14