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Low-energy ion accelerator to study the collisions of light ions with biomolecules

02.01.2012

A low-energy ion accelerator is being developed at the PTB to investigate the cross sections for ionization and fragmentation of biomolecules by light ions with energies below and in the vicinity of the stopping power maximum. This will be performed in a crossed beam experiment by means of low-energy angle-resolved electron spectrometry and time-of-flight spectrometry of the fragment ions. Protons, helium and carbon ions with kinetic energy of up to 150 keV/q will be used (where q is the charge of the ion) as primary particles.

Figure : Schematic drawing of the ion optics and SIMION simulation of 5 keV proton trajectories. (Red: deflected beam in the pulsing stage; Blue: undeflected beam). The beam size in the target region is 3 mm x 4 mm with a source emittance of 10 mm x 35 mrad (2°). The diameter of the ion beam is not to scale, it is shown oversized for better visibility of the trajectories.

The figure shows the general layout of the ion accelerator. Simion 8.0 [1] was used for the simulation of the ion beam transport and the ion optical elements. An extractor type ion source (SPECS IQE 12/38) [2] with a Wien mass filter (SPECS WF-IQE) [2] produces the q/m filtered ions, where m is the mass of the ion. This ion source allows tuning the primary energy over the range from 0.2 keV to 5 keV. With the ion accelerator it will be operated at its maximum energy of 5 keV. An electric pulsing stage and an electrostatic deflector are placed between the ion source and the electrostatic accelerator tube. The pulsing stage impresses a time structure on the ion beam required for time-of-flight measurements. The deflector aids the ion beam's alignment with respect to the accelerator tube’s axis. An electrostatic quadrupole triplet is used to focus the ion beam in horizontal and vertical direction to the target region. Another electrostatic deflector between the accelerator tube and quadrupole triplet assists the ion beam to enter the quadrupole along its axis. This is to avoid the steering of the ion beam with the quadrupole lens. A third electrostatic deflector is located behind the quadrupole to correct any beam deflection. The ion beam size in the target region is defined by an aperture. The AR 65 Angle-Resolved Electron/Ion Spectrometer (Omicron NanoTechnology) [3] will be used to measure the energy and angular distributions of the emitted electrons or ions. The length of the accelerator (without the ion source and the scattering chamber) is approximately 3.3 m.

Literature

  1. D. J. Manure and D. Dahl: SIMION 8.0 User Manual, 2008, Scientific Instrument Services, Inc.
  2. www.specs.de
  3. www.omicron.de