3 MV Tandem Pelletron Accelerator


FLC Region

Security Lab



4555 Overlook Avenue S.W.




FUNCTION: Generates high-energy ions for accelerator mass spectrometry (AMS), near-surface analysis, high energy ion implantation, and radiation effects studies.

DESCRIPTION: Negative ions are generated by Cs sputtering of a solid, or by attaching electrons onto neutral or positive ions by Cs or Rb vapor. Beam currents generated vary from μA to tens of μA depending on the source and ion chosen. Two Pelletron charging chains produce a terminal voltage up to 3 MV in the accelerator. Negative ions are injected at 10 to 70 keV, accelerated up to the terminal where they undergo collisions with a stripper gas or a carbon stripper foil and lose electrons, then are accelerated as positive ions back to ground potential. For AMS, the relative intensity of selected ions measures their concentration in the sample of interest located in the ion source. On the analysis beamline, the sample of interest is located at the end of the beamline, and a signal generated by scattering of incident high-energy ions indicates the composition of the sample. Incident high-energy ions can also be used todamage the surface of a sample of interest, or to introduce a dopant.

INSTRUMENTATION: Four ion sources: (1) National Electrostatics Corp. (NEC) Mutli-Cathode Source of Negative Ions by Cesium Sputtering (MC-SNICS) 40-cathode ion source; (2) custom multi-cusp gas-feed ion source under development for AMS; (3) front end of Cameca IMS 6f secondary ion mass spectrometer (SIMS); and (4) NEC Alphatross ion source to generate He ion beams. A unique Pretzel magnet coupled with an electrostatic analyzer (ESA) functions as a bandpass mass filter for the ion sources, covering a mass range from 1 to 240 amu. The tandem accelerator is an NEC model 9SDH-2 with a 3 MV terminal potential. An electrostatic bend selects the high-energy-ion charge state to be transmitted to the AMS beamline. This beamline provides mass-independent parallel transport through use of all-electrostatic components, including a 45° spherical ESA with an E/ Δ E of 800. Parallel mass analysis is performed with an Enge split-pole spectrograph having a 1.5-m-long focal plane covering a mass range of a factor of 8 with an M/ Δ M of about 2500. Intense beams are detected by well-shielded Faraday cups, weak beams are detected by position-sensitive microchannel plate detectors and energy detectors. Ions for non-AMS applications are transported through the spectrograph to a switching magnet that selects either (1) an analysis beamline with a variable-angle Si particle detector for Rutherford backscattering and elastic recoil detection analysis, manual and computer-controlled goniometers for ion channeling, and a 0.02 sr acceptance solid-angle, double-focusing, 180° magnetic spectrometer with 0.2% energy resolution; or (2) a high energy ion implantation beamline for uniform ion implantation over a 4-in.-diameter wafer, with heating and water or liquid nitrogen cooling of the sample. All beamlines have cryopumps or turbopumps for clean vacuum conditions.


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