Development of Carbon Stripper Foils

One of the key steps in AMS is the conversion of negative carbon ion beams (extracted from a sample) to the positive charge state in the terminal of a tandem accelerator. This process involves grazing collisions of the accelerated negative carbon ions with atoms of a "stripping" medium introduced into the high vacuum, leaving the ions stripped of several electrons. In most AMS systems, including the NOSAMS system, the stripping medium is argon gas at low pressure. This has the disadvantage of raising the gas pressure in the remainder of the system, thereby reducing beam transmission and mass resolution of the spectrometer.

Thin, solid-state stripper foils have long been used in tandem accelerators but have had a different set of problems: under ion beam bombardment they tend to either disintegrate prematurely or slowly thicken, leading to the complete loss or serious degradation of the positive ion beam after a few days of exposure.

Over the last decade, research on single-walled carbon nanotubes (cylindrical closed structures of graphitic carbon, ~1.2 nm diameter) has revealed remarkable properties: high electric and thermal conductivity and tensile strength far exceeding that of steel. That has led Karl von Reden and Enid Sichel to the idea of developing a durable carbon nanotube foil for electron stripping in accelerator mass spectrometry (AMS). The idea is to create thin mats of nanofibers into a mesh configuration that would perform like a “frozen gas”, keeping the stripping atoms largely stationary under ion beam bombardment and minimizing the structural damage known to occur in amorphous or graphitic films.

The mesh of nanotubes is expected to be able to easily conduct away the electron current or heat from the charge-exchanging ions. Preliminary work has identified two possible methods of creating the nanotube mats: matrix-assisted pulsed laser evaporation (MAPLE ) and electrospinning . Collaborations with scientists at these and other laboratories with comparable facilities have been initiated.

Development of a Carbon Nantube Stripper Foil
Presentation given at the SNEAP Conference
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