We demonstrate an atom detector based on field ionization and subsequent ion
counting. We make use of field enhancement near tips of carbon nanotubes to
reach extreme electrostatic field values of up to 9x10^9 V/m, which ionize
ground state rubidium atoms. The detector is based on a carpet of multiwall
carbon nanotubes grown on a substrate and used for field ionization, and a
channel electron multiplier used for ion counting. We measure the field
enhancement at the tips of carbon nanotubes by field emission of electrons. We
demonstrate the operation of the field ionization detector by counting atoms
from a thermal beam of a rubidium dispenser source. By measuring the ionization
rate of rubidium as a function of the applied detector voltage we identify the
field ionization distance, which is below a few tens of nanometers in front of
nanotube tips. We deduce from the experimental data that field ionization of
rubidium near nanotube tips takes place on a time scale faster than 10^(-10)s.
This property is particularly interesting for the development of fast atom
detectors suitable for measuring correlations in ultracold quantum gases. We
also describe an application of the detector as partial pressure gauge.Comment: 7 pages, 8 figure