Authors: Simeonov, L.I.
Reference: Proc.of the 6th Nat.Conf. with Intern.Part.on Contemporary
Problems of the Solar-Terrestrial Influences, Sofia, 188-191, 1999.
Abstract:
For the purposes of design and laboratory tests of a TOF mass
analyzer, an evaluation of the physical parameters of the carbon foil
was made. So far thin carbon foil is used exclusively in particle
analyzers for space research as a source of secondary electrons, whose
registration marks the Start and the Stop of the TOF measurement. The
thickness of the foil normally is chosen accordingly to the energy range
of the particles with concern to the preliminary evaluated energetic
loss and angle deflection after the foil. In laboratory the limitations
of the experimental setup are connected with the energy of the ions and
the thickness of the carbon foil. First, the carbon foil should be much
thinner, because of limitations in the energetic range of the available
ion sources. Additional ion acceleration in the device itself is also
limited because of possible electron leakage from conductive parts and
sharp edges. Leakage electrons create false signals and blend the signal
from secondary electron emission out of the carbon foil. On the other
side the energy of the incident particle should not be less than 20KeV,
because at lower energies the percentage of neutralized particles from
the foil is increased. With regard to secondary electron emission, the
general agreement is that despite of the very good explanations, an
overall quantitative theory is still lacking. The experimental data is
very scarce so far for energies around 20KeV. What is agreed is, that
most of the secondary electrons are created in foil depths of 20 to 50
Å, which is a limit to the thickness from below. Another requirement to
the laboratory experiment is to use ion sources with moderate emission
rate of 103-104 ions/sec, in order to provide a single particle
detection and analysis regime, in which the incidence of a second
particle in the device is not expected. In this case we rely, that a
single ion would produce in the carbon foil at least one secondary
electron, which is sufficient for the TOF measurement.