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.
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.