The Suprathermal Energetic Particle System (STEP)
Information from the EPACT home page
Above "is a picture of the STEP instrument. The protective
doors provide an extension of the sun shade at the entrance to each STEP telescope.
The measurement technique used in STEP is Time-of-flight vs Total E. In each of two
identical telescopes, incoming nuclei enter through a pair of thin nickel foils,
pass through a time-of-flight region and strike a surface barrier silicon solid state
detector located at the rear of the telescope. The detector is thick enough to stop
all particles of interest and the detector signal thus represents the total energy
of the particle. This energy signal is detected by a standard charge-sensitive amplifier,
shaped with a peaking time of 1 microsecond and run through a pair of discriminator
amplifiers and a pair of parallel linear amplifiers of differing gain. Two amplitude
discriminators set to 80 keV and 10 MeV allow us to select which of the two linear
amplifier outputs to analyze. The high gain linear amplifier generates a full scale
output for an input of about 12 MeV, the low gain channel handles up to about 60
In parallel with the energy, the time of flight is analyzed. As the incoming ion passes through the inner nickel foil, it knocks a few secondary electrons off the inner surface of the foil into the TOF region. These electrons are accelerated by a 1 kV potential within the chamber and are directed toward a chevron pair of microchannel plates (MCPs) which detects them and multiplies them by roughly 107, producing a measurable signal. The output signal is designated as START. A similar STOP signal is derived from the back-scattered secondary electroncs produced when the incoming ion strikes the front surface of the solid state detector. The time between the two signals equals the time taken by the ion to traverse the TOF chamber and is typically 2 - 100 nsecs. The START and STOP signals are processed through constant-fraction discriminators to reduce "walk" and drive a time-to-amplitude converter (TAC) whose output represents the time-of-flight of the ion. Logic circuitry is used to detect the coincidence of a STOP signal occurring within 100 nsec of a START and signals this coincidence by generating a VALID STOP pulse."