IMP-8 EECA Description

The Electrostatic Energy-Charge Analyzer (EECA) is the University of Maryland experiment on the IMP-8 Satellite. It uses electrostatic deflection and total energy measurements to determine the ionic charge and energy of particles in the range of approximately 100 to 1000 keV/charge.

Determining an ion's charge

The deflection of an ion traveling through any electrostatic field is a function only of the ratio of its energy to its charge. Thus from a simultaneous measurement of a particle's deflection in a known electrostatic field and its total kinetic energy, one can also determine its charge.

Deflecting the particles

In the EECA experiment, the deflection fields are created by applying voltages to two flat non-parallel plates, mounted on stand-off insulators in a grounded cavity. One plate is maintained at a potential of +14.5 kV relative to the ground plate, the other at +3.5 kV. The three field regions created in the deflection cavity have potential differences of 14.5 kV (top), 11 kV (center), and 3.5 kV (bottom), corresponding to fields of 7.25 kV/cm, 11 kV/cm, and 1.75 kV/cm, respectively. A passive collimator at the entrance of the cavity defines the arrival directions of incoming particles.

Measuring the deflection

A charged particle entering the sensor is deflected in one of the these three field regions. Its deflection is determined by the location of the one solid state detector in the array which is triggered as the particle strikes its surface while exiting the cavity. At the same time this detector measures the total energy of the particle by pulse height analysis of its output signal. The energy per charge range of each detector is defined by its position and width. The rates are determined by simply counting the number of particles which trigger each of the detectors. The charge and finer energy resolution for each detector is achieved by pulse height analysis of the signals from that detector.

Detector construction - front

The front of the detector array consists of seven thin silicon surface barrier detectors designated P1 to P4 and P6 to P8 in order of increasing energy-per-charge of the particles able to reach them. The detector P5 is covered with aluminum foil just thick enough to stop protons deflected to it. It is identical to P2 and P6, and its purpose is to measure the shape and intensity of the background spectrum. Continuous monitoring of the background spectrum is essential because the background spectral shape changes during different solar flare events.

Detector construction - back

Behind this front layer is an array of six thick Si (Li) detectors, positioned so that any particle which penetrates one of the front detectors must also strike one of these. All back detectors are wired in parallel and are referred to collectively as B. The requirement for pulse height analysis of a P detector Pn is an anti-coincidence of Pn and B since positive ions reaching these P detectors will be stopped in them. The B anti-coincidence also helps to reduce background due to penetrating particles.

Detector construction - the scintillator

A plastic anti-coincidence scintillator surrounds the detectors, preventing analysis of high energy penetrating particles and further reducing background. The scintillator provides greater than 2 pi anti-coincidence protection. In addition, the scintillator counting rate is a measure of the proton spectrum at high energies. The A1 scintillator responds to protons with energies above 40 MeV, and the A2 scintillator responds to protons above 20 MeV. Both of these have very large geometrical factors (~1100 and 700 cm² sr., respectively).

Table 1

IMP-8-EECA_DESC.HTML last modified on 8/30/12 at 1:05 PM by sel