Electron Proton Helium Instrument (EPHIN)

The EPHIN sensor is a multi-element array of solid state detectors with anticoincidence to measure energy spectra of electrons in the range 250keV to > 8.7 MeV, and of hydrogen and helium isotopes in the range 4 MeV/n to > 53 MeV/n. The associated signal processing electronics is housed in a separate unit. The units are interconnected by 22 double screened cables. Together, the two units have envelope dimensions of 35.5 x 21.9 x 19.1 cm³. The total mass of EPHIN is 3.55 kg, the total power consumption is 1.85 W and the telemetry rate after onboard data compression is 172 bits per second. The sensor aperture points in the dircetion of the nominal interplanetary magnetic field at 1 AU, 45° west of the spacecraft Sun line.

EPHIN Sensor Unit

The heart of the EPHIN sensor head consits of a stack of five silicon detectors, surrounded by an anticoincidence shield of plastic scintillator and a sixth silicon detector to distinguish between absorption and penetration mode. Two passivated ion-implanted detectors define the 83° full width conical field of view with a geometric factor of 5.1 cm² sr. These two detectors are divided into six segments. This coarse position sensing permits sufficient correction for path length variations (resulting from the large field of view) needed to resolve isotopes of hydrogen and helium. Another important advantage of segmentation is the capability to implement a commandable or self-adaptive geometric factor. On detection of high count rates in the centre segment the logic will disable all but the inner circular segment of both detectors, reduciing the effective geometric factor by a factor of 24 to permit measurements of fluxes as high as 10⁶ counts/(cm² s sr) without significant death time losses.

Three lithium-drifted silicon detectors stop electrons up to 10 MeV and hydrogen and helium nuclei up to 53 Mev/N. These large area detectors have thickness variations of less than 10 µm and diffused lithium contact dead layers of less than 50 µm silicon equivalent. A sixth ion-implanted detector will allow particles stopping in the telescope to be distinguished from penetrating particles. The fast plastic scintillation detector, viewed by a 1 inch photomultiplier and used in anticoincidence, helps to reduce background. The whole stack is mounted in an aluminium housing, the aperture being covered by two thin foils. The inner titanium foil of 2 µm thickness ensures light tightness and closes the electrical shielding of the sensor while the outer aluminized kapton foil of 8µm thickness is necessary for thermal control.

Taken from "THE SOHO MISSION" by Bernhard Fleck, Vicente Domingo and Arthur I. Poland; Kluwer Academic Publishers

Download the Complete Instrument Description (ZIP-Format, 1.8 MB, taken from "The SOHO Mission")