The Cosmic-Ray Energy Deposition Experiment (CEDEX) on the Phase-3D Satellite

Fabrication Responsibility:

Craig I. Underwood, G1WTW (GB)

Size:

270x200

Average Power Dissipation:

3 Watts

Einführung

Der Orbit des Phase 3-D Satelliten führt durch interessante Bereiche der Magnetospähre der Erde. Im Perigäum kreuzt der Satellit den Van Allen Strahlungsgürtel. Diese Region hat eine hohe Häufigkeit von hochenergetischen Protonen, welche Langzeit- und flüchtige Strahlungseinflüße auf die Elektronik des Satelliten haben. Im Apogäum befindet sich der Satellit außerhalb des abschirmenden Einflu§es der Magnetosphäre. In diesem Teil der Bahn herschen vor allem galactic cosmic-rays (GCRs) aus dem tiefen Weltraum und solar cosmic-rays (SCRs) verbunden mit solar flares.

Die Aufgabe des CEDEX-Experiments ist es, die von Phase 3-D vorgefundene Weltraumstrahlung zu bewerten. Diese Betrachtung wird sowohl im Lang- als auch im Kurzzeitbereich vorgenommen. Das Experiment belegt ein Modulgehäuse und enthält zwei verschiedene Systeme: Das Total Dose Experiment (TDE) und das Cosmic Particle Experiment (CPE).

CEDEX ist eine unveränderte Version des Cosmic-Ray Experiment (CRE), welches schon auf KITSAT-1 (KITSAT-OSCAR-23) und PoSAT-1 Mikrosatelliten im niedrigen Erdorbit (LEO, Low Earth Orbit) fliegt. Ein Vergleich der Ergebnisse dieser verschiedenen Missionen soll neue Erkenntnisse über die Vorgänge der Strahlungsgürtel der Erde bringen.

Beschreibung des Total Dose Experiment (TDE)

Each RADFET sensor consists of a matched pair of p-channel MOSFETs, one of which is biased during exposure (MEASURE MODE), whilst the other remains unbiased. In READ MODE, a constant current (6 microamps) is switched to each RADFET in turn, and the threshold voltage is measured. This voltage is a function of temperature as well as dose, but the temperature effect can be largely compensated for by noting the difference in threshold voltage change between the biased and un-biased RADFETs on a particular sensor. The gradual shift in this voltage difference is approximately proportional to the accumulated ionising radiation dose.

To maintain the correct biasing conditions, the TDE part of the CEDEX payload must be powered up as soon as possible after launch, and be kept powered-up during the entire mission.

During READ MODE, the data from the RADFETs and associated temperature and current sensors are collected by the internal CAN-controller, which formats the data and passes them on for storage in the RUDAK payload via the spacecraft's CAN bus.

The sub-multiplexed output of the is as follows:

Channel Number	Output
0	Switch 0	Constant Current Source
1-6	Switch 1-6	Not Used in CEDEX Version
7	Switch 7	RADFET 3 (biased)	CEDEX 1
8	Switch 8	RADFET 3 (un-biased)	CEDEX 1
9	Switch 9	RADFET 4 (biased)	CEDEX 2
10	Switch 10	RADFET 4 (un-biased)	CEDEX 2
11	Switch 11	RADFET 5 (biased)	CEDEX 3
12	Switch 12	RADFET 5 (un-biased)	CEDEX 3
13	Switch 13	RADFET 6 (biased)	CEDEX 4
14	Switch 14	RADFET 6 (un-biased)	CEDEX 4
15	Switch 15	Constant Current Source
16	Temperature Sensor 0		CEDEX 1
17	Temperature Sensor 1		CEDEX 2
18-21	Temperature Sensor 2-5	Not Used in CEDEX Version
22	RADFET Offset Voltage (approx. 4.5V)
23	Temperature Sensor Offset Voltage (approx. 1.4V)
24-31	= repeat of channels 16-23.

Beschreibung des Cosmic Particle Experiment (CPE)

Phase 3D stellt eine einmalige Möglichkeit zur Untersuchung eines Orbits mit hoher Inklination und Exzentrizität dar, welcher einem hohem Protonen-Fluss im inneren Van Allen Gürtel über dem Süd-Atlantik ausgesetzt ist. Die Daten werden sehr nützlich sein, um die Auswirkungen der Strahlung auf die elektronischen Nutzlasten von P3D beurteilen zu können.

The CPE consists of a single 30mm x 30mm PIN diode detector (900 mm² active area), 300 microns in depth, housed in a separate screened aluminium box within the CEDEX module box. This is connected to a charge amplifier and a pulse-shaping circuit which in turn is connected to an event-driven, hardware-logic controlled pulse-height multi-channel analyser. The experiment is controlled autonomously by a 87C51 microcontroller with its own data-storage RAM and built-in data-compression and communications software. This sends data to an internal CAN-controller which formats and sends them on to the RUDAK payload via the spacecraft's CAN bus.

Charged particles passing through the detector deposit energy within the silicon lattice primarily via coulomb interactions with the bound electrons of the silicon atoms. This energy appears as the formation of charge-carriers, and for silicon, one electron-hole pair is produced for every 3.6 eV of energy deposited. Thus, the charge induced in the detector is proportional to the energy deposited within the detector. This charge appears very promptly (governed by the transit-time of the particle - usually a few picoseconds), and can be swept-out of the detector at a rate which depends upon the mobility of the carriers and the response-time of the detector electronics (10s-100s of nanoseconds). This puts a physical upper-limit on the rate at which particles can be detected and counted.

The detector is chosen to be able to cope with both low and high flux conditions, and has a "dead-time" of approximately 5 microseconds (determined by the hardware used). This represents a maximum practical rate for a low-power, high resolution system.

The multi-channel analyser has 512 channels, each of equal width equivalent to approximately 0.046 pC of charge deposited in the detector (i.e. an equivalent normal-incidence LET of 14.4 MeV cm² g^-1)). The total charge-range of the instrument is approximately 0.185 pC to 23.7 pC, equivalent to a normal-incidence particle LET range of 57.7 MeV cm² g^-1) to 7370 MeV cm² g^-1). In operation, the charge-pulses from the PIN diode are amplified and shaped, and the pulse-height is recorded by a fast semi-flash 10-bit analogue-to-digital converter (ADC). The output of the ADC is used to address self-incrementing memory locations which can hold a count of up to 224 (16,777,215 counts). These counts are integrated over a fixed period of 600 seconds - internally sub-divided into four periods of 150 seconds to improve orbit-position/time resolution. Thus, 8K-bytes of data are recorded for each 600 second integration period, and are transmitted using a loss-free data-compression coding scheme to RUDAK via the CAN-bus interface. RUDAK will store and time-stamp the data, which will be downloaded each day.

Aus Gründen der Einfachheit werden die drei Byte Daten von jedem Kanal in vier Bytes gespeichert. Das vierte Byte enthält dabei ein festes Bitmuster, welches das Erkennen von Single-Event Upsets (SEUs) ermöglicht.

The aluminium shielding around the detector should limit the count-rates to around 10,000 counts per second in the most sensitive channel when passing through the heart of the South Atlantic Anomaly (SAA). The detector is designed to count particles up to a uniform rate of 200,000 particles per second (i.e. particles which arrive within 5 microseconds of each other cannot be distinguished). Der Detektor sollte damit eine Zähleffizienz von mehr als 95% haben.

Während des Betriebs erzeugt CEDEX alle 10 Minuten Pakete von CPE-Daten. Diese Pakete werden von dem internen CAN-Controller verarbeitet, der auch die 32 Kanäle des TDE sammelt.

RUDAK handles time-stamping and filing of CPE and TDE data, and data-files representing each day's activity are stored on-board until they are requested by the ground (usually each working day). Files stored in the RUDAK are protected against single-event upset (SEU) by software coding/wash routines, and the downlink is error-controlled by the packet communications protocols ensuring error-free data at the ground-station.

CPE sollte möglichst oft in Betrieb sein, damit seltene und ungewöhnliche Effekte (wie z.B. Solar-Flares) dokumentiert werden können. Da die Leistungsaufnahme mit 2,5 Watt jedoch relativ hoch ist, wird dieses Experiment eventuell nicht bei schlechten Beleuchtungsverhältnissen aktiviert sein.