ESA Science & Technology - Chilly test for INTEGRAL's spectrometer

Half a dozen technicians draped the large cylindrical instrument, 2.5 m high and 1.1 m in diameter, placed vertically on its support stand. The metallic structure was practically entirely hidden by the insulation. Just a couple of paces away, the heavy door of a large black chamber had been opened, ready to accommodate SPI for its thermal vacuum tests.

INTEGRAL's spectrometer has been developed for ESA under the responsibility of CNES, as prime contractor. With a 1.2 tonne mass, it is one of the largest space science instruments ever built by the French Space Agency.

SPI will be the first high resolution gamma-ray spectral imager to operate in the 20 keV to 8 MeV energy range with a resolution of 3 keV at 1.33 MeV, twenty times better than its predecessors. It is expected to make major breakthroughs in the field of gamma-ray spectroscopy and nuclear astrophysics.

Work on SPI began at CNES in 1993, and today the instrument is nearing the end of its Earth-bound existence. The INTEGRAL space observatory carrying this instrument and three others is to be launched in April next year.

After successfully completing all its functional tests, environmental tests started at the beginning of February in the INTESPACE facility (part of the CNES Technical Centre in Toulouse). Electro-magnetic compatibility tests first ensured that all the instrument's electrical systems are operating normally, without any interference with the satellite. Today thermal tests have begun.

The SPI "ice-box"

SPI's operating temperature is of crucial importance. To detect gamma rays, its camera - composed of 19 high-purity germanium detectors - is placed in a cold box at the base of the instrument, and will be operating at 90 Kelvin, - 183° Celsius. A low temperature minimises radiation damage to the detectors when close to the Earth and guarantees the resolution which worsens at higher temperatures.

To keep its detectors this cold, SPI is equipped with a sophisticated cooling system. It consists of two pairs of active coolers, "cryo-coolers" with compressors as in refrigerators, and a passive cooling unit. The cryo-coolers have been supplied by Astrium, Stevenage, United Kingdom. (Other SPI subsystems have been provided by a dozen European companies and institutes.)

A pair of large white radiators, and the space-facing sides of five boxes of electronics, are the only surfaces on the instrument not hidden by the gold-coloured insulation. These radiators will evacuate the heat generated by the cryo-coolers and the electronic systems that will control SPI and which will process its science data.

The payload also benefits from the spacecraft design. When launched, INTEGRAL's attitude in space will always keep the spectrometer on the deep-space side, facing away from the Sun. SPI will also be in the "shadow" of its larger-sized companion instrument, the IBIS gamma-ray imager.

"The detectors and cryo-coolers have already been fully bench-tested at sub-assembly level" explained Philippe Gari, ESA's systems engineer for SPI. "In this INTESPACE chamber, we will be operating SPI for the first time in the same cold and vacuum conditions it will encounter in space. A hundred test probes have been installed to calibrate the flight sensors; we will check that all the on-board equipment - notably at the detector plate and the electronics - are within the expected temperature ranges."

The 30 cubic-meter "SIMDIA" space simulator (built for and named after France's Diamant rocket programme in the 1960s) will be running at a vacuum (down to 10^-5 mbar) 24 hours a day. The temperature inside can be set to between -173 °C and + 126 °C. Ten small radioactive sources fastened to the front of SPI will also simulate celestial objects emitting gamma rays.

Satisfaction nearing the end of the road

One of SPI's two Co-Principal Investigators, Prof. Gilbert Vedrenne was present to see "his" instrument being prepared for the thermal test. The research institute to which he belongs, the Centre d'Etudes Spatiales des Rayonnements (CESR), is located just beside the CNES technical centre. (SPI's second co-Principal Investigator is Prof.Volker Schvnfelder, of the Max Planck Institut fur Extraterrestrische Physik in Garching, Germany).

"After having encountered many technological problems, I must say that I am relieved that we have reached the final stretch of a long road," said Vedrenne. "I think that when the thermal vacuum test is completed then we can go forward, with confidence, to the final task of calibrating the instrument next month."

The aim of the calibration is to measure the efficiency and imaging performance of the spectrometer over its entire energy range. It will be conducted in one of the particle accelerators at the Bruyhres-le-Chbtel facility, part of France's Atomic Energy Authority (CEA) on the outskirts of Paris. This is scheduled to last throughout April. CNES will then deliver the SPI instrument in May to INTEGRAL prime contractor Alenia Spazio in Turin, where the entire spacecraft is being assembled.

"We are approaching the end of the road with mixed feeling," says Marie-Anne Clair, SPI Project Manager at CNES. "On the one hand, SPI has for many of us been a slice of our lives and it will be sad to see our team being disbanded. But then, we all are greatly satisfied with the fruit of our efforts, and to see that the scientists are really pleased with the excellent performances of the instrument".

After being dressed in its sparkling MLI, the spectrometer was delicately tilted and installed horizontally on the door of the vacuum chamber. A week later on 8 March, the heavy door was shut tight and the imprisoned SPI resigned itself to 20 days in the bitter cold. It will be the same intense cold next year, when its much, much longer space journey begins.