INTEGRAL's SPI sees stars after a hefty dose of gamma rays
7 May 2001Calibration is a key moment in the life of a space telescope before launch. Those who have designed and built the instrument and the future users must precisely measure its performance before it starts to examine the Universe. Such calibrations can require considerable effort, and the use of unique installations. That has been the case for INTEGRAL's gamma-ray spectrometer SPI, which has just completed its calibration at a test centre belonging to France's Atomic Energy Commission (CEA).
The calibration campaign measured the efficiency and imaging performance of the spectrometer and covered SPI's entire energy range from 20 thousand electron volts (keV) to 8 million electron volts (MeV). At low energy levels (less than 3 MeV), it was possible to use standard compact radioactive sources as employed in many laboratories. But at levels greater than 3 MeV, it was necessary to harness the power of one of Europe's most powerful particle accelerators.
The 'gamma-ray gun'
The test centre at Bruyères-le-Châtel on the outskirts of Paris houses one such impressive machine. The Van de Graaf accelerator uses electrostatic principles to generate intense voltages and to produce an extremely high-energy beam of particles (up to 4 MeV). During the SPI calibration these fast moving protons were then fired into a thin piece of carbon-13 where carbon atoms, torn apart in a nuclear reaction, released a stream of gamma rays. This 2-millimetre diameter beam was steered and shot towards the SPI spectrometer.
A purpose-built clean-room next to the particle accelerator hall was ready for SPI when it arrived at Bruyères-le-Châtel from Toulouse on 31 March. The telescope was installed horizontally on a stand allowing it to be pointed towards the gamma-ray beam. It could also be rotated along its longitudinal axis and inclined, whilst all the time being connected, for these tests, to an external cooling plant which kept the germanium detectors at 90 Kelvin, -183° Celsius. (In space, SPI will use its own internal cooling system.)
"Calibration required that the instrument be manoeuvred with infinite precision," says Marie-Anne Clair, SPI project manager at CNES, the French Space Agency which is prime contractor for SPI. "We used state of the art ground support equipment but we are particularly proud of our axial rotation system, where we used a bicycle chain principle. Crude but remarkably efficient!"
Between 9 - 20 April, SPI was first subjected to gamma rays from several lower-energy radioactive sources placed 8 m away. Round-the-clock calibration runs then exposed the spectrometer to the 'gamma-ray gun'. These tests, totalling some 400 hours were completed on 4 May.
Star simulator produces world first
SPI was also subjected to a third series of performance tests which involved a particularly complicated logistical exercise. To evaluate the spectrometer's capacity to produce images of gamma-ray celestial sources, the calibration team from the CEA/Saclay Astrophysics Department, with its SPI co-investigators, developed a gamma-ray 'star simulator'.
This relied on small capsules of extremely radioactive material, such as cobalt-60 and sodium-24, which were placed outside the accelerator building, 125 m away from SPI. Given the coded-mask principle used by the spectrometer, these sources were considered to be at an infinite distance.
"It was a human and technical challenge," recalls Bertrand Cordier, co-investigator and scientific manager for SPI at CEA/Saclay. "The sodium-24, produced in the 'Osiris' nuclear research reactor at CEA/Saclay some 20 km away, has only a 15 hour radioactive half-life, that is to say the source's radioactivity is halved after 15 hours. So it was a race against the clock which we rehearsed many times, from transport logistics to convey the capsule in its 550 kg lead container quickly to Bruyères-le-Châtel, then losing no time installing and aligning it with the telescope."
To the great satisfaction of SPI's Principal Investigators Gilbert Vedrenne and Volker Schönfelder and their colleagues at Saclay, this and all the other calibrations were successfully completed and the SPI instrument is fully living up to their expectations.
For instance, the calibration carried out on the night 18-19 April using the sodium-24 source has resulted in a world premiere: the first ever gamma-ray image above 1 MeV of a simulated star-like source produced with the coded mask technique and with such a high angular resolution (less than 2 degrees). This SPI sodium-24 calibration was carried out at 2.75 MeV.
All these calibration operations were also carried out whilst fully complying with high-security and safety regulations to which the Bruyères-le-Châtel centre, as a military establishment, is well accustomed.
For example, although the particle beam from the accelerator itself is harmless, secondary radiation is produced by interaction with the installation's surrounding equipment. So rigorous checks ensure that no one is present when the accelerator is activated. The star-simulator calibrations also took place at night when the test centre was empty.
"We are greatly pleased to have allowed our accelerator and facilities to be used for the INTEGRAL mission and to have provided such a service to the scientific community," says the centre's director Daniel Verwaerde.
"A year before launch, we have been able to analyse the data from SPI exactly as if INTEGRAL was in orbit," explains Bertrand Cordier. "Although they were simulated, SPI has seen its first stars. We can now adapt our data processing software. With this calibration now completed, my task as a co-investigator ends, and my work as a scientist begins."
On 10 May, the SPI spectrometer is due to arrive at the premises of INTEGRAL's prime contractor Alenia Spazio in Turin where it will be installed on the spacecraft.
Our thanks to Bertrand Cordier, his colleagues at CEA/Saclay Service d'Astrophysique, and to the staff at Bruyères-le-Châtel CEA/DAM.