ESA Science & Technology - Discus thrower captured by gamma rays

The imager is currently being tested at LABEN in Milan, Italy, before it is delivered in November to ESTEC (ESA's research and technology centre in the Netherlands) where it will be placed in the INTEGRAL spacecraft.

ISGRI 'first light'

ISGRI is the first gamma-ray camera in the world equipped with semiconductor detectors operating at ambient temperature. Before it was delivered this summer to LABEN to be integrated with PICsIT, the CEA/Saclay team, who are providing this instrument, demonstrated its imaging capabilities with an original test.

The detection unit, with its eight modules, was assembled on a test platform. A bronze statuette, a copy of the famous 'Discobolus', was placed between ISGRI and a radioactive source, 60 centimetres away. The cobalt-57 source emits photons with an energy of 122 keV. (For ease of mounting, spaces were left between the modules. In ISGRI's final configuration, these 'dead' zones will be ten times smaller.)

Bronze is a strong absorber at this energy and photons can hardly pass through so that the statuette's shadow was cast upon the ISGRI detectors. The image shows however, that the actual discus, being thinner, lets some photons through.

"Our camera, with its 16 384 independent cadmium telluride detectors each only 4 millimetres square and 2 millimetres thick, is an exceptional device," says Dr. Frangois Lebrun, ISGRI principal investigator. "The subject of this image, the discus thrower, symbolises our team's intense efforts making it."

The image also highlights a number of dark points, corresponding to noisy detectors. They number a hundred, less than 0.6 per cent of the total, illustrating the superb quality of manufacturing. The specification had been set at no more than 5 per cent.

Gamma-ray ghost bottle

Building ISGRI's eight camera modules with its thousands of semiconductors has been a Herculean task at microscopic level. The road has been long and hard for the project team members.

That is where a bottle of champagne makes its appearance, as Frangois Lebrun explains: "Calibration of our detectors is essentially spectroscopic and we didn't really need to have an image. We decided to take a gamma-ray picture of a champagne bottle and glass not only to prove that ISGRI was working but also to remotivate the team when the going was tough."

This image was also produced with a cobalt-57 radioactive source placed 1 metre in front of an ISGRI module. The gamma rays feature three emission lines at 136 keV, 122 keV, and 14 keV. Enough to produce a 'gamma-ray ghost image' that again is unique.

"From a science standpoint, these images have little value," says Frangois Lebrun. "They were useful tests and the champagne picture at the time worked wonders on team moral. The Discobol image confirms our efforts. Now we eagerly await the day after launch in October next year when we will really celebrate, and with more than just one bottle of champagne!"

Note to Editors

The principal investigator for INTEGRAL's imager, IBIS, is Dr. Pietro Ubertini from the Istituto di Astrofisica Spaziale, Rome.

The upper detector unit on IBIS, ISGRI (INTEGRAL Soft Gamma-Ray Imager), has been provided by France's CEA/Saclay Astrophysics Department near Paris and will cover the 20 keV - 1 MeV range. Dr. Frangois Lebrun is principal investigator of ISGRI.

The other detector unit on IBIS, PICsIT (Pixellated Caesium-Iodide Telescope), for its part will measure the higher energy photons, above 150 keV. It has been provided by the Istituto di Tecnologie e Studio delle Radiazioni Extraterrestri (ITeSRE) in Bologna and built by LABEN under a contract from the Italian Space Agency, ASI. PICsIT's principal investigator is Dr. Guido Di Cocco.