After six years of sometimes painfully difficult
development, the construction of the world’s largest
and most sensitive gamma-ray imaging camera has finished.
Following calibration tests in Milan, the fully assembled
IBIS telescope has now been transported from Italy to
ESA’s European Space Research and Technology Centre in the Netherlands
to be integrated on the INTEGRAL spacecraft.

Principal investigator Dr. Pietro Ubertini often refers fondly to this state-of-the art imager as a ‘beast of an instrument’. The detector is in effect a voluminous cube-shaped instrument, with a hopper-like funnel on its upper surface. It has a total mass of some 380 kg, mainly due to the many large and heavy shielding units covering its other sides. These will identify stray cosmic rays which have to be eliminated from the detector data.

The instrument’s calibration, the final hurdle before delivery, took place at the LABEN facilities in Milan, Italy, where IBIS was assembled. For the tests lasting 12 days, IBIS was placed horizontally, dominating the low ceiling clean room. Through a large diameter flexible tube, a refrigerator breathed cool air into the instrument. Small radioactive sources, simulating gamma-ray sources in the sky, were positioned in front of the hopper to precisely measure how the instrument was functioning. The scene resembled a hospital theatre where a very stout patient is undergoing an operation.

Huddled together in front of their computer screens, more than a dozen scientists and engineers, members of the calibration team, concentrated on the data coming from the IBIS detectors.

“More than 200 people from eleven different institutes and space industry have worked on IBIS and all are represented here to see for the first time how the whole camera is working. It’s a great moment for all of us,” said Dr. Ubertini.

Twenty thousands pixels for super-sharp pictures

When installed at the base of INTEGRAL, IBIS will register in-coming gamma rays with sensitivity ten times greater than previous imagers. Before arriving on its two levels of detectors, the gamma-ray photons will have passed through a coded mask 3.2 m above at the summit of the spacecraft. The mask acts like a multi-hole pinhole camera producing shadows of celestial objects on the detectors. Gamma-ray sources will be located with a 1 arcminute precision – the equivalent of pinpointing the position of a person standing in a crowd situated over a kilometre away.

Each of the two detector layers is a minutely made and very sophisticated instrument. The upper level ISGRI (INTEGRAL Soft Gamma-Ray Imager) covers the 15 keV – 1 MeV energy range. It has been provided by France’s CEA/Saclay Astrophysics Department, under the responsibility of its principal investigator Dr. FranÁois Lebrun who is also a co-principal investigator on IBIS.

Below it, PICsIT (Pixellated Caesium Iodide Telescope) has been provided by the Istituto di Tecnologie e Studio delle Radiazioni Extraterrestri (ITeSRE) in Bologna and covers a higher energy range, from 170 keV to 10 MeV. It has been built by LABEN, under contract from the Italian Space Agency ASI, and its principal investigator is Dr. Guido Di Cocco, also an IBIS co-principal investigator.

“IBIS is like a domestic digital CCD camera but infinitely more complex. The two layers allow the paths of the photons to be tracked in 3D, as they scatter and interact with the elements” explains Pietro Ubertini. “ISGRI is made of 16 384 Cadmium-Telluride 4mm-square pixel detectors, while PICsIT consists of 4096 Caesium-Iodide pixels. Building their individual elements represented a considerable challenge for both detector planes. In particular for PICsIT, the glue used to make the first series proved to be insufficiently robust to withstand the launch conditions.”

Expecting the unexpected

Thermal and mechanical tests had preceded the assembly, on 10 0ctober, of the two detector levels. Electrical tests followed and the calibration of the complete IBIS instrument had begun at the start of November.

A dozen radioactive sources providing different energy rays were used for the tests. The single and multiple impacts of the rays on the detector pixels were recorded for subsequent analysis. The data, reaching several Gigabytes on each run, were visualised as multi-coloured images and graphs on the computer screens.

Engineers from LABEN were as pleased as the scientists. A sticker on the wall recalled that instruments for the BeppoSAX satellite, launched in 1996 and still operating, were built in this same clean room.

“IBIS has demonstrated that today we are detector builders from practically the raw materials upwards,” says Franco Monzani, IBIS production leader at LABEN. “Our company has gained great experience to the benefit not only of other space missions but also in medical fields such as tomography.”

Whilst LABEN prepared to transport IBIS to ESA’s technical centre in Noordwijk, the Netherlands, Pietro Ubertini looked forward to the INTEGRAL launch next October and the start of IBIS observations.

“We expect to discover many very exciting things with our camera, but the most exciting will be the unexpected. We really need this sophisticated instrument to understand the most complex physical processes in the Universe. With its wide field of view IBIS will look at big chunks of the sky and will act as a monitoring instrument. Then we will be able to zoom in to see the details. I am convinced that IBIS and the other instruments on INTEGRAL will truly open a new window on this Universe.”

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