INTEGRAL
The INTErnational Gamma-Ray Astrophysics Laboratory is a space telescope for observing gamma rays of energies up to 8 MeV. It was launched by the European Space Agency into Earth orbit in 2002, and is designed to provide imaging and spectroscopy of cosmic sources. In the MeV energy range, it is the most sensitive gamma ray observatory in space. It is sensitive to higher energy photons than X-ray instruments such as NuSTAR, the Neil Gehrels SWIFT Observatory, XMM-Newton, and lower than other gamma-ray instruments such Fermi and HESS.
Photons in INTEGRAL's energy range are emitted by relativistic and supra-thermal particles in violent sources, radioactivity from unstable isotopes produced during nucleosynthesis, X-ray binaries, and astronomical transients of all types, including gamma-ray bursts. The spacecraft's instruments have very wide fields of view, which is particularly useful for detecting gamma-ray emission from transient sources as they can continuously monitor large parts of the sky.
INTEGRAL is an ESA mission with additional contributions from European member states including Italy, France, Germany, and Spain. Cooperation partners are the Russian Space Agency with IKI and NASA.
Mission
Radiation more energetic than optical light, such as ultraviolet, X-rays, and gamma rays, and cannot penetrate Earth's atmosphere, direct observations must be made from space. INTEGRAL is an observatory, scientists can propose for observing time of their desired target regions, data are public after a proprietary period of up to one year.INTEGRAL was launched from the Russian Baikonur spaceport, in Kazakhstan. The 2002 launch aboard a Proton-DM2 rocket achieved a 3-day elliptical orbit with an apogee of nearly 160,000 km and a perigee of above 2,000 km, hence mostly beyond radiation belts which would otherwise lead to high instrumental backgrounds from charged-particle activation. The spacecraft and instruments are controlled from ESOC in Darmstadt, Germany, ESA's control centre, through ground stations in Belgium and California.
Fuel usage is much lower than predictions. INTEGRAL has far exceeded its 2+3-year planned lifetime, and is set to enter Earth atmosphere in 2029 as a definite end of the mission. Its orbit was adjusted in Jan/Feb 2015 to cause such a safe reentry, using half the remaining fuel then.
Spacecraft
The spacecraft body is a copy of the XMM-Newton body. This saved development costs and simplified integration with infrastructure and ground facilities. However, the denser instruments used for gamma rays and hard X-rays make INTEGRAL the heaviest scientific payload ever flown by ESA.The body is constructed largely of composites. Propulsion is by a hydrazine monopropellant system, containing 544 kg of fuel in four exposed tanks. The titanium tanks were charged with gas to 24 bar at 30 °C, and have tank diaphragms. Attitude control is via a star tracker, multiple Sun sensors, and multiple momentum wheels. The dual solar arrays, spanning 16 meters when deployed and producing 2.4 kW BoL, are backed up by dual nickel-cadmium battery sets.
The instrument structure is also composite. A rigid base supports the detector assemblies, and an H-shaped structure holds the coded masks approximately 4 meters above their detectors. The payload module can be built and tested independently from the service module, reducing cost.
Alenia Spazio was the spacecraft prime contractor.
Instruments
Four instruments with large fields-of-view are co-aligned on this platform, to study a targets across such a wide energy range of almost two orders of magnitude in energy. Imaging is achieved by coded masks casting a shadowgram onto pixelised cameras; the tungsten masks were provided by the University of Valencia, Spain.The INTEGRAL imager, IBIS observes from 15 keV to 10 MeV. Angular resolution is 12 arcmin, enabling a bright source to be located to better than 1 arcmin. A 95 x 95 mask of rectangular tungsten tiles sits 3.2 meters above the detectors. The detector system contains a forward plane of 128 x 128 Cadmium-Telluride tiles, backed by a 64 x 64 plane of Caesium-Iodide tiles. ISGRI is sensitive up to 1 MeV, while PICsIT extends to 10 MeV. Both are surrounded by passive shields of tungsten and lead. IBIS was provided by PI institutes in Rome/Italy and Paris/France.
The spectrometer aboard INTEGRAL is SPI, the SPectrometer of INTEGRAL. It was conceived and assembled by the French Space Agency CNES, with PI institutes in Toulouse/France and Garching/Germany. It observes radiation between 20 keV and 8 MeV. SPI has a coded mask of hexagonal tungsten tiles, above a detector plane of 19 germanium crystals. The high energy resolution of 2 keV at 1 MeV is capable to resolve all candidate gamma-ray lines.The Ge crystals are actively cooled with a mechanical system of Stirling coolers to about 80K.
IBIS and SPI use active detectors to detect and veto charged particles that lead to background radiation. The SPI ACS consists of a BGO scintillator blocks surrounding the camera and aperture, detecting all charged particles, and photons exceeding an energy of about 75 keV, that would hit the instrument from directions different from the aperture. A thin layer of plastic scintillator behind the tungsten tiles serves as additional charged-particle detector within the aperture.
The large effective area of the ACS turned out to be useful as an instrument in its own right. Its all-sky coverage and sensitivity make it a natural gamma-ray burst detector, and a valued component of the IPN.
Dual JEM-X units provide additional information on sources at soft and hard X-rays, from 3 to 35 keV. Aside from broadening the spectral coverage, imaging is more precise due to the shorter wavelength. Detectors are gas scintillators in a microstrip layout, below a mask of hexagonal tiles.
INTEGRAL includes an Optical Monitor instrument, sensitive from 500 to 580 nm. It acts as both a framing aid, and can note the activity and state of some brighter targets, e.g. it had been useful to monitor supernova light over months from SN2014J.
The spacecraft also includes a radiation monitor, INTEGRAL Radiation Environment Monitor, to note the orbital background for calibration purposes. IREM has an electron and a proton channel, though radiation up to cosmic rays can be sensed. Should the background exceed a preset threshold, IREM can shut down the instruments.