A high-resolution Ge spectrometer for Gamma-Ray burst astronomy

Abstract

The Transient Gamma-Ray Spectrometer (TGRS) to be flown aboard the WIND spacecraft is primarily designed to perform high resolution spectroscopy of transient γ-ray events, such as cosmic γ-ray bursts and solar flares over the energy range 25 keV to 8.2 MeV with an expected spectroscopic resolution of ∼ 3 keV at 1 MeV. The detector itself consists of a 215 cm3 high purityn-type Ge crystal kept at cryogenic temperatures by a passive radiative cooler. The geometric field of view defined by the cooler is ∼ 1.8 π steradian. To avoid continuous triggers by soft solar events, a thin BeCu Sun-shield around the sides of the cooler has been provided. A passive Mo/Pb occulter, which modulates signals from within ±5° of the ecliptic plane at the spacecraft spin frequency, is used to identify and study solar flares, as well as emission from the galactic plane and center. Thus, in addition to transient event measurements, the instrument will allow the search for possible diffuse background lines and monitor the 511 keV positron annihilation radiation from the galactic center. In order to handle the typically large burst count rates, which can be in excess of 100 kHz, burst data are stored directly in an onboard 2.75 Mbit burst memory with an absolute timing accuracy of ±1.5 ms after ground processing. The memory is capable of storing the entire spectral data set of all but the largest bursts. WIND is scheduled to be launched on a Delta II launch vehicle from Cape Canaveral on November 1, 1994. After injection into a phasing orbit, the spacecraft will execute a double lunar swing-by before being moved into a controlled ‘halo’ orbit about theL1 Lagrangian point (∼250R e towards the Sun). This will provide a 5 light-second light travel time with which to triangulate gamma-ray burst sources with Earth-orbiting systems, such as those on-board the Gamma-Ray Observatory (GRO). The response of instrument to transient γ-ray events such as GRB's and solar flares will be presented as well as the expected response to steady state point sources and galactic center line emission.