A GRB Detection System using the BGO-Shield of the INTEGRAL-Spectrometer SPI

The anticoincidence shield (ACS) of the INTEGRAL-spectrometer SPI consists of 512 kg of BGO crystals. This massive scintillator allows the measurement of gamma-ray bursts (GRBs) with a very high sensitivity. Estimations have shown that with the ACS some hundred gamma-ray bursts per year on the 5 σ level can be detected, having an equivalent sensitivity to BATSE. The GRB detection will be part of the real-time INTEGRAL burst-alert system (IBAS). The ACS branch of IBAS will produce burst alerts and light curves with 50 ms resolution. It is planned to use ACS burst alerts in the 4th interplanetary network (IPN) [1]. 1 The Anticoincidence Shield of SPI The spectrometer SPI [2] is one of the two main instruments on INTEGRAL, one of ESA’s next missions devoted to γ-ray research. Fig. 1 shows a drawing of the spectrometer SPI. The camera of SPI, which consists of 19 cooled highpurity germanium detectors, is shielded on the side walls and rear side by a large anticoincidence shield (ACS). The field of view of the camera is defined by the upper opening of the ACS. The imaging capability of the instrument is attained by a passive-coded mask on the top. Below the mask a plasticscintillator anticoincidence (PSAC) takes care for the reduction of the 511 keV background, which is mainly generated by particle interactions in the mask. The ACS consists of 91 BGO crystals which are arranged in 4 subunits. The units of the upper veto shield (UVS) consists of the upper collimator ring (UCR), the lower collimator ring (LCR) and the side shield assembly (SSA), each containing 18 crystals which are arranged hexagonally around the cylindrical axis of SPI. The lower veto shield (LVS), consisting of 36 crystals, is assembled as a hexagonal shell. The thickness of the crystals increases from 16 mm at the top (UCR) to 50 mm at the bottom (LVS). The total mass of BGO used for the ACS is 512 kg resulting in the obvious use of the ACS as a burst monitor. Each BGO crystal of the ACS (with one exception) is viewed by two photomultipliers (PMTs). Due to the redundancy concept used for the ACS, each of the 91 front-end electronic boxes (FEEs) sums the anode signals of two PMTs, which are viewing different BGO crystals (in most cases neighbouring crystals). This cross strapping of FEEs and BGO units leads in a failure case of one single PMT or FEE not to the loss of a complete BGO crystal. It emerges that a disadvantage of this method is an uncertainty in the energy-threshold value of individual FEEs, caused by a different light yield of neighbouring BGO-crystals and different PMT properties like quantum efficiency and amplification. A result 2 Andreas von Kienlin et al.