The trigger veto system of each spectrometer consists of one segmented trigger layer made from plastic scintillation detectors and a flexible system of additional veto layers and lead absorber layers. Electrons and muons leave characteristic signatures for providing a muon veto, electromagnetic shower detection, neutron background rejection, and trigger efficiency calibration. The single layers are mounted in form of a drawer system to guarantee practicability as well as setup flexibility.
The trigger layer of the detector is used for the fast timing and energy-loss measurements, and provides the basic hit and position information for a triggered readout of the time projection chamber.
Cosmogenic muons travel in the same direction as electrons inside the spectrometer. In order to reject these muons, the lead absorber layers are needed in which electrons and electron-initiated γ-rays are stopped, while the penetrating muons get detected after the absorbers. Beam induced background rates in the scintillation detectors, originating e.g. from neutrons, are counteracted by coincidence conditions between different scintillation detector layers.
The trigger layer consists of 22 plastic scintillator bars with a rhomboidal cross-sectional shape to ensure an overlap of the bars in the direction of the incoming particles. Each bar is read out coincidentally by two photomultiplier tubes (PMTs). The PMTs have an internal mu-metal shielding to minimize the effect of potential magnetic field tails caused by the spectrometer magnets. To avoid bulky high voltage cables, the PMTs are used with active bases which transform an applied control voltage of 0 - 5 V to the necessary high voltage for the PMT.
The veto layers are made from plastic scintillation detectors. In contrary to the trigger layer, the veto layers consist of only three segments and are read out by silicon photomultipliers (SiPMs). For these layers, the same readout concept is used as developed for the DarkMESA veto detectors. Therefore, the experience gained at DarkMESA is invaluable for MAGIX.
The lead absorber layers are designed with a width of 950 mm. Due to lead being a very soft material, the absorber layers are made of antimonial lead which is much more rigid compared to pure lead.