Trigger Veto System

The trigger veto system of each spectrometer consists of one segmented layer of plastic scintillation detectors and a flexible system of additional scintillation detectors and lead absorbers. 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 are used for the fast timing and energy-loss measurements and provide the basic hit and position information for a triggered read-out of the time projection chamber.
Cosmogenic muons travel in the same direction as electrons inside the spectrometer. In order to reject these muons, an absorber is needed in which electrons and electron-initiated γ-rays are stopped, while the penetrating muons get detected after the absorber. 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 segments with a rhomboidal cross section to ensure an overlap of the segments in the direction of the incoming particles. Each segment 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 remaining scintillation detector layers are also called veto layers. In contrary to the trigger layer, the veto layers consist of only four segments, and they are read out by silicon photomultipliers (SiPMs). For these layers we use the same readout concept as developed for the DarkMESA veto detectors. Therefore the experience gained with DarkMESA is invaluable for MAGIX.

The lead absorber layers are designed with a width of ~ 1 m. 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.