The chamber readout scheme consists of three parallel wire planes: the first facing the drift region, with horizontal wires (Induction I plane); the other two with the wires at ±60° from the horizontal direction (Induction II and Collection planes). The 3D image reconstruction of the event is provided by the signal of the electrons crossing the three wire planes, together with the measurement of the drift time (performed using a photomultiplier system). I NTRODUCTION A ND O BJECTIVES The ICARUS detector is a very massive liquid Argon Time Projection Chamber aimed at the study of some of the fundamental issues of astroparticle physics such as solar and atmospheric neutrino interactions, neutrinos following a Supernova explosion, neutrino oscillations with beams from particle accelerator,nucleon decay for some channel predicted by GUTs. The main challenge of the ICARUS trigger system is to be able to detect the events induced in the detector by rare phenomenon such as the Supernova explosions. For this purpose we have designed a segmented trigger system. The proposed design is based on three trigger levels. The first one has been tested on a small LAr TPC detector, the architecture and the design of the other two levels are presently being defined. A.G. Cocco*, M.Della Pietra*, A.Di Cicco*, P.Di Meo*, A.Ereditato*, G.Fiorillo*, P.Parascandolo*, R.Santorelli*, P.Trattino*, B.Baboussinov°, S.Centro°, F.Pietropaolo°, S.Ventura° *Università Federico II and INFN, Sezione di Napoli Via Cinthia, I Napoli, Italy °Università degli Studi di Padova and INFN, Sezione di Padova Via Marzolo, Padova, Italy The trigger scheme is based on three levels: the Local Trigger Control Unit (LTCU) which is remotely controlled by RS232, receives the Analogue Wire Sums from the CAEN V791 (front end board serving 32 input channels) and from PMTs and provides the first trigger level proposals. Each analogue crate can host one LTCU; the Trigger Control Unit (TCU) implements the actual trigger logic. It performs a topological analysis of each event distinguishing between local and global ones on the basis of number and pattern of fired pixels. The whole detector can be handled by four TCU modules; the Trigger Supervisor (TS) provides the trigger signal distribution (sending the trigger requests to the V783), control, analysis and statistical monitoring of the system. The analogue stage provides: filtering and discrimination. The digital stage provides: masking of the noisy inputs; setting of the comparator’s threshold; test pulsing; setting of the time windows for rate measurements. D ATA A CQUISITION O VERVIEW P HYSICS M OTIVATIONS A ND S EGMENTATION T RIGGER A RCHITECTURE T HE F IRST T RIGGER L EVEL -L OCAL T RIGGER C ONTROL U NIT - T HE S ECOND T RIGGER L EVEL -T RIGGER C ONTROL U NIT - The Trigger system has been designed to be able to detect some of the most interesting, but extremely rare, events such as those induced by Supernova explosion. Due to the high rate, such an event could not be handled if one aims at storing for each event the whole detector image. However, the events induced by a Supernova explosion are low energy events which have a little occupancy of the detector, as shown by Montecarlo Simulations. The proposed design has the capability to select the volume around the event and only readout the few corresponding channels (local events), leaving the other unaffected channels free to record further events, thus reducing the global dead time. The elementary unit which contributes to define the picture of the event is called pixel and it is defined by the coincidences between 9 AWS from the Collection plane and 9 AWS from the Induction II plane. The picture includes simulated neutrino interactions and passing muons. The energy releases in the detector as seen in the two projections (top) and as reconstructed by the trigger (bottom) are shown. The fired pixels are displayed in red. The Trigger Control Unit, which is a VME module, performs coincidences amongst the fired wires from the different planes and with PMTs and validates the trigger proposals passing their addresses to the next trigger level. It provides analysis of each incoming event distinguishing between global and local ones. It determines the nature of the event on the basis of the energy deposition, the detector occupancy, the 2D/3D pattern of fired pixels. The second trigger level is preliminarily implemented on the UNIBOARD. T HE T HIRD T RIGGER L EVEL -T RIGGER S UPERVISOR - The Trigger Supervisor which is a VME module, validates the trigger request according to the DAQ status, performs the trigger distribution and monitoring. In addition it performs statistical computations such as the number of valid triggers, the number of global triggers, the number of local triggers, etc. T EST P ERFORMANCE O F T HE L TCU W ITH A P ROTOTYPE D ETECTOR C ONCLUSIONS The Trigger system of the ICARUS experiment is based on three levels and a segmented architecture. The LTCU, that processes the signals from readout wire planes of the detector has been prototyped and tested on a TPC detector in order to evaluate the noise conditions and fake trigger rate due to electronic noise. A new version of the LTCU is presently being mounted. The architecture of the two higher trigger levels are presently being defined. We measured the LTCU efficiency on a LAr TPC detector exposed to cosmic-rays, using a PMT viewing the LAr scintillation and working in high efficiency conditions. It is used to provide the external trigger. The trigger proposals from the LTCU have been recorded by the DAQ. The efficiency has been evaluated as the fraction of trigger proposals generated wrt the total number of detected events as a function of the tracks incidence angle. The maximum measured efficiency is around 80%. This result can be ascribed to the noise of the board, estimated around 20 mV and due to the absence of a ground plane. The new board version, which will be soon available is based on eight layers layout and additional shielding against environmental noise. E ~11 MeV E ~12 MeV E ~900 MeV E  ~10 GeV 14th IEEE-NPSS Real Time Conference 2005, June , Stockholm-Sweden Presented by Adele Di Cicco