The VETO Power

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  • Topic: Data acquisition
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  • Published : March 18, 2016
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The Soudan Underground Laboratory is a 710 m (2090 mwe) deep laboratory in northern Minnesota, which has been operated by the University of Minnesota since 1980. It includes two experimental halls, each 15 m wide by 16 m high. The Cryogenic Dark Matter Search (CDMS II) occupies the 70 m long West Hall and the 5,500 ton MINOS Far Detector is located in the 82 m East Hall. The Soudan 2 Proton Decay experiment [1] stopped data-taking in 2001 and its kiloton calorimeter was finally removed from the back section of the West Hall in 2005, leaving its muon veto shield intact. This created a 13 m x 10 m x 40 m lab space located 2341 ft deep (2090 m.w.e.) surrounded by more than a thousand gas proportional tubes lining the walls, ceiling and floor. The veto tubes on the floor were removed since there is only ~1 upward-going muon per week and lots of gaps due to support structures. All the veto panels were pressure tested and run to HV under gas. Signals were observed from the preamps and noisy or dead channels were repaired. A new gas handling system was built, including gas checkers to monitor oxygen content in the input gas. In order to create a multi-user facility which could take advantage of a muon-shielded room, the CAMAC-based trigger logic was replaced by a PC-based system with custom electronics based on CPLDs, which then provides a database (GPS-based time stamp and track location) of every entering muon. It thus can be used as an offline muon veto for any experiment or screening device located inside its coverage and even has sensitivity to neutrons whose muons do not enter the cavern, via accompanying charged shower products at the cavern wall. In addition, as a large-area, moderate-granularity muon and electromagnetic shower fragment detector, it can be used to understand underground showers in general, and benchmark cosmogenic Monte Carlo simulations.

Figure 1. The layout of the Soudan Underground Laboratory, showing the location of the new muon-shielded experimental hall in relation to the running experiments. Experiments and screeners operating inside the shield are also shown.

Proportional Tubes
The veto shield panels are constructed from sheets of nested aluminum modules. Each module is an extruded form in which eight hexagonal wire chambers are arranged as a double layer of honeycomb cross section as shown in figure 2. The aluminum walls are ?? thick. Each wire chamber channel is ?? wide from flat-to-flat (inner dimension) and contains a single gold wire of ?? diameter strung down the center. The outer casing is grounded and the sense wire is held at an operating voltage between 2100 – 2500 volts, with a resulting gas gain of ?? The four wires of each layer are connected in series. Each module is thus a 21 cm wide double layer tube with one readout channel per layer. Most of the tubes are 7 m long, but shorter tubes are used to shadow openings or cover gaps. They filled with 90% - 10% Ar-CO2 at 1.12 bar.

Figure 2. Cross-section of the eight gas proportional chambers which compose one veto module or “tube”.

The end of each tube is connected to a two-channel preamplifier board, which reads out two layers independently. When a particle passes through the chamber, the resulting current ?? pulse is capacitively coupled to a ?? to produce a ?? signal in the preamp board. After amplification (how much??), if the signal is above the fixed threshold (?? mV) comparator, a one-shot creates a 1.2 μsec TTL pulse, which is then converted to TTL? differential signal and routed to separate readout electronics via 64-pin twisted pair ribbon cable. Each 64-pin cable is capable of accommodating 32 channels (16 tubes) of data. The preamp cards are also responsible for distributing the high voltage to the wire chambers. Both low voltage and high voltage are daisy-chained along the tubes and from module to module to form larger units called panels. Each wall panel has...
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