LAC Description
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Each LAC module has a multi layer structure with lead sheets and scintillator bars, a configuration that provides the best agreement between good energy resolution and high neutron detection efficiency requirements. It consists of 33 layers, each composed by a 0.20 cm thick lead foil and NE110A plastic scintillator bars with average width 10 cm and constant thickness 1.5 cm. The module thickness corresponds to 12.9 radiation lengths and 1.0 adsorption lengths. Teflon sheets with 0.2 mm thickness separate scintillators from lead while 0.2 mm thick Teflon strips between each pair of contiguous scintillators bars avoid optical crossover. Each layer is rotated by 90^deg to form a 40*24 matrix of 10*10 cm² cells.

The bar width increases going from the inner side toward the outer to guarantee the tapering required by the CLAS geometry. The surface exposed to particle fluxes is 217*400 cm². The module is vertically divided into an inner and an outer part to improve electron/pion discrimination. Scintillators lying (for the inner and outer part separately) one on top of the other with the same orientation form 128 different stacks. The electromagnetic shower originate in the lead sheets and propagate through the layers: the energy adsorbed in the active material produces a light pulse that is collected at both scintillator ends with a Lucite light guide coupled to scintillators with an air gap an a 7.0*0.4 cm² area. Being the coupling area smaller than the scintillator cross section, the remaining scintillator surface was protected with Teflon to prevent scratches from the aluminum structure.

The collected light is summed separately for each stack before a EMI 9945A photomultiplier. Therefore the light pulses emitted from 8 different scintillators are summed up on a single photomultiplier that is placed on the top surface of the LAC module several centimeters away from the scintillators. To reduce photocatode non homogeneity effects, the light guides are glued together before coupling to photomultiplier. Scintillators placed in the inner and the outer parts are coupled to different photomultipliers. Monte Carlo simulations showed that the LAC performances are strongly affected by the efficiency of light transmission and collection. For this reason, special attention was devoted to the production of high quality scintillators and light guides whose results have been already reported. In particular, we selected scintillators with a light attenuation length longer than 300 cm and light guides that provide about 5 photoelectrons/MeV when coupled to a EMI 9945A PMT.

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