Near Detector-Chiaki,



The near (front) detector was designed to make detailed studies of the neutrino beam spectrum and profile, and measure neutrino cross sections using different technologies.

The near detector complex is situated at 300m downstream from the horn-target. It consists of two major parts: 1kton water Cherenkov detector and fine-grained detector (FGD). The latter sits behind the former so that a part of escaping particles from the 1kton detector can be detected by the FGD. Both parts use water target just as Super-Kamiokande so that the uncertainty in the neutrino cross section on water can be cancelled out.

The 1kton detector is a 1kton water tank equipped with 680 20" photomultipliers (PMTs) facing inward in the inner part of the detector (inner detector) and 68 8" PMTs facing outward in the veto region (anti-detector). The inner detector observe Cherenkov light produced by charged particles that enter or are produced and measure their directions, energies and identification if possible while the anti-detector sevrves to veto charged particles that enter or are produced outside the inner detector. This way most of cosmic ray muons and neutrino beam induced muon produced in the dirt in front of the detector can be vetoed. This detector is a sort of mini-version of Super-Kamiokande which is a 50kton Cherenkov detector used as far detector. Because of this similarity between the 1kton detector and Super-Kamiokande, some of systematic uncertaintities can be eliminated. It can measure muon energy better than the FGD as long as the muon is contained in the inner volume. It also has a capability of detecting multi-particle events and measuring electron/gamma energy with a good accuracy.

The FGD is actually made of four components. The first major component is a scintillation fiber tracking device (scifi) that sandwich 20 6cm thick water targets. Each scifi plane of 2.4 x 2.4 m^2 have two sheets of 0.7mm diameter scintillating fibers (1142 fibers per sheet) for the x and y coordinates that are read by image intensifier tubes(IITs) with CCD camera. A study done with small prototype achieved a spatial resolution of 280 microns and an efficiency of over 99 %. Thanks to this excellent spatial resolution, this device has better capability of defining the fiducial volume than the 1kton detector. The scifi is surrounded by a pair of arrays of veto scintillation counters that veto cosmic rays and give timing information.

The scifi is followed by an array of leadglass calorimeter, which is made of 600 leadglass blocks salvaged from the Topaz experiment at TRISTAN. This calorimeter serves to identify electrons and measure their energies.

The last component of the FDG is a muon range detector which originally was used for the Venus experiment at TRISTAN. It measures muon energy by the range method up to 3 GeV/c with a resolution of 14% and is made of 7.6x7.6 m^2 wide 0.1/0.2 m thick iron plates that are sandwiched by 900 drift tubes.





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