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KamLAND is an experiment that will search and detect rare events such as antineutrinos emitted from nuclear reactors, from the Earth's radioactive elements, atmospheric neutrinos, solar neutrinos and nucleon decays. The success of the experiment and the quality of the data will heavily depend on how much the background can be suppressed and how well it will be understood. The central detector of KamLAND is surrounded by several thousand cubic meters of pure water which suppress the background in several ways. Passively, it absorbs the radioactivity coming from the walls. These will be coated with a special paint. Also, the water degrades the energy of the fast neutron coming from the rock. These neutrons originate from muon induced spallation. A priori, these neutrons can be a dangerous background. Indeed, they can sneak inside the counter, scatter on a proton, slow down and capture on another proton. Due to the difference of light emitted by a proton and an electron, the first proton might be misidentified as an electron and the whole scenario perfectly mimics an anti-nu reaction. Fortunately, only the neutrons produced in a 1 meter thick annulus of rock surrounding the experiment contribute to the background, the other have their energy degraded too much to be a problem, and it has been estimated that this process will therefore not contribute to the background. Of course, the muons will also create neutrons by spallation in the water surrounding the detector. However, these will be in coincidence with the muon which can be detected because of the Cerenkov light that it emits in the water. Therefore, these events will be actively vetoed but it is evident that the efficiency for this active veto must be high.

The Cerenkov veto

There is a major difference between the veto of KamLAND and the veto of the previous experiment Kamiokande. In the Kamiokande design, the Cerenkov light of the incoming muons was detected directly by an array of phototubes looking at the muons. They are mounted at the boundary of the fiducial volume and they are looking outside. The design of KamLAND is different. The PMT's of the veto are mounted against the walls of the pit and they are looking at the detector which is covered with a high reflectivity material: TYVEK. The Cerenkov light emitted by the incoming muons will be detected after they bounce back against the diffuser. The PMT's of the Kamiokande experiment will be reused to build this veto. Several hundreds of them will be mounted on the top, the bottom and the side of the veto counter; a total coverage of about 3%. These muons will cross the apparatus at a rate of about 0.4 Hz. They will typically go through 1 meter of water before entering the central detector. Assuming a 20% photocathode efficiency, one expects to detect about 100 photoelectrons, a number well above what is expected for the dark current counting rate.

The plastic veto

Because of the chimney, there is a large unvetoed area right on top of the detector where the cosmic ray muon flux is the highest. A 5 by 5 meter2 plastic veto will be built on top of it. The side of the chimney will also be surrounded by plastic plates.

The scintillator transparency monitor

A muon track going through the counter can be used to monitor the stability of the PMT's and the transparency of the scintillator. The Cerenkov PMT's will be operated in coincidence with the segmented plastic detector located on top of the chimney. Such a coincidence will therefore identify a well defined muon track and these events can be accumulated over the lifetime of the experiment to study the stability of the detector.

Last updated: September 8, 1999