The only way to reduce this background is to detect the cosmic muons which produce the neutrons. The detection has to happen far enough from the central detector in order to prevent neutrons produced by undetected muons from penetrating into the detector. This can be achieved if about 1 metre of steel sits between the veto counters detecting the muons and the central detector. Thus the only solution was to put large scale veto counters into the the walls and the roof of the iron bunker roughly in the middle of the shielding. Those veto counters are drawn in red in the above picture already.
All muon tracks shown cross the new veto counters and are detected.
Thus potential successive events in the central detector can be precisely identified and discarded. Detailed simulation calculations showed that a reduction of the background by a factor of 40 could be expected. Such a reduction of the background results in an enhancement of the sensitvity for the search for neutrino oscillations by roughly a factor of 10 assuming the same amount of measuring time. Within the 2-3 m thick walls of the 7000 ton heavy iron bunker a total of 300 square metres of plastic scintillators were installed to detect penetrating muons. The picture below shows a sketch of the iron bunker , partially cut away to show the new veto counters in red .