The Effect of Wavelength Shifters on Water Cherenkov Detectors

In recent years several experiments have been planned to measure the proton lifetime and, among those under construction, some detectors [ 1 3 ] use large amounts of water as a source of protons and Cherenkov light (CL) for detection of p-decay products. The dynamical interpretation of the events and the vertex reconstruction are the main problems in water Cherenkov detectors; these problems can be solved by measuring with good accuracy the energy and direction of p-decay products. In such a case, one c a n discriminate between real and background events, the main source of the latter being the interactions induced by atmospheric neutrinos deep underground. In addition, such detectors can be used as well to observe neutrinos from point cosmic sources, both through the ~e + P capture reaction and through the /;e + e elastic scattering reaction. Since the last reaction roughly maintains the direction of the incoming Pe flux, it is also important in this case to determine the e direction in order to correlate any event with a possible cosmic source. Usually, the efficiency of detection is improved by adding a wavelength shifter (WS) in water, which absorbs the ultraviolet CL and re-emits it in the blue region of the spectrum, at wavelengths where photocathodes are more sensitive. However, the re-emitted light is isotropically distributed and does not maintain the original direction, when the WS is added in the usual concentration of about 11 mg/1 of water [4,5].