There are two methods to measure the water transparency using real data in
Super-Kamiokande; decay electron events from stopping muon [this section] and penetrating muon events [next section].
Unlike the direct measurement described above, we can not measure the water transparency
dependent on light wave length in these methods. It can not directly be compared with the
direct measurement, but these measurements can be done in real time and continuously. The
value obtained by these methods is defined as the parameter which indicates the water
condition. From these methods, the stability of water quality can be checked, and the time
variation of water transparency measured by decay electron is used for determination of
Decay electron events from stopping muons are used for the measurement of the average water transparency. The selection criteria of a decay electron event are that it follows a stopping muon after 1.5 8sec and its , where Neff is the effective number of signals counted in units of photoelectron. (For strict definition of Neff see Appendix B.2 in Y.Koshio's PHD thesis on the publication page.) Fig 1 shows the typical event pattern of a decay electron event, and we can clearly see the Cherenkov ring. Only the hit-PMTs with hit timing in a 50nsec window and with opening angle from the reconstructed direction of the decay electron between 32 and 52 are used for getting the water transparency in order to exclude the reflected or scattered light. We can get the water transparency from the relation between the corrected Q of the each hit-PMT and the distance from the vertex to each hit-PMT. Here, corrected Q is a pulse height corrected for acceptance. Fig 2 shows the relation, and the water transparency is calculated to 8518cm. Fig 3 shows the water transparency as a function of time. After October in 1996, it is higher and higher, because we clean up the filter. This value is used for the correction of the energy reconstruction .
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