Measurement using real data in Super-Kamiokande
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
energy.
Measurement by decay electron
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 
8
sec 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 .
|
|
|
|
Figure 1: |
The typical event pattern of a parent muon event (upper), and decay electron event (lower). The time difference between these two events is 2.54 microsec. |
|
|
Figure 2: |
The relation between the corrected Q of the each hit-PMT and the distance
from the vertex to each hit-PMT. The calculated water transparency is 8518cm.  |
|
|
Figure 3: |
The water transparency as a function of time. The decay electron events from stopping muon are used for this calculation. |
