Decay electron events can also be used for the energy calibration. The selection criteria for the decay electron events are described in other section. The energy spectrum of decay electron is
|
(1) |
,where Ee is decay electron energy, G is Fermi coupling constant, and 
is muon mass.
The maximum energy of a decay electron is 53MeV, and the mean energy is 37MeV.
Fig 1 shows
Neff
distribution of observed decay electron events both in measurement and
in Monte Carlo simulation. The measured and Monte Carlo data are consistent except for
very low energy region. The difference of low energy region is caused by a gamma ray
emission from radioactive nuclei such as 16N generated by capture of a stopping 
to an oxygen
nucleus in water. The ratio of the peak positions of the Neff distributions between the
measured and the Monte Carlo data is as follows,
.
This confirms that energy scale measured by decay electron event is reproduced by Monte Carlo to within 1.2%, with no observed position dependence.
 |
Figure 1: Neff distribution of observed decay electron event both in measured and Monte Carlo simulation. Neff is the effective number of signals in units of photoelectron. |
