Precise energy calibration is an important characteristic of Super-Kamiokande and is essential for solar neutrino measurement. In these sections, three methods of energy calibration are described: using monoenergetic electrons from the LINAC [here]; using a radioactive source for what we call ``Nickel calibration''[next]; and using decay electron events from stopping muons [after next].
For MSW analysis using the solar neutrino energy spectrum, the systematic error of absolute energy scale must be less than 1%. In Super-Kamiokande, ``LINAC calibration'' is used for precise absolute energy calibration. The advantage of the LINAC is that the electron energy is monochromatic and can cover the full energy range relevant for solar neutrinos, 515MeV. Another advantage compared to the Nickel calibration, which is described in the next section, is that a direct calibration is available using electrons. Note that solar neutrinos are observed using electron signals from scattering. In addition, more precise position calibration and direction calibration is available using a collimated beam from the LINAC.
The schematic view of LINAC calibration system is shown in Fig 1. The LINAC, which was originally for medical purposes, is a model ML-15MIII from Mitsubishi. The beam pipe is evacuated to less than torr, and the desired beam size and momentum are obtained by collimators and magnets. On the top of the Super-Kamiokande tank holes are welded every 2.1m along the -x-axis. We insert vertical beam pipe into the detector through these, and the length of the vertical beam pipe is variable, so we can do LINAC calibration at various positions in the inner detector. The beam intensity is adjusted to 0.1 electron per bunch so that almost all events are single electron events. The bunch width is 2sec, and the maximum rate of bunches is 60Hz. Plastic scintillators acting as a trigger counter and veto counters are placed at the end of the beam pipe as shown in Fig 1.
The energy of electrons which travel through the beam pipe is measured by a Ge detector. For estimating the systematic error of the beam energy determination in the LINAC, the relation of observed momentum by Ge detector to the magnet field of magnet 1 is used, as shown in Fig 2. The observed momentum can be fitted by a line, and the deviation from that line is less than 0.3%.
The specifications of the LINAC are summarized in Table 1. The results of LINAC calibration, the position, angular, and energy calibrations, are described elsewhere (e.g. see Y.Koshio's PHD thesis, page 69, in the publications).
Table 1: LINAC specification.
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