Calibration

Relative gain of the PMTs

  The high voltage of each PMT is set in order to make the gain of each PMT the same. Each PMT has a standard high voltage value supplied from Hamamatsu Co. which was measured just after its production by using three methods: DC light source; Xe light source; single photo-electron distribution, However, we have re-calibrated the gain of all PMTs by ourselves in Super-Kamiokande, because a possible long term drift of the calibration system at Hamamatsu may have caused a systematic gain difference.

The relative gain was measured by the system shown in Fig 1. The light generated by a Xe-lamp is passed through an ultraviolet (U.V.) filter and injected into a scintillator ball via an optical fiber. The scintillator is an acrylic ball with BBOT scintillator (wavelength shifter) and MgO powder. This BBOT scintillator absorbs U.V. light and emits light the wavelength of which is near that of Cherenkov light. MgO powder is used for diffusing light in the ball. The intensity of the primary U.V. light is monitored by a monitoring system, and one of the outputs is used for a trigger, as shown in Fig 1.

In this system, the high voltage value of each PMT is set so that the corrected Q of each PMT is same as for all others. Here the ``corrected Q'' is pulse height corrected for light attenuation, acceptance, and uniformity of the scintillator ball. It is further normalized by Xe monitor pulse height. This measurement is done at various positions of the scintillator ball, and settings of the high voltage value. Fig 2 shows the corrected Q distribution of all PMTs after the adjustment. The relative gain spread is 7%. This remaining difference is later corrected in software.

xe-sys.eps.gif (13779 ?o?C?g)

Figure 1: The relative gain measurement system.

 

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Figure 2: The corrected Q distribution of all PMTs in the relative gain calibration.

 


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