Water transparency

  In Super-Kamiokande, the maximum distance of Cherenkov light traversal reaches 60m. The attenuation of Cherenkov light in water must be measured precisely, because the energy determination is affected by the water transparency.

The water transparency is measured two ways: direct measurement using laser and CCD camera, used for finding the absolute value of the water transparency; and using the decay electron events from stopping muons, or using penetrating muons observed in Super-Kamiokande to check the time variation of water transparency on an (1week) time scale.

Direct measurement

Fig 1 shows the system for the direct water transparency measurement, which consists of laser, diffuser ball, and CCD camera. For the light source, a N2 laser and a dye module are used. This dye module is able to produce mono chromatic laser output ranging from 337 nm to 600 nm. The light is split, one goes to a 2-inch PMT for monitoring the light intensity and another goes to an acrylic diffuser ball via an optical fiber. MgO is used for diffusing light in the ball. The light from the diffuser ball is received by a CCD camera, Fig 2 shows the picture of the light from the diffuser ball taken by CCD camera. The advantage of using CCD camera is that the effect of the scattering light is removed if only the region of the ball on a CCD view is used for measuring pulse height of the ball.

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Figure 1: The system of the direct water transparency measurement.


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Figure 2: The picture of the light from the source taken by CCD camera.

The total number of received photos by CCD camera is measured in various positions, and it is normalized by the monitoring PMT data. For example, Fig 3 shows the result for 400nm wavelength light. The derived water transparency is 72.09m. This measurement is applied to various wavelength light, and the result is summarized in Table 1.

watradat.eps.gif (5952 ?o?C?g) Figure 3: The result of the direct water transparency measurement. The horizontal axis is the distance between the diffuser ball and CCD camera, and the vertical axis is the normalized charge detected by CCD. The wavelength is 400nm.
Wavelength (nm) Water transparency (m)


Table 1: The water transparency obtained by the direct measurement in various wavelength light.


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