Speaker
Emanuele Leonora
(INFN-section of Catania)
Description
The detection element of many deep-sea Cherenkov detectors is the so-called
"optical module", which consists of a pressure resistant glass sphere with
photomultipliers which are optically coupled to the glass with optical gel.
Earlier studies indicated that source of noise for the optical module is not only the
photomultiplier itself but light could come from the glass of the pressure vessel
directly facing the photocathode of the pmt, produced by scintillation excited by
energy deposited through radioactive decays in the glass material itself or by
Cherenkov effect in the glass.
The main element in the glass composition that causes light production is Potassium
40, followed also by natural decay chains (Uranium, Thorium), Cerium, Iron and
other lanthanides.
A study has been conducted to quantify the contribution to noise pulses due to the
external glass sphere comparing rate and charge distribution of dark pulses and after
pulses measured on PMTs in two different configurations, naked and coupled into a
glass half-spheres by means of optical gel.
Measurements have been performed using 10-inch R7081 Hamamatsu PMTs, used
in optical modules with a single large area photomultipliers, and 3-inch R12199-02
Hamamatsu PMTs, which will be used for multi-PMT optical modules. Different glass
pressure-resistant vessels have also been tested. Main results obtained so far show
clearly as in a system composed of a PMT coupled with a glass vessel the light
generated internally in the glass influences the rate and the charge distribution of
the PMT noise pulses.
Authors
Emanuele Leonora
(INFN-section of Catania)
Sebastiano Aiello
(INFN-section of Catania)
Valentina Giordano
(INFN-section of Catania)
