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V
0.4
0.2
0
-0.2
-0.4
-0.6
-0.8
-1
0 500 1000 1500 2000 2500
ns
Figure 15: A typical one photo-electron pulse in AMANDA-B4
as such is affected by the presence of air bubbles, isotropizing photons near it. These bubbles
could be forming around it during the re-freezing of the melted ice following deployment.
The PM is housed in a 12-inches diameter sphere of 9 mm thick glass from Billings, rated for
660 atm. These spheres have less good optical qualities than the ones from Benthos, previously
used for AMANDA-A with EMI PMs. However, the Billings sphere had to be chosen, since
Benthos could not manufacture a sphere large enough to house a HAMAMATSU PM. Its transmissivity
is 90% from 400 nm and above, in the optical region, but it drops sharply with a 50%
cut-off at 364 nm whereas Benthos glass transmits light 30 nm further down (see Fig. 18). This
is unfortunate since in this wavelength range, the cathode still has a high quantum efficiency and
the Cherenkov light we wish to detect has a -spectrum (see Eq. 31). However, studies of
various wavelength-shifters are underway to remedy this loss [20, 41] in future OM deployments.
The optical contact between PM and glass is ensured with a ¥ 1 cm thick layer of optical
silicon gel from General Electrics. The only additional component of an optical module apart
from the PM itself is the base card holding the bleeder circuitry. A big advantage of this OM
is that all amplifiers, discriminators, ADCs, TDCs, etc. are located at the surface, connected to
the modules by 2000 m of cable. Thus, possible failure of the electronics has a higher chance
of being reparable and even upgrades are made possible over the years, adding flexibility to the
detector. Another advantage is the small need for the design and manufacturing of dedicated
electronics which must sit in the OMs. Once the PM has been potted in one of the two Billings
half spheres, these are sealed together with rubber tape and a metallic harness for mechanical
support is fitted around the OM.
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