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strain put on them.
4.3 Optical modules
An optical module is basically a photomultiplier enclosed in a sphere of thick glass. The
photomultiplier is a 20 cm Hamamatsu R5912-02, a 14-stage version of the 12-stage R5912 tube,
modified for AMANDA (see Fig. 13). This was done in order to operate it at the gain of
needed to get PM pulses through 2000 m of cable without further amplification. It has a bialkali
photocathode sensitive to photons with wavelengths between 300 nm and 650 nm and a peak
quantum efficiency of 23% at 420 nm (see Fig. 19).
This PM was also chosen for its other qualities, such as a large photocathode area, good time
resolution, and low dark count. The voltage-divider circuit designed by Hamamatsu is optimized
to yield an good sensitivity at the single photo-electron level, which gives a limited dynamic
range of typically ten p.e.’s. It is slightly modified in its actual implementation in order to be
used either with a coaxial or with a twisted-pair cable [36](see Fig. 14). In order to get a high
efficiency and a good time resolution, grids (F) are used to focus the cathode electrons before
passing through the 14 dynode stages (D). Seen from the front, the PM- cathode is circular-
Figure 13: The 14-stage R5912-02 Hamamatsu photomultiplier
shaped, with a radius of 10 cm, but its actual curvature radius is 13.1 cm. The timing properties
measured with a short cable in the laboratory yield a time jitter of ¥ 3 ns FWHM, with small
variations between individual PMs. This value is measured to ¥ 7–10 ns in-situ, with larger variations
between PMs, a degradation largely due to the longer cables, between ¥ 1600 and 2100
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