We consider 2" diameter photomultiplier tubes (PMT's) since they are well
matched to the cross sectional area of the scintillators, roughly 25 cm
.
We require a photocathode sensitive in the blue, and if we also require that a
signal of 10 photoelectrons (i.e. 
of the minimum ionizing peak in
Fig. 
) trigger a 15 mV, 50 
discriminator, then a
gain of approximately 
is called for.
A selection of Philips PMT's which satisfy these
requirements is presented in Table
Table: Options for detector phototubes. Prices are quoted for
lots of 200.
The gain of the XP2202 is too small and we do not consider it further. Timing is of some importance in this experiment, and even though the number of photons is high (in which case we likely trigger on photons from the center of the photocathode), the 5 ns transit time spread of the XP2212 may be uncomfortably large. This can only be confirmed with prototyping, but we might opt for the XP2282. This PMT not only has sufficient gain and small transit time spread, its rise time (1.6 ns) is even better than the very fast XP2020 and its mean transit time (19 ns) is 30% faster. (Of course, other manufacturers' PMTs will also be evaluated during prototyping.)
The magnetic field in the region of the photomultipliers is mainly transverse
(see Sec. ), and its magnitude is always less than 30 Gauss.
This means that the PMTs can be shielded with a standard off-the-shelf mu-metal
shield, for example the Magnetic Shield Corporation model 25P50
(
diameter by 5" long) which cost under $25 each for the
lots we are considering.
The mechanical and electrical design of the PMT base is straightforward. There are no special mechanical requirements, since there is plenty of room for the housing and we do not expect the bases to removed frequently. Electrically, the most important requirement is that the PMTs be powered with negative high voltage. This avoids the need for capacitive coupling at the output which leads to poorer timing and gain instability at high rate, although we need to electrically shield the photocathode from the surrounding grounded support structure. The voltage divider assembly depends on which PMT we choose, but this will also be prototyped on the basis of standard designs.