[Halld-pid] impact angle and paddle arrangement

[Beni Zihlmann]

Hi All,
In the previous email from me about the TOF I stated that one can expect 
impact angles
of tracks  up to 35 degree at the location of the tof.
Attached is a short sketch of two paddles 6cmx2.5cm in cross section and 
a circle around
with 6.6cm diameter indicating the PMT with shielding. In order to 
safely put them on
top of each other there needs to be a gap of about 5mm (roughly.) The 
red line indicates
a trajectory with a 35 degree impact angle.
 From this I would conclude that staggering is not an option. We need 
light guides that bend
sideways to provide enough space for the PMT+Shielding.
Moreover in the central region the rates are very high as shown in the 
CDR figure 2.1
http://argus.phys.uregina.ca/gluex/DocDB/0009/000994/004/TOF_cdr.pdf
which suggests that we should cut the innermost paddles in half so the 
hight is only
3cm. This would not only cut the rate in half but would increase the 
timing resolution
substantially I would assume. (Note that in fig. 2.1 only the full 
length paddles are shown).
A equal important argument if not more is Elton's point that if the PMT 
would be directly
on the scintillator hits that are very close to the edge of the paddle 
would generate
enormous signals in the PMT potentially damaging the PMT in the long run 
as well as
saturating the PMT/electronics chain for too long a period which is a 
problem at high
rates.

cheers,
Beni

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