Painted Crystal Signal Waveform
CsI crystal covered with the wavelength shifter laquer show a different
waveform. While the light going directly to the PMT is unaffected, the
light reflected at the surface undergoes the process of wavelength
shifting. Since this process takes a non-neglectible amount of time,
the part of the signal waveform coming from reflected light gets delayed
by ~5 to 10 ns. To verify this behaviour, the waveforms of three painted
and three unpainted crystals with cosmics were recorded. Each waveform
is an average of 20 events with approximately the same amplitude. Following
picture shows an overlay of the six signals (blue=unpainted, red=painted):
As can be seen, the maxima and the tails of the painted crystals are
delayed, but the risetime of the signal to about 50% of its maximum
value is unaffected. We do therefore not expect any timing degradation
due to the surface treatment with wavelength shifter laquer.
S. Ritt, 28 Jan 1997
