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