  Tetraphenyl-butadiene (TPB) is a widely used fluorescent wavelength-shifter.
A common application is in liquid-argon-based particle detectors, where
scintillation light is produced in the UV at 128 nm. In liquid argon
experiments, TPB is often employed to shift the scintillation light to the
visible range in order to allow detection via standard photomultiplier tubes.
This paper presents studies on the stability of TPB with time under exposure to
light. We also examine batch-to-batch variations. We compare
scintillation-grade TPB to 99% pure TPB response. In the 99% pure samples, we
report a yellowing effect, and full degradation of the TPB emission-peak, upon
extended exposure to light.
