Wavelength Shifters (WLS)

The Photo Multiplier Tubes (PMT) are used in SNO to detect the light emitted in the reactions described before. However, the PMT's can only detect certain wavelengths of light. In order to improve the frequency response range of the PMT's, Wavelength Shifters (WLS) can be added to the heavy water.

Wavelength shifters are molecules that shift photons from short to longer wavelengths by fluoresence. By adding WLS to SNO, UV light could also be detected. This would increase the number of neutrinos that SNO would detect, and would also help lower the energy threshold for photon detection.