Design of a highly sensitive position-sensitive detector based on redshifts in photoluminescence spectra

The degree of redshift in a photoluminescence spectrum can be applied for position sensing. Because a whole spectrum is not required, replacing a spectrometer with a combination of a dichroic mirror and two silicon photomultipliers (SiPMs) greatly improves its sensitivity. In experiment, a 1 mm-diameter scintillation fiber, a 3.2 mm-thick dichroic mirror and two 6 mm x 6 mm SiPMs were assembled. A 100 ns light pulse from a laser emitting at 377 nm was attenuated by neutral-density filters. When a single spot on the fiber was periodically excited, the output waveforms of the two SiPMs fluctuated. This was due to the statistical variation in the timing of the 377 nm photons entering the SiPMs. By correlating the pulse height of the averaged waveform and the distance between the excitation spot and the tip of the fiber, a calibration curve for position sensing over the distance of 60 cm was obtained. The geometrical efficiency for coupling the light reflected by the dichroic mirror to a SiPM was 0.919 and one for the transmitted light was 0.719. The loss came from the light spreading in the space around the dichroic mirror and the reflection on the mirror surfaces. This geometrical efficiency will be improved by using focusing lenses. Embedding the dichroic mirror and the tip of the fiber in a transparent medium will eliminate the reflection loss. This technology might be applicable for radiation detection.