Position detection of a beta particle emitter by utilizing self-absorption in a scintillation fiber

Self-absorption in a plastic scintillation fiber can be utilized to determine the incident position of single beta particles. A dichroic mirror directs scintillation photons with shorter wavelengths to one Si photomultiplier and those with longer wavelengths to another. An index calculated from the two signals is a monotonic function of the distance between the tip of the fiber and the incident point. Once this relation is known, one can determine the distance from the two measurables. In an experiment, such a calibration curve was acquired to detect the position of a ⁹⁰Sr source up to a distance of 240 cm. The average total number of photoelectrons for a single beta particle was about 15-17. Depending on the propagation distance in the scintillation fiber, they were unevenly divided by the two photodetectors.