Development of a compact multiprobe system for monitoring positron-emitting tracers in plant stems

Nondestructive monitoring of positron-emitting tracers in plant bodies at multiple points, including points separated by large distances, has been realised with the positron multiprobe system (PMPS) to investigate graminaceous plants, whose stems and leaves have simple shapes. Recently, the translocation of photosynthates into fruits has been studied intensively using a 11C tracer. The relatively complex shapes of the stems and leaves of these plants sometimes prevent the detector heads of the PMPS from approaching and being fixed to the target stem properly because of its relatively large and heavy detector heads based on photomultiplier tubes. Owing to the compactness, lightweightness and recent advances of silicon photomultipliers (SiPMs), fabricating compact and lightweight detector heads has become possible. In this study, we developed a compact PMPS (CPMPS) using SiPMs and successfully demonstrated its capability for monitoring a 11C tracer in strawberry stems. Moreover, we found that energy-window filtering markedly reduced noise events without radiation shielding. The dominant ionisation events detected by the CPMPS were Compton scattering and subsequent photoabsorption of a single 511 keV gamma ray, suggesting that the ionisation events of single-gamma-ray emitters, such as 42K, 43K, 54Mn, 59Fe and  65Zn, can be detected by the CPMPS. The developed CPMPS can also be applied to study the physiology of other plants with intricately shaped stems and leaves, such as the tomato and eggplant.