A method for monitoring γ-radiation from an extended source with uniform sensitivity

Quantitative in vivo measurement of carbohydrate flow within the phloem vasculature of a living plant can be made by labelling the carbohydrate with the short-lived isotope 11C. This requires measurement of a time sequence of total tracer within an extended region of the plant. Once tracer has been transported into the field of view any subsequent movement must not be registered as a change in observed count rate. Hence we require a method to monitor tracer level within an extended region with no sensitivity variation over this region. This can be achieved, for a planar region, by introducing a variable thickness attenuator between the radiation detector and the planar region, to compensate for sensitivity changes with distance. Using this method, a sensitivity variation of 30% was reduced to 5% for a detector placed 30 cm from a 30 cm diameter source plane.