Standardization of $^{18}$ F by Means of $4\pi({\rm PS})\beta-\gamma$ Plastic Scintillator Coincidence System

The present work describes the procedure developed for the standardization of
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F by means of a coincidence system using a plastic scintillator detector in 4pi geometry, named 47pi(PS)beta - gamma, which was developed at the Nuclear Metrology Laboratory in IPEN, Sao Paulo, Brazil. The main advantage of this detector system is the ability to perform primary standardizations without the need for a coating of a metal layer on the radioactive source film for rendering it conductive, as usually necessary for proportional counter measurements. The measurements were also performed with a conventional 47pi(PC)beta - gamma coincidence system, which makes use of a 4pi proportional counter for charged particles or X-ray detection coupled to a pair of Nal(Tl) scintillation counters. The gamma-ray window was set to measure the 511 keV gamma-rays produced by positron annihilation. The detector efficiency was changed by moving the electronic discriminator threshold. The observed activity values were extrapolated to 100% efficiency and the results showed good agreement between the two detector systems.