Standardisation of 18F by a coincidence method using full solid angle detectors.

A solution of 18F was standardised with a 4πβ–4πγ coincidence counting system in which the beta detector is a one-inch diameter cylindrical UPS89 plastic scintillator, positioned at the bottom of a well-type 5″×5″ NaI(Tl) gamma-ray detector. Almost full detection efficiency—which was varied downwards electronically—was achieved in the beta-channel. Aliquots of this 18F solution were also measured using 4πγ NaI(Tl) integral counting and Monte Carlo calculated efficiencies as well as the CIEMAT–NIST method. Secondary measurements of the same solution were also performed with an IG11 ionisation chamber whose equivalent activity is traceable to the Système International de Référence through the contribution IRA-METAS made to it in 2001; IRA's degree of equivalence was found to be close to the key comparison reference value (KCRV). The 18F activity predicted by this coincidence system agrees closely with the ionisation chamber measurement and is compatible within one standard deviation of the other primary measurements. This work demonstrates that our new coincidence system can standardise short-lived radionuclides used in nuclear medicine.