Activity Measurements of 3 H Using the TDCR Method and Observation of Source Stability L. Mo a,* , L. J. Bignell a, b , T. Steele a, c and D. Alexiev a a Australian Nuclear Science and Technology Organisation New Illawarra Road, Lucas Heights 2234 NSW Australia b The University of Sydney, Australia c Australian National University, Australia _________________________________________________________________________ ABSTRACT Activity measurements of 3 H were performed using the Triple to Double Coincidence Ratio method for the 2009 international comparison organized by the Bureau International des Poids et Mesures (BIPM). The activity obtained by using the FPGA (Field Programmable Gate Array) Acquisition System and Software Event Analysis was compared to that using MAC3 (Module d’Acquisition de Coincidences triples). Agreement to better than 0.4% was obtained. The stability of sources prepared in Ultima Gold TM , Ultima Gold TM LLT, Insta-Gel ® Plus and Optiphase “Hisafe”-3 over 140 days is presented. Key words: TDCR, 3 H , activity measurement, liquid scintillators, FPGA ______________________________________________________________________________________________________________ * Corresponding author. Tel.: +61-2-9717-9078; fax: +61-2-9717-9265. E-mail address: lmx@ansto.gov.au