Construction and implementation of the NIST triple-to-double coincidence ratio (TDCR) spectrometer

A description of the triple-to-double coincidence ratio (TDCR) spectrometer recently constructed at NIST and results of tests to assess its operating characteristics are presented. Measured activities for previously calibrated solutions of 3H (tritiated water) and 63Ni agreed with certified activity values to within 0.04% and 0.2%, respectively. Agreement between measurements of solutions containing 90Y using the TDCR and the CIEMAT/NIST efficiency tracing method was achieved to within 0.7% and within the respective experimental uncertainties. A difference of 5.3% was observed between TDCR measurements and the certified massic activity value for a calibrated solution containing 54Mn, but because of the large uncertainty on the TDCR result (3.6% relative standard uncertainty), the values agree to within the experimental uncertainties. Agreement to within experimental uncertainties (2%) for 103Pd was achieved with the TDCR and CIEMAT/NIST methods, but not between TDCR and isothermal calorimetric measurements. TDCR results for a 204Tl solution were in excellent agreement (maximum difference of 0.2% with any other method, with combined standard uncertainty of 0.45% on TDCR result) with results obtained using three other methods.