⁵⁵Fe is a low-energy radionuclide that is difficult to measure and decays to a ground state of ⁵⁵Mn through pure electron capture (EC), accompanied by the emission of Auger electrons and low-energy X-ray. As iron is the main component of nuclear reactor building materials, significant amounts of ⁵⁵Fe have been produced in nuclear reactors and other neutron-producing nuclear facilities.

This study aims to develop an ⁵⁵Fe nuclide standard through the absolute measurement of ⁵⁵Fe activity and provides activity traceability services for ⁵⁵Fe measuring instruments to ensure the accuracy and consistency of the measurement results of calibration instruments.

The liquid scintillation triple-to-double coincidence ratio (TDCR) method was applied to determining the activity of ⁵⁵Fe. First, based on nuclear and atomic data of ⁵⁵Fe, the electron deposition spectrum of ⁵⁵Fe in a scintillator was calculated using a random atomic rearrangement model. Second, the counting efficiency of single-energy electron was computed based on the free parameter model. The total efficiency curve of ⁵⁵Fe was then obtained by summing the efficiency of all deposited electrons. Finally, the experimental counting efficiency was derived by measuring the TDCR value and combining it with the total efficiency curve to realize an absolute measurement of ⁵⁵Fe activity.

The experimental results show that correction factors for the asymmetric effect of photomultiplier tube (PMT) quantum efficiency obtained on test samples are between 1.001 and 1.005. The measured specific activity of ⁵⁵Fe is 94.15 kBq?g^-1 with a relative standard uncertainty of 0.45%. Experimental efficiency is better than 63% for double coincidence logic sum of liquid scintillation counter.

This study demonstrates that low relative standard uncertainty of ⁵⁵Fe activity could be achieved using the liquid scintillation TDCR method with high detection efficiency, and more consistent measurement results can be obtained after applying the asymmetry correction of PMT quantum efficiency.