IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 54, NO. 3, JUNE 2007 677 Absolute Activity Determination of Au Solid Source Using — Coincidence Counting Corrected by Monte-Carlo Calculation L. Mo, H. Y. Wu, and C.Baldock Abstract—For the commissioning process of the OPAL nuclear reactor of the Australian Nuclear Science and Technology Organ- ization (ANSTO), the thermal neutron flux is measured through the activity measurement of an activated Au wire, Au-Al (0.112% of Au) alloy wire and Au foil. The absolute activities of Au in the form of Au wire, Al-Au wire and Au foil were determined using the conventional coincidence-counting method. Monte Carlo simulation technique was employed to simulate the complicated absorption and at- tenuation processes of electrons and gamma photon interactions with the surrounding materials. The Monte Carlo calculated probabilities of escape beta particles, internal conversion electrons and photon-interaction generated photoelectrons and Compton electrons were used to determine the correction term of the coincidence equation. The corrections for the Au wire (length: 8.000 mm, radius: 0.064 mm), Al-Au wire (length: 7.690 mm, radius: 0.255 mm) and Au foil (thickness: 0.025 mm, radius: 3.000 mm) were found to be 5.2% % % % and % % respectively. The study demonstrates that the Monte Carlo calculation for the correction term of the coincidence equation can be applied to the absolute activity determination of radionuclides with well-defined source geometries with an uncertainty of better than 1%. Index Terms— Au, coincidence-counting, activity, Monte Carlo simulation. I. INTRODUCTION T HERMAL neutron flux measurement is commonly car- ried out through the activity measurements of Au which is induced by the neutron reaction Au(n, ) Au. During the hot commissioning stage of the OPAL nuclear reactor at the Australian Nuclear Science and Technology Organization (ANSTO), three types of Au materials, namely pure Au wire (99.9971% of Au), Al-Au (0.112% of Au) alloy wire and pure Au foil (99.99% of Au) were used for the measurement of neu- tron flux distribution in the reactor core, irradiation facilities and neutron beam lines. This paper describes the work carried out on the determination of the absolute activity of Au in the form of Au wire, Al-Au alloy wire and Au foil. Manuscript received October 25, 2006; revised February 18, 2007. L. Mo is with the Australian Nuclear Science and Technology Organization, Lucas Heights NSW 2234, Australia and also with the Institute of Medical Physics, School of Physics, University of Sydney, Sydney NSW 2006, Australia (e-mail: lmx@ansto.gov.au). H. Y. Wu is with the Australian Nuclear Science and Technology Organiza- tion, Lucas Heights NSW 2234, Australia (e-mail: hwu@ansto.gov.au). C. Baldock is with the Institute of Medical Physics, School of Physics, Uni- versity of Sydney, Sydney NSW 2006, Australia (e-mail: c.baldock@physics. usyd.edu.au). Digital Object Identifier 10.1109/TNS.2007.895504 Fig. 1. Decay scheme of Au. Au has a simple decay scheme as shown in Fig. 1. It decays to Hg through 100% emission. The predominant branch (a branch) proceeds to the 411.802 keV excited state level of Hg, which promptly de-excites to ground state via emission of gamma ray of energy 411.802 keV (intensity 95.54%). This branch has an emission probability of 98.986% and maximum energy of 960.4 keV. The second branch (b branch) proceeds to the 1087.687 keV excited state level of Hg, which promptly de-excites to ground state via emission of gamma rays and of energy 675.885 keV (intensity 0.806%) and 1087.687 keV (0.159%) respectively. This branch has much lower emission probability of 0.989% and maximum energy 284.5 keV. The very weak branch (c branch) proceeds to the ground state of Hg, and is usually ignored when absolute activity is measured. The half-life of Au is 2.6944 days [1]. The coincidence-counting method [2] is an ideal method for standardizing such a nuclide. In the coincidence- counting system used in this work, a 4 gas-flow proportional counter was used for the event detection induced by -particles. A 3 by 3 inch thallium activated sodium iodide NaI(Tl) scintil- lator was used for the detection of -rays. For thick sources such as Au wires and foils, the problem of using the coincidence-counting method is the source self-absorption and interaction of gamma photons with source material which generates appreciable amount of photoelectrons and Compton electrons. The detection efficiency of -particles is less than 60%. For such low -efficiency, the efficiency ex- trapolation method [3] is not suitable. The activity can only be determined by applying a correction term to the basic coinci- dence equation [2], [4]. The correction term is due to the de- tection of unwanted radiations and charged particles, including 0018-9499/$25.00 © 2007 IEEE