Uncertainties in measuring trace amounts of cobalt and europium with low-flux neutron activation analysis
Authors: Burnham Steven , Moffit Greg , Faure Quentin , Jevremovic Tatjana 
Published: June 2017

Neutron activation analysis is widely used for identification of elements
and their quantities even in trace amounts in the samples of almost any
type. The challenges in detecting trace amounts of particular elements are
often associated with the neutron flux produced at the research reactors.
Low-flux neutron activation analysis usually presents the biggest challenge
when analyzing trace quantities of elements with lower magnitude of
radiative capture cross-sections. In this paper, we present the
methodology and the quantified uncertainties associated with the detection
of trace amounts of cobalt and europium, using as an example concrete aggregates.
Recent growing interest is in improving structural concrete
(increasing its strength but reducing its activation in nuclear power plant
environments). Aside from buildings, structural concrete is also used as a
biological shield in nuclear power plant that become radioactive after
exposure to neutron flux. Due to radiative capture interactions, artificial
radionuclides are generated to high enough concentrations that classify
concrete as low-level radioactive waste at the time of the plant's
decommissioning. Disposal of this concrete adds to the expense of nuclear
power plant financing and its construction. Three radionuclides, 60Co,
152Eu, and 154Eu, account for 99 % of total residual radioactivity of
nuclear power plant decommissioned concrete. IAEA document RS-G-1.7,
Application of the Concepts of Exclusion, Exemption, and Clearance,
specifies clearance levels of radionuclides specific activities: a specific
activity lower than 0.1 Bqg-1 for 60Co and 152Eu, and 154Eu allows for a
concrete to be recycled after decommissioning of the nuclear power plant.
Therefore, low-flux neutron activation analysis is used to test the
detection limits of trace elements in samples of cement, coarse, and fine
concrete aggregates. These samples are irradiated at the University of
Utah's 100 kW TRIGA Reactor at power levels of 10 kW, 30 kW, and 90 kW, with
the corresponding thermal neutron flux values of 1.5?108, 7.3?109, and
3.76?1011 cm-2s-1.The samples are irradiated for time periods of 1, 3, 30,
60, and 120 minutes. Different power levels and different irradiation times
are used to find if there is a threshold set of neutron activation analysis
parameters in detecting trace amounts of these isotopes. Each of the
samples is counted on a Canberra BEGe high purity germanium detector. Cement
samples are concurrently irradiated with a National Institute of Standards
and Technology coal fly ash standard reference material and coarse and fine
aggregates with Montana soil standard reference material to accurately
quantify the mass concentration of the isotopes in concrete samples. Final
results show that reactor power, irradiation, and detector measurement
times are heavily correlated to finding the optimum combination for a
low-flux neutron activation analysis approach in detecting trace contents of
elements, specifically cobalt and europium.