Written and developed by Prof. Tor Bjørnstad (IFE/UiO)
Although there are several types of neutron sources (reactors, accelerators, and radioisotopic neutron emitters) one can use for NAA, nuclear reactors with their high fluxes of neutrons from uranium fission offer the highest available sensitivities for most elements. Different types of reactors and different positions within a reactor can vary considerably with regard to their neutron energy distributions and fluxes due to the materials used to moderate (or reduce the energies of) the primary fission neutrons. However, as shown in the figure below, most neutron energy distributions are quite broad and consist of three principal components (thermal, epithermal, and fast).
A typical reactor neutron energy spectrum showing the various components used to describe the neutron energy regions.
The thermal neutron component consists of low-energy neutrons (energies below 0.5 eV) in thermal equilibrium with atoms in the reactor's moderator. At room temperature, the energy spectrum of thermal neutrons is best described by a Maxwell-Boltzmann distribution with a mean energy of 0.025 eV and a most probable velocity of 2200 m/s. In most reactor irradiation positions, 90-95% of the neutrons that bombard a sample are thermal neutrons. In general, a one-megawatt reactor has a peak thermal neutron flux of approximately 1neutrons per square centimeter per second.
The epithermal neutron component consists of neutrons (energies from 0.5 eV to about 0.5 MeV) which have been only partially moderated. A cadmium foil 1 mm thick absorbs all
thermal neutrons but will allow epithermal and fast neutrons above 0.5 eV in energy to pass through. In a typical unshielded reactor irradiation position, the epithermal neutron flux
represents about 2% the total neutron flux. Both thermal and epithermal neutrons induce (n,γ) reactions on target nuclei. An NAA technique that employs only epithermal neutrons to induce (n,γ) reactions by irradiating the samples being analyzed inside either cadmium or boron shields is called epithermal neutron activation analysis (ENAA).
The fast neutron component of the neutron spectrum (energies above 0.5 MeV) consists of the primary fission neutrons, which still have much of their original energy following fission. Fast neutrons contribute very little to the (n,γ) reaction, but instead induce nuclear reactions where the ejection of one or more nuclear particles - (n,p), (n,n'), and (n,2n) - are prevalent. In a typical reactor irradiation position, about 5% of the total flux consists of fast neutrons. An NAA technique that employs nuclear reactions induced by fast neutrons is called fast neutron activation analysis (FNAA).
The neutron source in this exercise is the JEEP II reactor at Institute for Energy Technology (IFE) at Kjeller. It is a heavy water moderated reactor with a thermal power of 2 MW, and the thermal neutron flux in the irradiation position is about 1.5 • 1013 n • cm-2 • s-1.
The NAA-laboratory at IFE is equipped with a “rabbit” system, which transports the sample pneumatically from the laboratory into the irradiation position. After a pre-programmed irradiation time the sample is returned pneumatically to the NAA-laboratory. The transfer time is 25-30 s over a distance of 200 m. This rabbit system is used for short irradiation times (< 3 h). For longer irradiation times the samples are loaded manually by a loading machine vertically from the reactor top.