NE 457: Flux Measurement Lab
Knowledge of the reactor flux is crucial to being able to both perform experiments and maintain an environment inside the reactor that is both predictable and controllable. As neutrons are the basis of how and why a nuclear reactor operates, it is logical to place value on information regarding where and when neutrons are present. Expanding upon this necessity is the realization that neutrons behave differently dependent on their energy level. It would then not seem to be a stretch to place additional value on information pertaining to which type of neutrons are present.
The main purpose of this experiment is to obtain that valuable information about neutron presence by measuring the thermal and epithermal neutron flux along the vertical axis of the OSTR. In this lab, neutron activation analysis is used to determine the thermal and epithermal fluxes present at various heights in the system. Simply put, two types of irradiations need be performed: irradiation of cadmium covered gold wire and bare gold wire. When the cadmium covered gold wire becomes irradiated, only neutrons that are above 0.4 eV (epithermal neutrons) will be attenuated by the gold. Since cadmium has a higher absorption cross section for lower energy neutrons than gold (see Fig. 1), neutrons below the energy of 0.4 eV, which are considered thermal neutrons, will be absorbed in the cadmium . Therefore, the cadmium covered gold wire will provide the epithermal flux profile. In contrast, the bare gold wire absorbs both neutrons that are above and below 0.4 eV. As such, the bare gold wire supplies both the thermal and epithermal flux profile. This means that in order to determine the thermal neutron flux, the cadmium ratio must be utilized.
Neutron Absorption Cross Section for Gold (blue) and Cadmium (green)
Another Introduction of the same Lab: