Neutron spectrum determination in a sub-critical assembly using the multi-disc neutron activation technique

• Published in: Radiation effects and defects in solids 2016-09, Vol.171 (9-10), p.766-774
• Main Authors: Koseoglou, P., Vagena, E., Stoulos, S., Manolopoulou, M.
• Format: Article
• Language: English
• Subjects: Activation; activation detectors; Capture cross sections; Disks; Fast neutrons; Foils; Mathematical analysis; neutron spectrum; Neutrons; Radiation; Reactor cores; SANDII; Sub-critical assembly; Thermal neutrons
• ISSN: 1042-0150; 1029-4953
• DOI: 10.1080/10420150.2016.1262370

Neutron spectrum of the sub-critical nuclear assembly-reactor of Aristotle University of Thessaloniki was measured at three radial distances from the reactor core. The neutron activation technique was applied irradiating 15 thick foils - disc of various elements at each position. The data of 38 (n, γ), (n, p) and (n, α) reactions were analyzed for specific activity determination. Discs instead of foils were used due to the relevant low neutron flux, so the gamma self-absorption as well as the neutron self-shielding factors has been calculated using GEANT simulations in order to determine the activity induced. The specific activities calculated for all isotopes studied were the input to the SANDII code, which was built specifically for the neutron spectrum de-convolution when the neutron activation technique is used. For the optimization of the results a technique was applied in order to minimize the influence of the initial-"guessed" spectrum shape SANDII uses. The neutron spectrum estimated presents a peak in the regions of (i) thermal neutrons ranged between 0.001 and 1 eV peaking at neutron energy ∼0.1 eV and (ii) fast neutrons ranged between 0.1 and 20 MeV peaking at neutron energy ∼1.2 MeV. The reduction of thermal neutrons is higher than the fast one as the distance from the reactor core increases since thermal neutrons capture by natural U-fuel has higher cross section than the fast neutrons.