Characterisation of neutron field in the polyethylene neutron irradiator.

The paper describes neutron field characterisation of low-flux multipurpose educational irradiator developed at Czech Technical University in Prague. The irradiator is aimed for demonstration experiments including neutron activation analysis, delayed neutron counting, or studies related to neutron and/or gamma detection and spectrometric devices. It may accommodate various neutron sources including 252Cf or AmBe radionuclide sources, D-D generator, or in-reactor irradiated nuclear material serving as delayed neutron source, or gamma sources including radionuclide ones or short-lived sources produced in the adjacent reactor. The characterisation was performed based on neutron activation technique using gold foils. It included two experimental parts. The first one verified the level of response symmetry in the four irradiation channels and characterisation of the axial distribution in the irradiation channels. Within the second one, the responses to thermal and epithermal neutrons and the cadmium ratio in the central irradiation position were determined. Whereas the former was determined solely with the 252Cf source, the latter was performed for all available sources: 252Cf, AmBe, and the D-D generator. The experimental results were further compared to calculations by the MCNP Monte Carlo code.

Investigation of mammoth remains using the neutron activation analysis at the Training Reactor VR-1.

At the time when the importance of the interdisciplinary research increases, the nuclear analytical techniques supported by the small research reactors represent a useful tool for investigation of human society, culture, history etc. The historical, archaeological, and palaeontological samples and objects of cultural heritage can be easily studied using the radioanalytical methods such as the neutron activation analysis. This paper deals with the detailed investigation of fragments of mammoth remains from the Mid-Upper Palaeolithic site Pavlov VI by means of the instrumental neutron activation analysis at the Training Reactor VR-1 of the Czech Technical University in Prague. Six mammoth hard tissue samples (fragments of bones, tusk, and molar) from the Institute of Archaeology of the Czech Academy of Sciences were irradiated in the dry vertical irradiation channel with thermal neutron field (φ=2×109cm-2s-1) at maximum reactor power (80 W). The activated mammoth samples were analysed employing the nuclear γ-spectrometry and semiconductor HPGe detector, and the composition of the remains was determined (qualitative and quantitative analysis). The presence of Na, Cl, K, As, Fe, Sr, Mn, Br, I, Ba, and U was revealed in studied mammoth samples. Based on obtained production rates, the concentrations of Fe, Sr, Na, K, As, and U were determined. The results presented in this paper show clearly that the low-power Training Reactor VR-1 is excellent tool for the neutron activation analysis experiments within the interdisciplinary research and can provide experimental data important for archaeologists and palaeontologists.

CROSS SECTIONS MEASURED BY QUASI-MONOENERGETIC NEUTRONS.

197Au, 209Bi, 59Co, natFe and 169Tm samples were irradiated several times with quasi-monoenergetic neutrons from the p+7Li reaction in the energy range of 18-34 MeV. The activities of the samples were measured with the HPGe detector and the reaction rates were calculated. The cross sections were extracted using the SAND-II code with the reference cross sections from the IRDFF database.

On similarity of various reactor spectra and 235U prompt fission neutron spectrum.

A well-defined neutron spectrum is an essential tool not only for calibration and testing of neutron detectors used in dosimetry and spectroscopy but also for validation and verification of evaluated cross sections. A new evaluation of thermal-neutron induced 235U PFNS was performed by the International Atomic Energy Agency (IAEA) in the CIELO (Collaborative International Evaluated Library Organisation Project) project; new measurements of Spectral Averaged Cross sections averaged in the evaluated spectrum are to be obtained. In general, a neutron spectrum in the core is not identical to the pure fission one because fission neutrons undergo many scattering reactions, but it can be shown that PFNS and reactor spectra become undistinguishable from a certain energy boundary. This limit is important for experiments, because when the studied reaction threshold is over this limit, the spectral averaged cross sections in PFNS can be derived from the measured reactions in the reactor core. The evaluation of the neutron spectrum measurements in three different thermal-reactor cores shows that this lower limit is around the energy of 5.5 - 6 MeV. Above this energy the reactor spectra becomes identical with the 235U PFNS. IAEA CIELO PFNS is within 5% of the measured PFNS from 10 to 14 MeV in a LR-0 reactor, while ENDF/B-VII evaluated PFNS underestimated measured neutron spectra.

NEUTRON FIELD MEASUREMENT OF P(35)+Be SOURCE USING THE MULTI-FOIL ACTIVATION METHOD.

Neutron field from the p+Be interaction was investigated at the NPI CAS for a proton beam energy of 35 MeV and thick beryllium target. Broad neutron spectra at close source-to-sample distances were determined using the multi-foil activation technique. Two large sets of dosimetry foils containing the Ni, Co, Au, In, Ti, Al, Y, Lu, Nb and Fe were irradiated at a distance of 74 mm at direct neutron beam axis and at a distance of 34 mm from beam axis. Supporting Monte-Carlo MCNPX calculations of the irradiation system were performed as well. From measured reaction rates, the neutron energy spectra at both positions were reconstructed employing the modified version of the SAND-II unfolding code and activation cross-section data from the EAF-2010 library. At the position of irradiated samples, the total fast neutron flux reaches the value up to 1010 cm-2 s-1, and the neutron field is utilizable for radiation hardness study and integral benchmark experiments within the International Fusion Material Irradiation Facility (IFMIF) program.