Abstract
The results of systematic evaluations of spectrum averaged cross section (SACS) measurements in the fission neutron fields of 252Cf and 235U are presented. The data form a complete database of high-threshold experimental SACS measured in the same installation under the same conditions and using the same high purity germanium gamma spectrometer. This is crucial to reduce the uncertainty of the ratio and the data scattering and therefore, to minimize discrepancies compared to cross section measured under different conditions in different laboratories. This new dataset complements and extends earlier experimental evaluations. The total emission of the 252Cf neutron source during the experiments varied from 9.5E8 to 4.5E8 neutrons per second. The emission was derived in accordance to the data in the Certificate of Calibration involving absolute flux measurements in a manganese sulphate bath. Concerning 235U fission neutron field, the irradiations were carried out in a specifically designed core assembled in the zero power light water LR-0 reactor. This special core has a well described neutron field. After the irradiation, the low volume irradiated samples to be measured by gamma spectrometry were placed directly on the upper cap of a coaxial high purity germanium (HPGe) detector in a vertical configuration (ORTEC GEM35P4). High volume samples were homogenized and strewn into the Marinelli beaker. The HPGe detector is surrounded by the lead shielding box with a thin inner copper cladding and covered with rubber for suppression of background signal and bremsstrahlung. The experimental reaction rates were derived for irradiated samples from the Net Peak Areas (NPA) measured using the semiconductor HPGe detector. The measured reaction rates are used to derive the spectrum-averaged cross sections. Furthermore, measured reaction rates are also compared with MCNP6 calculations using various nuclear data libraries, in particular IRDFF evaluations.
Highlights
All presented spectrum-averaged cross sections (SACS) and its ratios were measured under the same conditions and using the same gamma spectrometer
Prompt fission neutron spectra (PFNS) of 235U was taken from CIELO-g6 library and cross sections of the appropriate reactions was taken from IRDFF-1.05 library
The results significantly differ for 235U ENDF/B-VII.1 and CIELO-g6 prompt fission neutron spectrum (PFNS)
Summary
All presented spectrum-averaged cross sections (SACS) and its ratios were measured under the same conditions and using the same gamma spectrometer. This is important to minimize discrepancies compared to cross sections measured under different conditions and using different spectrometers in different institutes. The ratios of 252Cf and 235U spectral averaged cross sections can be use to specify high energy tail of 235U as the 252Cf spontaneous fission neutron spectrum is taken as a standard. The experiments were performed using a very well characterized light water pool type LR-0 reactor using a special core (6 fuel assemblies with enrichment of 3.3%) and isotopic 252Cf source with emission varying from 8.0E8 to 4.0E8 n/s during experimental work
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