M.J. Rapp

Beryllium and Graphite High-Accuracy Total Cross-Section Measurements in the Energy Range from 24 to 900 keV

Abstract This paper presents new measurements of the carbon and beryllium neutron total cross section in the energy range of 24 to 950 keV. The measurements were done using a pulsed neutron source driven by an electron LINAC. The neutron beam passed through a 30-cm-thick iron filter, which results in neutron transmission only in energies where resonance scattering and potential interference exist. The neutron filter removes most of the neutrons at other energies and significantly attenuates the gamma background resulting in 20 energy windows and a high signal-to-background ratio. The filtered beam was used for transmission measurements through graphite that results in ~1% accurate total cross sections that are in excellent agreement with current evaluations. The carbon measurement provides a verification of the accuracy of the filtered beam method. Measurements of three samples of different thicknesses of beryllium resulted in accurate total cross-section values that agree with one previous measurement and show discrepancies from current evaluations. The high accuracy of the new measurements can be used for improvement of future total cross-section evaluations of beryllium.

Simultaneous Measurement of 235U Fission and Capture Cross Sections From 0.01 eV to 3 keV Using a Gamma Multiplicity Detector

Abstract The neutron microscopic capture cross section for 235U is a critical parameter for the design and operation of nuclear reactors. The evaluated nuclear data libraries of ENDF/B-VII.1 and JENDL-4.0 have nearly identical values for the neutron capture cross section for neutron energies below 0.5 keV. In the most recent release of the JENDL library the onset of the unresolved resonance region was changed from 2.25 keV to 0.5 keV. In the energy region from 1.5 keV to 2.25 keV the average neutron capture cross section from ENDF/B-VII.1 is about 10% higher than that from JENDL-4.0. In an attempt to address the discrepancies between the libraries, a measurement of the neutron capture cross section of 235U was conducted at the Gaerttner LINAC Center located at Rensselaer Polytechnic Institute. This measurement used a 16-segment -multiplicity NaI(Tl) detector to detect the prompt gammas emitted from neutron interactions with a highly enriched 235U sample. Using the time-of-flight method, detected events were recorded and grouped based on the total gamma energy per interaction and observed multiplicity. A method was developed to separate fission from capture based on total energy deposition and gamma multiplicity. Application of this method in the thermal and resonance region below 0.5 keV for both the fission and capture produced cross sections that are in good agreement with both ENDF/B-VII.1 and JENDL-4.0 evaluations. The measurements support a lower 235U neutron capture cross section in the energy range 0.5 to 2.25 keV, which is closer to JENDL 4.0.

Quasi-Differential Neutron Scattering in Zirconium from 0.5 to 20 MeV

Abstract High-energy-neutron-scattering experiments for elemental zirconium were performed at the electron linear accelerator facility at Rensselaer Polytechnic Institute. The scattering experiments were performed in the energy region from 0.5 to 20 MeV using the time-of-flight technique. The scattering system is composed of an array of eight EJ301 liquid scintillator detectors coupled to photomultiplier tubes. The detector array collects data simultaneously at various angles. The raw signals from each detector were digitized and transferred to a personal computer hard drive for storage. The digitized data were postprocessed, and pulse-shape analysis was performed to determine whether the pulse was the result of a gamma ray or a neutron being detected. The experimental results were compared with Monte Carlo transport calculations that simulated the experiment. This comparison was a way to benchmark several nuclear data libraries used in the Monte Carlo code. Ratios of the calculated data to the experimental data (C/E values) are presented and used to compare the nuclear data libraries. Results show that the experimentally observed scattering cross section is smaller than the one used in the evaluated libraries at energies between 10 and 20 MeV. For all energies and angles, the investigated nuclear data libraries agree with the experimental data to within 9%. Overall, the JEFF-3.1 and JENDL-4.0 libraries provide the best match to the experimental data.

Neutron Capture Measurements and Resonance Parameters of Gadolinium

Abstract Neutron capture measurements were performed with the time-of-flight method at the Gaerttner LINAC Center at Rensselaer Polytechnic Institute (RPI) using isotopically enriched gadolinium (Gd) samples (155Gd, 156Gd, 157Gd, 158Gd, and 160Gd). The neutron capture measurements were made at the 25-m flight station with a 16-segment sodium iodide multiplicity detector. After the data were collected and reduced to capture yields, resonance parameters were obtained by a combined fitting of the neutron capture data for five enriched Gd isotopes and one natural Gd sample using the multilevel R-matrix Bayesian code SAMMY. A table of resonance parameters and their uncertainties is presented. We observed 2, 169, 96, and 1 new resonances in 154Gd, 155Gd, 157Gd, and 158Gd isotopes, respectively. Resonances in the ENDF/B-VII.0 evaluation that were not observed in the current experiment and could not be traced to a literature reference were removed. This includes 11 resonances from the 156Gd isotope, 1 resonance from 157Gd, 1 resonance from 158Gd, and 6 resonances from the 160Gd isotope. The resulting resonance parameters were used to calculate the capture resonance integrals in the energy region from 0.5 eV to 20 MeV and were compared to calculations obtained when using the resonance parameters from ENDF/B-VII.0 and previous RPI results. The present parameters gave a resonance integral value of 395 ± 2 b, which is ∼0.8% higher and ∼1.7% lower than that obtained with the ENDF/B-VII.0 parameters and with the previous RPI parameters, respectively.

Neutron Transmission and Capture Measurements of 133Cs from 0.01 To 600 eV

Abstract Neutron capture and transmission measurements were carried out from 0.01 to 600 eV on both solid and liquid samples containing elemental cesium (133Cs). Only s-wave resonances were observed in these measurements. These data were analyzed for resonance parameters utilizing the SAMMY Bayesian analysis code to simultaneously fit both the capture and transmission data. Parameters were obtained for 31 cesium resonances up to 600 eV. The thermal capture cross section and capture resonance integral were determined. The thermal capture cross section is 10% larger than the ENDF, JENDL, and JEFF evaluated values but lies within the uncertainty of the most recent measurement by Yoon and Lee [New Phys.: Sae Mulli (Korean Phys. Soc.)., Vol. 61, p. 7 (2011)]. The capture resonance integral has a statistical 1σ error of 2% and lies 1.4σ above the JENDL value, 5.5σ above the ENDF value, and 3.9σ above the JEFF value. The s-wave strength function was determined.