D. P. Barry

Neutron capture and total cross-section measurements and resonance parameters of gadolinium

Abstract Neutron capture and transmission measurements were performed by the time-of-flight technique at the Rensselaer Polytechnic Institute linac facility using metallic and liquid Gd samples. The liquid samples were isotopically enriched in either 155Gd or 157Gd. The capture measurements were made at the 25-m flight station with a multiplicity-type capture detector, and the transmission measurements were performed at 15- and 25-m flight stations with 6Li glass scintillation detectors. The multilevel R-matrix Bayesian code SAMMY was used to extract resonance parameters. Among the significant findings are the following. The neutron width of the largest resonance in Gd, at 0.032 eV in 157Gd, has been measured to be (9 ± 1)% smaller than that given in ENDF/B-VI updated through release 8. The thermal (2200 m/s) capture cross section of 157Gd has been measured to be 11% smaller than that calculated from ENDF. The other major thermal resonance, at 0.025 eV in 155Gd, did not display a significant deviation from the thermal capture cross section given by ENDF. In the epithermal region, the analysis provided here represents the most extensive to date. Twenty-eight new resonances are proposed, and other resonances previously identified in the literature have been revisited. The assignment of resonances within regions of complicated structure incorporated the observations of other researchers, particularly on the six occasions where ENDF resonances are recommended to be removed. The poor match of the ENDF parameters to the current data is significant, and substantial improvement to the understanding of gadolinium cross sections is presented, particularly above 180 eV where the ENDF resolved region for 155Gd ends.

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.

Neutron Transmission and Capture Measurements and Resonance Parameter Analysis of Neodymium from 1 to 500 eV

Abstract Neodymium is a 235U fission product and is important for reactor neutronic calculations. The aim of the present work is to improve upon the existing neutron cross-section data of neodymium. Neutron capture and transmission measurements were performed by the time-of-flight technique at the Rensselaer Polytechnic Institute (RPI) linear accelerator (LINAC) laboratory using metallic neodymium samples. The capture measurements were made at the 25-m flight station with a 16-segment NaI multiplicity detector, and the transmission measurements were performed at 15- and 25-m flight stations, respectively, with 6Li glass scintillation detectors. After the data were collected and reduced, resonance parameters were determined by combined fitting of the transmission and capture data with the SAMMY multilevel R-matrix Bayesian code. The resonance parameters for all naturally occurring neodymium isotopes were deduced within the energy range of 1 to 500 eV. The resulting resonance parameters were used to calculate the capture resonance integrals from this energy. The RPI parameters gave a resonance integral value of 32 ± 1 b that is ˜7% lower than that obtained with the ENDF/B-VI parameters. The current measurements significantly reduce the uncertainties of the resonance parameters when compared with previously published parameters.

Resonance Parameters and Uncertainties Derived from Epithermal Neutron Capture and Transmission Measurements of Natural Molybdenum

Abstract The electron linear accelerator facility at the Rensselaer Polytechnic Institute was used to explore neutron interactions with molybdenum in the energy region from 10 eV to 2 keV. Neutron capture and transmission measurements were performed by the time-of-flight technique. Resonance parameters were extracted from the data using the multilevel R-matrix Bayesian code SAMMY. A table of resonance parameters and their uncertainties is presented. Two transmission measurements were performed at a flight path of 25 m with a 6Li glass scintillation detector. The neutron capture measurements were performed at a flight path of 25 m with a 16-segment sodium iodide multiplicity detector. Nine different thicknesses of elemental molybdenum metal samples ranging from 0.051 mm (0.002 in.) to 6.35 mm (0.250 in.) were measured in either capture or transmission. Reductions in resonance integrals were observed when compared to ENDF/B-VII.0 for six of the seven stable isotopes. The largest reductions were 9% in 97Mo and 11% in 100Mo. The one measured increase in resonance integral relative to ENDF/B-VII.0 occurred in 95Mo, and it was significant (10%). The measured distribution of neutron widths for 95Mo and 97Mo are a better match to a Porter-Thomas distribution than those of ENDF/B-VII.0. Neutron strength functions for 95Mo and 97Mo were measured and compared to ENDF/B-VII.0. The strength of 95Mo and 97Mo are within uncertainties of each other. The measured radiation width distribution for 95Mo and 97Mo are compared to those of ENDF/B-VII.0 and to χ2 distributions. Significant aspects of this analysis are the assignment of radiation widths, the determination of the transmission resolution function, and the propagation of experimental uncertainties into resonance parameter uncertainties.

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.

Beryllium and Graphite Neutron Total Cross-Section Measurements from 0.4 to 20 MeV

Abstract The Gaerttner Laboratory electron linear accelerator at Rensselaer Polytechnic Institute was used in the measurement of the neutron total cross section of natural beryllium and carbon (graphite) in the energy range of 0.4 to 20 MeV. Neutron transmission measurements were made using the time-of-flight method with a 100-m flight path, fast detector response and electronics, and a narrow neutron pulse width to provide good energy resolution. A method was developed to determine the time-dependent background component associated with the transmission measurement using a combination of experimental data and Monte Carlo methods. The signal-to-background ratio combined with low counting statistics error resulted in low uncertainties and highly accurate data. The graphite measurement, showing excellent agreement with the current evaluations, provided a verification of the accuracy in the measurement and analytical methods used. The measurements of beryllium resulted in an accurate measurement of total cross section, showing some deviations with commonly used evaluations and better agreement with ENDF/B-VI.8. These results can be used for the improvement of future neutron cross-section evaluations of beryllium.

Hafnium Resonance Parameter Analysis Using Neutron Capture and Transmission Experiments

The focus of this work is to determine resonance parameters for stable hafnium isotopes in the 0.005-200 eV region, with special emphasis on the overlapping {sup 176}Hf and {sup 178}Hf resonances near 8 eV. The large neutron cross section of hafnium, combined with its corrosion resistance and excellent mechanical properties, make it a useful material for controlling nuclear reactions. Experiments measuring neutron capture and transmission were performed at the Rensselaer Polytechnic Institute (RPI) electron linear accelerator (LINAC) using the time of flight method. {sup 6}Li glass scintillation detectors were used for transmission experiments at flight path lengths of 15 and 25 m. Capture experiments were done using a sixteen section NaI(Tl) multiplicity detector at a flight path length of 25 m. These experiments utilized various thicknesses of metallic and isotopically-enriched liquid samples. The liquid samples were designed to provide information on the {sup 176}Hf and {sup 178}Hf contributions to the 8 eV doublet without saturation. Data analysis was done using the R-matrix Bayesian code SAMMY version M6 beta. SAMMY is able to account for experimental resolution effects for each of the experimental setups at the RPI LINAC, and also can correct for multiple scattering effects in neutron capture yield data. The combined capture and transmission data analysis yielded resonance parameters for all hafnium isotopes from 0.005-200 eV. Resonance integrals were calculated along with errors for each hafnium isotope using the NJOY [1] and INTER [2] codes. The isotopic resonance integrals calculated were significantly different than previously published values; however the calculated elemental hafnium resonance integral changed very little.

Hafnium Resonance Parameter Analysis using Neutron Capture and Transmission Experiments

The focus of this work is to determine resonance parameters for stable hafnium isotopes in the 0.005–200 eV region, with special emphasis on the overlapping 176Hf and 178Hf resonances near 8 eV. The large neutron cross section of hafnium, combined with its corrosion resistance and excellent mechanical properties, make it a useful material for controlling nuclear reactions.Experiments measuring neutron capture and transmission were performed at the Rensselaer Polytechnic Institute (RPI) electron linear accelerator (LINAC) using the time of flight method. 6Li glass scintillation detectors were used for transmission experiments at flight path lengths of 15 and 25 m. Capture experiments were done using a sixteen‐section NaI(Tl) multiplicity detector at a flight path length of 25 m. These experiments utilized various thicknesses of metallic and isotopically enriched liquid samples. The liquid samples were designed to provide information on the 176Hf and 178Hf contributions to the 8‐eV doublet without saturatio...

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.