R. R. Spencer

High-Resolution Transmission Measurements of 233U Using a Cooled Sample at the Temperature T=11 K

Abstract For the first time, high-resolution transmission data of 233U have been obtained using a cooled sample. The samples were cooled to T = 11 K using a cryogenic device, which reduced the Doppler broadening of resonances by 50% compared to room-temperature measurements. The measurements were carried out at the Oak Ridge Electron Linear Accelerator over the energy range from 0.6 eV to 300 keV at the 80-m flight path station. Corrections were made for experimental effects, and the average total cross section in this energy range was determined. Results are compared to previous measurements.

A BaF2 detector system for (n,γ) cross section measurements at ORELA

Abstract We have implemented a 4π BaF 2 detector system at the Oak Ridge Electron Linear Accelerator (ORELA) for making (n,γ) measurements of interest to nuclear astrophysics. This new detector will allow us to work with smaller samples and to avoid potential problems with the pulse-height weighting technique used in other systems. Also, our first measurements with this system demonstrate that as a result of the excellent time-of-flight (TOF) resolution at ORELA and the good pulse-height resolution of the detector, the background from sample-scattered neutrons will not be a serious problem in most cases.

New High-Resolution Fission Cross-Section Measurements of 233U in the 0.4-eV to 700-keV Energy Range

Abstract In support of the U.S. Department of Energy need for new data for criticality safety applications, new high-resolution fission cross-section measurements of 233U have been made at the Oak Ridge Electron Linear Accelerator. The measurements were carried out over a 0.4-eV to 700-keV energy range at the 80-m flight station using a fission chamber. Corrections were made for experimental effects to obtain the average fission cross section in this energy range. Results are compared to previous measurements.

R-Matrix Evaluation of 16O Neutron Cross Sections up to 6.3 MeV

We have evaluated 16O neutron cross sections in the resolved resonance region with the multilevel Reich-Moore code SAMMY. Resonance parameters were determined by a consistent analysis, including both Doppler and resolution broadening effects. To properly treat the α particle exit channel, an algorithm to calculate charged particle penetrabilities and shifts was incorporated into SAMMY.

New neutron capture and transmission measurements for 134,136Ba at ORELA and their impact on s-process nucleosynthesis calculations

We have made high-resolution neutron capture and transmission measurements on isotopically enriched samples of {sup 134}Ba and {sup 136}Ba at the Oak Ridge Electron Linear Accelerator (ORELA) in the energy range from 20 eV to 500 keV. Previous measurements had a lower energy limit of 3 - 5 keV, which is too high to determine accurately the Maxwellian-averaged capture cross section at the low temperatures (kT {approx} 6 - 12 keV) favored by the most recent stellar models of the {ital s}-process. Our results for the astrophysical reaction rates are in good agreement with the most recent previous measurement at the classical {ital s}-process temperature, kT = 30 keV, but show significant differences at lower 40 temperatures. We discuss the astrophysical implications of these differences.

Measurements of 142,144Nd(n,γ) cross sections at ORELA for astrophysical s-process studies

Abstract We have completed measurements of the 142,144 Nd(n,γ) cross sections from approximately 20 eV to 200 keV at the Oak Ridge Electron Linear Accelerator (ORELA) using a recently improved C 6 D 6 detector apparatus. 142 Nd is an s -only isotope, i.e. it is only formed during the s -process. It has a closed neutron shell and therefore defines a step in the 〈 σ 〉N s curve from which the mean s -process neutron exposure can be calculated. In addition, 144 Nd is the normalization point for the neodymium abundances. Also accurate (n,γ) cross sections would help to determine the r - and p -process residuals of these isotopes and will impact the interpretation of the recently discovered isotopic anomalies in silicon carbide grains from the Murchison meteorite. Our new (n,γ) cross sections also show that reaction rate extrapolations for nuclei near closed neutron shells from measured values at 30 keV down to 8 keV can be inaccurate.

Measurement of the energy dependence of prompt fission neutron emission from 233U, 235U, and 239Pu for e = 0.0005 to 10 mev relative to emission from spontaneous fission of 252Cf

Abstract A series of experiments has been performed to measure the dependence on the incident neutron energy of v p(E), the average number of prompt neutrons emitted per fission, of 233U, 235U, and 239Pu relative to v p (252Cf), the average number of prompt neutrons emitted in spontaneous fission of 252Cf. The incident energy range was 0.0005 to 10 MeV. A white neutron source was generated by the Oak Ridge Electron Linear Accelerator and the energies of the neutrons incident on the fissile samples were determined by time-of-flight techniques. The fissile sample and the 252Cf standard were contained in a fission chamber surrounded by a large volume (0.91 m3) liquid scintillator loaded with gadolinium. Fissions were detected by the fission chamber and the neutrons were detected by the scintillator. The results for 239Pu agree over most of the energy range with the results of the evaluation of Manero and Konshin. However, the present results are larger for 235U than those of Manero and Konshin and for 233U t...