ORELA Neutron-Induced Reaction
Measurement Abstracts

88Sr(n,gamma)
P.E. Koehler, R.R. Winters, K.H. Guber, T. Rauscher, J. A. Harvey, S. Raman, R. R. Spencer, J. C. Blackmon, D. C. Larson, D. W. Bardayan, T. A. Lewis
Phys. Rev. C62 (2000) 055803

Abstract:
We have made new and improved measurements of the neutron capture and total cross sections for 88Sr at the Oak Ridge Electron Linear Accelerator (ORELA). Improvements over previous measurements include a wider incident neutron energy range, better resolution, the use of metallic rather than carbonate samples, better background subtraction, reduced sensitivity to sample-dependent backgrounds, and better pulse-height weighting functions. Because of its small cross section, the 88Sr(n,gamma) reaction is an important bottleneck during s-process nucleosynthesis. Hence, an accurate determination of this rate is needed to better constrain the neutron exposure in s-process models and to better understand the recently discovered isotopic anomalies in certain meteorites. We performed an [script R]-matrix analysis of our capture and transmission data to extract parameters for 101 resonances between 100 eV and 350 keV. In addition, we fitted our transmission data alone to extract parameters for 342 additional resonances between 350 and 950 keV. We used this information to calculate average properties of the 88Sr + n system for comparison to previous work. Although previous data and resonance analyses were much less extensive, they are, in general, in good agreement with our results except that the average radiation widths as well as the p-wave correlation coefficients we determined are significantly smaller, and the s-wave correlation coefficient we determined has opposite sign from that reported in previous work. We used these resonance parameters together with a calculation of the small, but significant direct-capture contribution to determine the astrophysical reaction rate for the 88Sr(n,gamma) reaction to approximately 3% accuracy across the entire range of temperatures needed by s-process models. Our new rate is in good agreement with the results from a high-precision activation measurement at kT=25 keV, but it is approximately 9.5% lower than the rate used in most previous nucleosynthesis calculations in the temperature range (kT=6-8 keV), where most of the neutron exposure occurs in current stellar models of the s process. We discuss the possible astrophysical impact of this new, lower rate.

137Ba(n,gamma)
P.E. Koehler, K.H. Guber, S. Raman, J.A. Harvey, N.W. Hill, J.C. Blackmon, D.W. Bardayan, D.C. Larson, T.A. Lewis, D. Pierce, M.S. Smith, R.R. Spencer, R.R. Winters
Bull. Am. Phys. Soc. 42 (1997) 1680; Phys. Rev. C57 (1998) R1558.

Abstract:
We have made improved measurements of the 137Ba(n,gamma) and total cross sections over a sufficiently wide range of energies (100 eV - 280 keV) so that the reaction rate at s-process temperatures (kT = 6 - 26 keV) can be determined solely from the data. These rates are crucial for the interpretation of recently discovered anomalies of Ba isotopes in silicon carbide grains from the Murchison meteorite. Recent stellar models of the s process are in agreement with the meteoric anomaly data for Ba only if the 137Ba(n,gamma) reaction rate is 20% larger than the previously accepted rate. Our reaction rates at s-process temperatures are in agreement with the extrapolated reaction rate from the most recent previous measurement. Hence, our results uphold, and place on much firmer footing, the discrepancy between recent stellar models of the s-process and the meteoric anomaly data.

116,120Sn(n,gamma)
P.E. Koehler, R.R. Spencer, K.H. Guber, J.A. Harvey, N.W. Hill, R.R. Winters
Bull. Am. Phys. Soc. 42 (1997) 983.

Abstract:
We have made high-resolution (n, gamma) and transmission measurements on isotopically enriched samples of 116,120Sn at the ORELA in the energy range from 100 eV to 500 keV. Improved astrophysical reaction rates for these isotopes are of interest because current nucleosynthesis models are not able to reproduce the observed abundances in this mass region. The problem may lie in the (n, gamma) cross sections used in the models. Previous (n, gamma) measurements had a lower energy limit of 3 keV or higher which is too high to determine accurately the Maxwellian-averaged capture cross sections at the low temperatures (kT=6-8 keV) favored by the most recent stellar models of the s process. In addition, with our new transmission data for 116Sn we are able for the first time to calculate accurately the often substantial finite-thickness correction to the (n, gamma) data and hence obtain more accurate astrophysical reaction rates than in previous work. We will compare the results from the analysis of our new data to previous measurements and discuss their impact on calculations of the nucleosynthesis in this mass region.

142,144Nd(n,gamma)
K.H. Guber, R.R. Spencer, P.E. Koehler, R.R. Winters
Phys. Rev. Lett. 78 (1997) 2704.

Abstract:
We have implemented a 4pi BaF2 detector system at the Oak Ridge Electron Linear Accelerator (ORELA) for making (n, gamma) 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.

134,136Ba(n,gamma)
P.E. Koehler, R.R. Spencer, R.R. Winters, K.H. Guber, J.A. Harvey, N.W. Hill, M.S. Smith
Phys. Rev. C 54 (1996) 1463.

Abstract:
We have made high-resolution neutron capture and transmission measurements on isotopically enriched samples of 134Ba and 136Ba 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-average capture cross section at the low temperatures (kT approximately equal 8-12keV) favored by the most recent stellar models of the s process. By fitting the data with a multilevel R-matrix code, we determined parameters for 86 resonances in 134Ba below 11 keV and 92 resonances in 136Ba below 35 keV. Astrophysical reaction rates were calculated using these parameters together with our cross section data for the unresolved resonance region. Our results for the astrophysical reaction rates are in good agreement with the most recent previous measurement at the classical s-process temperature kT=30 keV, but show significant differences at lower temperatures. We determined that these differences were due to the effect of resonances below the energy range of previous experiments and to the use of incorrect neutron widths in a previous resonance analysis. Our data show that the ratio of reaction rates for these two isotopes depends more strongly on temperature that previous measurements indicated. One result of this temperature dependence is that the mean s-process temperature we derived from a classical analysis of the branching at 134Cs is too low to be consistent with the temperature derived from other branching points. This inconsistency is evidence for the need for more sophisticated models of the s process beyond the classical model. We used a reaction network code to explore the changes in the calculated isotopic abundances resulting from our new reaction rates for an s-process scenario based on a stellar model. These calculations indicate that the previously observed 20% discrepancy with respect to the solar barium abundance is reduced but not resolved by our new reaction rates.

7Li(n,gamma)
J.C. Blackmon, A.E. Champagne, J.K. Dickens, J.A. Harvey, M.A. Hofstee, S. Kopecky, D.C. Larson, D.C. Powell, S. Raman, M.S. Smith
Phys. Rev. C 54 (1996) 383.

Abstract:
The 7Li(n,gamma)8Li cross section is important in inhomogeneous big bang models, and as a constraint on model parameters used to determine the solar 7Be(p,gamma)8B reaction rate. Values of the 7Li(n,gamma0)8Li reaction cross section were measured for neutron energies between 1.5 and 1340 eV at the Oak Ridge Electron Linear Accelerator. The normalization of the cross section was determined by measuring the gamma-ray yield from the 7Li(n,gamma0)8Li reaction relative to that from the 10B(n,alpha gamma)7Li reaction. The cross section was found to have the inverse neutron-velocity relationship (l/v) indicative of s-wave capture. These results help resolve ambiguities in previous measurements.

A BaF2 Detector System for (n,gamma) Cross Section Measurements at ORELA
K.H. Guber, R.R. Spencer, P.E. Koehler, R.R. Winters
Nucl. Phys. A621 (1997) C254.

Abstract:
We have implemented a 4pi BaF2 detecotr system at the Oak Ridge Electron Linear Accelerator (ORELA) for making (n,gamma) 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.