Klaus H. Guber

A technical report describing the work done in performing a new resolved resonance region evaluation of {140,142}Ce

The U.S. Department of Energy's Office of Scientific and Technical Information

In order to maximally utilize the existing beam ports at the Spallation Neutron Source, developmentand installation of a new neutron instrument is proposed at beam port 16a within building 8700 at OakRidge National Laboratory. Said instrument will provide nominally equal neutron beam access for threemain science purposes; (1) as an alignment station for proposed single-crystal spectrometer samples to berun across the facility, (2) as a Near-Infrared (>100meV) filter analyzer spectrometer, nearly identical to thatwhich was previously housed at the Lujan Center in Los Alamos, and (3) as a high-throughput nuclear cross-section measurement station. These three experimental applications would require no substantial technicaldevelopments, are complimentary in their technical requirements, and provide a worth-while capabilitythat is infinitely beyond the current use of the 16a beam port at the First Target Station within the SNScomplex. All three methods would enhance the facility’s science contributions, while accommodating twoniche experiment methods that may not be strong enough to stand on their own.

Resonance parameter evaluations of the three stable silicon isotopes ^28;29;30Si, were performed at Oak Ridge National Laboratory in 2002 and the results of those evaluations were restored from the archives in 2018. The evaluations also accounted for the contribution of the direct component of the capture cross sections computed over the neutron resonance range. For the most abundant isotope, ²⁸Si, the value of the thermal capture cross sections was increased by 10% on the basis of analyses devoted to reevaluating the thermal radiative neutron capture cross sections. The contribution of the direct capture cross sections was recomputed for each isotope, and its energy-dependent behavior improved. Results of benchmark calculations performed at the International Atomic Energy Agency showed an excellent agreement with integral measurements.

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.