Klaus H Guber

The Karlsruhe 4π barium fluoride detector

A new experimental approach has been implemented for accurate measurements of neutron capture cross sections in the energy range from 5 to 200 keV. The Karlsruhe 4π barium fluoride detector consists of 42 crystals shaped as hexagonal and pentagonal truncated pyramids forming a spherical shell with 10 cm inner radius and 15 cm thickness. All crystals are supplied with reflector and photomultiplier, thus representing independent gamma-ray detectors. The energy resolution of the 4π detector is 14% at 662 keV and 7% at 2.5 MeV gamma-ray energy, the overall time resolution is 500 ps and the peak efficiency 90% at 1 MeV. The detector allows to register capture cascades with 95% probability above a threshold energy of 2.5 MeV. Neutrons are produced via the 7Li(p, n)7Be reaction using the pulsed proton beam of a Van de Graaff accelerator. A collimated neutron beam is passing through the detector and hits the sample in the centre. The energy of captured neutrons is determined via time of flight, the primary flight path being 77 cm. The combination of short primary flight path, a 10 cm inner radius of the spherical BaF2 shell, and the low capture cross section of barium allows to discriminate background due to capture of sample-scattered neutrons in the scintillator by time of flight, leaving part of the neutron energy range completely undisturbed. This feature, together with the high efficiency and good energy resolution for capture gamma-rays, allows to separate the various background components reliably enough, that the capture cross section ratio of two isotopes can be determined with an accuracy of ≤ 1.0%. The detector will be used for nuclear astrophysics to investigate the origin of the heavy elements in the slow neutron capture process.

Anomalous fluctuations of s-wave reduced neutron widths of 192,194Pt resonances

We obtained an unprecedentedly large number of s-wave neutron widths through R-matrix analysis of neutron cross-section measurements on enriched Pt samples. Careful analysis of these data rejects the validity of the Porter-Thomas distribution with a statistical significance of at least 99.997%.

Gamma-ray spectroscopy with a cooled barium fluoride crystal

Cooling a 1 l BaF2 crystal to 243 K yielded the same or even better gamma-ray energy resolution as is obtained NaI(Tl) detectors of comparable efficiency.

Improving explosive nucleosynthesis models via (n,α) measurements

In explosive environments, (gamma,alpha) reactions constitute some of the most important links in the nucleosynthesis chains. However. it is very unlikely that the rates for most of these reactions will be determined by direct experiments and these rates also are currently very poorly constrained by theory. In this paper, we demonstrate that low-energy (n,alpha) measurements constitute an important new global technique for considerably reducing the uncertainties in (gamma,alpha) reaction rates.

New Neutron Cross‐Section Measurements at ORELA for Improved Nuclear Data Calculations

The Oak Ridge Electron Linear Accelerator (ORELA) was used to measure neutron total and capture cross sections of aluminum, silicon, chlorine, fluorine, and potassium in the energy range from 100 eV to ~600 keV. These measurements were carried out to support the Nuclear Criticality Safety Program (NCSP). Concerns about the use of existing cross section data in nuclear criticality calculations have been a prime motivator for the new cross section measurements. Our results are substantially different from the evaluated nuclear data files of ENDF/B-VI and JENDL-3.2.

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.

High-Resolution Neutron Capture and Total Cross Section Measurements for 192, 194, 195, 196Pt and the Astrophysical Rates for the 192, 194, 195, 196Pt(n, γ) Reactions

We have made the first experimental determinations of the astrophysical rates for the 192, 194, 195, 196Pt(n, γ) reactions across the range of temperatures needed for astrophysical models of the s process. Both neutron capture and total cross sections were measured for 192, 194, 195, 196Pt at the Oak Ridge Electron Linear Accelerator (ORELA) from 10 eV to 500 keV. The data were fitted using the R-matrix code SAMMY to obtain resonance parameters. These parameters as well as the measured cross sections in the unresolved region were used to obtain the astrophysical reaction rates for all these isotopes. We compare our results to previous data and theoretical reaction rates and discuss their astrophysical impact. For example, a classical analysis of the s-process branching at 192Ir using our new rates yields a much lower neutron density than previous analyses of other branchings, in agreement with the long-sought freeze-out effect predicted by dynamical (e.g. stellar) models.

Neutron Cross Sections Measurements for Light Elements at ORELA and their Application in Nuclear Criticality

The Oak Ridge Electron Linear Accelerator (ORELA) was used to measure neutron total and capture cross sections of aluminium, natural chlorine and silicon in the energy range from 100 eV to ~600 keV. ORELA is the only high power white neutron source with excellent time resolution and ideally suited for these experiments still operating in the USA. These measurements were carried out to support the Nuclear Criticality Predictability Program. Concerns about the use of existing cross section data in the nuclear criticality calculations using Monte Carlo codes and benchmarks have been a prime motivator for the new cross section measurements. More accurate nuclear data are not only needed for these calculations but also serve as input parameters for s-process stellar models.

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.

Neutron Resonance Data Exclude Random Matrix Theory

Almost since the time it was formulated, the overwhelming consensus has been that random matrix theory (RMT) is in excellent agreement with neutron resonance data. However, over the past few years, we have obtained new neutron-width data at Oak Ridge and Los Alamos National Laboratories that are in stark disagreement with this theory. We also have reanalyzed neutron widths in the most famous data set, the nuclear data ensemble (NDE), and found that it is seriously flawed, and, when analyzed carefully, excludes RMT with high confidence. More recently, we carefully examined energy spacings for these same resonances in the NDE using the