J. A. Harvey

Fast neutron-induced changes in net impurity concentration of high-resistivity silicon

Resistivity changes produced by 1-MeV neutron irradiation at room temperature were measured in float-zone-grown n-type and p-type silicon with initial resistivities ranging from 1.8 to 100 k Omega -cm. Observed changes are discussed in terms of net electrically active impurity concentration. A model is presented which postulates escape of Si self-interstitials and vacancies from damage clusters and their subsequent interaction with impurities and other preexisting defects in the lattice. These interactions lead to transfer of B and P from electrically active substitutional configurations into electrically inactive positions (B/sub i/, P/sub i/, and E-center), resulting in changes of net electrically active impurity concentration. The changes in spatial distribution of resistivity are discussed, and the experimental data are fit by theoretical curves. Differences in the behavior of n-type and p-type material are explained on the basis of a faster removal of substitutional P and a more nonuniform spatial distribution of the original P concentration. >

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%.

Two detectors for (n,p) and (n,α) measurements at white neutron sources

Abstract We have developed two detectors for (n,p) and (n,α) cross section measurements at white neutron sources which make possible large increases in the sample size and geometric efficiency while at the same time reducing the potentially large background associated with the “gamma flash” to manageable levels. We present measurements of two (n,α) cross sections made with these detectors to energies as high as a few MeV.

The neutron total and capture cross sections of 92, 94Zr☆

Abstract The neutron total and capture cross sections of92,94Zr have been measured at the 80 and 40 m flight stations, respectively, on the Oak Ridge Electron Linear Accelerator (ORELA). Resonance analysis of the total cross sections gave values of s, p 3 2 and p 1 2 strength functions. Strong intermediate structure was observed in the p 3 2 cross section of 92Zr. Significant single particle transitions identified in the p-wave capture cross section indicate the importance of the valence process for both nuclei. It is noted that the intermediate structure in the p 3 2 total cross section leads to enhanced valence effects in the capture cross section. The effects of non statistical E1 transitions following particle-hole annihilation in the compound states were not observed in either the 92Zr or 94Zr capture cross sections as they had been in that for 90Zr.

Resonance structure of 33S+n from transmission measurements

Abstract The neutron total cross section for 33 S has been determined for energies from 10keV up to 2 MeV at the Oak Ridge Electron Linear Accelerator (ORELA) by a time-of-flight (TOF) transmission experiment. By combining these results with (n, α) data obtained at the Geel Electron Linear Accelerator (GELINA), resonance parameter ( E λ , J π , and Γ n ) have been determined up to 270 keV. Neutron strength functions for s- and p-wave resonances were derived. The p-wave strength function is much larger than for other nuclides in this mass region ( 30 Si, 32 S, and 34 S). This is discussed within the framework of the spherical optical model.

89Y + n resonances for E = 10–740 keV and intermediate structure

Abstract High-resolution neutron total-cross-section measurements on 89Y have been made from a few eV to 1 MeV using the pulsed neutron time-of-flight spectroscopy at ORELA with a neutron burst width of 5 ns and a nominal resolution of 0.025 ns/m at the highest energy. We have analyzed the data from 10–740 keV using a multilevel R-matrix code and obtained the values of resonance parameters (E0, Jπ, gΓn). From these results we have obtained the values of average quantities up to about 600 keV: Dl = 0 = 4.6 ± 0.3 keV, Dl = 1 = 1.35 ± 0.15 keV; S0 = 0.33 ± 0.045, S1 = 5.82 ± 0.38, in units of 10 −4 eV − 1 2 . The staircase plots of reduced neutron widths for s- and p-waves and the plots of the Lorentz-weighted strength functions averaged over different energy intervals show an evidence of several doorway states. A least-squares fit to the lorentzian smooth curves provides the best values of Ed (doorway energy) and γ2dc (escape reduced width of doorway). These values are in qualitative agreement with the theoretical predictions. The experimental ratio of s- to p-wave level spacings was found to be 3.43 ± 0.6. This is in disagreement with the theoretical prediction of 2.06 (for σ = 3.8) and indicates a parity dependence of the level density at high excitation.

Low-energy neutron resonances inPb206

Neutron resonance reactions on an enriched /sup 206/Pb target have been studied below 50 KeV using neutron time-of-flight techniques. The neutron and radiation widths of seventeen resonances were obtained from transmission and capture cross section measurements. Spin and parity assignments for eight resonances were made on the basis of the transmission measurements and the decay patterns of ..gamma.. rays measured with a Ge(Li)detector. The ..gamma..-ray spectra are strikingly simple, being composed of less than five primary ..gamma.. rays. The neutron separation energy for /sup 207/Pb was determined to be 6738.2 +- 0.7 KeV. The magnetic dipole transitions from several p-wave resonances to the first three excited states in /sup 207/Pb show some enhancement. A possible doorway state in the p/sub 1/2/ neutron channel has been observed in the /sup 206/Pb + n reaction at E/sub n/ approx. = 36 keV.

On the question of a giant M1 resonance near 8 MeV in 208Pb

Abstract Most of the ground-state radiative strength from levels near 8 MeV in 208 Pb is shown to be of E1 and not M1 character. In particular, a resonance reported to have B (M1 ↑) = 9 μ 0 2 must have ⩽2.1 μ 0 2 if present.

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.