Search

Elastic and inelastic neutron scattering differential cross sections and γ-ray production cross sections have been measured on 54,56Fe at several incident energies in the fast neutron region between 1.5 and 4.7 MeV. All measurements were completed at the University of Kentucky Accelerator Laboratory (UKAL) using a 7-MV Model CN Van de Graaff accelerator, along with the neutron production and neutron and γ-ray detection systems located there. The facilities at UKAL allow the investigation of both elastic and inelastic scattering with nearly mono-energetic incident neutrons. Time-of-flight techniques were used to detect the scattered neutrons for the differential cross section measurements. The measured cross sections are important for fission reactor applications and also for testing global model calculations such as those found at ENDF, since describing both the elastic and inelastic scattering is important for determining the direct and compound components of the scattering mechanism. The γ-ray production cross sections are used to determine cross sections to unresolved levels in the neutron scattering experiments. Results from our measurements and comparisons to model calculations are presented.

Inelastic neutron scattering measurements were performed at the University of Kentucky Accelerator Laboratory on enriched 76Ge and 76Se scattering samples. From measurements at incident neutron energies from 2.0 to 4.0 MeV, many new levels were identified and characterized in each nucleus; level lifetimes, transition probabilities, multipole mixing ratios, and other properties were determined. In addition, γ-ray cross sections for the 76Ge(n,n′γ) reaction were measured at neutron energies up to 5.0 MeV, with the goal of determining the cross sections of γ rays in 2040-keV region, which corresponds to the region of interest in the neutrinoless double β decay of 76Ge. Gamma rays from the three strongest branches from the 3952-keV level were observed, but the previously reported 2041-keV γ ray was not. Population cross sections across the range of incident neutron energies were determined for the 3952-keV level, resulting in a cross section of ~0.1 mb for the 2041-keV branch using the previously determined branching ratios. Beyond this, the data from these experiments indicate that previously unreported γ rays from levels in 76Ge can be found in the 2039-keV region.

The lifetime of the second 2+ state in 94Zr was redetermined by the Doppler-shift attenuation method following inelastic neutron scattering (DSAM-INS) from metallic Zr and ZrO2 samples of natural isotopic abundance. The new value for the level lifetime differs significantly from the previously published value, with the new lifetime found to be roughly twice that value. A reanalysis of the original γ-ray data from the enriched 94ZrO2 sample failed to expose the source of this discrepancy; however, powder X-ray diffraction and scanning electron microscopy performed on each scattering sample, including the enriched sample used previously, provide clues to an explanation and reveal the role of the chemical properties of the sample material in DSAM-INS lifetime determinations.

Low-lying states of 94Zr are populated following - decay of 94Y, and the emitted rays from 94Zr are detected using the 8p spectrometer composed of 20 Compton-suppressed HPGe detectors. High- statistics coincidence data have been used for the placement of very weak decay branches in the level scheme. Combining the results of level lifetimes from a previous experiment and the precisely measured branching ratio values of the weak decay branches from the present experiment, it is possible to extract the B(E2) values for all the possible decay branches from a given level. These values are helpful for proper identification of the collective and non-collective states of 94Zr. The experimental findings have been compared with predictions from shell-model calculations with a limited valence space; however, these calculations are inadequate in reproducing all of the measured spectroscopic quantities. [ABSTRACT FROM AUTHOR]

The low-spin structure of 93Nb has been studied using the (n,n’γ) reaction at neutron energies ranging from 1.5 to 2.6MeV and the 94Zr(p, 2nγ)93Nb reaction at bombarding energies from 11.5 to 19MeV. Excitation functions, lifetimes, and branching ratios were measured, and multipolarities and spin assignments were determined. The Jπ = 3/2- and 5/2- states at 1840 and 2013keV, respectively, are identified as mixed-symmetry states associated with the $\ensuremath{(2_{1,{\rm ms}}^{+}) \otimes 2_{p_{1/2}}}$ particle-core coupling. These assignments are in agreement with energy systematics, spins and parities, and the observed strong M1 transitions to the 2 p1/2 one-phonon structure. [ABSTRACT FROM AUTHOR]

The low-lying structure of 112Sn has been studied. γ-ray excitation functions, ranging from 2.5 to 4.0MeV, and angular distributions at 2.9 and 3.8MeV have been measured following the (n,n′γ) reaction to characterize the decays of the excited levels. Level lifetimes have been measured with the Doppler-shift attenuation method. Low-lying 1-, (2-), 3- and, 5- states have been identified as members of the heterogeneous quadrupole-octupole quintuplet. Decay properties and excitation energies are consistent with the structure formed by the coupling of the lowest quadrupole, 21+, and octupole, 31-, excitations. [ABSTRACT FROM AUTHOR]

The experimental signature in searches for the neutrinoless double-β decay of 76Ge is a peak near 2039 keV in the spectrum. Given the low probability of the process, it is important that the background in this region be well understood. Inelastic scattering reactions with neutrons from muon-induced interactions and (α,n) reactions in the surrounding materials or in the detector can provide contributions to the background. We have measured the production cross sections for γ rays from the 76Ge(n,n'γ) reaction in the 2039-keV region at incident neutron energies up to 4.9 MeV. In addition to determining that the cross sections of a previously known 2040.7-keV γ ray from the 3952-keV level in 76Ge are rather small, we find that a larger contribution arises from a 2037.5-keV γ ray which is attributed to a newly identified level at 3147 keV in 76Ge. A third contribution is also possible from another new level at 3577 keV. These results indicate that the 2039-keV region in 76Ge neutrinoless double-β decay searches is more complex than was previously thought. [ABSTRACT FROM AUTHOR]

Excited 0+ states in l60Gd have been examined with the (n,n'y) reaction at incident neutron energies up to 2.8 MeV. Gamma-ray excitation functions and angular distribution measurements allow the confirmation of the existence of 0+ states at 1379.70 keV and 1558.30 keV, but we reject the assignments of additional previously suggested 04 candidates. Limits on the level lifetimes of the observed 0+ states permit an evaluation of the collectivity of these states [ABSTRACT FROM AUTHOR]

The lifetime of the second excited 2+ state of 94Zr at 1671 keV was determined with the Doppler-shift attenuation method (DSAM) following the (n, n'y) reaction on scattering samples of Zr metal and Zr02 of natural isotopic abundance. The measured lifetime, r = 368-23+2 fs, is considerably longer than the previous measurement, as are measured lifetimes of other levels in 94Zr. However, lifetimes of low-lying, low-spin states in 90,9l,92,96Zr were also determined and found to be in good agreement with measurements employing non-DSAM methods. Possible reasons for the discrepant lifetimes measured for 94Zr are discussed and focus on the chemical properties of the scattering sample, specifically, the effects of using amorphous materials, or those composed of small particles as scattering samples. These properties have been investigated by x-ray powder diffraction and scanning electron microscopy. The effects of using these materials for lifetime measurements were investigated by employing, as scattering samples, Zr(OH)4 (a known amorphous material) and Zr02 with various crystalline domain sizes. [ABSTRACT FROM AUTHOR]

Based on results from a measurement of weak decay branches observed following the ß- decay of 94Y and on lifetime data from a study of 94Zr by inelastic neutron scattering, collective structure is deduced in the closed-subshell nucleus 94Zr. These results establish shape coexistence in 94Zr. The role of subshells for nuclear collectivity is suggested to be important. [ABSTRACT FROM AUTHOR]

Excited states in 150Nd have been investigated with the 150Nd(n,n'γ) reaction. In addition to the previously known Kπ = 0- band, a new Kπ = 2- band is established, and level lifetimes are determined for all the reported band members. These lifetime data reveal a pattern of enhanced E1 transition strengths, similar to that observed in 152Sm and unprecedented in other nuclei, thus suggesting a systematic pattern for octupole collectivity in the N = 90 isotones. The pattern lies outside of the various model descriptions that have been put forward for nuclei in this or any other region. [ABSTRACT FROM AUTHOR]

Measurements consisting of γ-ray excitation functions and angular distributions were performed using the (n,n'γ) reaction on 62Ni. The excitation function data allowed us to check the consistency of the placement of transitions in the level scheme. From γ-ray angular distributions, the lifetimes of levels up to ~3.8 MeV in excitation energy were extracted with the Doppler-shift attenuation method. The experimentally deduced values of reduced transition probabilities were compared with the predictions of the quadrupole vibrator model and with large-scale shell model calculations in the fp shell configuration space. Two-phonon states were found to exist with some notable deviation from the predictions of the quadrupole vibrator model, but no evidence for the existence of three-phonon states could be established. Z=28 proton core excitations played a major role in understanding the observed structure. [ABSTRACT FROM AUTHOR]

Inelastic neutron scattering was used to study the low-lying nuclear structure of 132Xe. A comprehensive level scheme is presented, as well as new level lifetimes, multipole mixing ratios, branching ratios, and transition probabilities. Comparisons of these data as well as previously measured E2 strengths and g factors are made with new shell-model calculations for 132,134,136Xe to explore the emergence of collectivity in the Xe isotopes with N < 82 near the closed shell. [ABSTRACT FROM AUTHOR]

The low-lying, low-spin levels of 76Se were studied with the (n,n'γ) reaction. Gamma-ray excitation function measurements were performed at incident neutron energies from 2.0 to 3.5 MeV, and γ-ray angular distributions were measured at neutron energies of 2.4, 3.0, and 3.7 MeV. From these measurements, level spins, level lifetimes, branching ratios, and multipole mixing ratios were determined. We established the 0+2 band, which supports the shape coexistence in 76Se predicted by large-scale shell model calculations and the interacting boson model. [ABSTRACT FROM AUTHOR]

The level structure of the N=82 nucleus 136Xe was studied with the inelastic neutron scattering reaction followed byγ-ray detection. A number of the spins and parities were reassigned, and many level lifetimes were determined for the first time using the Doppler-shift attenuation method. New shell-model calculations were also performed using both the fullZ=50-82 model space, and a reduced model space including only the1d5/2 and0g7/2 orbitals. This new information characterizing 136Xe was used to identify the seniority structure of the low-lying levels and to assign(π0g7/2)4υ=0,(π0g7/2)4υ=2,(π0g7/2)4υ=4,(π1d5/2)(π0g7/2)3υ=1, and(π1d5/2)2(π0g7/2)²υ=0 configurations to describe all observed states below 2.8 MeV. [ABSTRACT FROM AUTHOR]

The level structure of 134 Xe was studied with the inelastic neutron scattering reaction followed by γ-ray detection. A number of level lifetimes were determined for the first time with the Doppler-shift attenuation method and the low-lying excited states were characterized. From this new spectroscopic information, the third excited state, a 0+ level which had only been observed in a previous inelastic neutron scattering study, was verified. Reduced transition probabilities were calculated; comparisons were drawn with a vibrational description of the nucleus and found lacking. The 3 - octupole phonon has been confirmed, and the complete negative-parity multiplet resulting from the ν (1h11/22d3/2) configuration has also been tentatively identified for the first time in the N = 80 isotones. [ABSTRACT FROM AUTHOR]

Elastic and inelastic differential cross sections for neutron scattering from 56Fe have been measured for several incident energies from 1.30 to 7.96 MeV at the University of Kentucky Accelerator Laboratory. Scattered neutrons were detected using a C6D6 liquid scintillation detector using pulse-shape discrimination and time-of-flight techniques. The deduced cross sections have been compared with previously reported data, predictions from evaluation databases ENDF, JENDL, and JEFF, and theoretical calculations performed using different optical model potentials using the talys and empire nuclear reaction codes. The coupled-channel calculations based on the vibrational and soft-rotor models are found to describe the experimental (n,n0) and (n,n1) cross sections well. [ABSTRACT FROM AUTHOR]

160Gd(n,n'γ) experiments were performed with accelerator-produced monoenergetic neutrons. Excitation functions at neutron energies from 1.5 to 2.8 MeV aided in the placement of γ rays in the level scheme and angular distributions at three neutron energies resulted in the determination of 28 excited-level lifetimes or limits in 160Gd, including the lifetimes of several negative-parity levels attributed to octupole vibrations. [ABSTRACT FROM AUTHOR]

Excited states in 106Pd were studied with the (n,n'γ) reaction, and comprehensive information for excitations with spin ≤6ℏ was obtained. The data include level lifetimes in the femtosecond regime, spins and parities, transition multipolarities, and multipole mixing ratios, which allow the determination of reduced transition probabilities. The E2 decay strength to the low-lying states is mapped up to ≈2.4 MeV in excitation energy. The structures associated with quadrupole collectivity are elucidated and organized into bands. [ABSTRACT FROM AUTHOR]

The low-lying, low-spin levels of 76Ge were studied with the (n,n'γ) reaction. Gamma-ray excitation function measurements were performed at incident neutron energies from 1.6 to 3.7 MeV, and γ-ray angular distributions were measured at neutron energies of 3.0 and 3.5 MeV. From these measurements, level spins, level lifetimes, γ-ray intensities, and multipole mixing ratios were determined. No evidence for a number of previously placed levels was found. Below 3.3 MeV, many new levels were identified, and the level scheme was re-evaluated. The B(E2) values support low-lying band structure. The 2+ mixed-symmetry state has been identified for the first time. A comparison of the level characteristics with large-scale shell model calculations yielded excellent agreement. [ABSTRACT FROM AUTHOR]

Neutrinoless double-β decay (0νββ) searches typically involve large-scale experiments for which backgrounds can be complex. One possible source of background near the 0νββ signature in the observed spectra is γ rays arising from inelastic neutron scattering from the materials composing or surrounding the detector. In relation to searches for the 0νββ of 136Xe to 136Ba, such as the EXO-200 and KamLAND-Zen projects, inelastic neutron scattering γ-ray production cross sections for 136Xe and 134Xe are of importance for characterizing such γ rays that may inhibit the unambiguous identification of this yet-to-be-observed process. These cross sections have been measured at the University of Kentucky Accelerator Laboratory at neutron energies from 2.5 to 4.5 MeV. [ABSTRACT FROM AUTHOR]

The level structures of 130,132Xe were studied with the inelastic neutron scattering reaction followed by γ-ray detection. Level lifetimes were measured using the Doppler-shift attenuation method and low-lying excited states in these nuclei were characterized. With a focus on the decay properties of the 0+ states, these nuclei were examined as representations of the E(5) critical-point symmetry. [ABSTRACT FROM AUTHOR]