Daniel Edward Archer

Yrast superdeformed band in 194 Pb: J π and E x

The yrast superdeformed band in {sup 194}Pb has been populated using the {sup 174}Yb({sup 25}Mg,5n){sup 194}Pb reaction at E{sub beam}=130 MeV. Decay {gamma} rays were detected using the GAMMASPHERE array at the 88-Inch Cyclotron. Twelve {gamma}-ray transitions have been observed directly linking three members of the {sup 194}Pb yrast superdeformed band to low-lying normal deformed levels. Anisotropy measurements indicate that these linking decays include E1, M1, and mixed M1/E2 multipolarities. The radiative widths deduced are very inhibited, typically B(E1){approximately}10{sup {minus}8} Weisskopf units (W.u.) and B(M1){approximately}10{sup {minus}5} W.u. Without recourse to {ital a priori} assumptions J{sup {pi}}=6{sup +} and E{sub x}=4878.4(3) keV have been unambiguously assigned to the lowest-lying observed superdeformed state (the state populated by the 170-keV intraband transition). The intensity of the observed primaries accounts for 21(2){percent} of the superdeformed band population. {copyright} {ital 1997} {ital The American Physical Society}

Differential quadrupole moment measurements of the 1/2{sup +}[660] (i{sub 13/2}) neutron intruder band in {sup 133}Nd and {sup 135}Nd

Quadrupole moment measurements of the 1/2{sup +}[660] (i{sub 13/2}) bands in {sup 133}Nd (N=73) and {sup 135}Nd (N=75) were performed using the Doppler-shift attenuation method. These results, coupled with the previously measured Q{sub 0} for the same configuration in {sup 137}Nd (N=77), clearly demonstrate a trend of decreasing quadrupole deformation with increasing neutron number. The larger quadrupole moment in {sup 133}Nd compared with that in {sup 135}Nd and {sup 137}Nd offers evidence for the role played by the large shell gap at N=72 for {beta}{sub 2}{approximately}0.35{endash}0.40 in stabilizing the shape at enhanced deformation. The comparison of results from gating below and above the level of interest provides information on the time scale of the sidefeeding contributions to highly deformed structures in the A{approximately}130 region. {copyright} {ital 1999} {ital The American Physical Society}

Absolute Partial γ-ray Cross Sections in 238U(n, xny) Reactions

Measurements of discrete γ-ray spectra have been carried out as a function of incident-neutron energy for nuclei populated in 238U(n, xnγ (x ≤ 4) reactions. The GEANIE spectrometer, comprised of 26 Ge detectors (11 planar and 15 coaxial), was used to detect γ-rays. Neutrons were provided by the “white” neutron source of the Los Alamos Neutron Science Center’s WNR facility. The energy of the incident neutrons was determined using the time-of-flight technique. Absolute cross sections were determined for emission of several γ-rays from low-lying states of 235-238U isotopes (spin up to 10ħ and excitation energy up to ~1 MeV) as a function of incident-neutron energy (1 MeV < En 100 MeV). Uncertainties associated with the spectroscopic analysis of the data are discussed. Predictions of partial γ-ray cross sections from GNASH calculations up to neutron energy En = 30 MeV are compared to the experimental results and are generally in good agreement. Combining the experimental results with the predictions of the nuclear reaction modeling provides a measurement of the 238U(n, n′) reaction cross section and validate this technique for determining reaction cross sections using γ-ray spectroscopy.

Neutron-Induced Partial Gamma-Ray Cross-Section Measurements with GEANIE at LANSCE/WNR

GEANIE is the first large-scale Ge detector array used in conjunction with a high-energy neutron spallation source. GEANIE consists of eleven Compton-suppressed planar detectors, nine suppressed and six unsuppressed co-axial detectors. Spallation neutrons are provided by the LANSCE/WNR facility, and reaction neutron energies are determined via time-of-flight. neutron flux is monitored in-beam with a fission chamber. GEANIE at LANSCE/WNR currently emphasizes the measurement of partial gamma-ray cross sections as a function of neutron energy. Absolute cross section measurements require a complete understanding of array performance. Important effects include intrinsic detector efficiency, beam and detector geometry corrections, target attenuation, and deadtime. Measurements and calculations of these effects will be presented for the specific cases of iron and actinide targets. The use of radioactive targets incurs a large deadtime penalty. In order to increase data throughput they are making plans to move to a triggerless data acquisition system. These modifications and other improvements to the electronics for better timing will be discussed.

Rotational bands and isomeric states in 175Lu

Rotational bands in {sup 175}Lu have been extended through investigation with the (n,n{prime}{gamma}) reaction. Spallation neutrons bombarded Lu samples, and the resulting {gamma} rays were detected in a large-scale Compton-suppressed Ge detector array. Prompt- and delayed-{gamma}{gamma} coincidences have been used to extend most of the existing known bands, and to tentatively assign a new band, based on the 7/2{sup -}[523] configuration, from its band head to spin 13/2. The 3-quasiparticle K{sup {pi}} = 19/2{sup +} isomer is confirmed and its half life determined to be 984 {+-} 13(stat.) {+-} 30(sys.) {micro}s, in agreement with previous results.

Partial γ-Ray Cross Sections for the Reaction 239Pu(n,2nγi) and the 239Pu(n,2n) Cross Section

Absolute partial γ-ray cross sections for discrete γ-ray production in the 239Pu(n,2nγi)238Pu reaction have been measured. The experiments were performed at LANSCE/WNR on the 60R neutron flight path. Reaction γ-rays were measured using the large-scale Compton-suppressed array of Ge detectors, GEANIE. The motivation for this experiment, an overview of the partial γ-ray cross-section measurement, and an introduction to the main experimental issues will be presented. The energy resolution of the Ge detectors allowed identification of reaction γ rays above the background of sample radioactivity and fission γ rays. The use of planar Ge detectors with their reduced sensitivity to neutron interactions and improved line shape was also important to the success of this experiment. Absolute partial γ-ray cross sections are presented for the 61+ → 41+ member of the ground state rotational band in 238Pu, together with miscellaneous other γ-ray partial cross sections. The n,2n reaction cross section shape and magnitude as a function of neutron energy were extracted from these partial cross sections using nuclear modeling (enhanced Hauser-Feshbach) to relate partial γ-ray cross sections to the n,2n cross section. The critical nuclear modeling issue is the ratio of a partial cross section to the reaction channel cross section, and not the prediction of the absolute magnitude.

Two-phonon excitations in 170Er

Recent experiments at the GEANIE/WNR facility and the University of Kentucky accelerator have yielded strong evidence for a two-gamma excitation in 170 Er. This new case can be added to a handful of previously identified examples of two-gamma vibrations, all of them discovered in this decade. In this paper the experimental evidence for a two-phonon excitation 170 Er is presented and the current state of understanding of these structures is reviewed in the context of this and other recent findings.

Yrast superdeformed band in {sup 194}Pb: J{sup {pi}} and E{sub x}

The yrast superdeformed band in {sup 194}Pb has been populated using the {sup 174}Yb({sup 25}Mg,5n){sup 194}Pb reaction at E{sub beam}=130 MeV. Decay {gamma} rays were detected using the GAMMASPHERE array at the 88-Inch Cyclotron. Twelve {gamma}-ray transitions have been observed directly linking three members of the {sup 194}Pb yrast superdeformed band to low-lying normal deformed levels. Anisotropy measurements indicate that these linking decays include E1, M1, and mixed M1/E2 multipolarities. The radiative widths deduced are very inhibited, typically B(E1){approximately}10{sup {minus}8} Weisskopf units (W.u.) and B(M1){approximately}10{sup {minus}5} W.u. Without recourse to {ital a priori} assumptions J{sup {pi}}=6{sup +} and E{sub x}=4878.4(3) keV have been unambiguously assigned to the lowest-lying observed superdeformed state (the state populated by the 170-keV intraband transition). The intensity of the observed primaries accounts for 21(2){percent} of the superdeformed band population. {copyright} {ital 1997} {ital The American Physical Society}

Yrast superdeformed band in Pb-194: Jpi and Ex

The yrast superdeformed band in {sup 194}Pb has been populated using the {sup 174}Yb({sup 25}Mg,5n){sup 194}Pb reaction at E{sub beam}=130 MeV. Decay {gamma} rays were detected using the GAMMASPHERE array at the 88-Inch Cyclotron. Twelve {gamma}-ray transitions have been observed directly linking three members of the {sup 194}Pb yrast superdeformed band to low-lying normal deformed levels. Anisotropy measurements indicate that these linking decays include E1, M1, and mixed M1/E2 multipolarities. The radiative widths deduced are very inhibited, typically B(E1){approximately}10{sup {minus}8} Weisskopf units (W.u.) and B(M1){approximately}10{sup {minus}5} W.u. Without recourse to {ital a priori} assumptions J{sup {pi}}=6{sup +} and E{sub x}=4878.4(3) keV have been unambiguously assigned to the lowest-lying observed superdeformed state (the state populated by the 170-keV intraband transition). The intensity of the observed primaries accounts for 21(2){percent} of the superdeformed band population. {copyright} {ital 1997} {ital The American Physical Society}