25 Matching Results

Search Results

Advanced search parameters have been applied.

Shape transitions in neutron-rich Ru isotopes: spectroscopy of 109,110,111,112Ru

Description: The spectroscopy of neutron-rich {sup 109,110,111,112}Ru nuclei was studied by measuring the prompt {gamma} rays originated from fission fragments, produced by the {sup 238}U({alpha},f) fusion-fission reaction, in coincidence with the detection of both fragments. For {sup 109,111}Ru, both the negative-parity (h{sub 11/2} orbitals) and positive-parity (g{sub 7/2} and/or d{sub 5/2} orbitals) bands were extended to substantially higher spin and excitation energy than known previously. The ground-state and {gamma}-vibrational bands of {sup 110,112}Ru also were extended to higher spin, allowing observation of the second band crossing at the rotational frequency of {approx}450 keV in {sup 112}Ru, which is {approx}50 keV above the first band crossing. At a similar rotational frequency, the first band crossing for the h{sub 11/2} band in {sup 111}Ru was observed, which is absent in {sup 109}Ru. These band crossings most likely are caused by the alignment of the g{sub 9/2} proton pair. This early onset of the band crossing for the aligned {pi}g{sub 9/2} orbitals may be evidence of a triaxial shape transition from prolate to oblate occurring in {sup 111}Ru. The data together with a comparison of cranked shell model predictions are presented.
Date: June 29, 2005
Creator: Hua, H; Cline, D; Hayes, A B; Teng, R; Riley, D; Clark, R M et al.
Partner: UNT Libraries Government Documents Department

Order-to-chaos transition in rotational nuclei

Description: The authors have studied the narrow (valley-ridge) structure in the {gamma}-ray spectrum following a heavy-ion fusion reaction that produces several ytterbium nuclei. The intensity of this structure can be quantitatively related to the average chaotic behavior in these nuclei and they have traced this behavior from nearly fully ordered to nearly fully chaotic.
Date: May 13, 2004
Creator: Stephens, F.S.; Deleplanque, M.A.; Lee, I.Y.; Macchiavelli, A.O.; Ward, D.; Fallon, P. et al.
Partner: UNT Libraries Government Documents Department

Searching for E(5) behavior in nuclei

Description: The properties of even-even nuclei with 30 {le} Z {le} 82, A {ge} 60 have been examined to find examples displaying the characteristics of E(5) critical-point behavior for the shape transition from a spherical vibrator to a triaxially soft rotor. On the basis of the known experimental state energies and E2 transition strengths, the best candidates that were identified are {sup 102}Pd, {sup 106,108}Cd, {sup 124}Te, {sup 128}Xe, and {sup 134}Ba. The closest agreement between experimental data and the predictions of E(5) is for {sup 128}Xe and for the previously suggested example of {sup 134}Ba. It is proposed that {sup 128}Xe may be a new example of a nucleus at the E(5) critical point.
Date: January 1, 2004
Creator: Clark, R.M.; Cromaz, M.; Deleplanque, M.A.; Descovich, M.; Diamond, R.M; Fallon, P. et al.
Partner: UNT Libraries Government Documents Department

The N=90 transitional nuclei {sup 150}Nd and {sup 152}Sm revisited

Description: The purpose of this paper is to show that recent data on the ground-state band and excited states based on the 0{sub 2}{sup +} level in {sup 15}Nd and {sup 152}Sm, especially the measured B(E2) values, can be well described by including a {Delta}K = 0 coupling between rotational bands. This is contrary to recent statements in the literature. The experimental data are compared with models which have supported the widely differing interpretations of these transitional nuclei. These interpretations include describing excited states as rotational excitations of single-phonon states, the multiphonon ''phase coexistence'' picture, and the X(5) critical-point description.
Date: January 21, 2003
Creator: Clark, R.M.; Cromaz, M.; M.A., Deleplanque; Diamond, R.M.; Fallon, P.; Gorgen, A. et al.
Partner: UNT Libraries Government Documents Department

Damping in Yb nuclei

Description: In a mixture of three Yb nuclei, we find the rotational damping widths vary from 180 keV at 1.1 MeV {gamma}-ray energy to 290 keV at 1.5 MeV, and the average compound damping widths (or spreading widths) vary from 40 keV at 1.1 MeV {gamma}-ray energy to 60 keV at 1.3 MeV. The simulations also suggest extensive motional narrowing.
Date: July 1, 2002
Creator: Stephens, F.S.; Deleplanque, M.A.; Lee, I.Y.; Ward, D.; Fallon, P.; Cromaz, M. et al.
Partner: UNT Libraries Government Documents Department

Population of Nuclei Via 7Li-Induced Binary Reactions

Description: The authors have investigated the population of nuclei formed in binary reactions involving {sup 7}Li beams on targets of {sup 160}Gd and {sup 184}W. The {sup 7}Li + {sup 184}W data were taken in the first experiment using the LIBERACE Ge-array in combination with the STARS Si {Delta}E-E telescope system at the 88-Inch Cyclotron of the Lawrence Berkeley National Laboratory. By using the Wilczynski binary transfer model, in combination with a standard evaporation model, they are able to reproduce the experimental results. This is a useful method for predicting the population of neutron-rich heavy nuclei formed in binary reactions involving beams of weakly bound nuclei formed in binary reactions involving beams of weakly bound nuclei and will be of use in future spectroscopic studies.
Date: August 8, 2005
Creator: Clark, Rodney M.; Phair, Larry W.; Descovich, M.; Cromaz, Mario; Deleplanque, M.A.; Fall on, Paul et al.
Partner: UNT Libraries Government Documents Department

Benchmarking the External Surrogate Ratio Method using the (alpha,alpha' f) reaction at STARS

Description: We measured the ratio of the fission probabilities of {sup 234}U* relative to {sup 236}U* formed via an ({alpha},{alpha}{prime}) direct reactions using the STARS array at the 88-inch cyclotron at the Lawrence Berkeley National Laboratory. This ratio has a shape similar to the ratio of neutron capture probabilities from {sup 233}U(n; f) and {sup 235}U(n; f), indicating the alpha reactions likely formed a compound nucleus. This result indicates that the ratios of fission exit channel probabilities for two actinide nuclei populated via ({alpha}, {alpha}{prime}) can be used to determine an unknown fission cross section relative to a known one. The validity of the External Surrogate Ratio Method (ESRM) is tested and the results support the conclusions of Burke et al. [1].
Date: January 9, 2008
Creator: Lesher, S R; Bernstein, L A; Ai, H; Beausang, C W; Bleuel, D; Burke, J T et al.
Partner: UNT Libraries Government Documents Department

Population of Nuclei Via 7Li-Induced Binary Reactions

Description: The authors have investigated the population of nuclei formed in binary reactions involving {sup 7}Li beams on targets of {sup 160}Gd and {sup 184}W. The {sup 7}Li + {sup 184}W data were taken in the first experiment using the LIBERACE Ge-array in combination with the STARS Si {Delta}E-E telescope system at the 88-Inch Cyclotron of the Lawrence Berkeley National Laboratory. By using the Wilczynski binary transfer model, in combination with a standard evaporation model, they are able to reproduce the experimental results. This is a useful method for predicting the population of neutron-rich heavy nuclei formed in binary reactions involving beams of weakly bound nuclei and will be of use in future spectroscopic studies.
Date: August 9, 2005
Creator: Clark, R M; Phair, L W; Descovich, M; Cromaz, M; Deleplanque, M A; Fallon, P et al.
Partner: UNT Libraries Government Documents Department

Deducing the 237U destruction cross-sections using the Surrogate Ratio Method

Description: We have deduced the destruction cross section of {sup 237}U via the (n, {gamma}) and (n,2n) reactions over an equivalent neutron energy range of 0 to 20 MeV using a new form of the Surrogate Ratio method [1-4] . The relative fission and neutron-evaporation decay probabilities of excited {sup 238}U populated via the ({alpha},{alpha}{prime}) inelastic scattering were measured using the silicon telescope array for reaction studies (STARS) coupled to the Livermore Berkeley array for collaborative experiments (LIBERACE). These relative probabilities were then combined with the {sup 237} U(n,f) cross section deduced by Burke et al., [4] to deduce the (n, {gamma}) and (n,2n) cross sections in a model independent fashion. These cross sections are then compared to the compound reaction cross section calculated using an optical model calculation tuned to reproduce scattering data in the transactinide region. Our results presented and the prospects for using this technique to deduce (n,x) cross sections on radioactive nuclei are discussed.
Date: March 31, 2006
Creator: Bernstein, L A; Burke, J T; Ahle, L; Church, J A; Escher, J; Dietrich, F S et al.
Partner: UNT Libraries Government Documents Department

Deducing the 237U(n,f) cross-section using the Surrogate Ratio Method

Description: The authors have deduced the {sup 237}U(n,f) cross-section over an equivalent neutron energy range of 0 to 20 MeV using the Surrogate Ratio method. A 55 MeV {sup 4}He{sup 2+} beam from the 88 Inch Cyclotron at Lawrence Berkeley National Laboratory was used to induce fission in the following reactions {sup 238}U({alpha},{alpha}'f) and {sup 236}U({alpha},{alpha}'f). The {sup 238}U reaction was a surrogate for {sup 237}U(n,f) and the {sup 236}U reaction was used as a surrogate for {sup 235}U(n,f). The energies of the scattered alpha particles were detected in a fully depleted segmented silicon telescope array (STARS) over an angle range of 35{sup o} to 60{sup o} with respect to the beam axis. The fission fragments were detected in a third independent silicon detector located at backward angles between 106{sup o} to 131{sup o}.
Date: August 16, 2005
Creator: Burke, J T; Bernstein, L A; Escher, J; Ahle, L; Church, J A; Dietrich, F et al.
Partner: UNT Libraries Government Documents Department

Deducing the 237U(n,f) cross-section using the Surrogate Ratio Method

Description: The authors have deduced the cross section for {sup 237}U(n,f) over an equivalent neutron energy range from 0 to 20 MeV using the Surrogate Ratio method. A 55 MeV {sup 4}He beam from the 88 Inch Cyclotron at Lawrence Berkeley National Laboratory was used to induce fission in the following reactions: {sup 238}U({alpha},{alpha}{prime}f) and {sup 236}U({alpha},{alpha}{prime}f). The {sup 238}U reaction was a surrogate for {sup 237}U(n,f) and the {sup 236}U reaction was used as a surrogate for {sup 235}U(n,f). Scattered alpha particles were detected in a fully depleted segmented silicon telescope array (STARS) over an angle range of 35{sup o} to 60{sup o} with respect to the beam axis. The fission fragments were detected in a third independent silicon detector located at backward angles between 106{sup o} and 131{sup o}.
Date: December 29, 2005
Creator: Burke, J T; Bernstein, L A; Escher, J; Ahle, L; Church, J A; Dietrich, F S et al.
Partner: UNT Libraries Government Documents Department

Unsafe Coulomb excitation of {sup 240-244}Pu.

Description: The high spin states of {sup 240}Pu and {sup 244}Pu have been investigated with GAMMASPHERE at ATLAS, using Coulomb excitation with a {sup 208}Pb beam at energies above the Coulomb barrier. Data on a transfer channel leading to {sup 242}Pu were obtained as well. In the case of {sup 244}Pu, the yrast band was extended to 34{h_bar} revealing the completed {pi}i{sub 13/2} alignment, a ''first'' for actinide nuclei. The yrast sequence of {sup 242}Pu was also extended to higher spin and a similar backbend was delineated. In contrast, while the ground state band of {sup 240}Pu was measured up to the highest rotational frequencies ever reported in the actinide region ({approximately}300 keV), no sign of particle alignment was observed. In this case, several observable such as the large B(E1)/B(E2) branching ratios in the negative parity band, and the vanishing energy staggering between the negative and positive parity bands suggest that the strength of octupole correlations increases with rotational frequency. These stronger correlations may well be responsible for delaying or suppressing the {pi}i{sub 13/2} particle alignment.
Date: February 4, 1999
Creator: Ahmad, I.; Amro, H.; Carpenter, M. P.; Chowdhury, P.; Cizewski, J.; Cline, D. et al.
Partner: UNT Libraries Government Documents Department

Very extended shapes in the A~;110 region

Description: High-angular-momentum states in {sup 108}Cd were populated via the {sup 64}Ni({sup 48}Ca,4n) reaction at a beam energy of 207 MeV. Gamma rays were detected using the Gammasphere array. A rotational band has been observed with a dynamic moment of inertia and deduced lower limit of the quadrupole moment suggesting a major-to-minor axis ratio larger than 1.8:1, placing it among the most deformed structures identified in any nucleus, to date.
Date: September 12, 2001
Creator: Clark, R.M.; Fallon, P.; Gorgen, A.; Cromaz, M.; Deleplanque,M.A.; Diamond, R.M. et al.
Partner: UNT Libraries Government Documents Department

Aspects of the coriolis interaction in U235

Description: We have performed three separate experiments at LBNL's 88-Inch Cyclotron over a period of several years in which {sup 235}U (thick target) was Coulomb-excited. It involved stand-alone experiments with Gammasphere and with the 8PI Spectrometer using {sup 136}Xe beams at 720 MeV, and a CHICO-Gammasphere experiment with an {sup 40}Ar beam at 180 MeV. In addition to extending the known negative-parity bands to high spin, we have assigned levels in some seven positive-parity bands which are in some cases (e.g. [631]1/2, [624]7/2 and [622]5/2) strongly populated by E3 excitation. The CHICO data has been analyzed to extract E2 and E3 matrix elements from the observed yields. Additionally, many E1 and M1 matrix elements could be extracted from the {gamma}-ray branching ratios.
Date: January 1, 2005
Creator: Ward, D.; Clark, R.M.; Cromaz, M.; Deleplanque, M.A.; Diamond, R.M.; Fallon, P. et al.
Partner: UNT Libraries Government Documents Department

Quadrupole moments of wobbling excitations in 163Lu

Description: Lifetimes of states in the triaxial strongly deformed bands of {sup 163}Lu have been measured in a Gammasphere experiment using the Doppler-shift attenuation method. The bands are interpreted as wobbling-phonon excitations from the characteristic electromagnetic properties of the transitions connecting the bands. Quadrupole moments were extracted for the 0-phonon yrast band and, for the first time, for the 1-phonon wobbling band. The very similar results found for both bands suggest a similar intrinsic structure confirming the wobbling interpretation. While the in-band quadrupole moments for the bands show a decreasing trend towards higher spin, the strength of the inter-band transitions remains constant. Both features can be understood by a small increase in triaxiality towards higher spin. Such a change in triaxiality is also found in cranking calculations, to which the experimental results are compared.
Date: January 1, 2004
Creator: Gorgen, A.; Clark, R.M.; Cromaz, M.; Fallon, P.; Hagemann, G.B.; Hubel, H. et al.
Partner: UNT Libraries Government Documents Department

In-beam gamma-ray spectroscopy of target fragmentation

Description: Fragmentation reactions, typically performed at energies {approx}E/A>50 MeV, produce neutron-rich nuclei but leave little angular momentum in the residues. In this work we have examined the product distribution and angular momentum input for a {sup 12}C beam at 30 MeV/A on a thick (40 mg/cm{sup 2}) target of {sup 51}V, testing the feasibility of in-beam gamma-ray spectroscopy study of target fragments at this lower energy. This technique allows the study of some neutron-rich nuclei to moderate spins, complementing the beam fragmentation studies.
Date: October 6, 2004
Creator: Rodriguez-Vieitez, E.; Lee, I.Y.; Ward, D.; Fallon, P.; Clark, R.M.; Cromaz, M. et al.
Partner: UNT Libraries Government Documents Department

Very extended shapes in 108Cd: evidence for the occupation of 'hyper-intruder' orbitals

Description: High-spin states in {sup 108}Cd were studied following the reaction {sup 64} Ni({sup 48}Ca,4n) at a beam energy of 207 MeV. Gamma rays were detected using the Gammasphere array. Two rotational bands have been observed at very high angular momentum. Measurements of fractional Doppler shifts yielded lower limits for the quadrupole moments and showed that the observed structures are at least as deformed as the superdeformed structures e.g. in the A {approx} 150 region, and possibly exceed a 2:1 axis ratio. The existence of very extended shapes has been predicted by cranked Strutinsky calculations, and recent projected shell model calculations suggest that the {pi}i{sub 13/2}hyper-intruder orbital is occupied in these newly observed bands.
Date: July 1, 2002
Creator: Gorgen, A.; Clark, R.M.; Fallon, P.; Cromaz, M.; Deleplanque, M.A.; Diamond, R.M. et al.
Partner: UNT Libraries Government Documents Department

Surrogate Nuclear Reactions using STARS

Description: The results from two surrogate reaction experiments using the STARS (Silicon Telescope Array for Reaction Studies) spectrometer are presented. The surrogate method involves measuring the particle and/or {gamma}-ray decay probabilities of excited nuclei populated via a direct reaction. These probabilities can then be used to deduce neutron-induced reaction cross sections that lead to the same compound nuclei. In the first experiment STARS coupled to the GAMMASPHERE {gamma}-ray spectrometer successfully reproduce surrogate (n,{gamma}), (n,n'{gamma}) and (n,2n{gamma}) cross sections on {sup 155,156}Gd using Gd {sup 3}He-induced reactions. In the second series of experiments an energetic deuteron beam from the ESTU tandem at the Wright Nuclear Structure Lab at Yale University was used to obtain the ratio of fission probabilities for {sup 238}U/ {sup 236}U and {sup 237}U/ {sup 239}U populated using the {sup 236,238}U(d,d'f) and {sup 236,238}U(d,pf) reactions. Results from these experiments are presented and the implications for the surrogate reaction technique are discussed.
Date: October 26, 2004
Creator: Bernstein, L A; Burke, J T; Church, J A; Ahle, L; Cooper, J R; Hoffman, R D et al.
Partner: UNT Libraries Government Documents Department

Surrogate Reactions in the Actinide Region

Description: Over the past three years we have studied various surrogate reactions (d,p), ({sup 3}He,t), ({alpha},{alpha}{prime}) on several uranium isotopes {sup 234}U, {sup 235}U, {sup 236}U, and {sup 238}U. An overview of the STARS/LIBERACE surrogate research program as it pertains to the actinides is discussed. A summary of results to date will be presented along with a discussion of experimental difficulties encountered in surrogate experiments and future research directions.
Date: January 30, 2008
Creator: Burke, J T; Bernstein, L A; Scielzo, N D; Bleuel, D L; Lesher, S R; Escher, J et al.
Partner: UNT Libraries Government Documents Department

Observation of mean-spin barrier bump in sub-barrier fusion of sup 28 Si with sup 154 Sm

Description: We have measured the fusion excitation function and gamma-ray multiplicities, M{sub gamma}, for the {sup 28}Si + {sup 154}Sm system. We have also measured M{sub {gamma}} for the {sup 16}O + {sup 166} Er system that leads to the same compound nucleus, {sup 182}Os. This is used to calibrate the connection between M{sub {gamma}} and the first moment of the spin distribution of the compound nucleus, {l angle}{ell}{r angle}. We find that the deduced {l angle}{ell}{r angle} in {sup 28}Si + {sup 154}Sm agrees reasonably well with theoretical expectations, and in particular exhibits the barrier bump expected when another degree of freedom is strongly coupled to the relative motion. 17 refs., 2 figs.
Date: January 1, 1990
Creator: Gil, S.; Abriola, D.; DiGregorio, D.E.; di Tada, M.; Elgue, M.; Etchegoyen, A. et al.
Partner: UNT Libraries Government Documents Department