29 Matching Results

Search Results

Advanced search parameters have been applied.

Neutron Capture Cross Section Measurement on $^{238}$Pu at DANCE

Description: The proposed neutron capture measurement for {sup 238}Pu was carried out in Nov-Dec, 2010, using the DANCE array at LANSCE, LANL. The total beam-on-target time is about 14 days plus additional 5 days for the background measurement. The target was prepared at LLNL with the new electrplating cell capable of plating the {sup 238}Pu isotope simultaneously on both sides of the 3-{micro}m thick Ti backing foil. A total mass of 395 {micro}g with an activity of 6.8 mCi was deposited onto the area of 7 mm in diameter. The {sup 238}Pu sample was enriched to 99.35%. The target was covered by 1.4 {micro}m double-side aluminized mylar and then inserted into a specially designed vacuum-tight container, shown in Fig. 1, for the {sup 238}Pu containment. The container was tested for leaks in the vacuum chamber at LLNL. An identical container without {sup 238}Pu was made as well and used as a blank for the background measurement.
Date: February 14, 2011
Creator: Chyzh, A & Wu, C Y
Partner: UNT Libraries Government Documents Department

Bambino: a segmented silicon detector system for TIGRESS

Description: Bambino is a charge-particle detector system with sufficient energy and position resolutions for the differentiation between projectile-like and target-like particles and for the needed Doppler-shift corrections to the detected {gamma} rays in TIGRESS. It consists of two annular silicon detectors having an active inner diameter of 22 mm and outer diameter of 70 mm and a thickness about 150 {micro}m. They are placed 3.0 cm from the target and provide solid-angle coverage of 1.15{pi} sr. Each has 24 sectors in {theta} for the angle coverage between 20.1{sup o} and 49.4{sup o} and between 130.6{sup o} to 159.9{sup o} and has 16 sectors in {phi} for 2{pi} coverage. Three of those detectors and the matching preamplifiers, cables etc were ordered and received in 2005 at a cost about $50k funded by DOE/OS. The system was undergoing various tests at both LLNL and TRIUMF in the second quarter of 2006 and was successfully integrated into TIGRESS for the commission run in July/August 2006. A side-accessible spherical target chamber, used in the commission run, was designed and built in Rochester in the second quarter of 2006 to accommodate this detector system at a cost about $28k funded by NSF and AFOSR.
Date: August 17, 2006
Creator: Wu, C Y; Becker, J A & Cline, D
Partner: UNT Libraries Government Documents Department

Li/Li/sub 2/ supersonic nozzle beam

Description: The characterization of a lithium supersonic nozzle beam was made using spectroscopic techniques. It is found that at a stagnation pressure of 5.3 kPa (40 torr) and a nozzle throat diameter of 0.4 mm the ground state vibrational population of Li/sub 2/ can be described by a Boltzmann distribution with T/sub v/ = 195 +- 30/sup 0/K. The rotational temperature is found to be T/sub r/ = 70 +- 20/sup 0/K by band shape analysis. Measurements by quadrupole mass spectrometer indicates that approximately 10 mole per cent Li/sub 2/ dimers are formed at an oven body temperature of 1370/sup 0/K n the supersonic nozzle expansion. This measured mole fraction is in good agreement with the existing dimerization theory.
Date: January 1, 1977
Creator: Wu, C.Y.R.; Crooks, J.B.; Yang, S.C.; Way, K.R. & Stwalley, W.C.
Partner: UNT Libraries Government Documents Department

Photon and positive ion production from collisions of superthermal hydrogen atoms with lithium atoms and molecules. [1 to 10 eV, cross sections, emission spectra]

Description: Photon and positive ion production from superthermal collisions of hydrogen atoms with lithium atoms and molecules is reported. These are the first results in the difficult 1 to 10 eV collision regime obtained with recently developed high intensity plasma-arc source of H atoms. These superthermal H atoms (approximately 5 eV average kinetic energy but not velocity selected in these experiments) have been collided with an intense supersonic lithium beam (approximately 10 percent Li/sub 2/; Li/sub 3/ undetectable) with cold internal degrees of freedom (T/sub vib/ = 195/sup 0/K, T/sub rot/ = 70/sup 0/K). The other aspects of the apparatus have been given previously. Low resolution photon and positive ion production in the collision chamber scattering center and also medium resolution; photon production in the magnet chamber between the skimmer and the magnet were detected.
Date: January 1, 1978
Creator: Crooks, J.B.; Way, K.R.; Yang, S.C.; Wu, C.Y.R. & Stwalley, W.C.
Partner: UNT Libraries Government Documents Department

Unfolding the fission prompt gamma-ray energy and multiplicity distribution measured by DANCE

Description: The nearly energy independence of the {gamma}-ray efficiency and multiplicity response for the DANCE array, the unusual characteristic elucidated in our early technical report (LLNL-TR-452298), gives one a unique opportunity to derive the true prompt {gamma}-ray energy and multiplicity distribution in fission from the measurement. This unfolding procedure for the experimental data will be described in details and examples will be given to demonstrate the feasibility of reconstruction of the true distribution.
Date: October 16, 2010
Creator: Chyzh, A; Wu, C Y; Bredeweg, T; Couture, A; Jandel, M; Ullmann, J et al.
Partner: UNT Libraries Government Documents Department

Electrodeposition of Uranium and Plutonium on Thin Carbon and Titanium Substrates

Description: Preparation of Pu and U targets on thin natural C (100 {micro}g/cm{sup 2}) and ti (2 and 3 {micro}m) substrates is described. The Actinide material of interest was first purified using ion exchange chromatography to remove any matrix contaminants or decay products present in the parent stock solution. The actinide solution was prepared in 0.05 M HNO{sub 3} with a final aliquot volume not exceeding 100 {micro}L for the deposition procedure. The electroplating cells were developed in-house and were primarily made of Teflon. The source material deposited ranged from 125 to 400 {micro}g/cm{sup 2}. It was determined that multiple layers of U and Pu were required to produce thicker targets on Ti. Plating efficiency was greatly affected by the cell volume, solution aliquot size, pre-treatment of the foils, solution mixing during palting, and the fit of the electrode contact with the target substrate. The final procedure used for deposition is described in detail.
Date: February 18, 2011
Creator: Henderson, R A; Gostic, J M; Burke, J T; Fisher, S E & Wu, C Y
Partner: UNT Libraries Government Documents Department

Fission prompt gamma-ray multiplicity distribution measurements and simulations at DANCE

Description: The nearly energy independence of the DANCE efficiency and multiplicity response to {gamma} rays makes it possible to measure the prompt {gamma}-ray multiplicity distribution in fission. We demonstrate this unique capability of DANCE through the comparison of {gamma}-ray energy and multiplicity distribution between the measurement and numerical simulation for three radioactive sources {sup 22}Na, {sup 60}Co, and {sup 88}Y. The prospect for measuring the {gamma}-ray multiplicity distribution for both spontaneous and neutron-induced fission is discussed.
Date: August 24, 2010
Creator: Chyzh, A; Wu, C Y; Ullmann, J; Jandel, M; Bredeweg, T; Couture, A et al.
Partner: UNT Libraries Government Documents Department

Fabrication of a 238Pu target

Description: Precision neutron-induced reaction data are important for modeling the network of isotope production and destruction within a given diagnostic chain. This network modeling has many applications such as the design of advanced fuel cycle for reactors and the interpretation of radiochemical data related to the stockpile stewardship and nuclear forensics projects. Our current funded effort is to improve the neutron-induced reaction data on the short-lived actinides and the specific goal is to improve the neutron capture data on {sup 238}Pu with a half-life of 87.7 years. In this report, the fabrication of a {sup 238}Pu target for the proposed measurement using the DANCE array at LANL is described. The {sup 238}Pu target was fabricated from a sample enriched to 99.35%, acquired from ORNL. A total of 395 {micro}g was electroplated onto both sides of a 3 {micro}m thick Ti foil using a custom-made plating cell, shown in Fig 1. The target-material loaded Ti foil is sandwiched between two double-side aluminized mylar foils with a thickness of 1.4 {micro}m. The mylar foil is glued to a polyimide ring. This arrangement is shown partially in Fig. 2. The assembled target is then inserted into an aluminum container with a wall thickness of 0.76 mm, shown in Fig. 3. A derlin ring is used to keep the target assembly in place. The ends of this cylindrical container are vacuum-sealed by two covers with thin Kapton foils as windows for the beam entrance and exit. Shown in Fig. 4 is details of the arrangement. This target is used for phase I of the proposed measurement on {sup 238}Pu scheduled for Nov 2010 together with the DANCE array to address the safety issues raised by LANL. Shown in Fig. 5 is the preliminary results on the yield spectrum as a function of neutron incident energy ...
Date: November 16, 2010
Creator: Wu, C Y; Chyzh, A; Kwan, E; Henderson, R; Gostic, J & Carter, D
Partner: UNT Libraries Government Documents Department

Low-mass fission detector for the fission neutron spectrum measurement

Description: For the fission neutron spectrum measurement, the neutron energy is determined in a time-of-flight experiment by the time difference between the fission event and detection of the neutron. Therefore, the neutron energy resolution is directly determined by the time resolution of both neutron and fission detectors. For the fission detection, the detector needs not only a good timing response but also the tolerance of radiation damage and high {alpha}-decay rate. A parallel-plate avalanche counter (PPAC) has many advantages for the detection of heavy charged particles such as fission fragments. These include fast timing, resistance to radiation damage, and tolerance of high counting rate. A PPAC also can be tuned to be insensitive to particles, which is important for experiments with - emitting actinides. Therefore, a PPAC is an ideal detector for experiments requiring a fast and clean trigger for fission. In the following sections, the description will be given for the design and performance of a new low-mass PPAC for the fission-neutron spectrum measurements at LANL.
Date: October 20, 2010
Creator: Wu, C Y; Henderson, R; Gostic, J; Haight, R C & Lee, H Y
Partner: UNT Libraries Government Documents Department

Compact fission counter for DANCE

Description: The Detector for Advanced Neutron Capture Experiments (DANCE) consists of 160 BF{sub 2} crystals with equal solid-angle coverage. DANCE is a 4{pi} {gamma}-ray calorimeter and designed to study the neutron-capture reactions on small quantities of radioactive and rare stable nuclei. These reactions are important for the radiochemistry applications and modeling the element production in stars. The recognition of capture event is made by the summed {gamma}-ray energy which is equivalent of the reaction Q-value and unique for a given capture reaction. For a selective group of actinides, where the neutron-induced fission reaction competes favorably with the neutron capture reaction, additional signature is needed to distinguish between fission and capture {gamma} rays for the DANCE measurement. This can be accomplished by introducing a detector system to tag fission fragments and thus establish a unique signature for the fission event. Once this system is implemented, one has the opportunity to study not only the capture but also fission reactions. A parallel-plate avalanche counter (PPAC) has many advantages for the detection of heavy charged particles such as fission fragments. These include fast timing, resistance to radiation damage, and tolerance of high counting rate. A PPAC also can be tuned to be insensitive to {alpha} particles, which is important for experiments with {alpha}-emitting actinides. Therefore, a PPAC is an ideal detector for experiments requiring a fast and clean trigger for fission. A PPAC with an ingenious design was fabricated in 2006 by integrating amplifiers into the target assembly. However, this counter was proved to be unsuitable for this application because of issues related to the stability of amplifiers and the ability to separate fission fragments from {alpha}'s. Therefore, a new design is needed. A LLNL proposal to develop a new PPAC for DANCE was funded by NA22 in FY09. The design goal is to ...
Date: November 6, 2010
Creator: Wu, C Y; Chyzh, A; Kwan, E; Henderson, R; Gostic, J; Carter, D et al.
Partner: UNT Libraries Government Documents Department

Effect of pre-equilibrium spin distribution on neutron induced 150Sm cross sections

Description: Prompt {gamma}-ray production cross section measurements were made as a function of incident neutron energy (En = 1 to 35 MeV) on an enriched (95.6%) {sup 150}Sm sample. Energetic neutrons were delivered by the Los Alamos National Laboratory spallation neutron source located at the Los Alamos Neutron Science Center (LANSCE) facility. The prompt-reaction {gamma} rays were detected with the large-scale Compton-suppressed Germanium Array for Neutron Induced Excitations (GEANIE). Above E{sub n} {approx} 8 MeV the pre-equilibrium reaction process dominates the inelastic reaction. The spin distribution transferred in pre-equilibrium neutron-induced reactions was calculated using the quantum mechanical theory of Feshbach, Kerman, and Koonin (FKK). These preequilibrium spin distributions were incorporated into the Hauser-Feshbach statistical reaction code GNASH and the {gamma}-ray production cross sections were calculated and compared with experimental data. Neutron inelastic scattering populates 150Sm excited states either by (1) forming the compound nucleus {sup 151}Sm* and decaying by neutron emission, or (2) by the incoming neutron transferring energy to create a particle-hole pair, and thus initiating the pre-equilibrium process. These two processes produce rather different spin distributions: the momentum transfer via the pre-equilibrium process tends to be smaller than in the compound reaction. This difference in the spin population has a significant impact on the {gamma}-ray de-excitation cascade and therefore in the partial {gamma}-ray cross sections. The difference in the partial {gamma}-ray cross sections using spin distributions with and without preequilibrium effects was significant, e.g., for the 558-keV transition between 8{sup +} and 6{sup +} states the calculated partial {gamma}-ray production cross sections changed by 70% at E{sub n} = 20 MeV with inclusion of the spin distribution of pre-equilibrium process.
Date: April 16, 2007
Creator: Dashdorj, D; Kawano, T; Mitchell, G E; Becker, J A; Agvaanluvsan, U; Chadwick, M et al.
Partner: UNT Libraries Government Documents Department

Neutron induced inelastic cross sections of 150Sm for En = 1 to 35 MeV

Description: Cross-section measurements were made of prompt gamma-ray production as a function of incident neutron energy (E{sub n} = 1 to 35 MeV) on an enriched (95.6%) {sup 150}Sm sample. Energetic neutrons were delivered by the Los Alamos National Laboratory spallation neutron source located at the Los Alamos Neutron Science Center (LANSCE) facility. The prompt-reaction gamma rays were detected with the large-scale Compton-suppressed Germanium Array for Neutron Induced Excitations (GEANIE). Neutron energies were determined by the time-of-flight technique. The {gamma}-ray excitation functions were converted to partial {gamma}-ray cross sections taking into account the dead-time correction, target thickness, detector efficiency and neutron flux (monitored with an in-line fission chamber). Partial {gamma}-ray cross sections were predicted using the Hauser-Feshbach statistical reaction code GNASH. Above E{sub n} {approx} 8 MeV the pre-equilibrium reaction process dominates the inelastic reaction. The spin distribution transferred in pre-equilibrium neutron-induced reactions was calculated using the quantum mechanical theory of Feshbach, Kerman, and Koonin (FKK). These pre-equilibrium spin distributions were incorporated into a new version of GNASH and the {gamma}-ray production cross sections were calculated and compared with experimental data. The difference in the partial {gamma}-ray cross sections using spin distributions with and without pre-equilibrium effects is discussed.
Date: August 16, 2006
Creator: Dashdorj, D; Mitchell, G E; Kawano, T; Becker, J A; Agvaanluvsan, U; Chadwick, M B et al.
Partner: UNT Libraries Government Documents Department

Identification of a Pairing Isomeric Band in 152Sm

Description: A coexisting band structure is identified in {sup 152}Sm through {gamma}-ray coincidence spectroscopy following {beta} decay of {sup 152m,g}Eu and following multi-step Coulomb excitation. This structure is interpreted as a pairing isomer analogous to a similar band identified in {sup 154}Gd, based upon relative B(E2) values for transitions out of the band and two-neutron transfer reaction population of the 0{sup +} and 2{sup +} band members. Systematics for odd-A isotopes near N = 90 suggest that there should be a low-lying pairing isomer in {sup 156}Dy, and similar structures at higher energy in {sup 150}Nd and {sup 158}Er.
Date: May 13, 2005
Creator: Kulp, W D; Wood, J L; Garrett, P E; Allmond, J M; Cline, D; Hayes, A B et al.
Partner: UNT Libraries Government Documents Department

Progress on the Europium Neutron-Capture Study using DANCE

Description: The accurate measurement of neutron-capture cross sections of the Eu isotopes is important for many reasons including nuclear astrophysics and nuclear diagnostics. Neutron capture excitation functions of {sup 151,153}Eu targets were measured recently using a 4{pi} {gamma}-ray calorimeter array DANCE located at the Los Alamos Neutron Science Center for E{sub n} = 0.1-100 keV. The progress on the data analysis efforts is given in the present paper. The {gamma}-ray multiplicity distributions for the Eu targets and Be backing are significantly different. The {gamma}-ray multiplicity distribution is found to be the same for different neutron energies for both {sup 151}Eu and {sup 153}Eu. The statistical simulation to model the {gamma}-ray decay cascade is summarized.
Date: September 5, 2006
Creator: Agvaanluvsan, U; Becker, J A; Macri, R A; Parker, W; Wilk, P; Wu, C Y 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

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