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Time reversal tests in polarized neutron reactions

Description: This is the final report of a three-year, Laboratory-Directed Research and Development (LDRD) project at the Los Alamos National Laboratory (LANL). In recent years the nuclear weak interaction has been studied in the compound nucleus via parity violation. The observed parity-violating effects are strongly enhanced by nuclear structure. The predictions are that the interaction of polarized neutrons with polarized nuclear targets could be also used to perform sensitive tests of time-reversal-violation because of the nuclear enhancements. The author has designed experiments to search for time-reversal violation in neutron-nucleus interactions. He has also developed techniques to polarize neutrons with laser-polarized {sup 3}He gas targets. Using the polarized {sup 3}He neutron spin filter, he has performed two experiments at LANSCE: an absolute neutron beam polarization measurement with an accuracy of 0.2--0.3% and a neutron spin-rotation measurement on a {sup 139}La sample.
Date: November 1, 1998
Creator: Asahi, Koichiro; Bowman, J.D. & Crawford, B.
Partner: UNT Libraries Government Documents Department

Determining (n,f) cross sections for actinide nuclei indirectly: An examination of the Surrogate Ratio Method

Description: The validity of the Surrogate Ratio method for determining (n,f) cross sections for actinide nuclei is examined. This method relates the ratio of two compound-nucleus reaction cross sections to a ratio of coincidence events from two measurements in which the same compound nuclei are formed via a direct reaction. With certain assumptions, the method allows one of the cross sections to be inferred if the other is known. We develop a nuclear reaction-model simulation to investigate whether the assumptions underlying the Ratio approach are valid and employ these simulations to assess whether the cross sections obtained indirectly by applying a Ratio analysis agree with the expected results. In particular, we simulate Surrogate experiments that allow us to determine fission cross sections for selected actinide nuclei. The nuclei studied, {sup 233}U and {sup 235}U, are very similar to those considered in recent Surrogate experiments. We find that in favorable cases the Ratio method provides useful estimates of the desired cross sections, and we discuss some of the limitations of the approach.
Date: May 22, 2006
Creator: Escher, J E & Dietrich, F S
Partner: UNT Libraries Government Documents Department

Estimation of (n,f) Cross-Sections by Measuring Reaction Probability Ratios

Description: Neutron-induced reaction cross-sections on unstable nuclei are inherently difficult to measure due to target activity and the low intensity of neutron beams. In an alternative approach, named the 'surrogate' technique, one measures the decay probability of the same compound nucleus produced using a stable beam on a stable target to estimate the neutron-induced reaction cross-section. As an extension of the surrogate method, in this paper they introduce a new technique of measuring the fission probabilities of two different compound nuclei as a ratio, which has the advantage of removing most of the systematic uncertainties. This method was benchmarked in this report by measuring the probability of deuteron-induced fission events in coincidence with protons, and forming the ratio P({sup 236}U(d,pf))/P({sup 238}U(d,pf)), which serves as a surrogate for the known cross-section ratio of {sup 236}U(n,f)/{sup 238}U(n,f). IN addition, the P({sup 238}U(d,d{prime}f))/P({sup 236}U(d,d{prime}f)) ratio as a surrogate for the {sup 237}U(n,f)/{sup 235}U(n,f) cross-section ratio was measured for the first time in an unprecedented range of excitation energies.
Date: April 21, 2005
Creator: Plettner, C.; Ai, H.; Beausang, C. W.; Bernstein, L. A.; Ahle, L.; Amro, H. et al.
Partner: UNT Libraries Government Documents Department

Compound-Nucleus Formation Following Direct Interactions to Highly-Excited Final States

Description: When direct reactions populate highly excited, unbound configurations in the residual nucleus, the nucleus may further evolve into a compound nucleus. Alternatively, the residual system may decay by emitting particles into the continuum. Understanding the relative weights of these two processes as a function of the angular momentum and parity deposited in the nucleus is important for the surrogate-reaction technique. A particularly interesting case is compound-nucleus formation via the (d, p) reaction, which may be a useful tool for forming compound nuclei off the valley of stability in inverse-kinematics experiments. We present here a study of the compound formation probability for a closely-related direct reaction, direct-semidirect radiative neutron capture.
Date: February 5, 2008
Creator: Dietrich, F S
Partner: UNT Libraries Government Documents Department

Giant Dipole Resonance in the hot and thermalized 132Ce nucleus: damping of collective modes at finite temperature

Description: The {gamma} decay of the Giant Dipole Resonance in the {sup 132}Ce compound nucleus with temperature up to {approx} 4 MeV has been measured. The symmetric {sup 64}Ni + {sup 68}Zn at E{sub beam} = 300, 400, 500 MeV and the asymmetric reaction {sup 16}O + {sup 116}Sn at E{sub beam} = 130, 250 MeV have been investigated. Light charged particles and {gamma} rays have been detected in coincidence with the recoiling compound system. In the case of the mass symmetric {sup 64}Ni induced reaction the {gamma} and charged particle spectral shapes are found to be consistent with the emission from a fully equilibrated compound nuclei and the GDR parameters are extracted from the data using a statistical model analysis. The GDR width is found to increase almost linear with temperature. This increase is rather well reproduced within a model which includes both the thermal fluctuation of the nuclear shape and the lifetime of the compound nucleus.
Date: June 16, 2006
Creator: Wieland, O; Bracco, A; Camera, F; Benzoni, G; Blasi, N; Brambilla, S et al.
Partner: UNT Libraries Government Documents Department

Computer Program for Calculating the Relative Yields of Isomers Produced in Nuclear Reactions

Description: A three-part Fortran program for IBM 704 computation of the relative cross sections of isomers produced in nuclear reactions is described. The three subprograms compute the partial compound nucleus cross sections and the normalized initial compound nucleus spin distribution, the normalized spin distribution following particle emission, and the normalized spin distribution following gamma emission. The Fortran listing of the program and cards for a sample problem is included. (D.C.W.)
Date: December 1, 1962
Creator: Hafner, W. L. Jr.; Huizenga, J. R. & Vandenbosch, R.
Partner: UNT Libraries Government Documents Department

Dynamics of the dinucleus

Description: Damped reactions between heavy nuclei have revealed the temporary formation and evolution of a novel nuclear system, the dinucleus. This system possesses macroscopic degrees of freedom not present in the more ordinary mononucleus. Most important are: the energy associated with the relative dinuclear motion, the partition of the microscopic excitation energy (heat) and the total mass and charge among the two parts of the dinucleus, and their angular momenta. This paper briefly highlights the characteristic features of the dynamics of these dinuclear degrees of freedom and our current understanding of them. 19 refs., 3 figs.
Date: June 1, 1985
Creator: Randrup, J.
Partner: UNT Libraries Government Documents Department

Study of parity nonconservation with epithermal neutrons

Description: The authors describe a new generation of experiments studying the weak interaction between nucleons. Measurements of the effect of this interaction are few in number and the significance of the observed effects are generally small. It is well known that the weak interaction violates parity. This was first experimentally established by C.S. Wu through measurement of an asymmetry of electrons emitted in the beta-decay of polarized {sup 60}Co. The measured asymmetry was large because beta decay is a weak interaction process. For a process in which the - strong interaction can contribute, we expect much smaller asymmetries, of order 10{sup -7}.
Date: September 1996
Creator: Seestrom, S. J.; Bowman, J. D.; Knudson, J. N. & Yuan, V.
Partner: UNT Libraries Government Documents Department

Particle structure function and subbarrier fusion in hot nuclei

Description: The study of particle evaporation spectra can provide information about shape polarization phenomena induced by the nascent particle on the residual nucleus, and about optical modulations felt by the particle as it is preformed inside the nucleus. These aspects can be studied as a function temperature. Preliminary experimental evidence about these features has been obtained.
Date: February 1, 1997
Creator: Moretto, L.G.; Jing, K.X.; Phair, L. & Wozniak, G.J.
Partner: UNT Libraries Government Documents Department

Emission of complex fragments for compound nuclei formed far from the stability line for A{approx}120

Description: Experiments and calculations are presented for the emission of complex fragments (Z>2) from compound nuclei around mass 120 and for bombarding energies below 15 MeV/nucleon. It is shown that a strong enhancement in the emission of complex fragments (especially for Z=6) is expected for compound nuclei formed close or at the N=Z line. Several predictions are given that could be tested with the radioactive beams soon to be available. Also results from a recent experiment that utilizes the emission of complex fragments for {gamma}-ray nuclear spectroscopy studies are presented.
Date: October 1, 1996
Creator: Gomez Del Campo, J.
Partner: UNT Libraries Government Documents Department

Model-space approach to parity violation in heavy nuclei

Description: The model-space approach is the basis of both shell model and statistical spectroscopy analyses of nuclear phenomena. The goal of this session is to bring out the main theoretical issues involved in its application to parity violation in the compound nucleus. Section 1 of the current paper sets the stage for the session, and Sect. 2 introduces and explores the model-space formulation as it underlies quantitative connections that are being made between the mean-square matrix element M{sup 2} measured in polarized neutron scattering from compound nuclei and the underlying parity violating interaction. This is followed in the paper by Tomsovic by a description of how statistical spectroscopy is applied to this problem, and in the paper by Hayes by a discussion of shell-model aspects of parity violation in the compound nucleus.
Date: June 1, 1996
Creator: Johnson, M.B.
Partner: UNT Libraries Government Documents Department

Stochastic resonance in a periodically modulated dissipative nuclear dynamics

Description: A fission decay of highly excited periodically driven compound nuclei is considered in the framework of Langevin approach. The authors have used residual-time distribution (RTD) as the tool for studying of dynamic features in a presence of periodic perturbation. The structure of RTD essentially depends on the relation between Kramers decay rate and the frequency {omega} of the periodic perturbation. In particular, intensity of the first peak in RTD has a sharp maximum at certain nuclear temperature depending on {omega}. This maximum should be considered as first-hand manifestation of stochastic resonance in nuclear dynamics.
Date: February 1, 2001
Creator: Berezovoy, V.P.
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

Recent developments of the CEM2K and LAQGSM codes.

Description: Recent developments of the Cascade-Exciton Model (CEM) of nuclear reactions are briefly described. The improved cascade-exciton model as implemented in the code CEM97 [l] differs from the CEM95 version [4] by incorporating new approximations for the elementary cross sections used in the cascade, using more precise {approx} values for nuclear masses and pairing energies, using corrected systematics for the I level-density parameters, and several other refinements. Algorithms used in many i subroutines have been improved, decreasing the computing time by up to a factor of 6 for heavy targets. A number of further recent improvements and changes to CEM97, motivated by new data on isotope production measured at GSI will be presented. This leads us to CEM2k [2], a new version of the CEM code. CEM2k has a longer cascade stage, less preequilibrium emission, and evaporation from more highly excited compound nuclei compared to earlier versions. CEM2k also has other improvements and allows us to better model neutron, radionuclide, and gas production in Accelerator Transmutation of nuclear Wastes (ATW) spallation targets. The improved CEM97 code was recently used both to study fundamental nuclear physics problems like the role of nuclear medium effects in transport of 7r mesons in nuclei [5] and fission processes at intermediate energies [6], and was incorporated in the well known transport code MCNPX (LANL) to solve applied problems. The CEM95 version [4] of the CEM was incorporated in the MARS (FNAL) and CALOR95 (ORNL) transport codes, and its preequilibrium part was incorporated in many other transport codes like GEANT4 (CERN, see, e.g., [7]), HETC-3STEP (JAERI), HADRON (IHEP, Protvino), CASCADE (JINR, Dubna), SONET (RPCPI, Minsk), etc. The latest version of the CEM code, CEM2k, is still under development. The increased accuracy and predictive power of the code CEM2k will be shown by several examples. Further necessary work ...
Date: January 1, 2002
Creator: Mashnik, S. G. (Stepan G.); Gudima, K. K. (Konstantin K.) & Sierk, A. J. (Arnold J.)
Partner: UNT Libraries Government Documents Department

The role of phase space in complex fragment emission from low to intermediate energies

Description: The experimental emission probabilities of complex fragments by low energy compound nuclei and their dependence upon energy and Z value are compared to the transition state rates. Intermediate-mass-fragment multiplicity distributions for a variety of reactions at intermediate energies are shown to be binomial and thus reducible at all measured transverse energies. From these distributions a single binary event probability can be extracted which has a thermal dependence. A strong thermal signature is also found in the charge distributions. The n-fold charge distributions are reducible to the 1-fold charge distributions through a simple scaling dictated by fold number and charge conservation.
Date: June 1, 1996
Creator: Moretto, L.G.; Ghetti, R.; Jing, K.; Phair, L.; Tso, K. & Wozniak, G.J.
Partner: UNT Libraries Government Documents Department

Neutron resonances in the compound nucleus: Parity nonconservation to dynamic temperature measurements

Description: Experiments using epithermal neutrons that interact to form compound-nuclear resonances serve a wide range of scientific applications. Changes in transmission which are correlated to polarization reversal in incident neutrons have been used to study parity nonconservation in the compound nucleus for a wide range of targets. The ensemble of measured parity asymmetries provides statistical information for the extraction of the rms parity-violating mean-square matrix element as a function of mass. Parity nonconservation in neutron resonances can also be used to determine the polarization of neutron beams. Finally the motion of target atoms results in an observed temperature-dependent Doppler broadening of resonance line widths. This broadening can be used to determine temperatures on a fast time scale of one microsecond or less.
Date: August 1, 1997
Creator: Yuan, V.W.
Partner: UNT Libraries Government Documents Department

Experiments on parity violation in the compound nucleus

Description: Results from experiments that measure parity-violating longitudinal asymmetries in the scattering of epithermal neutrons from compound-nuclear resonances at the Manuel Lujan Neutron Scattering Center at Los Alamos are discussed. Parity non-conserving asymmetries have been observed for many p-wave resonances in a single target. Measurements were performed on several nuclei in the mass region of A-100 and A-230. The statistical model of the compound nucleus provides a theoretical basis for extracting mean-squared matrix elements from the experimental asymmetry data, and for interpreting the mean-squared matrix elements. The constraints on the weak meson-exchange couplings calculated from the compound-nucleus asymmetry data agree qualitatively with the results from few-body and light-nuclei experiments. For all nuclei but {sup 232}Th measured asymmetries have random signs. For {sup 232}Th eight of eight measured asymmetries are positive. This phenomenon is discussed in terms or doorway models.
Date: September 1, 1996
Creator: Bowman, J.D.
Partner: UNT Libraries Government Documents Department

A compilation of information on the {sup 31}P(p,{alpha}){sup 28}Si reaction and properties of excited levels in the compound nucleus {sup 32}S

Description: This report documents a survey of the literature, and provides a compilation of data contained therein, for the {sup 31}P(p,{alpha}){sup 28}Si reaction. Attention is paid here to resonance states in the compound-nuclear system {sup 32}S formed by {sup 31}P + p, with emphasis on the alpha-particle decay channels, {sup 28}Si + {alpha} which populate specific levels in {sup 28}Si. The energy region near the proton separation energy for {sup 32}S is especially important in this context for applications in nuclear astrophysics. Properties of the excited states in {sup 28}Si are also considered. Summaries of all the located references are provided and numerical data contained in them are compiled in EXFOR format where applicable.
Date: November 1, 1997
Creator: Miller, R.E. & Smith, D.L.
Partner: UNT Libraries Government Documents Department

Spin distributions for {sup 64}Ni+{sup 100}Mo with the Argonne/Notre Dame BGO-array

Description: The experimental representation of the fusion barrier distribution from the second derivative, d{sup 2}(E{sigma}{sub fus})/dE{sup 2}, was the basis for a series of precise measurements of the fusion excitation function. As an alternative approach, it has been proposed recently that the first derivative of the compounds nucleus (CN) spin distribution is equivalent to d{sup 2}(E{sigma}{sub fus})/dE{sup 2}. Multiplicity distributions for the reactions {sup 64}Ni+{sup 100}Mo and {sup 32}S+{sup 110}Pd have been measured making use of the Argonne/Notre Dame-BGO-array and GASP, respectively. In particular, the influence of fission on the high spin tail of the spin distribution and the consequences for the experimental representation of the distribution of barriers is discussed for {sup 64}Ni+{sup 100}Mo.
Date: September 1, 1997
Creator: Ackermann, D.; Back, B.B.; Betts, R.R. & Carpenter, M.
Partner: UNT Libraries Government Documents Department

I. Excluded Volume Effects in Ising Cluster Distributions and Nuclear Multifragmentation II. Multiple-Chance Effects in Alpha-Particle Evaporation

Description: In Part 1, geometric clusters of the Ising model are studied as possible model clusters for nuclear multifragmentation. These clusters may not be considered as non-interacting (ideal gas) due to excluded volume effect which predominantly is the artifact of the cluster's finite size. Interaction significantly complicates the use of clusters in the analysis of thermodynamic systems. Stillinger's theory is used as a basis for the analysis, which within the RFL (Reiss, Frisch, Lebowitz) fluid-of-spheres approximation produces a prediction for cluster concentrations well obeyed by geometric clusters of the Ising model. If thermodynamic condition of phase coexistence is met, these concentrations can be incorporated into a differential equation procedure of moderate complexity to elucidate the liquid-vapor phase diagram of the system with cluster interaction included. The drawback of increased complexity is outweighted by the reward of greater accuracy of the phase diagram, as it is demonstrated by the Ising model. A novel nuclear-cluster analysis procedure is developed by modifying Fisher's model to contain cluster interaction and employing the differential equation procedure to obtain thermodynamic variables. With this procedure applied to geometric clusters, the guidelines are developed to look for excluded volume effect in nuclear multifragmentation. In part 2, an explanation is offered for the recently observed oscillations in the energy spectra of {alpha}-particles emitted from hot compound nuclei. Contrary to what was previously expected, the oscillations are assumed to be caused by the multiple-chance nature of {alpha}-evaporation. In a semi-empirical fashion this assumption is successfully confirmed by a technique of two-spectra decomposition which treats experimental {alpha}-spectra has having contributions from at least two independent emitters. Building upon the success of the multiple-chance explanation of the oscillations, Moretto's single-chance evaporation theory is augmented to include multiple-chance emission and tested on experimental data to yield positive results.
Date: May 16, 2005
Creator: Breus, Dimitry E.
Partner: UNT Libraries Government Documents Department