Search Results (20 total)

Transverse beam profiles are observed to broaden with increasing intensity in the Proton Storage Ring at the Los Alamos Neutron Scattering Center. Measured profiles are simulated with an H^- injection model that includes a 2D particle-in-cell space charge calculation. Inclusion of space charge effects in the simulation improves the agreement between the experimentally observed profiles and the calculated profiles. The comparisons are made for a range of injected intensities.

Numerical calculations for the Spallation Neutron Source accumulator ring indicate that lattice resonances excited by the space-charge potential can increase a mismatch significantly by deforming the beam distribution in phase space. Hence increased mismatch leads to enhanced envelope oscillations that are driving the 2:1 parametric resonance leading to halo formation, even for initially matched beams. We have observed this behavior for the 2ν_x-2ν_y=0 resonance and for the 4ν_y=23 resonance. This mechanism for halo formation peculiar to rings through resonance driven mismatch is very sensitive to the tunes, which emphasizes the importance of a careful choice of operating point in tune space.

Uncontrolled beam losses due to space-charge-induced halo generation are a concern in high intensity rings, which are characterized by high beam intensities and low uncontrolled beam loss requirements. It is therefore important to investigate the dynamics of space charge in high intensity rings. We report here the results of extensive calculations using a particle-tracking approach with a self-consistent particle-in-cell model and alternatively with a particle core model. We find that the inclusion of space charge forces provides agreement between calculated and experimentally observed beam profile shapes in the high intensity proton storage ring. We also confirm computationally the extension to rings of the accepted dynamics of halo generation with rms beam mismatch exciting the parametric resonance. In addition, we propose a new two-stage mechanism for halo production in rings in which space-charge-driven lattice resonances generate beam mismatch that excites the parametric resonance. Because of its dependence on lattice resonances, this mechanism is peculiar to rings and is capable of generating halo even from initially matched beams. It is also very sensitive to the operating point in tune space, as we show in the results of a vertical tune scan simulating injection into the Spallation Neutron Source accumulator ring. Our results extend and enhance the understanding of fundamental space charge physics, which has been developed for linear accelerators, to rings.

The analysis of a recent high-resolution neutron capture measurement at 152 m has provided the first evidence that the strong fission resonance at 721 eV is a class-II resonance. This conclusion is based on the measured capture width of 4.7±0.6 meV, which is considerably smaller than the average capture width of 23.5 meV for the neighboring resonances. Furthermore, after analyzing the fission widths for the 721- and 1211-eV clusters, we conclude for the J^π=1 / 2⁺ fission barrier in ²³⁹U that the inner barrier is lower than the outer barrier by ∼ 1.5 MeV.

A measurement of the ²³⁸U neutron-induced fission cross section has been performed in the neutron energy range between 5 eV and 3.5 MeV. The favorable signal-to-background ratio and high resolution of this experiment resulted in the identification of 85 subthreshold fission resonances or clusters of resonances in the neutron energy region between 5 eV and 200 keV. The fission data below 100 keV are characteristic of a weak coupling situation between Class I and Class II levels. The structure of the fission levels at the 720- and 1210-eV fission clusters is discussed. There is an apparent enhancement of the fission cross section at the opening of the 2⁺ neutron inelastic channel in ²³⁸U at 45 keV. An enhancement of the subthreshold fission cross section between 100 and 200 keV has been tentatively interpreted in terms of the presence of a Class II, partially damped vibrational level. There is a marked structure in the fission cross section above 200 keV up to and including the plateau between 2 and 3.5 MeV.

NUCLEAR REACTIONS, FISSION ²³⁸U(n,f). E=5 eV-3.5 MeV; measured σ(E); deduced parameters second well fission barrier.

The complex poles and widths of the transition T matrix are determined by the trajectory equations which consist of a set of first order nonlinear differential equations. A heirarchy of approximate solutions to the trajectory equations is developed by iterative methods. The results of this formalism are compared with exact solutions for the case of some strongly interacting pairs of resonances in two iron isotopes. In the presence of intermediate structure the average neutron reaction cross section is interpreted in terms of a resonant strength function which exhibits peaks at neutron energies corresponding to "doorways" levels.

NUCLEAR REACTIONS Complex poles and widths, intermediate structure.

The ⁸⁶Kr and ⁸⁸Kr nuclei have been studied by the ^84,86Kr(t,p) reactions at E_t=17.0 MeV. Energy levels up to 6400 and 4400 keV excitation in ⁸⁶Kr and ⁸⁸Kr, respectively, were observed. Differential cross sections for many excited states have been extracted from 12° to 50° c.m. for ⁸⁸Kr and 12° to 55° c.m. for ⁸⁶Kr. The data are compared with DWBA calculations in order to obtain the angular momentum (L) transfer as well as nuclear structure information. The excited L=0 strength leading to ⁸⁶Kr in the region where the pairing vibration is expected is found to be fractionated and about 2 / 3 of the intensity of the ⁸⁶Kr → ⁸⁸Kr (g.s.) transition strength. Strong L=2 transitions are found above this excited L=0 strength in ⁸⁶Kr and appear to have their parentage in the pairing quadrupole states represented by the first 2⁺ level in ⁸⁸Kr. The ⁸⁶Kr spectrum and transition strengths as found in the (t,p) reaction are compared with the ⁸⁶Kr(p,p^′) and ⁸⁷Rb(t,α) experiments.

NUCLEAR REACTIONS ^{86, 88}Kr(t,p) E_t=17.0 MeV; measured σ(θ) and level energies; deduced J^π and enhancement factors from DWBA.

Differential cross sections have been measured at a proton bombarding energy of 39.8 MeV for ²⁰Ne(p, t) transitions to the 0₁⁺, 2₁⁺, and 4₁⁺ states of ¹⁸Ne at 0.0, 1.89, and 3.38 MeV, respectively, and to the (0₂⁺, 2₂⁺) doublet in ¹⁸Ne at (3.58, 3.62) MeV. These cross sections are compared with zero-range coupled-channel Born-approximation calculations in which the coexistence-model wave functions of Benson and Flowers have been used. The inclusion of inelastic effects improves the agreement between experiment and theory; however, the calculated (p, t) cross sections are quite sensitive to the nature of the inelastic processes in the t+¹⁸Ne channel, and, until these processes are better understood, it is not possible for the calculation to provide a sensitive test of the mass-18 nuclear wave functions used.

NUCLEAR REACTIONS ²⁰Ne(p, t), E=39.8 MeV; measured σ(θ) and calculated σ(θ) with CCBA and DWBA.

The level structures of ^79,81,83Kr have been investigated via the (d, p) stripping reaction using isotopically enriched gas targets at an incident deuteron energy of 11 MeV. Proton groups leading to about 14 states with excitation energies up to approximately 3 MeV have been identified in each of the above nuclei. Differential cross sections were measured from 20° to 160°, and were fitted with zero-range distorted-wave-Born-approximation calculations. Excitation energies, l values, spectroscopic factors, and the implied values of J^π are given. Comparisons are made with states in the selenium and germanium isotopes having the same number of neutrons and, respectively, one and two protons less than the krypton isotopes. Comparisons are also made between the present work and studies of ⁷⁹Kr via ⁷⁹Rb decay measurements. Two unresolved discrepancies are found between the two sets of measurements.

NUCLEAR REACTIONS ^78,80,82Kr(d, p)^79,81,83Kr, E=11.0 MeV, measured σ(θ) and level energies; deduced l, S with DWBA analysis.

Differential cross sections for elastic and inelastic scattering of 12.0-MeV protons from ⁸⁴Kr and ⁸⁶Kr have been measured. Optical-model parameters obtained from the elastic cross sections were used in distorted-wave-Born-approximation and coupled-channel calculations in order to determine the spin, parities, and deformation parameters of excited states. The deformation parameters found are β₂=0.128 and 0.108 for the first 2⁺ states in ⁸⁴Kr and ⁸⁶Kr, respectively, and are β₃=0.158 and 0.145, respectively, for the first 3^- states. Most of the scattering strength to the "two-phonon" states proceed through their one-phonon components. The spins of several higher excited states are suggested.

NUCLEAR REACTIONS ^84,86Kr(p, p^′), E=12.0 MeV; measured σ(θ) and level energies; deduced optical-model parameters, J^π and β from DWBA and coupled channels.

We have measured relative x-ray-production cross sections for Ca Kα satellite and hypersatellite transitions produced from 24.0- to 48.0-MeV oxygen bombardment of thick targets. All cross-section ratios as functions of projectile energy for initial defect configurations K^kL^l, where k=1, 2 and l=0, 1, 2, 3, 4, are in qualitative agreement with configuration-space binary-encounter-approximation predictions.

Silicon and silicon dioxide K x-ray spectra produced by 0.8-MeV hydrogen, 3.2-MeV helium, and 13.0- and 35.0-MeV oxygen bombardment were measured with a high-resolution crystal spectrometer. The resulting Kα and Kβ, diagram and satellite, chemical energy shifts observed with these ions and those from electron excitation are compared and discussed. In general, the chemical shifts obtained from the different projectiles are similar and are dependent upon the amount of additional L-shell ionization. However, the oxygen-produced Kβ satellite shifts are anomalously large. The spectra indicate that perhaps these shifts are due to transitions from the conduction band of silicon dioxide.

30 states in ⁸⁵Kr were investigated via the ⁸⁴Kr(d , p)⁸⁵Kr reaction using 11.0-MeV deuterons. At least 22 of these states have not been previously reported. Angular distributions were measured from 20 to 160° for transitions to 25 of these states, and were fitted with zero-range distorted-wave Born-approximation calculations. Excitation energies, l values, spectroscopic factors, and the implied values of J^π are given. Comparisons are made with states of other nuclei having both one neutron less and one neutron more than the N=50 shell closure.

[NUCLEAR REACTIONS ⁸⁴Kr(d ,p), E=11.0 MeV; measured σ(θ), levels; deduced l ,S with DWBA analysis.]

The excitation of states in A1 with single K-plus multiple L-shell vacancies formed by collisions with 0.4-3.0-MeV helium ions is studied by observing the x-ray decay associated with filling of the K-shell vacancy. The observed K x rays come from initial states with one K-shell and 0, 1, 2, and 3 L-shell vacancies. The absolute cross section for each of these transitions is determined and compared to a calculation using multiple Coulomb ionization. An important result of this experiment is the observation that the ratio of double to single ionization exhibits a maximum at a bombarding energy near the maximum for L-shell ionization.

The effects of inelastic excitations on the ²²Ne(p,t)²⁰Ne reaction are investigated. Differential cross sections at a proton energy of 39.8 MeV were measured for ²²Ne(p,t)²⁰Ne transitions to the 0⁺, 2⁺, and 4⁺ members of the ground-state rotational band, to the 2^- and 3^- members of the excited K^π=2^- band; and to the 0⁺ and 2⁺ members of the first excited K^π=0⁺ band. The cross sections to members of the ground-state band are compared with distorted-wave Born-approximation (DWBA) and coupled-channel-Born-approximation (CCBA) calculations using both pure Nilsson and pairing-mixed Nilsson rotational wave functions. Only the CCBA calculation with pairing provides a reasonable description of the experimental data. The effects of multistep processes are found to be as large for neon as for rareearth nuclei. The shape and relative strength of the measured cross section to the 2^- state, a transition which is forbidden by a direct single-step process, are reproduced well by the CCBA calculation, which allows the excitation of the 2^- state by inelastic processes in the entrance and exit channels.

The K x-ray spectrum of Mg produced by 30-MeV oxygen ions is observed to consist of four regions of K x-ray excitations. These regions consist of the Kα satellite lines, the Kβ satellite lines, the Kα hypersatellite lines, and the Kβ hypersatellite lines. The last region is observed for the first time and corresponds to 1s→3p transitions in an atom consisting of double K-shell vacancies as well as multiple L-shell vacancies.

Spectra have been measured [using a crystal (LiF;200) spectrometer] of L x rays produced by 2.0-MeV proton, 3.2-MeV α-particle, and 30.0-MeV oxygen bombardment of thick tin targets. L x-ray lines are observed in the α-particle-produced spectrum from initial configurations with up to four additional M-shell electron vacancies. The energy resolution of 8 eV (full width at half-maximum) did not allow any detailed structure to be resolved in the oxygen bombardment; an almost continuous structure is observed from multiply ionized tin. The ratios of x-ray yields from L+M vacancies to yields from L vacancies are shown to have the correct order of magnitude for a simultaneous direct Coulomb ionization process for proton and α-particle impact.

We compare the differential cross section at a proton energy of 39.8 MeV for the transition Ne²²(p,t)Ne²⁰ (2^-, 4.97 MeV), which is forbidden by the usual selection rules for a single-step process, with a coupled-channel Born-approximation calculation using rotational wave functions of the adiabatic form. The calculated results are in good agreement with the experimental data.

The shape of the α-particle continuum from the Li⁶(p, α) reaction at a proton energy of 20.0 MeV (lab) has been investigated for structure due to excited states of He³. α-particle spectra were measured with thin surface-barrier detectors from 15° to 85° (lab) in steps of 5°. The maximum He³ excitation energy which could have been observed with this experimental technique depended upon the lab angle and varied from 14.3 to 10.1 MeV. In order to continuously monitor the target composition, protons elastically scattered at 90° (lab) were observed simultaneously with the α-particle spectra. No clear structure attributable to the formation of excited states of He³ was observed. In particular, an upper limit of 300 μb/sr (lab) can be placed on the differential cross section at 20° (lab) for the excitation of a 12-MeV state in He³ having a natural width of 0.9 MeV (c.m.).