Search Results (13 total)

Direct density measurements were made from shock-loaded aluminum and copper samples by combining plate-impact experiments with proton radiography at the Los Alamos Neutron Science Center. Flyer plates were accelerated using a 40 mm bore powder gun to create a shock wave in a sample. The sample material was then interrogated in real time using the proton radiography facility. The increase in density behind the shock front causes a measurable change in the transmission of protons through the sample, which can then be quantified as a density value in the material. Hugoniot values were calculated using more traditional techniques to evaluate the accuracy of the radiographically obtained density measurements.

The High Resolution Fly’s Eye (HiRes) experiment has observed the Greisen-Zatsepin-Kuzmin suppression (called the GZK cutoff) with a statistical significance of five standard deviations. HiRes’ measurement of the flux of ultrahigh energy cosmic rays shows a sharp suppression at an energy of 6×10¹⁹  eV, consistent with the expected cutoff energy. We observe the ankle of the cosmic-ray energy spectrum as well, at an energy of 4×10¹⁸  eV. We describe the experiment, data collection, and analysis and estimate the systematic uncertainties. The results are presented and the calculation of the statistical significance of our observation is described.

We have made a systematic study of rubidium optical pumping in a simple cylindrical cell geometry with a high-power 10  W diode laser array, low magnetic fields, and buffer-gas pressures of less than 50  torr. We have determined rubidium polarizations experimentally for H₂, N₂, He, and Ar buffer gases, with Rb number densities from 10¹²  to  10¹³  cm⁻³. Comparison to a relatively simple optical pumping model allows us to extract useful information about radiation trapping and quenching effects.

We have measured the cosmic ray spectrum above 10^17.2   eV using the two air-fluorescence detectors of the High Resolution Fly’s Eye observatory operating in monocular mode. We describe the detector, phototube, and atmospheric calibrations, as well as the analysis techniques for the two detectors. We fit the spectrum to a model consisting of galactic and extragalactic sources.

We report an experimental confirmation of the power-law relationship between the critical anisotropy parameter and ion number for the linear-to-zigzag phase transition in an ionic crystal. Our experiment uses laser cooled calcium ions confined in a linear radio-frequency trap. Measurements for up to ten ions are in good agreement with theoretical and numeric predictions. Implications on an upper limit to the size of data registers in ion trap quantum computers are discussed.

Asymmetries for the (π^-,π⁰) reaction on polarized ³He were measured using a 200-MeV pion beam at LAMPF. The π⁰’s were detected with the neutral meson spectrometer in coincidence with the recoiling tritons. A recoil triton detector, consisting of scintillation-counter telescopes and a wire chamber, was used to measure the time-of-flight, energy loss, and direction of the tritons. The polarized gaseous ³He target, developed at TRIUMF, was modified to run with two diode lasers; the polarization reached 65%. The asymmetries between θ_lab=60° and 105° were found to be strongly angle dependent. The results are compared with theoretical calculations.

We report the successful trapping of 6×10^{6 82}Rb (t_1/2=75 s) atoms with a trap lifetime of ∼30 s in a magneto-optical trap that is coupled to an off-line mass separator. Efficient sample introduction is achieved by implanting ⁸²Rb ions into a small yttrium catcher foil located inside the trapping cell. Upon heating, the radioactive atoms are released from the foil and trapped without significant gas loading. This advancement makes a variety of high-precision electroweak interaction experiments possible, including the measurement of β-decay correlations associated with spin-polarized ⁸²Rb nuclei.

Asymmetries A_y for π⁺ and π^- elastic scattering from polarized ³He were measured at incident energies passing through the π nucleon P₃₃, or Δ(1232), resonance. The data were taken at the Clinton P. Anderson Meson Physics Facility using the high-energy pion channel P³ and a large acceptance spectrometer to detect the scattered pions. The polarized ³He target, originally developed at TRIUMF, was modified for these experiments by the addition of diode lasers. Completely unexpected negative asymmetries were observed at and below the resonance in π⁺ scattering near 50°–60°, which cannot be reproduced by multiple scattering theory and Faddeev wave functions. A hybrid model, which adds a Δ-neutron spin-spin interaction term to the conventional calculations, provides a good description of the π⁺ data. For π^- scattering this term predicts a negligible effect on A_y and the data do not show the anomalous A_y seen with π⁺.

Asymmetries for π⁺ elastic scattering from polarized ³He were measured across the Δ(1232) π-nucleon resonance region at incident energies T_π  =  142, 180, and 256 MeV. Large discrepancies were found between the data and calculations that use the multiple scattering formalism and Faddeev wave functions. Inclusion of a Δ-neutron spin-spin interaction term in the calculations gives a greatly improved representation of the data.

We have measured the analyzing power A_NO and the spin transfer K_NN for np-elastic scattering from about 60° to 170° c.m. at 485, 635, and 788 MeV. The new data clarify previous discrepancies and complete the first-order determination of nucleon-nucleon elastic scattering at these energies.

We have formed highly polarized muonic helium by stopping unpolarized negative muons in a mixture of unpolarized gaseous He and laser polarized Rb vapor. The stopped muons form muonic He ions which are neutralized and polarized by collisions with Rb. Average polarizations for ³He and ⁴He of (26.8±2.3)% and (44.2±3.5)% were achieved, representing a tenfold increase over previous methods. Relevant cross sections were determined from the time evolution of the polarization. Highly polarized muonic He is valuable for measurements of the induced pseudoscalar coupling g_p in nuclear muon capture.

Calculations are presented on the effect of radiation trapping on the electron-spin polarization that can be produced by optical pumping of an alkali-metal vapor in a weak magnetic field. Radiation trapping severely limits the polarization that can be obtained in a weak magnetic field.

Calculations are presented of the limitations imposed by radiation trapping on the electron spin polarization produced in an alkali-metal vapor by optical pumping in a large magnetic field. It is found that electron spin polarizations of 90% are possible with Na densities up to 10¹⁹ atoms/m³ and ground-level relaxation times of 150 μs in a large magnetic field using a cylindrical geometry of radius 7.5×10^-3 m.