Search Results (26 total)

We present a measurement of the spin-dependent cross sections for the ³He→(e→,e^′)X reaction in the quasielastic and resonance regions at a four-momentum transfer 0.1≤Q²≤0.9  GeV². The spin-structure functions have been extracted and used to evaluate the nuclear Burkhardt-Cottingham and extended Gerasimov-Drell-Hearn sum rules for the first time. The data are also compared to an impulse approximation calculation and an exact three-body Faddeev calculation in the quasielastic region.

Isospin is a good quantum number under the assumption that the nuclear interaction is charge independent. An analogous structure of excited states is expected for nuclei with the same mass number A but with different z components T_z of the isospin T, where T_z=(N-Z)/2. The analogous structure has been studied for the isobaric nuclei ⁵⁸Ni and ⁵⁸Cu by comparing the transitions from the ⁵⁸Ni ground state (initial isospin T_i=1 and J^π=0⁺) to the M1 and the Gamow-Teller (GT) states (J^π=1⁺) in ⁵⁸Ni and ⁵⁸Cu, respectively. For this purpose, proton inelastic scattering (p,p^') at E_p=160 MeV and the charge-exchange (³He,t) reaction at 140 MeV/nucleon were both measured at 0^°, exciting final states with isospin T_f=1 and 2 and T_f=0, 1, and 2, respectively. High energy and scattering-angle resolutions were achieved by applying complete beam matching techniques. On the basis of the correspondence between excitation energies and transition strengths, isospin values T_f=1 and 2 of analog GT and M1 states were identified. The distribution of T_f=2 states was also compared with results of ⁵⁸Ni(d,²He), ⁵⁸Ni(t,³He), and ⁵⁸Ni(n,p) experiments, in which only T_f=2 states are excited. The obtained GT strength distribution is compared with the results of shell-model calculations.

We have measured the transverse asymmetry A_T^' in the quasielastic ³He→(e→,e^') process with high precision at Q² values from 0.1 to 0.6  (GeV/c)². The neutron magnetic form factor G_Mⁿ was extracted at Q² values of 0.1 and 0.2  (GeV/c)² using a nonrelativistic Faddeev calculation which includes both final-state interactions (FSI) and meson-exchange currents (MEC). Theoretical uncertainties due to the FSI and MEC effects were constrained with a precision measurement of the spin-dependent asymmetry in the threshold region of ³He→(e→,e^'). We also extracted the neutron magnetic form factor G_Mⁿ at Q² values of 0.3 to 0.6  (GeV/c)² based on plane wave impulse approximation calculations.

We present new accurate measurements of the differential cross section σ(θ) and the proton analyzing power A_y for proton-³He elastic scattering at various energies. A supersonic gas-jet target has been employed to obtain these low-energy cross-section measurements. The σ(θ) distributions have been measured at E_p=0.99, 1.59, 2.24, 3.11, and 4.02 MeV. Full angular distributions of A_y have been measured at E_p=1.60, 2.25, 3.13, and 4.05 MeV. This set of high-precision data is compared to four-body variational calculations employing realistic nucleon-nucleon (NN) and three-nucleon (3N) interactions. For the unpolarized cross section, the agreement between the theoretical calculation and data is good when a 3N potential is used. The comparison between the calculated and measured proton analyzing powers reveals discrepancies of approximately 50% at the maximum of each distribution. This is analogous to the existing “A_y puzzle” known for the past 20 years in nucleon-deuteron elastic scattering.

The generalized forward spin polarizabilities γ₀ and δ_LT of the neutron have been extracted for the first time in a Q² range from 0.1 to 0.9  GeV². Since γ₀ is sensitive to nucleon resonances and δ_LT is insensitive to the Δ resonance, it is expected that the pair of forward spin polarizabilities should provide benchmark tests of the current understanding of the chiral dynamics of QCD. The new results on δ_LT show significant disagreement with chiral perturbation theory calculations, while the data for γ₀ at low Q² are in good agreement with a next-to-leading-order relativistic baryon chiral perturbation theory calculation. The data show good agreement with the phenomenological MAID model.

A measurement of the n-³He coherent scattering length using neutron interferometry is reported. The result, b_c=(5.8572±0.0072)  fm, improves the measured precision of any single measurement of b_c by a factor of eight; the previous world average, b_c=(5.74±0.04)  fm, now becomes b_c=(5.853±0.007)  fm. Measurements of the n-p, n-d, and n-³He coherent scattering lengths have now been performed using the same technique, thus allowing one to extract the scattering length ratios: parameters that minimize systematic errors. We obtain values of b_n³He∕b_np=(−1.5668±0.0021) and b_nd∕b_np=(−1.7828±0.0014). Using the new world average value of b_c and recent high-precision spin-dependent scattering length data also determined by neutron optical techniques, we extract new values for the bound singlet and triple scattering lengths of b₀=(9.949±0.027)  fm and b₁=(4.488±0.017)  fm for the n-³He system. The free nuclear singlet and triplet scattering lengths are a₀=(7.456±0.020)  fm and a₁=(3.363±0.013)  fm. The coherent scattering cross section is σ_c=(4.305±0.007)  b and the total scattering cross section is σ_s=(5.837±0.014)  b. Comparisons of a₀ and a₁ to the only existing high-precision theoretical predictions for the n-³He system, calculated using a resonating group technique with nucleon-nucleon potentials incorporating three-nucleon forces, have been performed. Neutron scattering length measurements in few-body systems are now sensitive enough to probe small effects not yet adequately treated in present theoretical models.

We have measured the spin structure functions g₁ and g₂ of ³He in a double-spin experiment by inclusively scattering polarized electrons at energies ranging from 0.862 to 5.058 GeV off a polarized ³He target at a 15.5° scattering angle. Excitation energies covered the resonance and the onset of the deep inelastic regions. We have determined for the first time the Q² evolution of Γ₁(Q²)=∫₀¹g₁(x,Q²)dx, Γ₂(Q²)=∫₀¹g₂(x,Q²)dx, and d₂(Q²)=∫₀¹x²[2g₁(x,Q²)+3g₂(x,Q²)]dx for the neutron in the range 0.1≤Q²≤0.9  GeV² with good precision. Γ₁(Q²) displays a smooth variation from high to low Q². The Burkhardt-Cottingham sum rule holds within uncertainties and d₂ is nonzero over the measured range.

We have performed the first high precision measurement of the coherent neutron scattering length of deuterium in a pure sample using neutron interferometry. We find b_nd=(6.665±0.004)   fm in agreement with the world average of previous measurements using different techniques, b_nd=(6.6730±0.0045)   fm. We compare the new world average for the nd coherent scattering length b_nd=(6.669±0.003)   fm to calculations of the doublet and quartet scattering lengths from several modern nucleon-nucleon potential models with three-nucleon force (3NF) additions and show that almost all theories are in serious disagreement with experiment. This comparison is a more stringent test of the models than past comparisons with the less precisely determined doublet scattering length of ²a_nd=(0.65±0.04)   fm.

We have performed high-precision measurements of the coherent neutron scattering lengths of gas phase molecular hydrogen and deuterium using neutron interferometry. After correcting for molecular binding and multiple scattering from the molecule, we find b_np=(-3.7384±0.0020) fm and b_nd=(6.6649±0.0040) fm. Our results are in agreement with the world average of previous measurements, b_np=(-3.7410±0.0010) fm and b_nd=(6.6727±0.0045) fm. The new world averages for the n-p and n-d coherent scattering lengths, including our new results, are b_np=(-3.7405±0.0009) fm and b_nd=(6.6683±0.0030) fm. We compare b_nd with the calculations of the doublet and quartet scattering lengths of several nucleon-nucleon potential models and show that almost all known calculations are in disagreement with the precisely measured linear combination corresponding to the coherent scattering length. Combining the world data on b_nd with the modern high-precision theoretical calculations of the quartet n-d scattering lengths recently summarized by Friar et al., we deduce a new value for the doublet scattering length of ²a_nd=[0.645±0.003(expt)±0.007(theory)] fm. This value is a factor of 4, more precise than the previously accepted value of ²a_nd=[0.65±0.04(expt)] fm. The current state of knowledge of scattering lengths in the related p-d system, ideas for improving by a factor of 5 the accuracy of the b_np and b_nd measurements using neutron interferometry, and possibilities for further improvement of our knowledge of the coherent neutron scattering lengths of ³H, ³He, and ⁴He are discussed.

A high precision measurement of the transverse spin-dependent asymmetry A_T^′ in ³He→(e→,e^′) quasielastic scattering was performed in Hall A at Jefferson Lab at values of the squared four-momentum transfer, Q², between 0.1 and 0.6 (GeV/c)². A_T^′ is sensitive to the neutron magnetic form factor, G_Mⁿ. Values of G_Mⁿ at Q²=0.1 and 0.2 (GeV/c)², extracted using Faddeev calculations, were reported previously. Here, we report the extraction of G_Mⁿ for the remaining Q² values in the range from 0.3 to 0.6 (GeV/c)² using a plane-wave impulse approximation calculation. The results are in good agreement with recent precision data from experiments using a deuterium target.

We present data on the inclusive scattering of polarized electrons from a polarized ³He target at energies from 0.862 to 5.06 GeV, obtained at a scattering angle of 15.5°. Our data include measurements from the quasielastic peak, through the nucleon resonance region, and beyond, and were used to determine the virtual photon cross-section difference σ_1/2-σ_3/2. We extract the extended Gerasimov-Drell-Hearn integral for the neutron in the range of four-momentum transfer squared Q² of 0.1–0.9   GeV².

We present the first precision measurement of the spin-dependent asymmetry in the threshold region of ³H→e(e→,e^′) at Q² values of 0.1 and 0.2 (GeV/c)². The agreement between the data and nonrelativistic Faddeev calculations which include both final-state interactions and meson-exchange current effects is very good at Q²  =  0.1 (GeV/c)², while a small discrepancy at Q²  =  0.2 (GeV/c)² is observed.

We have measured the transverse asymmetry A_T^′ in ³He→(e→,e^′) quasielastic scattering in Hall A at Jefferson Laboratory with high precision for Q² values from 0.1 to 0.6 (GeV/c)². The neutron magnetic form factor G_Mⁿ was extracted based on Faddeev calculations for Q²  =  0.1 and 0.2 (GeV/c)² with an experimental uncertainty of less than 2%.

Electron magnetic-resonance (EMR) spectra observed at liquid-helium temperatures in the one-dimensional Ising ferromagnet [(CH₃)₃NH]FeCl₃⋅2H₂O result from transitions within the first excited set of cluster states. Improved fits of the EMR data in the range 10–35 GHz are presented both with and without the inclusion of the magnetic dipolar interaction within a ferrous chain. In the absence of spin canting, an expression independent of the chain length is derived for the matrix elements of the dipolar interaction. Justification for the omission of spin canting is obtained from the rotational behavior of the 17-GHz spectrum, which supports a canting angle of less than 10°.

A precision measurement of spin transfer in pp elastic scattering at 197.8 MeV has been completed recently at the Indiana University Cyclotron Facility. The new data span a kinematic regime chosen specifically to maximize sensitivity to the neutral πNN coupling constant. Our results provide strong support for modern potential models of the NN interaction in which a relatively weak pion coupling ( g₀²≈13.6, f₀²≈0.075) is employed, but disagree significantly with predictions of older models in which g₀²≈14.4. Calculations in a one-boson-exchange framework indicate that most of these latter differences can be removed by reducing g₀² from 14.4 to 13.6 in these models.

The ²⁸Si(α,⁸He)²⁴Si reaction has been used to measure the energies of the first two excited states in ²⁴Si. The excitation energies were found to be 1.879±0.011 and 3.441±0.010 MeV. These data allow for the first time the calculation of the stellar reaction rate of ²³Al(p,γ)²⁴Si on the basis of experimental information. This reaction is of considerable interest, since it might lead to a decrease of the ²²Na production in nova explosions, and solve the discrepancy between the ²²Na yield predictions of nova models and recent COMPTEL observations. We show, however, that the temperatures and densities required for a significantly reduced ²²Na yield are not reached in current nova models.

Angular distributions of the analyzing powers iT₁₁, T₂₀, and T₂₂ for ¹H(d→,γ)³He at 40<~E_c.m.<~110 keV and the angular distribution of the analyzing power A_y for ²H(p→,γ)³He at 70<~E_c.m.<~210 keV have been measured for the first time. In addition, absolute differential cross sections for proton-deuteron capture have been determined for E_c.m.= 75, 108, 133, and 173 keV. Thick ice or heavy ice targets and two large-volume, high-purity Ge γ-ray detectors were used. Results are in general agreement with an exact three-body calculation utilizing a realistic nucleon-nucleon potential. The vector-polarization observables are found to be especially sensitive to meson-exchange-current effects. The extracted S(0) value for proton-deuteron capture is ∼25% lower than that presently used in astrophysical calculations. An expression for the thermonuclear reaction rate below 10 GK is given.

The electron-paramagnetic-resonance technique has been used to obtain an unambiguous identification of resonances between spin-cluster states from the distinctive satellite spectra observed in the ranges 10–36 GHz and 2.6–4.2 K from a large single crystal of the one-dimensional Ising ferromagnet [(CH₃)₃NH]FeCl₃⋅2H₂O, known as FeTAC. The frequency and temperature dependencies of the spectra are explained by the addition of the term ΔS_x to the Zeeman term in the effective S=1/2 spin Hamiltonian describing the first set of excited cluster states. The spin-cluster levels are the eigenvalues of the resulting tridiagonal matrix, and lead to a satellite structure in FeTAC which is independent of the chain length for chains of more than about 30 Fe²⁺ ions.

Angular distributions of the tensor analyzing powers A_zz and A_xx-A_yy for the reactions ²H(d,p)³H and ²H(d,n)³He are presented at deuteron energies of 25, 40, 60, and 80 keV. The analyzing powers for the two reaction channels are quite similar at these energies. The data have been included in an R-matrix analysis of the four nucleon system. According to this analysis, transitions from entrance-channel quintet-S states are important in these reactions, so that the use of polarized fuels would not result in a neutron-lean fusion reactor.

Modulated-microwave-absorption spectra associated with the intermediate state have been observed at 15 GHz in a carefully prepared, pure mercury sample. Corrections to the measured critical fields were based on the assumption that the superconducting system follows the resultant of the sweep and modulation fields while the signal position is given by the sweep field only. The theory was tested on the behavior of a modulation-dependent transient signal amplitude observed below 2 K, which was associated with the onset of the truncated spectrum caused by ‘‘supercooling’’ of the normal state for downward sweeps. The values T_c=(4.159±0.002) K and H₀=(419±2) Oe were obtained by fitting the corrected data to the parabolic law.

Modulated microwave absorption spectra associated with the intermediate state have been observed at 15.1 GHz for an oblong mercury sample. The use of a liquid-helium bath eliminated the thermal instabilities associated with modern, gas-flow cooling systems. Differences in the absorption signals observed with low modulation amplitudes, which depend on the direction of passage through the intermediate region, could be due to either the supercooling of the normal state below H_c or the dependence of the domain topology on the thermodynamic path.

Low-temperature microwave absorption measurements at 9.3 GHz in ceramic YBa₂Cu₃O_7-δ were made on the broad modulated signal observed after maximum flux had been trapped. Noise occurred on the extrema of this signal if the modulation frequency and amplitude were sufficiently high and the temperature was close to the lowest that could be attained when the limiting factor was the microwave heating of the sample. The source of the noise is proposed to be thermal instabilities, which produce small flux avalanches in the superconducting grains.

We have measured the positron-lifetime and the Doppler-broadening spectra in the high-temperature superconductor YBa₂Cu₃O_7-δ as a function of temperature between 77 and 295 K. Positron-lifetime and Doppler-broadening data show new features in the superconducting phase. These results are interpreted in terms of positron localization in lattice distortions and positron surface states.

The 34-GHz EPR spectrum of Cu²⁺ in ZnTiF₆·6H₂O shows a Jahn-Teller effect with a transition from a single-line spectrum at high temperatures to a multiline anisotropic spectrum. The transition temperature on cooling varied with Cu concentration from 172 K for a sample containing 0.2 at.% Cu to roughly 90 K for a 46-at.% Cu sample. For dilute samples, the single-line spectrum was isotropic at 300 K with g=2.223±0.005, but showed axial symmetry about the trigonal axis at 180 K with g_∥^′=2.226±0.005 and g_⊥^′=2.223±0.005. At 4.2 K, a "static" Jahn-Teller effect was observed with six axially symmetric Cu²⁺ spectra, each with g_∥=2.470±0.005, g_⊥=2.100±0.005, |A_∥|≃106×10^-4 cm^-1, and |A_⊥|≃30×10^-4 cm^-1. The z axis of these spectra was found to lie along the fourfold axes of two cubes with a common [111] axis, rotated by 40°±2° with respect to each other about this axis. Analysis of the 4.2-K data leads to the values q≃0.50 for the Ham reduction factor and κ≃0.26 for the Fermi contact parameter, with A_∥A_⊥<0. An activation energy of about 100 cm^-1 was deduced from the gradual increase of the anisotropy of the spectrum on cooling in the low-temperature region.

An EPR spectrum with orthorhombic symmetry of U³⁺ in SrF₂ has been investigated. The principal axes and characteristic values of the g-tensor ellipsoid were determined. Twelve equivalent, yet distinguishable, spectra were resolved. The 12 X axes lie in {110} planes and are 15° from <110> directions. The Y axes lie in the same {110} planes 90° from the corresponding X axes (i.e., ± 15° from <100> directions). The 12 Z axes were found to be in the <110> directions perpendicular to the {110} plane which contains the corresponding X and Y axes. These principal axes were determined to within ± 0.5°. The principal g values are g_x=1.3276±0.0005, g_y=2.88±0.01, and g_z=3.183±0.002. A rotating sample holder which facilitated this investigation will be discussed briefly.