Search Results (26 total)

Frequency-domain spectroscopy requires long pulses, whereas time-domain spectroscopy requires short pulses. This Letter demonstrates both theoretically and experimentally that simultaneous detection in frequency and time generates well-resolved spectra using intermediate-length pulses. In the case of coherent Raman spectroscopy, typical femtosecond pulses lie between the time and frequency domains. To demonstrate this method, a high-resolution Raman spectrum of nitrobenzene is obtained from 60 fs pulses. Phase control, pulse shaping, or pulses of widely differing duration are not required.

Evaporation residue cross sections and spin distributions have been measured for ²⁰⁰Pb compound nucleus formed in ¹⁶O+¹⁸⁴W reaction at the laboratory beam energies of 84, 92, 100, 108, 116, and 120 MeV. The evaporation residues have been selected using the recoil mass spectrometer, HIRA and detected using a 2D position sensitive silicon detector. The evaporation residue spin distributions have been measured by detecting gamma rays with 14 element BGO multiplicity filter. Measured evaporation residue cross sections and spin distributions are compared with the values predicted by a standard statistical model code. Comparison shows that, in the energy region studied, the nuclear viscosity parameter γ=3 is required to explain total evaporation residue cross sections and evaporation residue spin distributions.

The half-life of exohedral ⁷Be-C₆₀ complex and that of ⁷Be implanted in a gold foil have been found to be about the same within ≈0.2%. Using a radiochemical technique, we also measured that the probability of formation of endohedral ⁷Be@C₆₀ complex by nuclear implantation technique was (5.6 ± 0.45)%. We also find that the half-life of endohedral ⁷Be@C₆₀ complex is shorter than that of exohedral complex by more than 1%. An analysis of these results using linear muffin-tin orbital method calculations indicates that most of the implanted ⁷Be ions in fullerene C₆₀ stay at a distance of ≈5.3  Å from the centers of nearest C₆₀ molecules forming exohedral compounds and those who enter the fullerene cages go to the centers of the cages forming endohedral ⁷Be@C₆₀ compounds.

Quasi-elastic-scattering and transfer reaction cross-section measurements were made for the ⁷Be+²⁷Al system at E_lab=17, 19, and 21 MeV in the angular range θ_c.m.=12^°-43^°. An optical model (OM) analysis of the quasi-elastic scattering data was carried out. The fusion cross sections were derived at these energies by subtraction of the integrated transfer cross sections from the reaction cross sections obtained from the fits to quasi-elastic-scattering data. These fusion cross sections were found to be consistent with those obtained from the coupled-channels calculations. Elastic scattering and fusion cross sections were measured for the ⁷Li+²⁷Al system at E_lab=10, 13, 16, 19, and 24 MeV. For elastic scattering the angular coverages were in the θ_lab=12^°-72^° range and for fusion the α-evaporation spectra from the compound nucleus were measured in the angular range θ_lab=52^°-132^° (142° at 10 MeV). The elastic-scattering angular distributions were subjected to OM analysis. The α-evaporation spectra were reproduced with the statistical model calculations, and the fusion cross sections were extracted from them. The fusion cross sections were also extracted by subtraction of the integrated inelastic-scattering cross sections from the reaction cross sections obtained from the OM fits to the elastic-scattering data, and these fusion data were found to be consistent. The CCDEF calculations describe these data quite well. A comparison of the fusion data for the ⁷Be+²⁷Al and ⁷Li+²⁷Al systems shows a similar and consistent behavior.

Angular distribution measurements of ²H(⁷Be,⁷Be)²H and ²H(⁷Be,⁸B)n reactions at E_c.m.~4.5 MeV were performed to extract the astrophysical S₁₇(0) factor using the asymptotic normalization coefficient (ANC) method. For this purpose a pure, low emittance ⁷Be beam was separated from the primary ⁷Li beam by a recoil mass spectrometer operated in a novel mode. A beam stopper at 0° allowed for the use of a higher ⁷Be beam intensity. Measurement of the elastic scattering in the entrance channel using kinematic coincidence, facilitated the determination of the optical model parameters needed for the analysis of the transfer data. The present measurement significantly reduces errors in the extracted ⁷Be(p,γ) cross section using the ANC method. We get S₁₇ (0) = 20.7 ± 2.4 eV b.

Excited states of ⁹⁰Nb were investigated via prompt and delayed γ decays and the recoil-isomer tagging technique. The level scheme of ⁹⁰Nb has been extended up to J=19ℏ and E_x=8.4 MeV. Half-lives of the 11^- and 17/2^- isomeric levels in ^90,91Nb were measured to be 0.47±0.01 and 3.3±0.4 μs, respectively. The results are compared with the predictions of large-basis shell model calculations. The effects of truncation of the valence model space on the calculated results are discussed.

The odd-odd ¹⁴⁶Tb nucleus has been investigated using the ¹¹⁵In(³⁴S,3n) reaction at 140 MeV incident energy using an array of eight Compton-suppressed Clover detectors. Based on the observed γ-γ coincidences, the level structure of ¹⁴⁶Tb has been extended up to E_x∼10  MeV. Linear polarization measurements have been combined with angular correlations of the observed γ rays to assign the electromagnetic nature to the transitions. Probable origin of a cascade of M1 transitions observed around 8 MeV excitation energy has been discussed.

The recoil-isomer tagging technique has been used to study the isomeric states of nuclei in the vicinity of the N=50 shell closure. The nuclei of interest were separated from the projectilelike nuclei and other evaporation residues and transported to the focal plane of a recoil separator. The decay of the tagged isomer was studied at the focal plane using a high-purity Ge detector. The prompt transitions feeding the isomer were detected in an array of Clover Ge detectors placed around the target. The measured half-lives of the 8⁺ isomers in ⁸⁸Zr and ⁹⁰Mo are 1.41_(−0.09)^(+0.12)  μs and 1.17_(−0.07)^(+0.19)  μs, respectively. The corresponding transition probabilities are in reasonable agreement with the predictions of shell model calculations. The systematics of effective charges (e_p and e_n) for N=48 nuclei are qualitatively understood in these calculations.

A halo generation mechanism in the nonperiodic lattices such as the SNS (Spallation Neutron Source) linac MEBT [medium-energy beam transport between radio-frequency quadrupole and DTL (drift tube linac)] is reported. We find that the nonlinear space charge force resulting from large transverse beam eccentricity ∼2:1 in the ∼1.6 m-long MEBT chopper section is responsible for halo formation. As a result, the beam distribution, based on the front end emittance measurements and multiparticle simulation studies, develops halo that leads to beam loss and radioactivation of the SNS linac. Designing lattices with transverse beam eccentricity close to 1:1 suppresses this kind of halo generation. Modifying the MEBT optics and introducing adjustable collimators in the MEBT significantly reduced beam losses in the coupled cavity linac, which is a preferred scheme for mitigating halo. It turns out that the DTL collimation does not effectively remove halo and presents a risk of overheating drift tubes.

Measurements are presented for several mixtures of the spin observables C_SS,C_SL=C_LS, C_LL, and C_NN for neutron-proton elastic scattering. These data were obtained with a free polarized neutron beam, a polarized proton target, and a large magnetic spectrometer for the outgoing proton. The neutron beam kinetic energies were 484, 567, 634, 720, and 788 MeV. Combining these results with earlier measurements allows the determination of the pure spin observables C_SS, C_LS, and C_LL at 484, 634, and 788 MeV for c.m. angles 25°≤θ_c.m.≤180° and at 720 MeV for 35°≤θ_c.m.≤80°. These data make a significant contribution to the knowledge of the isospin-0 nucleon-nucleon scattering amplitudes. © 1996 The American Physical Society.

A measurement of Δσ_L(np), the difference between neutron-proton total cross sections for pure longitudinal spin states, is described. Data were taken at LAMPF for five neutron beam kinetic energies: 484, 568, 634, 720, and 788 MeV. The statistical errors are in the range of 0.64–1.35 mb. Various sources of systematic effects were investigated and are described. Overall systematic errors are estimated to be on the order of 0.5 mb and include an estimate for the uncertainty in the neutron beam polarization. The Δσ_L results are consistent with previous results from PSI and Saclay. These data, when combined with other results and fitted to a Breit-Wigner curve, are consistent with an elastic I=0 resonance with mass 2214±15 (stat) ±6 (syst) MeV and width 75±21±12 MeV. Because of a lack of Δσ_T(np) data between 500 and 800 MeV, it is not possible to differentiate between a singlet or coupled-triplet partial wave being responsible.

Polarized proton beams and a liquid-deuterium target were used to measure the analyzing power for quasifree p→-n and p→-p scattering in the forward direction at 643 and 797 MeV. Alternate use of a liquid-hydrogen target provided a comparison of free and quasifree p→-p scattering. The precision of the data is in general ±0.01 to ±0.02, although some of the p→-p data for 797 MeV are at the precision level of ±0.005. For c.m. angles θ^*≥22° there is no difference between quasifree and free results at either energy. The data are compared with the predictions of several phase-shift analyses. In a search for more pronounced quasifree scattering effects, a small amount of p→-‘‘p’’ data were taken with a carbon target.

Results are presented for the spin-spin correlation parameters C_SS and C_LS for free np elastic scattering at neutron beam kinetic energies of 484, 634, 720, and 788 MeV and c.m. angles between 25° and 80°. The measurements were performed with a polarized neutron beam and a polarized proton target. These are the first measurements of this type to be reported in the forward angular region with a free polarized neutron beam. The observables C_SS and C_LS are both small at all energies, except for C_LS at 788 MeV, which is larger than phase-shift analysis predictions by more than one standard deviation for most of the measured points.

Final results are presented for the spin-spin correlation parameters C_SL and C_LL for np elastic scattering with a polarized neutron beam incident on a polarized proton target. The beam kinetic energies are 484, 634, and 788 MeV, and the c.m. angular range is 80°-180°. These data will contribute significantly to the determination of the isospin-0 amplitudes in the energy range from 500 to 800 MeV.

The spin-correlation observable A_LL for p-p elastic scattering has been measured at energies 589, 640, 692, 743, and 793 MeV, over a c.m. angular range between 20° and 100°. The spin observable A_SL was also measured in this angular range at energies 640 and 793 MeV. At 488 MeV both the spin observables were measured, but only near the c.m. angle of 90°. The data are compared with the predictions of several phase-shift analyses and previous measurements. The energy dependence of A_LL for the c.m. angle of 90° is also presented and shows no anomalous behavior. These data provide better angular coverage over five of the energies, with comparable or better statistical precision, than previous measurements of A_LL.

The momentum dependence of the analyzing power A_y in proton-proton elastic scattering has been measured in small steps using an internal target during polarized beam acceleration from 1 to 3 GeV/c. The momentum bin size ranges from 5 to 18 MeV/c. The relative uncertainty of A_y is typically less than 0.01 for each momentum bin. Narrow structures have been found in the two-proton invariant mass distribution of A_y.

A measurement of the analyzing power for n-p scattering has been made at center-of-mass angles, 8.8°, 15.0°, and 20.7° with a nearly monoenergetic polarized neutron beam peaked at 790 MeV. These angles represent an acceptance from 5° to 30° in the center of mass, and therefore extend to the smallest angles yet reached for measurement of this observable. The data are compared with the predictions of several phase shift analyses, and with the results from previous measurements that extend to larger angles for both free and quasifree n-p scattering.

The mixed spin-spin correlation parameter C_σσ≈0.5C_SS-0.8C_SL for np elastic scattering was measured for incident-neutron-beam kinetic energies of 484, 634, and 788 MeV over the center-of-mass angular range 75°-180°. These C_σσ data are important for determining the I=0 nucleon-nucleon amplitudes and provide strong constraints on the phase-shift solutions. It was found that the ¹P₁, ³S₁, and ³D₁ isospin-0 partial waves are most strongly affected.

The spin-correlation parameter A_nn for free n-p elastic scattering has been measured for the first time for incident-neutron-beam energy E_n=790 MeV and c.m. angles 48°≤θ^*≤149°. The data are compared with the widely differing predictions of several phase-shift analyses, clearly favoring one of them. They also are compared with recently published quasifree A_nn data for the more limited c.m. angular region 98°≲θ^*≲122°.

Angular distributions of the spin-correlation parameters A_sl and A_ll for the reaction pp→π⁺d have been measured at pion center-of-mass angles 40°≤θ_π^+*≤130° at incident energies of 500, 650, and 800 MeV. Additional measurements of A_ll were made at 600, 700, and 750 MeV. The results of the experiment are compared with the predictions of several unified coupled-channel calculations and partial-wave analyses. While the latest partial-wave analyses were found to fit the data reasonably well, all except one of the various model predictions not only do not fit the data well, but also tend to be in disagreement with each other. The data show no clear sign of a need for proposed dibaryon resonances.

The spin-spin correlation parameters C_LL=(L,L;0,0)=A_LL and C_SL=(S,L;0,0)=A_SL for np elastic scattering were measured for incident polarized-neutron–beam kinetic energies of 484 and 634 MeV over the center-of-mass angles from ≃80° to 180°. The data are important for determining the I=0 nucleon-nucleon amplitudes. These results are compared with phase-shift calculations.

The polarization transfer parameter D_t(180°) for n-p scattering has been measured at 50 MeV for the first time. Polarized neutrons produced in the ²H(darrow,narrow) ³He reaction were scattered from the hydrogen in a polyethylene target and the polarization of the recoil protons emitted at 0° was measured in a carbon polarimeter. The result of this measurement tests the prediction of D_t from a phase shift analysis of the N-N data and that of a theoretical proposal concerning n-p charge exchange.

Measurements of A_LL between 500 and 800 MeV and spin dependent cross sections derived from these measurements are presented for the reaction pp→πd at c.m. angles θ_π^* of 50°, 70°, and 90°. Angular distributions for A_SL are also presented for energies 500, 650, and 800 MeV. These are the first measurements of spin-correlation parameters in this channel above 590 MeV, and are in substantial disagreement with theoretical predictions of the triplet amplitudes above 600 MeV.

The vector analyzing power A_y(E_n) for the breakup reaction ²H(p→,n)2p at laboratory angle θ_n=18° has been measured for E_p=21.3 MeV as a function of neutron energy. The experimental A_y(E_n) values are compared with the predictions of various three-nucleon Faddeev calculations.