Inverse beta decay of arbitrarily polarized neutrons in a magnetic field

We calculate the cross-section of the inverse beta decay process,v _e+n → p+e, in a background magnetic field which is much smaller than m _p ^2/e . Using exact solutions of the Dirac equation in a constant magnetic field, we find the cross-section for arbitrary polarization of the initial neutrons. The cross-section depends on the direction of the incident neutrino even when the initial neutron is assumed to be at rest and has no net polarization. Possible implications of the result are discussed.     

Extended optical model analyses for the 11B + 209Bi system at near-Coulomb-barrier energies

In the framework of an extended optical model approach in which the polarization potential is decomposed into direct-reaction (DR) and fusion parts, simultaneous c² \chi^{{2}}_{} analyses of elastic scattering and fusion cross-section data for the ¹¹B+ + ²⁰⁹Bi system at near-Coulomb-barrier energies are performed. We show that the DR and fusion potentials extracted from c² \chi^{{2}}_{} analyses separately satisfy the dispersion relation and that the threshold anomaly exhibits in both DR and fusion parts. We find that the analyses using only elastic scattering and fusion data can produce reliable predictions of cross-sections even though the direct reaction cross-section data are not complete. By using the extracted DR potential, we investigate the effects of the projectile breakup on fusion cross-sections.     

Effect of <Emphasis Type="Italic">β</Emphasis><Subscript>6</Subscript> deformation parameter on fusion cross-section and barrier distribution

The effect of hexacontatetrapole deformation parameter on both the fusion cross-section and the barrier distribution have been studied for U²³⁸ + O¹⁶ nuclear pair using microscopically derived heavy ion (HI) potential. A method was described to extend the calculation of HI potential between two spherical nuclei using density dependent finite range NN forces to the spherical-deformed interacting pair. Density dependent and density independent effective NN forces were used in the generalized double folding model to derive the HI potential. We found that positive β₆ has large effect on both the fusion cross-section and the barrier distribution in the presence of a small value of hexadecapole deformation parameter β₄. In this case the fusion cross-section is less sensitive to the negative value of β₆. In the presence of a large value of β₄, negative and positive β₆ values affect the fusion cross-section and have a small effect on the barrier distribution.     

Distribution of fusion barriers

Heavy ion fusion cross sections and compound nucleus average spin values obtained from distribution of fusion barriers are discussed. Various shapes of distribution functions are studied using a truncated Gaussian distribution function (TGD). It is shown that fusion cross section and average spin values are less sensitive to different parametrization of TGD function, whereas the second derivative of the product of energy and fusion cross sections (w.r.t. energy), obtained from the corresponding TGD functions are significantly different depending on the shape of the barrier distribution function. It is also shown byχ ² analysis of fusion cross section data that some systems favour a narrow Gaussian distribution function whereas others, for which the vibrational and rotational collective states are less important, favour a flat barrier distribution. A physical interpretation of the dynamical process that gives rise to different barrier distribution is given in the framework of microscopic coupled channel calculations.     

动量空间中能质子-12C弹性散射截面和自旋量的研究

在KMT理论框架下，应用微观的动量空间一级光学势，包括了库仑修正，自旋关联，NN振幅反对称，离壳效应，核子反冲和结合能转换，Lorentz不变的角变换.在整个中能区域系统地计算了质子-¹²C弹性散射微分截面和自旋观测量，并与实验数据及Glauber理论框架下或已有的其他理论计算结果做了比较，其结果显示，在200—1000MeV，该理论与实验结论符合程度较好.     

L-dependent heavy ion fusion potentials

The energyE and angular momentuml dependence of optical potential for fusion of¹⁶O+²⁰⁸Pb system, observed by Christleyet al [5], is expressed as a function of radial kinetic energy (ɛ) instead of explicitE andl dependence. It is shown that the effects of different channel couplings, which result in different effective potentials, can also be parametrized as a function ofɛ. A correlation is obtained between the energy dependent part of this effective potential and the maximum of the spin enhancement around the Coulomb barrier and both these quantities depend on the details of the channel couplings.     

Spin polarization of photoelectrons from solid cesium by circularly polarized light

The spin polarization of photoelectrons excited in cesium metal by circularly polarized light is discussed using group theoretical methods. Calculations of the spin polarization have been performed for direct transitions at the-point and between, and states. The results suggest that photoelectrons excited in monocrystalline Cs could have a high degree of spin polarization which depends both on the wave vector of the electrons and on the energy of the absorbed photon. Measurement of this polarization should allow the spin-orbit splitting of different energy bands to be determined.     

Proton polarization in 16O(p,p0)16O and states in 17F1 (II)

The proton polarization in ¹⁶O(p, p₀)¹⁶O has been measured for laboratory proton energies between 10.0 and 16.0 MeV. Measurements were made for 18 or more angles at 60 energies. In addition, excitation functions of polarization were measured at 6 angles in 50 keV steps from 12.8 to 16.0 MeV. Both optical-model and phase-shift analyses were performed using published cross-section data in conjunction with the polarization data. The energy dependence of the complex phase shifts yielded spin, parity, and width assignments for 10 levels in ¹⁷F and possible assignments for 10 additional levels. A review of the energy levels of ¹⁷F below 16 MeV is given.     

Fusion, resonances and scattering in12C+12C reaction

The variation of fusion cross-section (σJ_fus) with energy in the¹²C+¹²C collision is linked to the underlying resonance phenomenon through the behavior of reaction cross-section (σ) of which σ_fus is taken as a part. The calculation of σ_fus is done through an energy-dependent imaginary potential in the optical model potential (OMP). Through dispersion relation, such an imaginary potential gives rise to energy-dependent real potential which is incorporated in the OMP. In our calculation, a form of potential for the nuclear part which has a soft repulsive in-built core is introduced based on similar works done earlier. The calculated results of σ_fus are used to explain the oscillatory structure, astrophysical S-factor and the decreasing trend at higher energies of the experimental σ_fus data in the case of¹²C+¹²C system with remarkable success. The potential used for fusion calculation is tested for fitting elastic scattering data at some energies and is found good in forward angles. Further improvement of the fitting of these data is obtained by incorporating a coupling potential in the surface region. About twenty resonances are observed in our calculation in the specific partial waves and some of them are found close to the experimentally identified resonances in¹²C+¹²C reaction. Thus, we provide an integrated and comprehensive analysis of fusion, resonance and scattering data in the best studied case of¹²C+¹²C reaction within the framework of optical potential model.     

Relativistic analysis of nuclear ground state densities at 135 to 200 MeV

A relativistic analysis of p +⁴⁰Ca elastic scattering with different nuclear ground state target densities at 135 to 200 MeV is presented in this paper. It is found that the IGO densities are more consistent in reproducing the data over the energy range considered here. The reproduction of spin-rotation-function data with the simultaneous fitting of differential cross-section and analyzing power, and the appearance of wine-bottle-bottom shaped Re U_eff(r) in the transition energy region, sensitively depends on the input nuclear ground state densities and are not solely the relativistic characteristic signatures. We also found that the wine-bottle-bottom shaped Re U_eff (r) is preferred by the spin observables in the transition energy region (i.e. 181 MeV to 200 MeV)     

The muon capture in <Superscript>16</Superscript>O: the angular and polarization correlations

Longitudinal polarization of the daughter nuclei ¹⁶N which arises in μ⁻ capture on ¹⁶O as a function of the recoil angle, together with the angular distribution and the alignment of the recoil nucleus are calculated. The neutrinos born escape mainly along the muon spin. The polarization is found to vary from zero (recoil momentum counter to the muon spin direction) up to 50% (along the muon spin direction). The results can be applied to the experimental tests of T conservation, to the analysis of the projects of constructing the powerful mono-energetic neutrino sources, to the experimental study of the pseudo-scalar form factor and the K-electron capture, and to other spin-polarization correlation experiments.     

Spin-Polarized 3He–HeII Mixtures in the Static Fluctuation Approximation

We have used the so-called static fluctuation approximation (SFA) to calculate the thermodynamic properties of spin-polarized ³He–HeII mixtures at low temperature, T < 0.025 K. This approximation is based on the replacement of the square of the local-field operator with its mean value. A closed set of nonlinear integral equations is derived for spin-up and spin-down systems. This set is solved numerically by an iteration method for a realistic interhelium potential. The mean internal energy per unit volume, the pressure, the entropy per unit volume, and the specific heat per unit volume increase with increasing temperature. The mean internal energy per unit volume, the pressure increase with increasing spin polarization; while the entropy per unit volume and the specific heat per unit volume are weakly–dependent on spin polarization.     

Effect of top quark spin on the unparticle couplings in $\gamma\gamma \to t\bar{t}$

We investigate the potential of γ γ collisions to probe scalar unparticle couplings via top–antitop quark pair production. We find 95% confidence level limits on the unparticle couplings with an integrated luminosity of 500 fb⁻¹ and an energy of  TeV. We investigate the effect of the top quark spin polarization on the unparticle couplings. It is shown that spin polarization of the top quark leads to a significant improvement in the sensitivity limits.     

C12(d,p)C13和Ca40(d,p)Ca41基态反应质子极化的研究

本实验测量了C¹²(d,p)C¹³和Ca⁴⁰(d,p)Ca⁴¹两个基态反应质子群在六个角度上的极化值。在C¹²的反应中,小角区的结果和前人的工作相近,和半经典符号规则j_n=l_n±1/2,P=(±)一致。θ_L=115°的数据是前人没有测量过的,我们得到P=0.529±0.068,在Ca⁴⁰的反应中,小角区数据的符号也和半经典符号规则一致,和Немeц及Boschitz的实验结果相同,而和Hird,Takeda及Bercaw的实验结果相反,看来这是由于入射能量不一样所致。因为在这些实验中,三个较高入射能量和三个较低入射能量的结果,都分别有一致的符号,把已发表的极化实验数据进行比较,可以看出,半经典符号规则还是有一定的参考价值,或者,可能找出一个修改后的符号规律,以供核能谱学应用。在有些情况下,我们看到,随着氘核能量的增加,极化角分布的图形似乎有向小角区移动的趋势,这可能是直接反应的一个特性。关于截面和极化角分布间的位置对应关系,我们认为,截面角分布的极小,除了可对应于极化的变号以外,还可能对应于极化绝对值的极大,截面角分布的极大也可能对应于极化的变号,对于这些现象,我们用粗糙的扭曲波理论进行了讨论。     

Elastic scattering of intermediate energy protons on4He and12C

The elastic angular distribution and polarization of ～1 GeV protons on⁴He and¹²C have been analysed using the recently proposed correlation expansion for the Glauber amplitude by retaining terms up to three-body density only. The calculations include Coulomb and spin effects. Using realistic form factors for target nuclei it is found that we need to consider only up to the second-order density term to provide a satisfactory explanation of both kinds of experimental data in the available momentum transfer region. The contribution of the three-body density term is only marginal except in⁴He differential cross-section in the cm angular range 45°<θ _cm<65°.     

On the calculation of <H 2> molecular integrals

The density matrix equations of motion arising in the triplet mechanism of chemically induced electron spin polarization are solved exactly without the imposition of the Redfield approximation. It is shown that the triplet spin relaxation time occurring in the final expression is not the true relaxation time because the spectral density involved depends both on the rotational correlation time and on the quenching rate. The effective spin relaxation time differs only slightly from the true time. The equations are extended to the case where the initial triplet passes on its polarization to the secondary triplet and exact solutions for the polarizations of the latter's doublets are obtained in the form Π_B = cΠ_A; an explicit expression for c is presented. The consequences of the secondary triplet being able to pass back its polarization to the initial triplet are explored and a ‘coherence effect’ on the polarization on the first triplet's doublets is analysed.     

Dynamical decay process of <Superscript>219, 220</Superscript>Ra<Superscript>*</Superscript> formed in <Superscript>10, 11</Superscript>B + <Superscript>209</Superscript>Bi reactions

The excitation functions for both the evaporation residue and fission have been calculated for ¹⁰B + ²⁰⁹Bi and ¹¹B + ²⁰⁹Bi reactions forming compound systems ^{219, 220}Ra^* , using the dynamical cluster-decay model (DCM) with effects of deformations and orientations of the nuclei included in it. In addition to this, the excitation functions for complete fusion (CF) are obtained by summing the fission cross-sections, neutron evaporation and charged particle evaporation residue cross-sections produced through the axn\ensuremath \alpha xn and pxn\ensuremath pxn (x = 2, 3, 4) emission channels for the ²¹⁹Ra system at various incident centre-of-mass energies. Experimentally the CF cross-sections are suppressed and the observed suppression is attributed to the low binding energy of ^{10, 11}B which breaks up into charged fragments. The reported complete fusion (CF) and incomplete fusion (ICF) excitation functions for the ²¹⁹Ra system are found to be nicely fitted by the calculations performed in the framework of DCM, without invoking a significant contribution from quasi-fission. Although DCM has been applied for a number of compound nucleus decay studies in the recent past, the same is being used here in reference to ICF and subsequent decay processes along with the CF process. Interestingly the main contribution to complete fusion cross-section comes from the fission cross-section at higher incident energies, which in DCM is found to consist of an asymmetric fission window, shown to arise due to the deformation and orientation effects of formation and decay fragments.     

The 3701 keV neutron g 9/2 single-particle state in 57Ni

The neutron g _9/2 single-particle state in ⁵⁷Ni has been unambiguously identified in a combined measurement of the linear polarization, angular distribution, and angular correlation of γ rays following the fusion-evaporation reaction ²⁸Si(³²S,2pn)⁵⁷Ni at a beam energy of 90 MeV. The linear polarization was measured with a Euroball cluster detector and a prototype of an encapsulated six-fold segmented hexaconical Ge-detector. The spin of the 3701 keV level in ⁵⁷Ni was confirmed to be I= 9/2 and its parity determined to be positive. Received: 24 September 1999 / Revised version: 22 October 1999     

On high-energy elastic scattering of protons by nuclei

The two coupled channel formalism for high energy elastic scattering [1] is extended to include spin and isospin effects. For a spin and isospin zero nucleus these manifest themselves by additional spin-orbit terms in the potentials. Explicit formulas for these potentials are obtained in terms of the fully spin and isospin dependent nucleon-nucleon scattering amplitude, the ground state nuclear form factor and the state dependent correlation functions. The coupling potential except for a small term arising from double spin and isospin flip process involving nuclear excitation depends only upon the pair correlations.Numerical calculations are performed for the elastic scattering of 1 GeV protons incident on ⁴He. Various phenomenological dynamical two-body correlations as well as correlations generated from the Reid soft-core and Tabakin potentials in an approximate Brueckner-Hartree-Fock calculation are considered. The angular distribution beyond its first diffraction minimum as well as the polarization in the same angular range are shown to be sensitive to these correlations. However, the present accuracy of the experimental data and the lack of knowledge of the nucleon-nucleon scattering amplitude prevent any definitive conclusion about their nature.