s- and p-wave neutron spectroscopy. Xe. Intermediate structure in 90Y and the generalized R-matrix theory

The resonance structure observed in the ⁸⁹Y(n, n)⁸⁹Y total cross section measurements in the range of 0.3 to 1.2 MeV incident energy was investigated using the generalized R-matrix theory of nuclear reactions and the doorway interpretation of intermediate structure. The energies and wave functions of the doorway resonances were calculated in a 2-particle and 3p-1h basis of the shell model. The model space and the parameters of the model calculation chosen were consistent with other shell model calculations in the mass-90 region. Several strong p-wave doorways with J^π = 0⁺, 1⁺, and 2⁺ were predicted by the model in the energy range studied. This is due to proximity of p-wave giant resonance. The escape widths Γ^↑ and the spreading widths Γ^↓ for these states were evaluated using the model wave functions and the R-matrix formalism. The calculated energy dependence of the total cross section shows that most of the predicted doorways are in general agreement with the observed anamolies with similar relative strength. More significantly, the underlying p-wave gross structure representing a grand average is of very similar shape in both theory and experiment. As expected in the mass 90-region, the s- and d-wave doorways contribute less significantly to the calculated resonance structure.     

Inclusive two-particle correlations in 40 GeV/c π−p collisions and particle production mechanisms

Inclusive two-particle correlations in rapidity variables between π⁺π^?, π⁺π⁺ and π^?π^? obtained in 40 GeV π^?p collisions are discussed in the framework of the Mueller Regge model and the multiperipheral cluster model. The data show features similar to the data reported at much higher energies from the CERN ISR. This behaviour is more easily understood from the multiperipheral cluster model. Data on transverse momentum correlations in the same reactions can be qualitatively understood from the same model.     

On the Q 2 dependence of nuclear structure functions

The recent high statistics NMC data on the Tin to Carbon structure function ratio seems to indicate, for the first time, a significant Q ² dependence, especially at small values of Bjorken x, x < 0.05, and Q ² > 1 GeV². A purely log(Q ²)-type dependence of the structure functions, which is consistent with the free nucleon data, yields a fairly flat ratio with little or no Q ² dependence. In view of this seeming contradiction, we re-examine the applicability of such a model to nuclear structure functions in such a kinematical regime. We find that the model is consistent with all data, within experimental errors, without any need for introducing additional Q ² dependences or higher twist contributions. The model correctly reproduces the Q ² dependence of the Carbon structure function as well. We also critically examine the Q ² dependence of the corresponding spin dependent structure functions.     

Measurement and statistical model analysis of pre-fission neutron multiplicities

The average number of neutrons preceding fission (ν_pre) was measured for the compound systems ¹⁶⁸YB, ¹⁷⁸W, ¹⁸⁸Pt, ¹⁹²Pb, ¹⁹⁸Pb, ²⁰⁰Pb, ²¹⁰Po, ²¹³At and ²⁵¹Es formed by reactions induced by ¹⁶O, ¹⁸O, ¹⁹F, ²⁸Si or ³⁰Si projectiles with energies (E) between 4.9 and 7.2 MeV/A In some cases ν_pre is seen to increase with increasing E above a threshold energy (E_th) whereas the statistical model indicates that it should decrease. For a given projectile, this threshold decreases with increasing fissility, becoming equal to the Coulomb barrier around A_CN∼213 for ¹⁶O projectiles. Below E_th the variation of ν_pre with E is consistent with statistical model predictions. The deviations above E_th have been attributed to dissipative effects not included in the model. Extensive statistical model and Xχ² analyses of the pre-fission data below E_th and of fission and fusion excitation function data, previously measured, were made. The diffuseness parameters of the fusion spin-distributions agreed reasonably well with those suggested by the zero-point motion model. The ratios of level densities at saddle and equilibrium deformations (a_f/a_v) were found to be consistent with a value of unity, and the fission barriers (E_f) consistent with the predictions of the finite-range rotating liquid-drop model. However these values for a_f/a_ν and E_f may not represent the true values. Inclusion of dissipation requires higher values, whilst inclusion of the temperature dependence of E_f in statistical model calculations is shown to result in a reduction in the value of a_f/a_ν. Since reliable theoretical calculations are unavailable for either effect the consistency of the data with the finite range fission barriers can only be demonstrated to within 10–15% and values for a_f/a_ν, have an uncertainty of at least 5%.     

On the recombination mechanism of quarks into a baryon in proton fragmentation

The similarity and difference between our quark cascade model with recombination mechanism and Van Hove's quark fragmentation-recombination model are discussed. Van Hove's fragmentation-recombination model is extended to the model which describesx-dependence of the hadron spectra as well as the baryon multiplicities by using our diffusion equation. The recombination probability in Van Hove's model can be related with the recombination factor λ′ in our diffusion equation. Thex-dependence of the hadron spectra causes another restriction on the recombination probability in Van Hove's model. The inclusive spectra of mesons, proton andΔ ⁺⁺ in proton fragmentation and baryon multiplicities except∑ ^? multiplicity are explained by both our model with recombination mechanism and the model with recombination mechanism a la Van Hove. But the ration(∑ ^?)/n(∑ ^+;) may not be explained by naive quark-parton model with recombination mechanism without considering quark spin.     

Simple tests of quark-nucleon reciprocity

The inclusive cross-section for the production of a single hadron in deep inelastic electroproduction is studied in a dual resonance model. The Bjorken scaling behaviour in the virtual photon fragmentation region for finite x (≡ 2p_L^c.m./√s) is (1/σ^{T,L))d3σT,L/E?}d³p ～ (1/q²) F(x,p_⊥²/q²) and thus the transverse momentum grows like q², whereas in the parton model (1/σ^T,L)d³σ^T,L/E^?1d³p ～ F(x,p_⊥²). A related effect is the absence of two-jet structure in e⁺e^? annihilation. We believe that dual model results may give a more reliable indication of the deep inelastic behaviour for composite hadrons than the parton model.     

QCD jets ine + e −-annihilation and the transition into hadrons

A model has been developed describing QCD jets in the leading logarithmic approximation (LLA) and the subsequent transition into hadrons via a chain decay model. Besides of the production of mesons, the model is also able to describe baryon production. Agreement with recent data frome ⁺ e ^?-annihilation is found. This includes average multiplicities, average transverse momenta and transverse momentum distributions, longitudinal momentum distributions as well as particle production ratios π^α:K ^π : p(0070), charge compensation probabilities and energy flux correlations. The model is compared to similar models developed by other authors.     

A modified born term model for π± Δ electroproduction

An electric Born type model is used to describeπ ^± Δ electroproduction at high energies. The electric pion form factor is taken from VDM; the contact term form factor and theΔ form factor are set equal to the proton form factor (or equal to zero) which is left as an arbitrary parameter, and is determined by a fit of the model to the recentπ ⁺ δ ⁰ andπ ^? Δ ⁺⁺ electroproduction data.     

A study of 80,82,84Sr by (α, 2nγ) reactions

The ^AKr(α, 2nγ)^A+2Sr reactions have been studied by in-beam γ-ray spectroscopy for A = 78, 80, and 82. States with spins up to 10⁺ in ^82,84Sr and 6⁺ in ⁸⁰Sr are identified. The Sr nuclei undergo a transition from spherical to deformed ground states as neutrons are removed from the N = 50 shell. The systematics of these nuclei are compared with calculations based upon the shell model and interacting boson model.     

Solar neutrino-induced reactions

The ⁸B solar neutrino-induced ³⁷Cl(ν_e, e^?)³⁷ Ar cross section is obtained using weak matrix elements from a recent model calculation to complement existing experimental data: ω_8Bmodel=(1.27 ± 0.01) x 10^?42cm². More generally, constraints that can be placed on this cross section by a model-independent analysis are considered and yield ω_8Bmodel=(1.27 ± 0.23) x 10^{?42 cm2}.     

A Semi-empirical tight-binding theory of the electronic structure of semiconductors2

A nearest-neighbor semi-empirical tight-binding theory of energy bands in zincblende and diamond structure materials is developed and applied to the following sp³-bonded semiconductors: C, Si, Ge, Sn, SiC, GaP, GaAs, GaSb, InP, InAs, InSb, AlP, AlAs, AlSb, ZnSe, and ZnTe. For each of these materials the theory uses only thirteen parameters to reproduce the major features of conduction and valence bands. The matrix elements exhibit chemical trends: the differences in diagonal matrix elements are proportional to differences in free-atom orbital energies and the off-diagonal matrix elements obey the d^?2 rule of Harrison et al. The lowest energy conduction bands are well described as a result of the introduction of an excited s state, s¹. on each atom. Examination of the chemical trends in this sp³s¹ model yields a crude but “universal” sp³s¹ model whose parameters do not depend explicitly on band gaps, but rather are functions of atomic energies and bond lengths alone. The “universal” model, although cruder than the sp³s¹ model for any single semiconductor, can be employed to study relationships between the band structures of different semiconductors; we use it to predict band edge discontinuities of heterojunctions.     

Cluster Model Analysis of Pion Elastic and Inelastic Scattering from 12C

Angular distributions of differential cross sections for the ¹²C(π ^±, π ^±)¹²C and ¹²C(π ^±, π ^±)¹²C^* reactions at pion kinetic energy ranging from 50 to 260 MeV have been analyzed with the 3α-particle model of ¹²C. The model provides good fits to a wide range of data. Differential cross sections for inelastic transitions to the (2^?+?; 4.44 MeV) and (3^???; 9.64 MeV) states in ¹²C are computed and the deformation lengths δ ₂ and δ ₃ are extracted. It is found that the extracted deformation lengths are sensitive to the nuclear model used and similar to the corresponding values found with other probes and nuclear models.     

The amplitude structure of the processesK - p→ϱ-∑ (1385) andK - p→Φ ∑0 (1385) at 4.2 GeV/c

We present model-independent transversity amplitudes for the processesK ^- p→?^-∑ (1385) andK ^- p→Φ ∑⁰ (1385) at 4.2 GeV/c. We also perform a model dependent analysis and compare the resulting helicity amplitudes with the predictions of a simple exchange model. Implications for future model building are discussed.     

Model for nucleon valence structure functions at allx,allp ⊥ and allQ 2 from the correspondence between QCD and DTU

We propose a simple parametrization of the nucleon valence structure functions at allx, allp _⊥ and allQ ². We use the DTU parton model to fix the parametrization at a reference point (Q ₀ ² =3 GeV²) and we mimic the QCD evolution by replacing the dimensioned parameters of the DTU parton model by functions depending onQ ². Excellent agreement is obtained with existing data.     

Inner and outer shell ionization for atomic iodine: calculation of cross-sections and angular distribution parameters

This work presents results relating to photoemission cross-sections and angular distribution parameters for atomic iodine, where the photon energy varies over an energy range large enough to involve the ionization of the outermost 5p⁵ Subshell, as well as the inner 5s² and 4d¹⁰ electrons. The calculations were performed with several models, of varying sophistication according to the number of channels which are coupled to describe the continuum final-state wave function. They show that it is necessary to use a model which couples all the channels opened by the multiplet structure in order to show that the 4d shape resonance of iodine looks very much like the large 4d resonance of xenon. Anisotropie effects, whose size is reflected in the splitting among the various angular distribution parameters for alternative final ionic states, are found to be negligible within the framework of the most elaborate model, which couples all the channels corresponding to the residual ion configuration 4d⁹5p⁵; this holds less true as the angular momentum of the ionized electrons decreases. Whereas a model introducing the interchannel interactions within the ionization of one single subshell is sufficient to account for the resonances shifted beyond the thresholds for both 4d and 5p ionization, another model taking intershell interactions (5s + 5p) into account is necessary to obtain a minimum in the 5s cross-section.     

Analysis of scattering and reactions of aligned7Li-nuclei in a double folding model

Previously measured angular distributions of the differential cross section and the tensor analyzing powerT ₂₀ for the scattering of⁷Li from⁵¹V in the energy range from 10 to 18 MeV were analyzed with the double folding model using the effective nucleon-nucleon interaction M3Y as well as a zero range effective nucleon-nucleon interaction. Due to the spectroscopic deformation of the⁷Li-nucleus the interaction potential contains a central and aT _R -tensor part. The latter one is the origin of the observed tensor analyzing powers. Within the same model the scattering of aligned⁷Li-nuclei from⁵⁸Ni at 14.2 and 20.3 MeV is well described. The strength of theT _R -tensor potential in relation to the spectroscopic mass deformation of⁷Li is discussed. Using the real interaction potential obtained by the double folding model the energy dependence of the total reaction cross sectionσ ^r and its tensor analyzing powerT ₂₀ ^r for the system⁷Li?⁵¹V is well described within a barrier penetration model.     

Structure functions of nuclei for all x and Q 2

We propose a modified version of the quark cluster model to describe the structure functions of nuclei over the complete kinematical region of the variables x = Q ²/2m _N ν and Q ². The model accounts for shadowing effects at very small x and anti-shadowing in the region 0.1 ≤ x ≤ 0.3. The experimental observation that the ratio of structure functions F ₂ ^A /F ₂ ^D is less than unity in the region x ? 0.5 is related to the influence of multiquark clusters. The model agrees with all available data on nuclear structure fuctions for a large variety of nuclei, and can be used to make predictions for kinematical regions not yet studied experimentally.