122891342
Antisymmetrization and density-dependent effects within the folding model approach to α-nucleus scattering
Abstract The real α-nucleus interaction is investigated within the framework of the folding model. First the non-local α-nucleon interaction is calculated in the Hartree-Fock approximation and then folded in with the target matter density. The resulting α-nucleus potential is non-local but a local equivalent potential is obtained within the Perey-Saxon approximation. The calculations are performed both with density-independent and density-dependent forces. The rearrangement terms, arising from the density dependence of the force, which are usually neglected, are calculated and found to be significant at small distances. The local density appearing in the density-dependent part of the force is evaluated using a prescription which avoids explicit contradiction of the Pauli principle, in contrast to many earlier calculations where it is calculated as the sum of the densities of the colliding nuclei. However our prescription is expected to underestimate the density-dependent effects. The calculated potentials are used to fit elastic α-particle scattering from 40 Ca at 104 MeV incident energy. Good agreement with experimental data is obtained after renormalizing the theoretical potentials by a factor ranging from 0.6 to 0.9. Finally the E - and A -dependences of the volume integral per nucleon pair of the calculated potentials are investigated and found to be in good qualitative agreement with the available phenomenological results.
42989902
Potential description of the positive- and negative-energy properties of the alpha + 40Ca system and alpha -cluster structure of 44Ti.
123154942
Elastic and inelastic scattering of alpha particles from Ca-40, Ca-44 over a broad range of energies and angles
Angular distributions for alpha -particle elastic scattering by /sup 40,44/Ca and excitation of the 3.73 MeV 3/sup -/ collective state of /sup 40/Ca were measured for incident energies ranging from 40 to 62 MeV. An extensive optical model analysis of these elastic scattering cross sections and other available data, using squared Woods-Saxon form factors, results in potentials with fixed geometry for both real and imaginary parts and depths with smooth energy behavior over a broad incident energy range. These results are discussed in the frame of the semi-classical approximation developed by Brink and Takigawa (1977). The sensitiveness of the calculated elastic scattering cross sections to the real part of the potentials as a function of the projectile-target distance has been investigated by means of a notch test. Distorted-wave Born-approximation calculations for the excitation of the 3.73 MeV 3/sup -/ state of /sup 40/Ca are presented.
119969931
Optical-model Description of Alpha+o-16 Elastic-scattering and Alpha-cluster Structure in Ne-20
O16(,) elastic scattering angular distributions have been measured for incident energies 39.3, 49.5, and 69.5 MeV. These data, and previous measurements at 32.2, 104, and 146 MeV, have been subjected to a global optical model analysis. The deduced global potential has two energy-dependent parameters which are found to vary smoothly with energy and it is uniquely determined by the data. Backward angular distributions measured between 40 and 54 MeV are also presented and shown to be nicely reproduced by the model. The sensitivity of the cross sections to the various regions of the real potential has been investigated as a function of energy using the notch test technique. The low energy behavior of the differential cross sections can be understood in terms of the semiclassical decomposition of Brink and Takigawa. A natural extrapolation of the global potential below 30 MeV is shown to reproduce the wide bump observed in the experimental excitation functions around 20 MeV. This bump is shown to be due to an l=8 shape resonance and is interpreted as the J=8+ member of the K=04+ rotational band of Ne20, in contradiction with the current attribution. Other bound and quasibound states supported by the potential are discussed in the light of orthogonality condition model-type arguments and shown to be consistent with the well-known K=01+ and 0- bands, and with the first three states of the K=04+ band of Ne20. NUCLEAR REACTIONS O16(,), measured (), E=39.3, 49.5, 69.5 MeV; global optical model analysis, E=32-146 MeV; semiclassical decomposition of the scattering amplitude; investigation of the compatibility of the potential description with existing low energy data and comparison with cluster models. © 1983 The American Physical Society.
35383178
Molecular interpretation of the oscillations of the fusion excitation function for the alpha +40Ca system.
The oscillations observed in the experimental ..cap alpha..+ /sup 40/Ca fusion excitation function between E/sub ..cap alpha../ -- 10 and 27 MeV are described within the frame of the optical model. Use is made of an existing optical potential giving a precise description of the elastic scattering data for E/sub ..cap alpha../>24 MeV on a broad angular range, which is extrapolated to lower incident energies. A correct description of elastic scattering experimental excitation functions between E/sub ..cap alpha../ -- 12 and 18 MeV, as well as of the fusion excitation function, is achieved by using different imaginary geometries for both processes. The oscillations appearing in the calculated fusion excitation function are due to maxima in the even-l transmission coefficients, for l ranging from 6 to 12; these maxima correspond to shape resonances in the underlying potential, which are associated with states belonging to an excited positive parity band with N -- 14 in the ..cap alpha..+ /sup 40/Ca system. The same potential predicts an N -- 12 positive parity band of states whose energies are in good agreement with the /sup 44/Ti experimental ground state band. It is shown that the correct reproduction of the spacing between the fusion oscillationsmore » achieved here results from the use of a deep real potential, which automatically yields a decoupling between positive and negative parity bands.« less
42706318
Alpha-cluster spectroscopy in 40Ca and in the sd-shell closure region.
The low-energy {sup 36}Ar({alpha},{alpha}) elastic scattering data of Gaul {ital et} {ital al}. are analyzed within the frame of the optical model, using energy-independent Woods-Saxon squared or model-independent geometries for the real part of the potential. The optical-model scattering amplitude is decomposed into its barrier and internal wave components to understand the rapid and complicated evolution of the angular distributions with incident energy. The properties of the low-energy bound and quasibound states supported by this potential are compared with those of the states of the deformed 4p-4h rotational band in {sup 40}Ca, built on the {ital J}{sup {pi}}=0{sup +}, {ital E}{sub {ital x}}=3.35 MeV state. The possibility of extending this local potential model approach to neighboring nuclei near the {ital sd}-shell closure is discussed. A negative-parity band of states with appreciable {alpha}-cluster character, starting around the {alpha}-particle threshold, is expected to be systematically present in this mass region.
123416974
Theory of pulsar magnetospheres
There is a wide range of fundamental physical problems directly related to how pulsars function. Some of these are independent of the specific pulsar mechanism. Others relate directly to the physics of the pulsar and already shed some light on the properties of matter at high density (\ensuremath{\sim}${10}^{15}$ g/cc) and in strong magnetic fields (\ensuremath{\sim}${10}^{12}$ G). Pulsars are assumed to be rotating neutron stars surrounded by strong magnetic fields and energetic particles. It is somewhere within this "magnetosphere" that the pulsar action is expected to take place. Currently there has been considerable difficulty in formulating an entirely self-consistent theory of the magnetospheric behavior and there may be rapid revisions in the near future, which is all the more surprising since many of the issues involve "elementary" problems in electromagnetism. One interesting discovery is that charge-separated plasmas apparently can support stable static discontinuities.
119697750
Gravitationally induced electric fields in conductors.
An estimate for the gravitationally induced electric field is presented that is based on a statistical model for the electrons in a metal in which their long-range electrostatic interaction with a differentially compressed lattice of positive ions is taken into account. The field is in the upward direction and is estimated to be of the order of magnitude $E\ensuremath{\approx}\frac{\mathrm{Mg}}{e}$, where $M$ is the atomic mass, $e$ is the electronic charge, and $g$ is the acceleration due to gravity. For copper, $E\ensuremath{\approx}{10}^{\ensuremath{-}7}$ V/m. An earlier estimate by Schiff and Barnhill gives a field in the downward direction equal to $\frac{\mathrm{mg}}{e}$, where $m$ is the mass of the electron. The difference between our result and theirs is due to different treatments of the effect of lattice compressibility.
121467224
PARITY NONCONSERVATION IN NUCLEI
BS>The influence of the known weak interactions on the parity of nuclear states is discussed. Derivation of a parity nonconserving interaction rests on the assumption of a current-current hypothesis for the weak interactions. Consequently, observation of parity impurity effects would be an important confirmation of this hypothesis. A simple approximate method of treating the nuclear parity impurity is developed and applied in an effort to find experimental situations favorable to observation of effects due to such impurity. Parity-forbidden alpha decay from excited states of light nuclei and certain electromagnetic transitions in the heavy nuclei appear to be promising. Special attention was paid to the internal conversion electrons from the 123kev transition nn Lu/sup 173/ whose polarization is estimated to be about 0.4%. An effect on polarized neutrons analogous to optical rotation is also discussed. (auth)
118960042
The state of pulsar theory
Abstract I summarize the status of pulsar theory, now 35 years after their discovery. Although progress has been made in understanding the relevant processes involved, there are several widely held misconceptions that are inhibiting further advances. These include the idea that plasma “must” be accelerated from the magnetic polar caps (the basis for the “Hollow Cone Model”) and the idea that winds would be driven away by centrifugal forces, with large amplitude electromagnetic waves playing no role whatsoever. However, recent theoretical work is converging on a picture that closely resembles the latest HST and Chandra images, providing hope for the future. No less than three groups have recently confirmed the early Krause-Polstorff–Michel simulations showing that the fundamental plasma distribution around a rotating neutron star consists of two polar domes and an equatorial torus of trapped nonneutral plasma of opposite sign charges. Unless a lot of new physics can be added, this distribution renders the Goldreich–Julian model irrelevant (i.e., along with most of the theoretical publications over the last 33 years).
123629832
Intense Coherent Submillimeter Radiation in Electron Storage Rings
Energetic electron bunches in storage rings produce pulsed bursts of incoherent synchrotron radiation. It is pointed out that they should also produce a roughly comparable power output of coherent radio-frequency radiation. Thus electron storage rings, in addition to their intended purpose and their secondary use as intense sources of x rays, might additionally serve as pulsar simulators, producing a similar spectrum of coherent emission, the properties and modification of which could be studied in the laboratory. A spontaneous bunching of electrons (artificially bunched here) might be evidenced as ''superbunching.''