Medium-weight Nuclei Research Articles

Spreading Widths of Deeply Bound Hole States in Medium Weight Nuclei

Spreading Widths of Deeply Bound Hole States in Medium Weight Nuclei

Nuclear sensitivity to time-reversal non-invariance: Traditional detailed balance

We investigate the most basic fluctuation measure relevant to traditional detailed balance studies with compound nucleus reactions. The treatment is exact to leading order in the hypothetical parity-even time-reversal-odd part of the nuclear interaction and applies from the regime of isolated resonances through to that of strongly-overlapping resonances. We infer approximate expressions in which the scaling with basic compound nucleus parameters is manifest and identify the presence of two enhancements: admixture and lifetime. We confirm the validity of assumptions implicit in the previous theoretical treatment of the strongly-overlapping regime. For the purpose of order of magnitude estimates, this treatment is adequate throughout the overlapping resonance regime. We find that, in the resolved resonance regime, the correlations due to unitarity between the symmetric and anti-symmetric parts of the S-matrix are strong. With regard to future measurements, we present a revised sensitivity estimate for the class of optimal isolated-resonance regime measurements identified recently and we argue in favour of two observables constructed entirely from energy averages (which have some practical advantages). We conclude that, for the case of (p, α 0) (α, p 0) - reactions , one of our observables constructed from energy averages is only marginally less sensitive than optimal isolated-resonance regime measurements. Related findings are that: conventional measurements with (p, α 0) (α, p 0) - reactions in the overlapping resonance regime of medium-weight nuclei ( A ∼ 100) can possess a sensitivity comparable to that of optimal isolated-resonance regime measurements; on-resonance measurements in the isolated resonance regime are of interest in as much as they benefit from dynamic and structural enhancement.

Classical European-Muon-Collaboration effect from few-body systems to nuclear matter: Can binding effects explain it?

It is shown that if the effects of nucleon binding on deep inelastic scattering are considered within many-body realistic descriptions of nuclei which include nucleon-nucleon correlations, the European-Muon-Collaboration effect in light and medium weight nuclei and nuclear matter can be accounted for in the region 0.2{le}{ital x}{le}0.5, but a systematic discrepancy between theory and experiment remains to be explained for 0.5{le}{ital x}{le}0.9.

Can nuclear binding explain the classical EMC effect?

It is shown that if the effects of nucleon binding on Deep Inelastic Scattering are considered within many-body realistic descriptions of nuclei which include nucleon-nucleon correlations, the EMC effect in medium weight nuclei and nuclear matter can be accounted for in the region 0.2⩽×⩽0.5, but a systematic dicrepancy between theory and experimental remains to be explained for 0.5⩽×⩽0.9.

The Ericson-Ericson Lorentz-Lorenz correction

The Lorentz-Lorenz correction is briefly reviewed for the electromagnetic system and then the arguments of the Ericsons for the pion-nucleus many-body system are given. A transparent discussion by Torleif Ericson is repeated. The importance of the Ericson-Ericson Lorentz-Lorenz correction is discussed. In particular, it is shown that with the classical value of this correction, multiple scattering of the pions is highly suppressed. Experimental data confirms such a suppression, indicating that in pionic units the value of the EELL parameter cannot be far from the classical (g′ 0 ) N Δ = 1 3 . No stiffness is seen in the longitudinal response of 208Pb to polarized protons, indicating that ( g′ 0) NN must be substantially greater than 1 3 , seeming to require (g′ 0) NN ⪆0.9 . This poses a problem, however, because reanalysis of the Bonn potential in nucleon-nucleon scattering, together with a G-matrix calculation for nuclear matter, does not allow ( g′ 0) NN to be larger than ∼0.525. This value would, however, give pionic contributions which would strongly disagree with EMC and especially, Drell-Yan experiments which measure the change in quark sea in going from deuterium to medium weight nuclei. Some of the discrepancy is removed by noting that the p-meson exchange contribution to ( g′ 0) NN is increased in medium by the factor ( m n m n ∗) 2 , where m n nucleon effective mass. Since, however, the proton scattering favors the nuclear surface where m n ∗ is not much smaller than m n, this enhancement is not large for this experiment. It is pointed out that when the decrease with momentum of screening by isobar-hole excitations and the increase in p-exchange tensor contribution with angle is taken into account, ( g′ 0) NN becomes a function of momentum. Its value is estimated to be ⪆0.9 for the momentum transfer k = 1.75 fm −1 where the experiments were performed. Possible reasons why ( g′ 0) N Δ is so much smaller than ( g′ 0) NN are discussed.

Study of ion-crystal interaction using the blocking technique for scattered recoils

Experimental data are presented on orientation effects observed in swift heavy-ion exposures of diamond. Si and Ge crystals by recording recoil nuclei. The volume capture of medium-weight nuclei to channeling has been revealed and studied. The ion damaging power is systematized and the anomalously low damaging power of Xe ions is established. The crystal-media response to the passage of highly ionizing particles is discussed. On the basis of the crystal-blocking technique, a variant of the experimental estimate of the de-excitation time for inelastic nuclear collision products is described.

Exciton model comparison of the activation and the integrated 14 MeV neutron radiative capture cross sections

The exciton model for the γ-emission was tested in its abilities to describe the radiative capture of 14 MeV neutrons over a wide scale of atomic numbers (Al-27, Sc-45, V-51, Mn-55, I-127, Pr-141, Pb-208, and Bi-209). Special attention was paid to the difference between the activation cross section and the cross section integrated over the prompt capture γ-ray spectra. Relatively good agreement of theoretical and experimental data was obtained in this case, while the spectra agree in their gross structure only for medium weight and (possibly) heavy nuclei.

On theJ-dependence effect of (d,p) reaction angular distributions

The (d,p) reaction on some medium-weight nuclei between40Ca and62Ni have been analysed with FR-DWBA formalism including spinorbit term both in the entrance and in the exit channel of the optical potentials, in an effort to describe experimentally observed «dip» in the 1/2− angular distributions. The calculations to the data show up clearly the «dip» in the angular distributions as observed experimentally. Besides, the asymmetry arising out of spin difference between the two states is well described by calculations.

Direct and β-delayed particle decays near closed shells

We have been involved in the investigation of nuclear structure properties of nuclei close to the proton drip line. In these studies we have used on-line isotope separators at Oak Ridge (UNISOR) and Berkeley (OASIS), the ORNL velocity filter, and a fast helium-gas-jet transport system at the LBL 88-in. cyclotron. We describe briefly three areas of our research that have dealt with particle decays: (1) α-decay in medium-weight nuclei, (2) α-decay rates for even-even isotopes, and (3) ß-delayed-proton decay of N = 81 precursors.

Mechanism of s-wave and p-wave neutron resonance capture in light and medium-weight nuclei

Capture gamma-ray spectra of 16O, 28Si and 32S have been measured to investigate the mechanism of neutron capture in s-wave and p-wave resonances with large reduced neutron width. Observed gamma-ray transitions from the 434-keV p3/2-wave resonance of 16O exhibit typical features of valence neutron capture in light nuclei. For the 565-keV and 806-keV p3/2-wave resonances of 28Si, a particle-vibrator coupling model indicates that in these resonance states core excitation is essential to explain the observed partial radiative widths. There is also some possibility of core excitation in the 203-keV p1/2-wave resonance capture in 32S. Moreover, a strong correlation between partial radiative widths and spectroscopic factors of the final states was found for gamma-ray transitions from the 188-keV s-wave resonance of 28Si and the 103-keV s-wave resonance of 32S. A discussion follows on the particle-core coupling scheme in these resonance capture transitions.

DWBA (e,e′) cross section and coulomb sum rule

The Coulomb sum rule is studied in 12C, 40Ca and 208Pb in impulse approximation including the distortion of the electron wave. The effects are sizeable even in medium weight nuclei. Transverse responses are also discussed.

Isovector pairing collective motion: Generator-coordinate-method approach.

Isovector pairing collective motion is treated by means of the generator coordinate method. In this scheme, the isospin and number projection is performed analytically by the recognition of symmetry properties in the generalized Bardeen-Cooper-Schrieffer wave functions. Among the results obtained, our generator-coordinate-method values of energy and spectroscopic amplitude are shown to be comparable to those of shell model calculations. This is indeed encouraging, especially in view of the fact that they were reached using a simple approximation. The great simplicity of the present method, as compared with earlier complicated versions, suggests that they might prove useful in the study of isovector pairing collective states which are strongly populated by pair transfer reactions in medium weight nuclei.

Tensor interaction in elastic scattering of polarized deuterons from medium-weight nuclei near Ed = 22 MeV

Differential cross section σ(θ), vector analyzing power iT 11( θ) and all three tensor analyzing powers T 20( θ), T 21( θ), and T 22( θ) have been measured for elastic scattering of deuterons from twelve medium-weight nuclei 40Ca, 58,60,62,64Ni, 90Zr, 92,100Mo, and 104, 106, 108, 110 near E d = 22 MeV. Scattering angles were covered over a wide range from θ = 30° to 170°. The data were analyzed in terms of an optical model potential including a complex tensor interaction of the type T r. The tensor analyzing power T 21( θ) and the quantity X 2 T 22(θ)-√ 3 2 T 20(θ) play an essential role in extracting the tensor interaction and its strength. The obtained T r tensor potential has the real (imaginary) part with well depth, radius, and diffuseness parameters of 1.0 ~2.5 MeV (1.0~ 2.5 MeV), 1.0 ~1.5 fm (0.9~1.2 fm), and 0.4~0.6 fm (0.8 ~ 1.7 fm), respectively. The imaginary T r potential has larger diffuseness than the real one. In extracting these parameter values the T 21( θ) data of the backward angular region (100° ≲ θ ≲ 170°) are indispensable. The optical potential thus obtained is compared with a folding model potential. The effect of the tensor potentials of the types T P and T L was investigated.

Recent advances and future trends in neutron resonance spectroscopy

Abstract Neutron resonance spectroscopy contributes primarily to two areas of nuclear physics: In medium weight and heavy nuclei with a high level density it tests their statistical properties; in nuclei with a sufficiently low level density, i.e. light nuclei (A ≤ 50) and nuclei around 208Pb, it investigates nuclear structure at several MeV excitation energy. In the first field, recent years have seen growing knowledge and understanding of nuclear level densities and their spin and parity dependence. Several questions basic to the statistical properties of nuclei, although extensively studied in the past, are still open: the statistical distribution of partial widths; possible narrow energy variations of the average partial widths; and correlations between partial widths for different reaction channels. The major progress has occured and will continue to take place in the field of light nuclei: Improved resolution of neutron time-of-flight spectrometers yields detailed resonance data over an extended ene...

Enhancement of Parity Violation for Overlapping Nuclear Resonances

Proton scattering from a medium weight nucleus and the possibility of finding tWQ resonances, with the same spin but opposite parity, overlapping is studied. As a guide to the experimental study of this possibility we have constructed a simple separable potential model involving spin ~-spin 0 scattering that contains the required resonances for adjacent I values in the absence of a parity-violating (PV) interaction. A phenomenological PV interaction is then introduced and the magnitude of the longitudinal asymmetry is studied as a function of the model parameters. It is found that the magnitude of the asymmetry is greatest at zero separation for the case of narrow resonance levels.

Systematic properties of proton single-particle energies

Single-particle energies of protons in the 1f7/2, 2p3/2, 2p1/2, 1f5/2 and 1g9/2 shells of medium-weight nuclei were determined from proton pickup and stripping experiments. The data reveal a simple linear dependence on mass number A and isospin To of the target nuclei which can be interpreted in terms of an extended Bansal-French model.

Isospin mixing in the decay of theT>giant dipole state

The magnitude of the isospin mixing in the decay of the ${T}_{>}$ giant dipole resonance has been estimated, using the ($\ensuremath{\gamma}$, n) and ($\ensuremath{\gamma}$,p) cross sections available for the medium-weight nuclei $^{60}\mathrm{Ni}$, $^{88}\mathrm{Sr}$, $^{89}\mathrm{Y}$, $^{90}\mathrm{Zr}$, and $^{92}\mathrm{Mo}$. The deduced values show a fair correspondence with the existing data for mixing between compound states. From these results the mean mixing Coulomb matrix elements between compound states could also be derived.

Behaviour of the real part of the nucleon optical potential for medium-weight nuclei

A recent paper has shown that empirical values of J, the volume integral per nucléon of the real part of the nucleon optical potential, for intermediate-mass nuclei do not follow the predictions of the Lane model. We calculate the optical potentials using first-order Brueckner theory, obtain the corresponding values of J, and compare these with the empirical values. Agreement with some of the empirical features is obtained but major qualitative discrepancies remain. We trace these discrepancies to the method used to extract values of J from the experimental scattering cross sections.

Depolarization in proton inclusive inelastic scattering and in the ( [formula omitted], [formula omitted]) reaction on medium-weight nuclei

The energy-averaged depolarization parameter K y y′ has been measured for the inelastic scattering of 18 MeV protons from 54Fe, 63Cu and 92Mo at 45°, 90° and 135°, and for 14.35 MeV protons from 63Cu at 45° and 135°. In all cases K y y′ varies from approximately unity for scattering with low energy loss to approximately zero for inelastic scattering to high excitation energies. The change from one of these values to the other occurs over a region ≈ 6 MeV wide centered at about 5 MeV excitation. A simple two-component model fits both the K y y′ and inelastic crosssection data. K y y′ has also been measured for the 54Fe( d , p )Fe reaction with 16 MeV deuterons incident. Here K y y′ changes from approximately the maximum possible value, 2 3 , to about zero in a 6 MeV region centered at roughly 13 MeV excitation. The ( d , p ) data can be fitted by an extension of the model used for the proton scattering data.

中重核におけるフォノン模型の検討

中重核におけるフォノン模型の検討