Effect Of Nuclear Deformation Research Articles

Effect of Nuclear Deformation on Fast-Neutron Total Cross Sections

Effect of Nuclear Deformation on Fast-Neutron Total Cross Sections

Neutron Scattering from Nonspherical Nuclei

The scattering of neutrons from nuclei in the energy region where only a few nuclear levels may be excited is studied with a combined nonspherical optical model and statistical compound-nucleus theory. The nuclei studied are even-even nuclei with a low-lying ${2}^{+}$ state and range from the very light (${\mathrm{C}}^{12}$) to the very heavy (${\mathrm{U}}^{238}$). The effects of nuclear deformation on the scattering properties are illustrated, and the importance of direct interactions at relatively low incident-neutron energies is demonstrated.

The effect of nuclear deformation on hyperfine structure anomaly

A recent measurement of the hyperfine structure anomaly of deformed 171, 173Yb isotopes is interpreted in terms of the Nilsson model. It turns out that the anomaly is best reproduced with deformations which are consistent with those obtained by other, more direct, methods.

A note on the interpretation of isotope shifts

It is pointed out that in tin observed irregular changes in the mean square radius of the nuclear charge distribution cannot be reconciled with the common assumption that such irregular changes are due solely to the effects of nuclear deformation.

Hyperfine Structure in Europium Metal

M\"ossbauer resonance experiments were performed on europium metal, using the 21.7-keV gamma ray in ${\mathrm{Eu}}^{151}$. The N\'eel point of 87\ifmmode^\circ\else\textdegree\fi{}K was confirmed. Above this temperature a single line was observed and attributed to paramagnetic relaxation. At 4.2\ifmmode^\circ\else\textdegree\fi{}K a six-line spectrum was found. Analysis yielded a spin of $\frac{7}{2}$ and a moment of +2.53\ifmmode\pm\else\textpm\fi{}0.03 nm for the 21.7 keV excited state, an internal field of 264\ifmmode\pm\else\textpm\fi{}8 kG, and an isomeric chemical shift of -0.81\ifmmode\pm\else\textpm\fi{}0.03 cm/sec, relative to ${\mathrm{Eu}}_{2}$${\mathrm{O}}_{3}$. This is consistent with an electronic level $4{f}^{7}6{s}^{2}$, $^{8}S_{\frac{7}{2}}$ for the europium atom. The effect of nuclear deformation on isomeric chemical shifts is considered, and it is concluded that ${\mathrm{Eu}}^{151}$, with a moderately deformed ground state, has a less deformed excited state at 21.7 keV.

Helium-Ion-Induced Fission of Bi, Pb, Tl, and Au

The fission cross sections have been measured with solid-state detectors for helium-ion-induced fission of bismuth, lead-206, thallium, and gold. The measurements were made at several helium-ion projectile energies between 30 and 43 Mev. The fission cross sections of bismuth, lead-206, thallium, and gold with 42.8-Mev helium ions are 7.3, 1.8, 0.65, and 0.28 mb, respectively, and the cross sections decrease rapidly with reduced-energy projectiles. The competition between fission and neutron emission as a function of excitation energy is compared with theoretical predictions of $\frac{{\ensuremath{\Gamma}}_{f}}{{\ensuremath{\Gamma}}_{n}}$ and some comments are made on the effect of nuclear deformation on the Fermi gas level density parameter $a$. Fission thresholds for ${\mathrm{At}}^{213}$, ${\mathrm{Po}}^{210}$, ${\mathrm{Bi}}^{207,209}$, and ${\mathrm{Tl}}^{201}$ of 15.8\ifmmode\pm\else\textpm\fi{}2.0, 18.6\ifmmode\pm\else\textpm\fi{}2.0, 20.6\ifmmode\pm\else\textpm\fi{}2.0, and 19.9\ifmmode\pm\else\textpm\fi{}2.0 Mev are derived. The saddlepoint masses of these nuclei relative to Cameron's reference mass surface lie on a smooth curve with the heavy element data, indicating that the shell structure is completely destroyed during the distortion from equilibrium to saddle-point deformation. An empirical equation for fission thresholds is deduced from the saddle-point mass surface which is thought to be valid for nuclei with $\frac{{Z}^{2}}{A}$ between 32 and 40.

Gross structure in reactions of deformed nuclei

The Nilsson model is applied to determine the effect of nuclear deformation on gross structure in direct reaction spectra among intermediate nuclei. While the gross structure known for spherical nuclei is obscured to low resolution observations, it is replaced by an equally distinctive pattern whose major features are fixed relative to single nucleon levels in the undeformed well. Definite minima in the spectrum persist at magic number gaps even for fairly large deformations. Location of the gross structure pattern on a nuclear excitation scale is determined by deformation and nucleon population numbers, so that shifts of the structure among spectra of neighbouring nuclei are definite and predictable. The angular distribution in each part of the spectrum should be dominated by a few specific l min values even though the single particle l is not a quantum number and parities are mixed. The effects are illustrated numerically with respect to the (n, p) reaction on target isotopes of Rh, In and Sb.

Semiempirical atomic mass law

New terms are proposed for the atomic mass law which incorporate effects of nuclear deformation and shell structure including interference effects between unfilled neutron and proton shells. In addition, a surface-symmetry term is included. Coulomb terms are fixed by recent radii determinations in electron scattering. The standard deviation of calculated masses is reduced by the addition of the new terms as is the error in extrapolating from one side of the valley of beta stability to the other. The usefulness of the law for calculating the mass and binding energy of atoms with large neutron excess is discussed. A table giving mass excess, neutron, proton and alpha-particle binding energies, and beta decay transition energies is appended.

Nuclear Deformation and the Photodisintegration Giant Resonances

The effects of nuclear deformation and shell closure upon the photodisintegration giant resonances have been investigated by measuring the (γ,n) cross sections for seven isotopes with 82 ≤ N ≤ 108 using the bremsstrahlung from the 33 MeV Canberra electron synchrotron. The induced radioactivities were measured with a NaI (T1) scintillation spectrometer. The most accurate results were obtained for the 181Ta (γ,n) reaction leading to the 8.15 hour isomeric level of 180Ta, the yield of which was measured to ½% statistical accuracy at 250 kev intervals over the rising part of the activation curve. The giant resonance for the reaction measured in this way has a width of 5.3 MeV and shows no evidence for the splitting into two components predicted by recent calculations on the photodisintegration of spheroidal nuclei. In qualitative agreement with these calculations, it was found that the (γ,n) cross sections for the other distorted nuclei investigated, 150Nd, 154Sm and 160Gd, have widths of 5.7, 6.5 and 5.3 MeV respectively, while for the closed-shell nuclei 141Pr, 142Nd and 144Sm, the corresponding (γ,n) cross sections have widths of 4.3, less than 5.0 and 4.0 MeV. In addition, it was found that the (γ,n) cross sections for closed-shell nuclei have substantially larger high-energy tails than do those for spheroidal nuclei.