Spin Cut-off Factor Research Articles

Untersuchung von (n, 2n) anregungsfunktionen

Investigation of (n, 2n) excitation-functions. The activation method was used to measure (n, 2n) excitation-functions in the energy region 12.6–19.6 MeV for 52Cr, 75As, 85Rb, 106Cd, 112Sn, 121Sb, 140Ce, 142Ce, 141Pr, 144Sm, 165Ho and 181Ta. Further isomeric (n, 2n) cross-section ratios were obtained for 85Rb and 121Sb in the energy region stated above and for 123Sb and 144Sm at 14.1 MeV. The total (n, 2n) excitation-functions were compared with results from statistical theory calculations using optical model absorption cross sections and the Fermi-gas model nuclear level density ω(E) = C(E+t) −2 exp(2(aE) 1 2 ) . Particularly at lower excitation energies (near reaction thresholds) better agreement between theoretical and experimental cross sections has been obtained when the pairing effect in the level density was accounted for by using appropriate values for the factor C according to Brown and Muirhead instead of shifting the nuclear ground state by the pairing energy δ as usually is done in the literature. In most cases good agreement has been obtained with level density coefficients a from Abdelmalek and Stavinsky. From the isomeric cross section ratios spin cut-off factors σ were deduced applying the theory of Vandenbosch and Huizenga. These values of σ lead to moments of inertia for the nuclei considered which only amount to 20–50% of the rigid body values.

The spin cut-off factor for60Co

Nine experimental isomeric cross-section ratios for the reactions59Co(n, γ)60Co,60Ni(n,p)60Co, and63Cu(n,α)60Co are analysed in terms of statistical theory by means of the method ofHuizenga andVandenbosch. Theγ-cascades are treated by a new model which was recently published. The results confirm the energy dependence of the spin cut-off factor for60Coσ∼U1/4 which should not be neglected. As a mean value (σ=4.3±0.3 is obtained forU=Bn=7.5 MeV in good agreement with a nuclear moment of inertia for a rigid sphere.

Systematic properties of neutron resonances in the rare earth region

The S-wave neutron strength functions, level spacings and potential scattering radius parameters have been measured for a number of nuclides in the rare earth region A = 155 to A = 176. The 〈Γ n 0〉 D and R′ are compared to rotational optical model calculations. No dependence of strength function on target nucleus spin is found. From a fit of level spacing to the Fermi gas model formula, a spin cut-off factor of 2.8 ± ± 0.2 is determined.

A study of the spin dependence of the nuclear level density by means of the 89Y(n, γ)90g, 90mY reactions with fast neutrons

The 89Y(n, γ)90mY and 89Y(n, γ)90gY cross sections have been determined at 12 neutron energies between 0.14 and 1.22 MeV. Neutrons were produced by the 7Li(p, n)7Be reaction. The full neutron energy spread was about 65 keV. The activation cross sections were measured relative to both the 235U(n, f) cross section and the 197Au(n, γ)198Au activation cross section. The results for the isomer ratios have been compared with the predictions of the statistical theory of nuclear reactions together with a Fermi gas, level-spacing formula in order to obtain the spin cut-off factor σ in the spin dependence of the nuclear level density. The effects of variations in the parameters in the theory have been studied. Very good agreement was obtained with σ = 3.3, assuming a γ-ray multiplicity of 4. The 89Y(n, γ)90mY cross section has also been measured at 14.8 MeV to be 0.50±0.07 mb. Finally, the half-life of 90mY has been found to be 3.19±0.01 h.

Co59(p, α)Fe56andFe56(p, p′)Reactions

Evaporation spectra of $\ensuremath{\alpha}$ particles from the ${\mathrm{Co}}^{59}(p, \ensuremath{\alpha}){\mathrm{Fe}}^{56}$ reaction and protons from the ${\mathrm{Fe}}^{56}(p, {p}^{\ensuremath{'}})$ reaction were investigated with silicon surface-barrier detectors for proton-bombarding energies of 9 to 13.5 MeV. The $\ensuremath{\alpha}$-particle spectra are in good agreement with previous results. However, the proton spectra differ somewhat from earlier measurements. The anisotropy of the $\ensuremath{\alpha}$ particles from the ${\mathrm{Co}}^{59}(p, \ensuremath{\alpha})$ reaction was measured as a function of $\ensuremath{\alpha}$-particle energy for proton-bombarding energies of 9 to 13.5 MeV. The level density and the spin-cutoff factor $\ensuremath{\sigma}$ were calculated from the experimental results as a function of excitation energy in the range 3 to 9 MeV. The level density varies irregularly with excitation energy at the lower excitation energies and increases smoothly in an exponential manner at higher excitation energies. The nuclear temperature is in the range of 1.10 to 1.45 MeV and appears to increase slightly with excitation energy. The spin-cutoff factor $\ensuremath{\sigma}$ increases from ${3.5}_{\ensuremath{-}0.3}^{+0.6}$ to ${3.8}_{\ensuremath{-}0.3}^{+0.5}$ in the excitation-energy range of 4 to 8 MeV.

Le « spin cut-off factor » et l'énergie de corrélation pour 29Si

The spin cut-off factor and correlation energy for /sup 29/Si have been estimated from the fluctuations observed in the /sup 28/Si(n alpha )/sup 25/Mg reaction. (auth)

Interpretation of Isomeric Cross-Section Ratios for (n, γ) and (γ, n) Reactions

The relative probability of forming each member of a pair of nuclear isomeric states has been compared with theoretical predictions in order to learn which nuclear parameters can be determined from these data. For thermal neutron capture reactions, the observed ratios do not give much information about the dependence of the nuclear level density on spin, but they are consistent with a spin cutoff factor, $\mathrm{exp}[\ensuremath{-}\frac{{(J+\frac{1}{2})}^{2}}{2{\ensuremath{\sigma}}^{2}}]$, where $\ensuremath{\sigma}\ensuremath{\le}5$. The calculations are sufficiently consistent with experiment to make their predictions usable as a guide for assigning spins to the compound states formed in thermal or resonant energy neutron capture. For ($\ensuremath{\gamma}, n$) reactions, the calculations reproduce the energy dependence of the experimentally observed isomeric cross-section ratios. In order to obtain quantitative information about the spin dependence of the nuclear level density, it is necessary to consider reactions where particles are emitted which can carry off enough angular momentum to reach many spin states of the residual nucleus.