Partial width fluctuation method of determining nuclear level density

A new method of determining the nuclear level density is presented. This method is based on the statistical analysis of the partial width fluctuations appearing in an excitation function of the radiative proton capture. The method was applied in the case of the ⁸⁸Sr(p, γ₀)⁸⁹Y and⁸⁹Y(p, γ₀)⁹⁰Zr reactions. The density of levels with spin 1^? in ⁹⁰Zr and the densities of levels with spins $\text{1}\text{2}{}^{\mathrm{+}}$ and $\text{3}\text{2}{}^{\mathrm{+}}$ in ⁸⁹Y at excitation energies from 10.9 to 11.6 MeV and from 9.3 to 10.8 MeV respectively, were determined with an uncertainty of about 35%.     

Effects of particle evaporation on the angular momentum of the emitting nucleus for deep inelastic and compound nuclear reactions

A model is developed which allows one to calculate analytically the angular momentum removed, and the angular momentum misalignment created by the evaporation of light particles from an excited nucleus. The mass, temperature and angular momentum of the emitting nucleus are explicitly considered. The formalism applies equally well to heavy-ion and compound nuclear reactions.     

Isospin dependence of the nuclear level density

Experiments on $\text{(ft)}{}^{\mathrm{+}}\text{/(}\text{ft)}{}^{\mathrm{?}}$ for mirror Gamow-Teller β-decay are reanalyzed in the light of recent work and new values for the Kubodera, Delorme and Rho second class parameters are deduced: ξ = ? (0.8 ± 2.0) × 10^?3MeV^?1; λ = (3.4 ± 3.9) × 10^?3. These values are compared with recent correlation experiments.     

Reaction dependence of nuclear decay linewidths

Various light- and heavy-ion reactions, 20 < E < 100 MeV, have been used to study the reaction dependence of α-decay widths for ⁸Be¹(2 ⁺ , 2.9 MeV) and ¹⁶O¹(1^?, 9.6 MeV). Although slight differences (< 20 %) are found for the observed line shapes (Γ), the resonance widths inferred (Γ_R) are self-consistent and indicate little if any reaction dependence (< 10 %). Near a decay threshold one may expect Γ < Γ_R by 20 % or more, however, and thus care must be taken in comparing decay widths inferred from nuclear reactions with those from scattering resonances. Reduced formal α-decay widths of γ_λ² = 680 ± 100 keV (s = 4.8 fm) and γ_λ² = 350 ± 50 keV(s = 5.4 fm), corresponding to θ_λ² = 0.50 and θ_λ² = 0.49 are deduced for ⁸Be¹(2 ⁺ , 2.9 MeV) and ¹⁶O¹(1^?, 9.6 MeV) using the nuclear-reaction Γ_R values and a particular set of α-nucleus potentials.     

Isospin mixing in 12C

We observe a yield to the 1⁺T = 1 level in ¹²C at 15.11 MeV in the isospin forbidden ¹²C(d, d') and ¹⁰B(α, d) reactions at about 1 % of the yield to the 1⁺T = 0 level at 12.71 MeV. Observed yields to the T = 12⁺ level at 16.11 MeV in both reactions at about half the yield to the 15.11 MeV level preclude attributing this observed isospin violation entirely to final state mixing. From the ratios of the spectroscopic factors for the 12.71 and 15.11 MeV levels in ¹²C and the ground state of ¹²B from the ¹³C(d, t) and ¹³C(d, ³He) reactions, we find a charge dependent matrix element between the 1⁺ states in ¹²C of 179 ± 75 keV.     

Measurement of compound nuclear lifetimes by the crystal blocking technique

The crystal blocking technique has been used to measure the lifetime of the 12.44 MeV 2⁺ level in ²⁸Si excited in the ²⁷Al(p, α)²⁴Mg resonance reaction at E_p = 885 keV. Both 〈110〉 axial and {111} planar blocking effects in Al single crystals have been investigated. Measurements have been made for two reaction depths, 1000 and 4000 A. Various methods of extracting the lifetime, involving both analytical calculations and computer simulation, are compared. The theoretical change in volume of an axial blocking dip due to a lifetime effect has been calculated in the continuum approximation. The results obtained from the different methods of analysis agree, the average value of the lifetime being 28 as. Although the results show no systematic dependence on reaction depth, pronounced depth effects are evident in computer simulations of the planar blocking dips.     

Isospin violation in α-induced reactions

A study has been made of the reactions ¹²C(α, α')¹²C and ¹⁰B(α, d)¹²C which lead to the 12.71 MeV T = 0 and the 15.11 MeV T = 1 levels in ¹²C. Significant structure is observed in the yield curves of the reactions ¹²C(α, α') ¹²C for both the isospin allowed and forbidden reactions up to the maximum available α-energy of 27 MeV; this structure is attributed to compound nucleus formation. No significant structure is observed in the reaction ¹⁰B(α, d)¹²C.     

Isospin components of the giant dipole resonance in several germanium isotopes

Measurements of the (γ, np) cross sections of ⁷⁰Ge, ⁷²Ge, ⁷⁴Ge and ⁷⁶Ge and the (γ, p) cross section of ⁷⁴Ge are reported. The (γ, np) reaction is discussed as a channel for the observation of the T = T₀ + 1 component of the giant dipole resonance of the nuclear photo effect, and experimental evidence is presented in support of this conjecture in Ge isotopes.     

Lifetime measurements on the compound nucleus 236U by means of the shadow effect

The lifetime of the ²³⁶U nucleus is measured in the range of excitation energies E_x = 6.7–11.5 MeV by a method based on the shadow (blocking) effect. The results obtained, as well as those of an earlier measurement of the lifetime for the ²³⁹U nucleus in the range E_x = 6.4–9.1 MeV, are compared with the results of calculations based on the level density ρ(E_x) in the Fermi-gas model and on the empirical level density ρ(E)_x) calculated as a result of an analysis of(n, γ) and (n, n') cross sections. An increase of excitation energy produces a substantial reduction in the rate of decrease of the lifetime at E_x ? 7.5 MeV for both compound nuclei as compared to the theoretical dependence τ(e_x).     

Systematics of nuclear level density parameters of nuclei deficient in one neutron

Photoneutron mean energies of 38 elements were measured as a function of peak bremsstrahlung energy for elements with 23 ≦ Z ≦ 83. Results are compared with neutron mean energies calculated from statistical theory, using for nuclear level densities modified Fermi gas formulae with and without pairing corrections and a constant temperature formula. Except near closed shells the Fermi gas formula with pairing corrections gives reasonable to good correlation between experimental and theoretical data. Derived values of the nuclear level density parameter a-except near Z = 82-are in quantitative agreement with those from recent neutron resonance data.     

Isospin mixing observed in the reaction 12C(6Li, α)14N

The reaction ¹²C(¹²Li, α)¹⁴N was studied to investigate the isospin mixing of high-lying levels in ¹⁸F. Excitation functions and angular distributions of the α-transitions to the ground, first and second excited states in ¹⁴N were measured for bombarding energies from 3.2 to 8.0 MeV. The isospin-forbidden cross section for the excitation of the lowest T = 1 state in ¹⁴N at 2.31 MeV was found to lie between 1–2 % of that of the allowed transitions. A partial wave analysis of the α₁ angular distribution data revealed a strong resonance with J^π = 2⁺ at E_x = 15.99 MeV. Arguments are presented which tentatively identify this resonance as being due to two close-lying 2⁺ levels with different isospin.     

Isospin dependence of nuclear radius

Isospin dependence of proton density radius is proposed. The fit to the experimental isotopic shifts of the mean square charge radius of the even-even nuclei with 38Z78 shows that the radius is less dependent on the neutron number than is usually assumed. Uniform density distribution was assumed and the ground state deformations were taken from the microscopic calculations.This work is partly supported by the Polish Committee of Scientific Research under contract No. 203119101 and the French Ministry of Scientific Research and Education     

Neutron-fission competition near threshold: The influence of shells and pairing on the decay of the 241Cm compound nucleus

The excitation functions of the reactions ²³⁸Pu(α, 2n) and ²⁴¹Am(p, 2n), both leading to ²⁴⁰Cm under fission competition, are measured between threshold and 8 MeV excess energy. The two identical excitation functions show a bump-like structure 5 MeV above threshold, which is impossible to explain by the use of statistical reaction theories with standard level density expressions. A compound reaction calculation based on shell dependent level densities for the neutron and fission channels, respectively, with the inclusion of pairing leads to a detailed understanding of the observed structure.     

Deuteron break-up in the field of the nucleus

The break-up of deuterons in Coulomb and nuclear fields is calculated in the framework of a distorted-wave Born approximation (DWBA). The matrix element in the post-interaction formulation is evaluated using a partial-wave decomposition. The results are in very good agreement with recent experiments, but only if nuclear forces are included. In addition, other theoretical models for the disintegration of deuterons are reviewed critically.     

Spectroscopy of low-lying positive parity states in 30P from the study of the reaction (3He,d) at 14.0 MeV

The reaction ²⁹Si(³He, d)³⁰P was studied at an incident energy of 14.0 MeV. Spectroscopic strengths for 14 positive parity states up to an excitation of 4.50 MeV have been obtained using DWBA analysis. The incident channel optical-model parameters for the DWBA calculations were extracted from elastic scattering cross sections measured also at 14.0 MeV.     

Isospin dependence of the nuclear binding energy

In this paper,we study the symmetry energy and the Wigner energy in the binding energy formula for atomic nuclei.We simultaneously extract the I2 symmetry energy and Wigner energy coefficients using the double difference of "experimental" symmetry-Wigner energies,based on the binding energy data of nuclei with A≥16.Our study of the triple difference formula and the "experimental" symmetry-Wigner energy suggests that the macroscopic isospin dependence of binding energies is explained well by the I2 symmetry energy and the Wigner energy,and further consideration of the I4 term in the binding energy formula does not substantially improve the calculation result.     

Direct and compound nuclear contributions to the 13C(6Li,t) reaction at Elab = 25 MeV

The reaction ¹³C(⁶Li, t)¹⁶O has been studied in the incident energy range 24–26 MeV. Complete angular distributions have been measured at E_⁶Li, = 25 MeV in the angular range θ_lab = 8°–172°, with the reaction ⁶Li(¹³C, t)⁶O being used for the backward angle measurements. Cross sections for evaporation residues from the fusion of the ⁶Li + ¹³C system have been measured in the incident ⁶Li energy range 9.2–35.1 MeV. Compound nuclear contributions to the transfer cross sections have been calculated using the Hauser-Feshbach statistical theory with the assumption that the compound-nucleus formation cross section is equal to the measured fusion cross section. By comparison of the compound nuclear calculations with backward angle data it is found that the sharp cutoff approximation commonly used to represent the initial angular momentum distribution of the compound nucleus is not adequate for the ¹³C(⁶Li, t)¹⁶O reaction. Good fits to the backward angle data can be obtained by using a smooth cutoff approximation. The forward angle cross sections have been compared with exact finite-range distorted-wave Born approximation calculations to extract transferred angular momenta and spectroscopic strengths. The present results differ from those of an earlier study. These differences are due to the inclusion of forward angle data in the present study.     

The compound nucleus reaction 12C(11B, 2α) 15N

Energy spectra and differential cross sections of nitrogen products formed in the reaction 28 MeV ¹¹B + ¹²C have been measured using a ΔE?E counter telescope. The energy spectra are smooth and therefore indicate that the nitrogen products were formed by a compound nucleus mechanism, via the formation and decay of the compound nucleus ²³Na. The experimental results are compared with statistical model calculations and good agreement is obtained. This result provides further evidence for the importance of the compound nucleus mechanism in heavy ion reactions with light nuclei and also gives added validity to the statistical model for light compound systems.     

Isospin dependence of nuclear multifragmentation in statistical model

The evolution of nuclear disintegration mechanisms with increasing excitation energy, from com- pound nucleus to multifragmentation, has been studied by using the Statistical Multifragmentation Model (SMM) within a micro-canonical ensemble. We discuss the observable characteristics as functions of excitation energy in multifragmentation, concentrating on the isospin dependence of the model in its decaying mechanism and break-up fragment configuration by comparing the A<,0> = 200, Z<,0> = 78 and A<,0> = 200, Z<,0> = 100 systems. The calculations indicate that the neutron-rich system (Z<,0> = 78) translates to a fission-like process from evaporation later than the symmetric nucleus at a lower excitation energy, but gets a larger average multiplicity as the excitation energy increases above 1.0 MeV/u.