Multiparameter study of 1H, 3H and 4He in fast neutron induced fission of 235U

The emission probabilities per fission of α-particles, tritons and protons have been measured in fast neutron induced fission of ²³⁵U. The measurements were carried out at neutron energies of 120, 180, 230 and 550 keV. AΔE-E semiconductor detector telescope was used to identify different light charged particles and the fission fragments were detected with an ionization chamber. The three-parameter data corresponding to the pulse heights from the ΔE-E detectors and the ion-chamber were recorded event by event on magnetic tape and were analyzed off-line by computer. No significant variation in the most probable energy ($\text{E}$) and the standard deviation (σ_E) of the energy spectra of different light charged particles with incident neutron energy was observed, although $\text{E}{}_{\mathrm{\alpha }}$ was seen to have a slightly higher value beyond E_n = 230 keV. The yield of α-particles in fission induced by neutrons of E_n ～ 200 keV was found to be higher by about 20 % than that in thermal neutron induced fission. The yields of tritons and protons were found to increase significantly with neutron energy.     

Quasi-fission and other strongly damped collisions between 63Cu ions and 197Au nuclei

The interaction of ⁶³Cu ions with ¹⁹⁷Au nuclei have been studied experimentally at incident energies of 365 and 443 MeV (1.1 and 1.4 times the Coulomb barrier). Mass and kinetic energy distributions of reaction products have been measured at several angles. Near the grazing angle, a continuous transition was found from elastic events to partially damped (PD) events, and to fully damped events (quasi-fission, QF). Away from the grazing angle a clean separation between elastic and QF events was observed. Events that may be due to fission following fusion (CF) were also obtained. Results are discussed in terms of decomposition into PD, QF, and CF components. The QF kinetic energy is independent of the incident energy (implying full damping of the initial relative motion). It is lower than the Coulomb barrier and close to the kinetic energies from the fission of similar systems. The angular distribution is peaked somewhat forward of the grazing angle for low mass transfers. For large mass transfers the yield increases slowly with decreasing angle. At 443 MeV a large contribution from negative angles is present. σ_QF accounts for more than 65 % of the reaction cross section σ_R at 443 MeV and for more than 50 % at 365 MeV. The upper limit on CF is about 10 % of σ_R, and σ_PDis of the order of 25 % of σ_R.     

Doubly radiative neutron capture by 2H, 3He, 16O and 208Pb

Cross-sections for doubly radiative thermal neutron capture on ²H, ³He, ¹⁶O and²⁰⁸Pb are calculated to be 21, 1200, 41 and 50 nb, corresponding to branching ratios of σ(n, γγ)/σ(n, γ) = 4.0 × 10^?5, ～ × 10^?2, 2.2 × 10^?4 and 1.0 × 10^?4, respectively.     

Fission of 232Th, 238U and 235U induced by negative muons

The absolute yields of prompt and delayed fission induced by negative muons in ²³²Th, ²³⁸U and ²³⁵U have been measured. The delayed fission yields are much lower than could be predicted from Γ_n/Γ_f systematics for 15–20 MeV nuclear excitation. The systematics of prompt fission yields are compared with recently obtained photofission data. It is suggested that prompt fission can be used for investigating the channel structure of the fission barrier.     

High resolution 13C NMR in undoped cis- and trans-polyacetylene

We have applied ¹³C cross-polarization (CP) powder- and MAS-NMR to cis- and trans-polyacetylene. All three elements of the chemical shift tensor (σ₁₁, σ₂₂, σ₃₃) were determined in p.p.m. with respect to TMS as: (a) Cis; σ₁₁ = ?228, σ₂₂ = ?139, σ₃₃ = ?17; (b) Trans; σ₁₁ = ?234, σ₂₂ = ?146, σ₃₃ = ?34.     

Measurements of evaporation residues and fission for 40Ar + 110Pd

Fusion cross sections for the reaction ⁴⁰Ar + ¹¹⁰Pd have been measured in the bombarding energy range 164–262 MeV. Evaporation residues and fission fragments have been detected by ΔE-E telescopes. We compare the fusion cross sections to various theoretical models with special attention to the high-energy data. An analysis is also given for entrance-channel spin zones for evaporation residues and fission.     

Angular momentum transfer in 12C-, 20Ne- and 40Ar-induced fission

Angular momentum transfer in a variety of ¹²C-, ²⁰Ne- and ⁴⁰Ar-induced fission reactions has been investigated using γ-ray multiplicity techniques. Fission fragments were detected in coincidence using a pair of solid-state detectors. The fragment masses were deduced from the kinetic energies and emission angles using two-body kinematics. The γ-ray multiplicities (M_γ) of the fission fragments were measured utilizing an array of eight NaI detectors. For most of the systems studied, M_γ is nearly independent of the exit-channel mass asymmetry. The strongest dependence on mass is observed in the systems ¹⁵⁴sm + 240 MeV ⁴⁰Ar, where a minimum exists at symmetry, and ¹⁹⁷Au + 164 MeV ²⁰Ne, where nuclear structure effects are suggested by the data. For all the reactions the quantity M_γ tends to decrease gradually with increasing fragment kinetic energy. The magnitude of M_γ generally appears to be larger than expected on the basis of rigid rotation, suggesting a spin enhancement effect. The data are compared with a simple model which assumes the statistical excitation of a variety of angular momentum bearing collective modes. Reasonable agreement is obtained with the experimental results. The roles of other collective effects, such as shape fluctuations and angular momentum fractionation, are also considered.     

Doubly radiative thermal neutron capture in 2H and 16O: Experiment and theory

Measurements and calculations are presented for the cross sections for two-photon emission following thermal neutron capture in ²H and ¹⁶O. Upper limits for σ_2γ were measured in both cases. For ²H, σ_2λ = 8 ± 15 μb, for γ-rays in the energy region 700 < E_γ < 5550 keV. A detailed threeparticle calculation gives σ_2γ^tot = 26 nb. For ¹⁶O, the experimental result is σ_2γ = 3 ± 19 μb for 1200 < E_γ < 2943 keV. A single-particle, direct-capture calculation for ¹⁶O gives σ_2γ^tot = 41 nb. Contributions from excitation of the giant dipole state of the core change this result by ± 16 %. In a separate measurement the total cross section for ¹⁶O(n, γ)¹⁷O was measured to be 202 ± 28 μb. Branching ratios of (82 ± 3)% and (18 ± 3)% were determined for decays to the 3055 and 871 keV levels of ¹⁷O, respectively.     

On the fusion cross section for 16O + 12C

The total fusion cross section σ_f for ¹⁶O + ¹²c.m. has been measured in 250 keV steps between 17 and 28 MeV c.m. energy via γ-ray techniques. Individual yields for 10 nuclides in the evaporation chain were determined. The oscillations in σ_f, which agree in gross structure but not in detail with previous work, appear to be most closely associated with reactions leading to ²⁰Ne. It is observed that the reaction yield is >80% dominated by nuclides having at least one α particle in the evaporation chain.     

1+ state and effective g-factor

The effects of nuclear field r²(Y₂σ₁)_1υ on magnetic properties of single-particle states in odd-A nuclei (^208±1Pb) are considered. The coupling constant associated with this type of field is estimated by an argument that realtes it to the coupling constant for the field (γ₀σ₁)_1υ. The effects of including the r²(Y₂σ₁)_1υ field on the M1 moments and transitions are estimated.     

16O, 24, 26Mg, 28Si, 32S, 40Ca(α, 12C) and multi-α-cluster transfer reactions

The (α, ¹²C) reaction has been studied on a variety of nuclei, A = 16 to 40, at E_α = 90.3 MeV. The data indicate a rapid fall-off of cross sections with increasing target mass, approximately as A_t^{?5 ± 1}. This and other systematics are used to estimate cross sections for multi-α-cluster transfer reactions in heavy nuclei and suggest σ_T < 10^?34 cm² consistent with present experimental limits. The data for ²⁴Mg(α, ¹²C)¹⁶O has been studied in more detail and indicates a selective population of final states including ¹⁶O g.s., with oscillatory angular distributions in some instances. Finite-range distorted-wave Born approximation calculations for direct ⁸Be pickup have been performed utilizing cluster overlap amplitudes obtained with zero-order SU(3) wave functions. The calculations are in qualitative, and often quantitative, agreement with shapes and absolute magnitudes of the measured angular distributions although the cross sections for certain α-cluster states (2⁺, E_x ≈ 7 MeV; 4⁺, E_x ≈ 10.3 MeV) are greatly overestimated with this model. Other more complicated mechanisms, such as successive α-transfer, cannot be excluded. The systematics of the calculated ⁸Be cluster overlaps and the calculated and measured (α, ¹²C) cross sections are investigated, and implications for multi-α-cluster transfer reactions are discussed.     

Time-of-flight measurement of the evaporation residues from fusion of 5.9 mev/u 84kr and 27al

Mass and Z-distributions of the evaporation residues from compound-nucleus formation in the reaction 5.9 MeV/u ⁸⁴Kr on ²⁷Al were measured using a time-of-flight ΔE ? E telescope, which is described in detail. The high recoil velocity attained by choosing the heavy reaction partner as projectile made it possible to resolve the reaction products by mass and atomic number. Data were taken in the angular range from 1.5 to 6°. The residue distributions are compared to evaporation calculations assuming the statistical decay by fission and particle evaporation of the compound nucleus ¹¹¹Mn formed at an excitation energy of 108 MeV with angular momenta up to $\text{L}{}_{\mathrm{<mtext>CN</mtext>}}\text{≈ 69}\text{h}\text{?}$. The data are consistent with the assumption of statistical equilibrium. Details of the de-excitation process, in particular the fission competition and the influence of nuclear deformations at high angular momenta, are discussed.     

The energy dependence of fusion in the 9Be+28Si system

The angular distributions of the energy spectra of the light charged particles (p, d and α) from the ⁹Be + ²⁸Si reaction have been measured in the energy range 12 ≦ E_lab ≦ 30 MeV. The particle evaporation spectra and the angular distributions were analyzed with a spin dependent statistical model. Angular distributions of ⁹Be ions elastically scattered on ²⁸Si have been measured at the energies 12 MeV, 17 MeV, 23 MeV and 30 MeV and were analysed, together with previously measured cross sections, with the optical model. The fusion cut-off angular momentum l_fus(E), the fusion cross section σ_fus(E) and the ratio σ_fus/σ_R^OM(E) were deduced. The excitation function for fusion was analyzed with the Glas and Mosel model. The parameters obtained from the fusion excitation function were compared with the corresponding ones from the ⁹Be + ²⁸Si optical-model interaction potential.     

The (6Li, 6He) spin-isospin-flip reaction

A formulation of the (⁶Li, ⁶He) spin-isospin-flip scattering problem is presented. The method is applicable to quasielastic collisions between⁶Li described in pureLS coupling scheme and any target nucleus which can be described in a shell model or an adiabatic deformed-nucleus model picture. The effective interaction responsible for the (⁶Li, ⁶He) reaction involves an integration of the (σ₁ · σ₂) (ρ₁ · ρ₂) compo nent of the central part and the (τ₁· τ₂) tensor part of the nucleon-nucleon interaction over the internal wave function of the projectile. The ⁴⁸Ca(⁶Li,⁶He)⁴⁸Sc reaction populating the ($\text{πf}{}_{\mathrm{<mtext>7</mtext><mtext>2</mtext>}}\text{νf}{}_{\mathrm{<mtext>7</mtext><mtext>2</mtext>}}{}^{\mathrm{?1}}$) multiplet in ⁴⁸Sc is analysed. It that the tensor force makes some contribution to the transitions to high spin states.     

13C NMR investigation of adsorbed hydrocarbons

By means of Fourier transform techniques ¹³C nmr spectra and longitudinal ¹³C magnetic relaxation times of various butene isomers sorbed on NaY and NaX zeolites have been studied. In contrast to ¹H resonance where only two broad lines can be observed, the ¹³C spectra show separated sharp lines. The shift Δσ = σ_ads-σ_free of the carbon-13 lines of adsorbed molecules with respect to the resonance positions of free molecules and the longitudinal relaxation times indicate a peculiar behaviour of the groups  CH and = C

of the adsorbed n-butene and isobutene molecules, respectively. The resonance deviations Δσ to lower fields in these groups decrease in the order isobutene (δσ = ? 10.3 ppm), but-1-ene (δσ ≈ ?5.7 ppm) and but-2-enes (Δσ ≈ ? 2.2–2.4 ppm), whereby the spectra of cis-but-2-ene and trans-but-2-ene show similar differences as found in the liquid state. When adsorbed on NaY and NaX types (silica/alumina ratios 5 and 3.5) the ¹³C nmr line positions of but-1-ene molecules are the same within the limits of experimental error, which demonstrates that electrostatic fields have no noticeable influence on the molecular parameters in these systems. Therefore one may conclude that chemical bonds to Na⁺ cations have the dominant effect on the line shift Δσ. CNDO-MO calculations based on such a model have shown that a charge transfer occurs from the π-electrons and the electrons of the methyl group hydrogen atoms to the cation.     

Fission of medium mass elements induced by 126 MeV 14N ions

Ni, Se, Mo, Ag, Ho and Au targets were bombarded by 126 MeV ¹⁴N ions in order to study compound nucleus fission. The large angular momentum brought in by the projectile implies much greater fission probabilities than those obtained when the same compound nuclei are created with light projectiles. We employed two surface-barrier detectors. The results obtained are fission cross sections, angular correlations and kinetic energy and mass distributions. Fissilities are found to decrease exponentially with the ratio Z²/A of the fissioning nucleus until Z²/A = 19 (molybdenum) and then to increase again for lower Z²/A. The calculations of fissilities performed with the fission barrier of Myers and Swiatecki, and including angular momentum effects in evaporation-fission competition, gives the same variation of fissilities as the experimental one, but the results obtained are lower than the experimental values. The total kinetic energies measured are higher than the predictions of the liquid drop model by around 10 MeV (Nix). The shapes of the mass distributions indicate that the value of x_BG (Businaro-Gallone point) is lower than 0.4 (silver).     

The β− decay of 102Mo

The ¹⁰²Mo activity was obtained by photofission of natural U and thermal-neutron induced fission of ²³⁵U, with subsequent chemical separation of the molybdenum fraction. In addition, nuclei with mass 102 were separated from fission products of ²³⁵U(n_th, f) using the mass separator LOHENGRIN. A decay scheme and absolute γ-intensities are deduced from measured γ-ray, X-ray and γ-coincidence spectra. Logftvalues are calculated. Shell-model calculations for a $\text{(πlg}{}_{\mathrm{<mtext>9</mtext><mtext>2</mtext>}}\text{)}{}^3$$\text{(νlg}{}_{\mathrm{<mtext>7</mtext><mtext>2</mtext>}}\text{)}{}^3$ multiplet have been carried out using effective nucleon-nucleon interactions. The γ-decay between low-lying, low-spin members of this multiplet was studied in detail and compared with the experimental data.     

Fragment angular distributions in electron-induced fission of 232Th

Fission fragment angular distributions have been observed in electron-induced fission of ²³²Th for electron energies 8.7 MeV ≦ E_e ≦ 30 MeV. For low energies (though above the fission threshold) the angular distributions contain both a dipole and a quadrupole component. The (90°/0°) anisotropy decreases rapidly for higher electron energies but reveals smaller peaks after the onset of second-, third- and fourth-chance fission suggesting that the effective fission barriers for ²³¹Th and ²²⁹Th in second- and fourth-chance fission, respectively, are both characterized by $\text{K =}\text{1}\text{2}$.     

Photoinduced fission of the doubly even uranium isotopes 234U, 236U and 238U

A measurement of the angular distributions and yields of fission fragments in the photofission of ²³⁴U has been performed between 5.2 and 6.4 MeV. As γ-source, the bremsstrahlung from a microtron was used. For the detection of the fission fragments, solid-state track detectors were used. The present data for ²³⁴U have been analysed together with earlier obtained data for ²³⁶U and ²³⁸U. The values of the fission barrier parameters obtained are compared to results in theoretical macroscopic and microscopic fission potential energy calculations.     

Correlation between fragment mass-distribution fine structure,charge distribution and nuclear structure for thermal-neutron-induced fission of 233U and 235U

Masses corresponding to observed fine-structure peaks in the fragment mass distributions for thermal-neutron-induced fission of ²³³U and ²³⁵U are shown to correspond to average measured masses for even-even nuclear charge splits. Evidence is presented that the yield enhancement for even-Z fragments is not restricted just to fission events with higher-than-average total kinetic energy. The anomalously high yield of fragments with mass 134 in ²³⁵U(n, f) as opposed to ²³³(n, f) is tentatively correlated with rapidly changing nuclear structure properties as a function of the mass of the complementary light (Z = 40) fragments.