Study of the reaction238U (α, α′f) atE α=480 MeV

The excitation function of the fission probability P _E E _x) for²³⁸U has been measured in the reaction²³⁸U(α, α′ f) at 480 MeV bombarding energy. The reaction mechanism of this reaction is discussed for excitation energies belowB _nf , the threshold for second chance fission, and aboveB _nf up toE _x =37 MeV. In comparing with results from fission induced by photons and by particle transfer reactions the (α, α′f) reaction gives too low values for the fission probabilityP _f at excitation energies well aboveB _nE . The role of the quasi-elastic knock-out process in this reaction is discussed.     

The probability of prompt and delayed fission of muonic237Np

Fission fragments from the reaction²³⁷Np(μ ^?,γ,f) have been measured in coincidence with muonic X-rays. The efficiency of the fission fragment detector is determined from (μ ^?,γ,f)-data of the same experiment. The total fission probability perμ-stopP _t has been measured as well as the fission probabilities P_f of the non-radiative muonic (3d→1s)- and (2p→1s)-transitions; the latter has been divided into two parts leading to different mean excitation energiesE:P _t =(54±17)%,P _f (3d→1s)=(41±21)%,P _f (2p→1s,E=6.218 MeV)=(61±19)%, andP _f (2p→1s,E=6.525 MeV)=(57±18)%. The influence of the muon on the fission barrier is discussed. The fission probability after muon capture is compared with a calculated value using a distribution of nuclear excitation energies following muon capture and the fission probability as measured in a²³⁸U(³He,αf)-reaction.     

Nuclear dissipation,fission probability and neutron multiplicity prior to fission

We discuss the effects of nuclear dissipation on fission probabilities that are characteristic of a diffusion model of the fission process. Reproducing the experimental fission probabilities at low excitation energies fixes the ratioa _f/a_n of the level density parameters for a given strength of the reduced dissipation coefficientβ. These low energy constraints ona _f/a_n andβ balance the effects of transients on neutron multiplicities prior to fission at higher excitation energies. For the competitive decay of¹⁵⁸Er formed in the reaction¹⁶O+¹⁴²Nd at 207 MeV we show that dueto transients only the multiplicity of pre-fission neutrons is enhanced with respect to the prediction of the statistical model in a manner consistent with our earlier general analysis.     

A kinetic study of the thermal decomposition process of potassium metabisulfite: Estimation of distributed reactivity model

The thermal decomposition kinetics of potassium metabisulfite was studied by thermogravimetric (TG) and differential thermogravimetric (DTG) techniques using non-isothermal experiments. The apparent activation energy (E_a) is determined using the differential (Friedman) isoconversional method. The results of the Friedman's isoconversional analysis of the TG data suggests that the investigated decomposition process follows a single-step reaction and the observed apparent activation energy was determined as 122.4±2.1 kJ mol⁻¹. A kinetic rate equation was derived for the decomposition process of potassium metabisulfite with contracting area model, f(α)=2(1−α)^1/2, which is established using the Malek's kinetic procedure. The value of pre-exponential factor (A) is also evaluated and was found to be A=1.37×10¹² min⁻¹. By applying the Miura's procedure the distributed reactivity model (DRM) for investigated decomposition process was established. From the dependence α versus E_a, the experimental distribution curve of apparent activation energies, f(E_a), was estimated. By applying the non-linear least-squares analysis, it was found that the Gaussian distribution model (with distribution parameters E₀=121.3 kJ mol⁻¹ and σ=1.5 kJ mol⁻¹) represents the best reactivity model for describing the investigated process. Using the Miura's method, the A values were estimated at five different heating rates and the average A values are plotted against E_a. The linear relationship between the A and E_a values was established (compensation effect). Also, it was concluded that the E_a values calculated by the Friedman's method and estimated distribution curve, f(E_a), are correct even in the case when the investigated decomposition process occurs through the single-step reaction mechanism.     

Prompt mass distributions in fission

A previously developed structure-sensitive statistical model, incorporating the nuclear structure and deformation effects, has been utilised to study the prompt mass distributions in fision and other related characteristics. The principle of detailed balance has been applied to calculate the yield probabilities. The fusion barriers and the penetrabilities of the fragments were estimated from a nuclear plus Coulomb interaction potential. The prompt fragment mass yields were then obtained as a product of three quantities: the product level densities of the conjugates at their most probable excitation energies, the barrier penetrabilities and the isobaric charge yields. The calculations are free from any arbitrary parameters. Comparison with experimental data for²²⁶Ra(p, f) atE _p =11.1MeV,²³²Th(n, f) atE _n =1.0 MeV,²³⁵U(n, f) and²³⁹Pu(n, f) at thermal neutron energies and for the spontaneous fission of²⁵²Cf shows fairly good agreement.     

Collinear cluster tripartition channel in the reaction 235U(n th, f)

Investigation of the ²³⁵U(n _th, f) reaction using the miniFOBOS double-arm time-of-flight spectrometer of fission fragments confirmed manifestations of the earlier unknown many-body, at least ternary, decay involving almost collinear decay-product escape, which were first observed in the spontaneous fission of ²⁵²Cf(sf). The use of variables sensitive to the nuclear charge of fission fragments allowed the reliability of identification of decay events to be increased and new decay modes to be revealed.     

Observation of new nuclides37Si,40P,41S,42S produced in deeply inelastic reactions induced by40Ar on238U

A non-relativistic quantum-mechanical system is studied which consists ofN identical bosons interacting by pair potentials of the form 〈r¦V¦r ¹〉=?π/2ν ₀ a ^?3 f(r/a)f ^*(r ¹/a). General upper and lower bounds to the ground-state energyE _N are provided for alla, V ₀ andN, and detailed results are given in the case of the Yamaguchi potential for whichf(x)=e ^?x/x. It is shown that the ratioE _N /E ₂ diverges both under the limit (i) a↓0,E ₂ =arbitrary constant <0, and (ii) (V ₀ a ²)↓(V ₀ a ²)_c, where (V ₀ a ²) _c corresponds toE ₂=0. The results complement recent studies of the Efimov effect via scattering theory.     

Prompt neutron emission spectra and multiplicities in the thermal neutron induced fission of235U

The emission spectra of prompt fission neutrons from mass and kinetic energy selected fission fragments have been measured in²³⁵U(n _th,f). Neutron energies were determined from the measurement of the neutron time of flight using a NE213 scintillation detector. The fragment energies were measured by a pair of surface barrier detectors in one set of measurements and by a back-to-back gridded ionization chamber in the second set of measurements. The data were analysed event by event to deduce neutron energy in the rest frame of the emitting fragment for the determination of neutron emission spectra and multiplicities as a function of the fragment mass and total kinetic energy. The results are compared with statistical model calculations using shell and excitation energy dependent level density formulations to deduce the level density parameters of the neutron rich fragment nuclei over a large range of fragment masses.     

Mass dependence of statistical fission parameters

Statistical fission parameters B_f and $\text{a}{}_{\mathrm{<mtext>f</mtext>}}\text{a}{}_{\mathrm{\nu }}$ are extracted from analyses of recently measured heavy-ion induced fission and evaporation residue excitation functions for medium mass systems using a formalism with spin dependent level densities and multiple particle/fission competition. Results are compared with those of the less rigorous s-wave approximation as treated in the code ALICE.     

Effect of the initial excitation energy on the average fission lifetime of nuclei

The dependence of the fission time on the initial nuclear excitation energy E _{tot 0} ^* is studied on the basis of a refined combined dynamical and statistical model. It is shown that this dependence may be nonmonotonic, in which case it features a broad maximum. It turns out that the form of the average fission time 〈t _f 〉 as a function of E _{tot 0} ^* depends greatly on the orbital angular momentum L _n carried away by prescission neutrons.     

Fission following complete or incomplete fusion of 135 MeV16O and238U

From measurements of the laboratory angular distribution and the projected range distribution for each of 26 individual radioactive fission products from the interaction of 135 MeV¹⁶O on²³⁸U, we have determined for each product quantities including the average velocity,ν, of the parent fissioning system and the anisotropy,ω, of the fission angular distribution. We have interpreted the results in terms of two competing mechanisms: complete fusion and a process involving transfer of anα-particle from projectile to target followed by fission of²⁴²Pu. The values ofν lie betweenν _cn, the complete fusion value, and 0.3ν _cn, and can be taken as a measure of the relative importance of the two processes competing in populating each observed product. These two contributions to each measured product yield can then be separated, and understood using a simple model of the fission process. The data suggest that approximately 24% of the overall fission yield is due to incomplete fusion processes. The anisotropy,ω, is strongly correlated with the value ofν, indicating anisotropies of 1.4 and 2.8 for the two individual processes.     

High spin shell model excitations in149Gd

The high spin level structure of the three-neutron nucleus¹⁴⁹Gd has been investigated by in-beamγ-ray and electron spectroscopy with (α, xn) reactions. The observed levels are characterized as members of the shell model multipletsνf ₇ ^2/3 ,νh _9/2 f ₇ ^2/2 ,νf ₇ ^2/3 ×3^?,νh _9/2 f ₇ ^2/2 ×3^?, and tentativelyνf ₇ ^2/3 ×(3^?)². The energies of theν f ₇ ^2/3 states agree only moderately with those calculated using empirical two-nucleon interactions taken from¹⁴⁸Gd, which indicates the importance of long range contributions already atN=85.     

Effect of entrance channel parameters on fission fragment angular momentum in medium energy fission

Independent isomeric yield ratios of¹³²I were radiochemically determined in alpha particle induced fission of²³⁸U in the energy range 25–44 MeV. Fission fragment angular momenta were deduced from the measured isomeric yield ratios using spin dependent statistical model analysis. It was seen that angular momentum of¹³²I increases with increase of excitation energy and angular momentum of the fissioning nucleus. Comparison of the present data on¹³²I in²³⁸U(α,f) with the literature data for the same product in²³⁸U(p, f) and²³⁸U(γ, f) at various excitation energies show that fragment angular momentum strongly depends on the input angular momentum in the range of excitation energy considered. Experimental fragment angular momentum at all excitation energies were seen to be in agreement with the theoretical values calculated based on thermal equilibration of the various collective rotational degrees after considering the occurence of multichance fission. Thus, strong effect of input angular momentum as well as the statistical equilibration among the various collective rotational degrees of freedom in medium energy fission is corroborated.     

Shell model and octupole states in148Gd from in-beam experiments

Through (α, 4n) and (τ, 3n) reactions the high-spin states in the two-neutron nucleus¹⁴⁸Gd were populated up toI ^π=21? at 7.2 MeV, including numerous states above the yrast line. The¹⁴⁸Gd energy spectrum is interpreted in terms of the spherical shell model. Identification of the (νf _7/2 i _{1 3/2})_10? state gives the νi _13/2 single particle energy free of octupole admixtures as 2.1(1) MeV. Eight high-spin states between 1.2 and 3.7 MeV were identified as the couplings of the two valence-particles to the¹⁴⁶Gd octupole phonon, and three above-lying levels are assigned as double-octupole excitations including a 12⁺ state which decays by anE3-E3 stretched cascade. All these octupole levels can be quantitatively predicted from the one-particle x phonon spectrum of¹⁴⁷Gd. The high-spin states above 3 MeV are four-quasiparticle excitations ofπ ⁺¹ π ^?1 ν ² andπ ² ν ² type and their energies are in good accord with shell model estimates.     

High resolution analysis of the ν12 and ν17 fundamental bands of acrolein, CH2CHCHO, in the 600 cm region

Synchrotron radiation from the new Canadian Light Source facility has been used to obtain a high resolution (0.0012 cm⁻¹) absorption spectrum of acrolein vapor in the 550-660 cm⁻¹ region. Almost 2000 transitions have been included in a detailed analysis of the ν₁₂ (∼564 cm⁻¹) and ν₁₇ (∼593 cm⁻¹) fundamental bands which yielded precise values for the band origins, rotational and centrifugal distortion parameters. The analysis included the a- and b-type Coriolis interactions connecting ν₁₂ and ν₁₇, as well as an a-type Coriolis interaction between ν₁₇ and a “dark” perturbing state, identified as 4ν₁₈. We believe that this is the first high resolution infrared study of acrolein.     

Puzzling features of heavy-ion fission at sub-barrier energies

The heavy-ion induced fission fragment angular distributions measured for systems with Th, U and Np as targets have revealed “anomalous” values of anisotropies at energiesE≤V _B (fusion barrier) and this feature is observed to be independent of the entrance channel mass-asymmetry. While this puzzling feature is exhibited by the deformed targets like Th, U and Np, most of the fission data measured for the spherical targets like Pb and Bi can be satisfactorily explained using the standard saddle point statistical model with moderate correction for pre-fission neutron emission. Plausible reasons for this anomalous behaviour are explored.     

Cross section for the subthreshold fission of 236U

The cross section for ²³⁶U fission in the neutron-energy range E _n = 0.001–20 keV was measured by using the INR RAS (Institute of Nuclear Research, Russian Academy of Sciences, Moscow) LSDS-100 neutron spectrometer of the lead slowing-down spectrometer type. The resonance fission areas of the resonances at 5.45 eV and 1.28 keV were found, and the fission widths of these resonances were evaluated. The cross section for the ²³⁸U(n, f) fission process was measured, and the threshold sensitivity of the LSDS-100 to small values of fission cross sections was estimated. The well-known intermediate structure in the cross section for the neutron-induced subbarrier fission of ²³⁶U was confirmed.     

A model for electrical conductivity in (TMTSF)2ClO4, (TMTSF)2ReO4 and related materials

To describe quantitatively the diverse variations of the temperature-dependent electrical conductivities of tetramethyltetraselenafulvenium (TMTSF) salts, a model based on small Fermi (E_f) and small gap (E_g − E_f) energies and T⁻ⁿ dependence of scattering is presented. The difference between the conductivities of quenched and slowly cooled (TMTSF)₂ClO₄ is described by different values of E_f and E_g − E_f in the two cases. The model illustrates, in terms of variables E_f and E_g − E_f, the effect of pressure on the metal-insulator transition in (TMTSF)₂ReO₄, for example, and indicates a relation between E_f and E_g − E_f at the onset of superconductivity.     

Calculations of vacancy formation energies and divacancy binding energies for noble metals using pseudopotential formalism

Vacancy formation energies (E^f_1v) and divacancy binding energies (E^B_2v) for noble metals have been calculated using the Moriarty's pseudopotential. While the calculated E^f_1v are coincident with the experimental values except Cu, E^B_2v agree with them for all the metals tolerably.