Light particle accompanied quasifission in superheavy composite systems

The dynamics of the excited superheavy system with Z = 118 in the reaction ⁸⁶Kr + ²⁰⁸Pb at E _Kr = 460, 500, and 600 MeV has been investigated. The mass and kinetic energy of binary fragments were measured by the time-of-flight method. Double-differential distributions of protons and α particles were measured in coincidence with fragments. The proton spectra can be described considering only evaporation from fragments. Evidence of the neck fragmentation was obtained from analysis of double-differential α spectra. Properties of the α-particle neck fragmentation component are close to those known from the ternary fission of actinide nuclei, but the multiplicity is much larger than can be expected from extrapolation of the ternary fission data. The text was submitted by the authors in English.     

Energy and angular momentum transfers in equilibrium and pre-equilibrium 158Gd(α, xn) reactions

Neutron time-of-flight spectra and γ-ray multiplicities were measured in coincidence with discrete γ-rays for specific exit channels in the ¹⁵⁸Gd(α, xn) reactions at E_α = 70 MeV. The neutron energy spectra and angular distributions were analysed in terms of equilibrium and non-equilibrium decay processes. Significant emission of non-equilibrated neutrons was found, amounting to about 40 %, 25 % and 15 % of the total neutron emission in the 4n, 5n and 6n exit channels, respectively. The corresponding average kinetic energies in this precompound phase were around 12, 8 and 4.5 MeV. The angular momentum carried away by the neutrons was found to be rather constant for all exit channels and on the average about 3? units. It is found that the total energy and angular momentum removed by the neutrons and γ-rays agrees within the experimental errors with the calculated values for the compound nucleus.     

Cross sections for 4He interaction with protons at 5 GeV/c

The total and topological ⁴Hep cross sections, as well as the cross sections for individual αpinteraction channels and the differential cross sections dσ/dt for elastic αp scattering, were measured by using a 2-m hydrogen bubble chamber exposed to a separated beam of 5-GeV/c α particles from the ITEP synchrotron (the kinetic energy of initial protons in the nuclear rest frame was T _p=620 MeV). The data obtained are compared with the results of previous experiments and with theoretical predictions based on Glauber-Sitenko multiple-scattering theory.     

Neutron-emission mechanisms in the <Superscript>115</Superscript>In(<Emphasis Type="Italic">α, xn</Emphasis>) reactions

Spectra and angular distributions of neutrons from the ¹¹⁵In(α, xn) reactions were measured at α-particle energies of 16, 18, 27, and 45 MeV. The measurements were performed with time-of-flight neutron spectrometers at pulsed accelerators of charged particles. The data obtained in this way were analyzed within the models of equilibrium, preequilibrium, and direct reaction mechanisms. The exact formalism of Hauser-Feshbach statistical theory was used in the calculations, level densities in residual nuclei excited in the reactions under consideration being found from the neutron evaporation spectra in the (p, n) reactions on tin isotopes. Contributions from equilibrium, preequilibrium, and direct neutron emission were studied in a wide range of α-particle energies.     

Cumulative production of nucleons and extremely light nuclei in 4Hep interactions at an incident momentum of 5 GeV/c

The invariant cross sections for the cumulative production of protons, neutrons, and extremely light nuclei (d, ³H, and ³He) are determined on the basis of experimental data obtained by exposing the 2-m liquid-hydrogen bubble chamber of the Institute of Theoretical and Experimental Physics (ITEP, Moscow) to a 5-GeV/c α-particle beam, the kinetic energy of primary protons in the rest frame of the nucleus involved being T _p = 620 MeV. The results obtained in this way are compared with the predictions of the Frankfurt-Strikman model, which takes into account short-range few-nucleon correlations, and with the predictions of the nuclear coalescence model.     

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.     

Correlation between energies and angles of α-particles emitted in thermal-neutron fission of U

The energy distribution and yields of the α-particles emitted in the thermal-neutron fission of ²³⁵U were measured with the same detector system for the cases when the average angles between fragments and α-particles were 90°, 46°, 27° and 11°. The data were analysed by the Monte Carlo method to take into account the effects of the finite size of the source and the various detectors, and the following results were obtained: (i) Even at small angles with respect to the fission axis the yield of the α-particles is about 2–3 % of the yield at 90°, and does not go to zero as would be expected for a true Gaussian angular distribution of a variance which fits the data around 90°. (ii) At these angles where most of the α-particle yield belongs to the non-Gaussian component, the α-particle energy distribution has a significantly higher most probable energy but the FWHM of the energy distribution is not significantly different, (iii) The rms width σ_θ of the Gaussian angular distribution is found to increase for very high (E_α > 20 MeV) α-particle energies and also to a lesser extent for very low (E_α < 15 MeV) energies. The origin of the yield of the α-particles at small angles, and the dependence of the rms width σ_θ on the energy are discussed.     

Aspects of alpha-particle scattering and structure of the nuclear surface

The interaction of α particles above 5 MeV with a 2s-1d target is dominated by resonances. It cannot be described only in terms of a mean-field one-body potential. An analysis of the elastic α-particle scattering by ²⁸Si encourages the comprehension of the resonance states to be mainly fragments of a mixed-parity band. In the present article, the angular distributions of particles scattered by ³²S are analyzed in terms of such bands. The analysis of new data from an experiment made at Florida State University reveals the existence of states that do not belong to the above bands. This follows from a coupled-channel analysis of the elastic and inelastic (2⁺) cross sections. An α-particle structure at the nuclear surface is suggested.     

Analysis of fragment mass distribution in asymmetric area at fission of <Superscript>235</Superscript>U induced by thermal neutrons

The fragment mass yields in fission of ²³⁵U induced by thermal neutrons for A = 145–160 and E_K = 50–75 MeV were measured using a mass spectrometer. The fine structure is observed at A = 153, 154 and E_K = 50–60 MeV. The obtained results were described in the framework of a model based on the dinuclear system concept. The analyzed correlation between the total kinetic energy and mass distribution of fission fragments is connected with the shell structure of the formed fragments of fission. From this correlation and the time dependence of the calculated mass distribution of the binary reaction products, one can conclude that the descent time from a saddle point to a scission point for the more deformed fragments is longer than that for fragments of more compact shape.     

Mass and kinetic energy distribution in cold fission of233U,235U and239Pu induced by thermal neutrons

The time-of-flight technique was used to measure the mass and kinetic energy distribution of fragments from fission of²³³U,²³⁵U and²³⁹Pu, induced by thermal neutrons at the Grenoble High Flux Reactor. The data array is presented as equal probability lines in the high kinetic energy regions. The fluctuations observed in those experimental lines are explained by a static scission configuration model, in which the most important influence comes from the Coulomb interaction energy between the two fragments. The highest values of total kinetic energy are obtained for fragmentations with heavy fragmentsZ=50–52,N=80–82 and light fragmentsZ=40–42,N=60–64.     

The 56Fe(α, γ)60Ni reaction cross section

The yield of 1.332 MeV γ-rays from the reaction ⁵⁶Fe(α, γ)⁶⁰Ni has been measured as a function of α-particle bombarding energy in the range 4.2–7.1 MeV. This energy region includes the (α, n) threshold at E_α = 5.46 MeV. The energy dependence of this γ-ray cross section, as well as the (α, nγ) cross section, is compared with statistical model calculations. Good agreement is achieved between these data and calculations using global optical model parameters to determine the transmission functions. In particular, the pronounced threshold effect in the (α, γ) yield, at the opening of the (α, n) channel, is well represented.     

Reactions of 40Ar with 159Tb, 142Nd and 144Sm; New α-activities from 189Bi, 173Pt and 177Au

Reactions of ⁴⁰Ar ions with targets of ¹⁵⁹Tb, ¹⁴²Nd, and ¹⁴⁴Sm have been studied at energies below 300 MeV with a helium gas-jet system. Excitation functions for (Ar, xn) reactions, where x = 5–10, were obtained for the radioactive products that decay by α-emission. Based on the characteristics of these excitation functions and on the systematics of α-decay, evidence is presented for the existence of the nuclides ¹⁸⁹Bi with α-particle energy E_α = 6.67±0.01 MeV and half-life < 1.5 sec, and ¹⁷³Pt with E_α = 6.19±0.01 MeV; and for the emission from ¹⁷⁷Au of an α-particle with E_α = 6.15±0.01 MeV.     

Identification of the nuclide 226U

Identification of the uranium isotope ²²⁶U is reported. This nuclide was produced in bombardments of ²³²Th with 140 MeV ⁴He ions. It is observed to decay by α-particle emission with a half-life of 0.5±0.2 sec and has an α-particle energy of E_α = 7.43±0.03 MeV, giving a Q-value of Q_α = 7.56±0.03 MeV. Detection was achieved by on-line counting and pulsed-cyclotron beam techniques. It is also observed that the maximum yields for (α, p(x ? 1 )n) reactions are about ten times greater than those for (α, xn) reactions at these energies.     

Determination of angular momenta in the incomplete-fusion channels of the reactions Ta+22Ne and Ir+12C

By using the γ-ray-multiplicity technique, experiments were carried out to determine the input angular momenta lⁱⁿ corresponding to the heavy-ion induced incomplete-fusion reactions involving the emission of fast charged particles. The α-particle and Li-nuclei emission channels were investigated in the reaction ¹⁸¹Ta + ²²Ne at the average Ne-ion energy of 155 MeV and for the α-particle emission channel in the reaction ^natIr+¹²C at E = 100 MeV. The separation of the reaction channels was carried out using an X-ray spectrometer. The angular distributions of the fission fragments were measured in the reaction ¹⁸¹Ta(²²Ne, αf). The data obtained indicate that input angular momenta are equal to about 60 ? and 50 ? for the channels of Li emission and α-emission, respectively, being practically independent of the particle energy. For the α-emission channel the lⁱⁿ values are the same for both reactions within experimental errors. The lifetime of the system of interacting target and projectile nuclei, prior to the emission of an α-particle, has been estimated to be equal to 10^?20s.     

The 238U(n, α)235Th reaction with thermal neutrons

The thermal neutron induced (n, α) reaction cross section of ²³⁸U was measured using the highly pure thermal neutron beam from the 87 m curved neutron guide at the High Flux Reactor of the ILL (Grenoble). The energy spectrum showed an α-particle line with E_α = 9.05±0.06 MeV and σ(n, α) = 1.3±0.6 μb. The α-particle energy was used to calculate the ²³⁵Th mass of 235.04700±0.00008 amu, the Q_α value of 9.20±0.06 MeV for the ${}^{238}\text{U(n}\text{, α)}{}^{235}\text{Th}$ reaction and the Q_β value of 1.44±0.08 MeV for the β-decay of ²³⁵Th. The cross-section data are compared with the results obtained with the statistical model calculation.     

Alpha emission in coincidence with fission-like fragments from Cl+Ag reactions at 10 MeV/n

Energy and angular correlations between alpha particles and fission-like fragments in the reaction of³⁵Cl+Ag at 10 MeV/n have been measured. The coincident yield is dominated by two mechanisms: sequential emission from both fragments (M _α =0.137±0.014) and alpha emission from the composite system prior to scission (M _α =0.124±0.012). The observation of nuclear shadowing for sequential emission from the fragments allowed us to estimate the “lifetime” of the 218 MeV excited¹⁴³Gd composite system as (1–3)×10^?20s. It is argued that this rather long time reflects the slow evolution in shape of the composite system and might be termed “sticking time,” since statistical-model lifetimes are typically an order of magnitude shorter. Observed also are small contributions of nonequilibrium emission in the forward direction (M _α =0.005±0.001) and a component displaying the characteristics of emission from the neck (M _α =0.010±0.002).     

Comparison of the spontaneous fission of 244Cm and 252Cf: (I). Fragment masses and kinetic energies

The energy of both fission fragments of ²⁴⁴Cm and ²⁵²Cf, respectively, was measured in coincidence with the prompt neutrons (see part II). Energy calibration of the surface-barrier detectors was done after the method of Schmitt et al. with the ²⁵²Cf fragments. Mean values and rms widths of the mass and energy distributions of both isotopes are calculated and compared with the results of other authors. The total kinetic energy of ²⁴⁴Cm fragments is at least as high as that of ²⁵²Cf fragments.     

Energy loss of tens keV charged particles traveling in the hot dense carbon plasma

The energy loss of charged particles, including electrons, protons, and α-particles with tens keV initial energy E0, traveling in the hot dense carbon(C) plasma for densities from 2.281 to 22.81 g/cm3 and temperatures from 400 to 1500 eV is systematically and quantitatively studied by using the dimensional continuation method. The behaviors of different charged particles are readily distinguishable from each other. Firstly, because an ion is thousands times heavier than an electron, the penetration distance of the electron is much longer than that of proton and α-particle traveling in the plasma. Secondly, most energy of electron projectile with E0 100 keV deposits into the electron species of C plasma, while for the cases of proton and α-particle with E0 100 keV,about more than half energy transfers into the ion species of C plasma. A simple decreasing law of the penetration distance as a function of the plasma density is fitted, and different behaviors of each projectile particle can be clearly found from the fitted data.We believe that with the advanced progress of the present experimental technology, the findings shown here could be confirmed in ion-stopping experiments in the near future.     

Energy deposition and GDR emission in the reaction 209Bi(α,α′) at 240 MeV

Neutron fold distributions measured for the reaction ²⁰⁹Bi(α,α′) at 240 MeV have been analyzed with the help of Statistical Model calculations to determine the distribution of excitation energy, E_x, in the primary target fragments as a function of the projectile energy loss, EL. The reconstructed distributions in excitation energy feature a plateau which extends from the kinematical limit E_x = EL to very small excitations, indicating a variety of interactions of the beam particles with the target nucleus. The requirement of an additional coincidence with a light charged particle leads to the selection of a significant higher average excitation energy. Those results are extrapolated to explore the effects of including the excitation energy distributions in the analysis of previous GDR measurements in ²⁰⁸Pb. Corrections of the derived GDR parameters due to the partial transfer of excitation energy are suggested.