Study of fusion and nucleon transfer channels in the <Superscript>197</Superscript>Au + <Superscript>6</Superscript>He reaction in an energy range of <Superscript>6</Superscript>He to 20 Mev/A

New data on the cross sections of reactions occurring during the interaction of ⁶He nuclei with ¹⁹⁷Au at energies of ⁶He from 40 to 120 MeV are presented. The experiments were performed in the ACCULINNA secondary beam separator of the Laboratory of Nuclear Reactions, Joint Institute for Nuclear Research. To identify the reaction products, the activation method for measuring the gamma activity of the target assembly of thin foils was used. The excitation functions for fusion reactions involving the evaporation of up to ten neutrons from the compound nucleus as well as reactions with the emission of charged particles and nucleon transfer in the investigated energy region are obtained. Data analysis was carried out using two codes: ALICE-MP and NRV. The cross sections for the (⁶He, xn) reactions occurring through the compound nucleus are mostly in agreement with the results of model calculations based on the statistical approach. It is shown that, up to the energies of 114 MeV, the cross-section drop in the complete fusion reactions is negligible. The experimental excitation functions of reactions leading to the formation of isotopes of mercury and gold (transfer reaction) indicate that the main contribution to their formation is made by direct processes and that evaporation reactions (⁶He, pxn) and (⁶He, αxn) play a minor role, as is evidenced by a comparison of the measured cross sections with the calculation results.     

Excitation functions for complete-fusion and transfer reactions in <Superscript>4</Superscript>He interaction with <Superscript>197</Superscript>Au nuclei

Excitation functions are measured for the fusion reactions ¹⁹⁷Au(⁴He, xn)^201?xn Tl that are induced by alpha-particle interaction with gold nuclei in the energy range 14–36 MeV and in which x neutrons (0 ≤ x ≤ 3) are evaporated. The stack-activation technique was used to record and separate reaction products. Experimental data on the fusion reactions followed by evaporation of one to three neutrons agree with results of previous studies. For the radiative-capture reaction ¹⁹⁷Au(⁴He,γ)²⁰¹Tl, the upper limit on the cross section proved to be much lower. The excitation functions for the reactions subjected to measurements are compared with the results of calculations based on the statistical model and with the results of an experiment performed previously in a ⁶He beam.     

Excitation of <Superscript>3</Superscript><Emphasis Type="Italic">F</Emphasis><Superscript>o</Superscript> levels of nickel atom by collisions with slow electrons

Electron-impact excitation of odd ³ F ^o levels of the nickel atom has been studied experimentally. A total of 45 NiI line emission cross-sections at 50-eV exciting-electron energy were measured. Eight optical excitation functions in the 0–200 eV electron-energy range were recorded.     

Measurement of the excitation function for the <Superscript>7</Superscript>Li(t,p)<Superscript>9</Superscript>Li reaction using detection of delayed neutrons

The excitation function for the ⁷Li(t, p)⁹Li reaction is measured at the EGP-10 electrostatic tandem accelerator (VNIIEF) at the incident triton energies E _t = 5–11.3 MeV. The measurements were based on detection of delayed neutrons resulting from the decay of ⁹Li nuclei. The neutrons were detected by the 4π detector consisting of ³He counters enclosed in a polyethylene moderator. A pulsed mode was used for irradiating the LiF target (210 μg cm^?2) of natural isotopic composition on the tantalum backing. Absolutization of the excitation function was performed against the ⁷Li(p, n) reaction cross sections.     

Dynamical decay process of <Superscript>219, 220</Superscript>Ra<Superscript>*</Superscript> formed in <Superscript>10, 11</Superscript>B + <Superscript>209</Superscript>Bi reactions

The excitation functions for both the evaporation residue and fission have been calculated for ¹⁰B + ²⁰⁹Bi and ¹¹B + ²⁰⁹Bi reactions forming compound systems ^{219, 220}Ra^* , using the dynamical cluster-decay model (DCM) with effects of deformations and orientations of the nuclei included in it. In addition to this, the excitation functions for complete fusion (CF) are obtained by summing the fission cross-sections, neutron evaporation and charged particle evaporation residue cross-sections produced through the axn\ensuremath \alpha xn and pxn\ensuremath pxn (x = 2, 3, 4) emission channels for the ²¹⁹Ra system at various incident centre-of-mass energies. Experimentally the CF cross-sections are suppressed and the observed suppression is attributed to the low binding energy of ^{10, 11}B which breaks up into charged fragments. The reported complete fusion (CF) and incomplete fusion (ICF) excitation functions for the ²¹⁹Ra system are found to be nicely fitted by the calculations performed in the framework of DCM, without invoking a significant contribution from quasi-fission. Although DCM has been applied for a number of compound nucleus decay studies in the recent past, the same is being used here in reference to ICF and subsequent decay processes along with the CF process. Interestingly the main contribution to complete fusion cross-section comes from the fission cross-section at higher incident energies, which in DCM is found to consist of an asymmetric fission window, shown to arise due to the deformation and orientation effects of formation and decay fragments.     

Investigations of Properties of <Superscript>89</Superscript>Y Levels with Reactor Fast Neutrons

By employing a beam of reactor fast neutrons, the spectrum of gamma rays up to an energy of 4.6MeV and their angular distributions with respect to the neutron-beamaxis aremeasured in the reaction ⁸⁹Y(n, n'γ). The multipolarities and multipole-mixture parameters for 34 gamma transitions and the spin–parities J^π of states excited in this reaction are determined. The lifetimes of the lowest 32 levels of 89Y were measured by the Doppler shift attenuation method, and the reduced probabilities for the respective gamma transitions were calculated. Levels of the K^π = +5/2⁺ and K^π = ?7/2⁺ bands associated with, respectively, prolate and oblate deformation shapes are found in ⁸⁹Y at low excitation energies.     

Nuclear level densities in <Superscript>208</Superscript>Bi and <Superscript>209</Superscript>Po from the neutron spectra in the (<Emphasis Type="Italic">p,n</Emphasis>) reactions on <Superscript>208</Superscript>Pb and <Superscript>209</Superscript>Bi nuclei

The spectra of neutrons from the (p, n) reactions on the ²⁰⁸Pb and ²⁰⁹Bi nuclei were measured in the proton-energy range 8–11 MeV. These measurements were performed by using a time-of-flight spectrometer of fast neutrons on the basis of the pulsed tandem accelerator EGP-15 of the Institute of Physics and Power Engineering (Obninsk, Russian Federation). A high resolution and stability of the time-of-flight spectrometermade it possible to identify reliably low-lying discrete levels alongwith the continuum section of the neutron spectra. The measured data were analyzed on the basis of the statistical equilibrium and preequilibrium models of nuclear reactions. The respective calculations were performed by using the precise formalism of Hauser-Feshbach statistical theory together with the generalizedmodel of a superfluid nucleus and the back-shifted Fermi gas model for the nuclear-level density. The nuclear-level densities in ²⁰⁸Bi and ²⁰⁹Po were determined along with their energy dependences and model parameters. Our results are discussed together with available experimental data and recommendations of model systematics.     

Evidence for a K<Superscript>π</Superscript> = 1/2<Superscript>+</Superscript> isomer in neutron-rich <Superscript>185</Superscript>Ta

In this paper we present for the first time the measured delayed neutron (DN) yields and time spectra from high-energy protons interacting with thick ^natPb targets. The 1GeV protons from the accelerator impinged on targets of different thicknesses producing a huge number of spallation-fission products, some of which can be DN precursors. After the beam is switched off, the DNs were detected with optimized ³He counter. The production yields of light DN precursors as ¹⁷N and “usual” fission products as ⁸⁷Br and ⁸⁸Br, which dominate the total DN activity, are obtained both for thin and thick targets. These new data are of great interest for the new generation high-power spallation targets based on liquid-metal technologies. Our findings also should help to constrain the physics models within the simulations codes.     

Neutron multiplicity in deep inelastic collisions: 400 MeV Cu+Au system

We have detected in nine different positions of space the neutrons associated with the collision of ⁶³Cu on ¹⁹⁷Au at 400 MeV bombarding energy. The deep inelastic products were detected at two different angles: close to the grazing angle and 30° forwards of it. Their masses were measured using a time-of-flight technique. The neutrons were detected in coincidence with the fragments - the efficiency of the neutron detectors was measured relatively to a ²⁵²Cf source during beam time. The neutron threshold was set at 300 keV. Within an accuracy of 10% all the emitted neutrons are evaporated by the fully accellerated deep inelastic fragments. It is shown that the excitation energy is shared between the fragments in proportion to their masses and that the relaxation time for internal equilibration of the composite system is very short (～10^?22 s).     

Dynamical study of the Cs<Superscript>+</Superscript>(<Superscript>1</Superscript>S<Subscript>0</Subscript>)+Mg(3 <Superscript>1</Superscript>S<Subscript>0</Subscript>) non adiabatic collision system in the few keV energy range

The dynamics of collisional processes between Mg atoms and caesium ions is studied using the hemiquantal (HQ) approach with special attention to the collisional channels leading to Mg(3 ¹P) and Cs(6 ²P) states, for which the corresponding emission excitation functions have been previously measured in our laboratory. The radial and angular non-adiabatic couplings between the manifold of quasimolecular states have been determined using an ab initio configuration interaction calculation. The cross-sections for the different channels, as a function of the laboratory collisional energy, are compared with experimental values. The dynamical calculations indicate that, for the inelastic processes considered, the range of relevant impact parameters is small, active collisions being of the head-on type. .     

Search for the emission of π<Superscript>0</Superscript> mesons from the neutron-induced fission of <Superscript>235</Superscript>U nuclei

A nuclear-reactor experiment on the search for the emission of neutral pions from the neutron-induced fission of ²³⁵U nuclei has been carried out. To this end, an experimental setup for searching for the emission of π⁰ mesons with high sensitivity to this process has been designed and produced. This setup consisted of two Cherenkov total-absorption spectrometers for the detection and measurements of the energy of two gamma-ray photons from π ⁰-meson decay. The spectrometers were placed at the exits of two coaxial horizontal experimental channels. To protect the detectors from low-energy β and γ rays and neutron fluxes, 3.5-m-long water filters were situated in the channels. To reject the cosmic background, large-area scintillation counters operating in the anticoincidence mode were placed over each spectrometer. The energy and time resolution of the spectrometers, as well as the efficiency of detecting charged particles by scintillation counters, were tested on the secondary electron beam of the Tomsk electron synchrotron Sirius. Runs of measurements of the effect on the operating reactor (duration of 805 h) were alternated with runs of measurements of background on the stopped reactor (duration of 403 h). Statistical processing of the experimental results yields an upper limit of ≤5.3×10 ^?12 with a 90% confidence level for the probability of the emission of neutral pions from the neutron-induced fission of ²³⁵U nuclei. This result and results of other works carried out with sources of spontaneously fissioning ²⁵²Cf nuclei show that the probabilities of the emission of π⁰ mesons from spontaneous fission and fission induced by fission-spectrum neutrons are equal to each other.     

Alpha-cluster states in <Superscript>18</Superscript>O

The excitation function of elastic α-particle scattering on ¹⁴C has been measured in the laboratory energy range 16.3–19.2 MeV using a backscattering technique with a thick target. These data were analyzed together with the old low-energy data of G.L. Morgan et al. in the framework of the R-matrix formalism. Spin-parity assignments were made for 32 states in ¹⁸O in the excitation range 9–20 MeV. The estimates of the widths of the states are also presented. The 0⁺ and 0^?α-cluster bands appeared to be well separated by 5.6 MeV (as in ¹⁶O and ²⁰Ne). We have not found a confirmation of existence of the negative-parity molecular states proposed by M. Gai et al. We observed an effect of a doubling of α-cluster levels in ¹⁸O similar to that found in ²²Ne.     

Structure of the <Superscript>6</Superscript>He nucleus in the three-particle model

The properties of the ⁶He nucleus were investigated in a three-particle approximation (a alpha-particle cluster plus two neutrons) on the basis of the variational approach by using a Gaussian basis. For nn and nα interactions, potentials that make it possible to describe S-wave phase shifts for elastic scattering and, simultaneously, to reproduce faithfully the energy and the size of the ⁶He nucleus were proposed. Characteristic structural features of this nucleus that are manifested in the density distributions, elastic form factor, pair correlation functions, and momentum distributions of particles constituting the ⁶He nucleus in the three-particle model were revealed.     

Momentum distributions of <Superscript>4</Superscript>He nuclei from the <Superscript>6</Superscript>He and <Superscript>6</Superscript>Li breakup

Experimental data on the momentum distributions of ⁴He nuclei originating from ⁶He and ⁶Li breakup on various targets are presented over a wide beam energy range. The experiment with ⁶He was performed at the DRIBs accelerator complex for radioactive beams at the Joint Institute for Nuclear Research (JINR, Dubna). The intensity of the ⁶He beam used was 5 × 10⁶ particles per second and its energy was 10 MeV per nucleon. The momentum distributions of breakup products were measured by means of the MSP-144 magnetic spectrometer. The distribution width was shown to be virtually independent of the target mass. A small value of this width, σ ～ 28 MeV/c, confirms the presence of a halo in ⁶He. The measurements performed with ⁶Li beams of energy 18 and 46 MeV per nucleon at the U-400M accelerator yielded a width value of σ ～ 50 MeV/c for the momentum distributions of ⁴He nuclei, which is intermediate between that for ⁶He and those for stable nuclei. A compilation of the widths of the momentum distributions of fragments originating from the breakup of various nuclei is presented versus the binding energy of one or two neutrons in these nuclei, the target mass and the projectile energy.     

Angular Gamma Dose Rate Distribution at the Surface of Injected Ducted Concrete Shield

The shielding problems that arise due to the irregular penetrations such as neutral beam injection ducts should be treated carefully to aid in the shield design. The present work was undertaken to describe the effects arising due to radiation streaming through the neutral beam injector ducts (NBID) on the angular distribution of the total gamma ray doses at the outer surface of illmenite concrete shield (? = 4.6g/cm³). The shield is pierced with NBID of different diameters and lengths. The measurements were performed using a collimated beam of both gamma rays and neutrons emitted from one of the horizontal channels of the ET-RR-1 reactor. The measurements were carried out using ⁷LiF teflon thermoliminescent dosimeters. Generally the obtained data reveal that the presence of the total dose increase at the centerline of NBID and which in turn tends to decrease with the increase of scattered angle. An empirical formula describing the differential dose rate ratio is predicted. The experimental data obtained reveal good agreement with the calculated ones.     

Theoretical study of structural and energy parameters of the van der Waals complex of the Li<Superscript>+</Superscript> cation with the N<Subscript>2</Subscript> molecule

The three-dimensional vibrational problem for the isolated van der Waals complex formed by the Li⁺ cation with the N₂ molecule is solved by the variational method. The potential energy and dipole moment surfaces are calculated using different basis sets of atomic functions and different approaches for taking electron correlation into account. The anharmonic effects caused by the interaction of vibrational degrees of freedom of the complex are consistently considered for the first time. The energy levels of three-dimensional vibrational states are determined. The frequency shift of the N₂ molecule vibration upon complexation and the fundamental transition intensity for this vibration in the complex are calculated. The frequencies and intensities for a number of spectral transitions between the states associated with excitation of low-frequency modes are determined. The average values of geometrical parameters and their variances are calculated for the ground state and excited vibrational states of the complex.     

Cross section for the subbarrier fission of <Superscript>244</Superscript>Cm

The cross section for ²⁴⁴Cm fission induced by neutrons of energy in the range between 0.07 eV and 20 keV was measured by using the lead slowing-down spectrometer (LSDS-100) of the Institute for Nuclear Research (Russian Academy of Sciences, Moscow). The parameters of the resonance areas were determined for the lowest eight s-wave neutron resonances, and the respective fission widths were evaluated. Also, the parameters of the intermediate structure in the cross section for the subbarrier fission of ²⁴⁴Cm nuclei were evaluated. The results were compared with available data and recommendations based on evaluations.     

Luminescence and energy transfer mechanism in Eu<Superscript>3+</Superscript>/Tb<Superscript>3+</Superscript>-co-doped ZrO<Subscript>2</Subscript> nanocrystal rods

Nanocrystal rods of Eu³⁺/Tb³⁺-co-doped ZrO₂ were synthesized using a simple chemical precipitation technique. Both ions were successfully doped into the Zr⁴⁺ ion site in a mixed structure containing both monoclinic and tetragonal phases. The Eu³⁺ or Tb³⁺ singly doped zirconia produced red and green luminescence which are characteristics of Eu³⁺ and Tb³⁺ ions, respectively. The co-doped zirconia samples produced blue emission from defect states transitions in the host ZrO₂, red and green luminescence from dopant ions giving cool to warm white light emissions. The phosphors were efficiently excited by ultraviolet and near-ultraviolet/blue radiations giving white and red light, respectively. The decay lifetime was found to increase with increasing donor ion concentration contrary to conventional observations reported by previous researchers. Weak quadrupole–quatdrupole multipolar process was responsible for energy transfer from Tb³⁺ (donor) ion to Eu³⁺ ion. No energy back-transfer from Eu³⁺ to Tb³⁺ ion was observed from the excitation spectra. Temperature-dependent photoluminescence shows the presence of defects at low temperature, but these defects vanished at room temperature and beyond. The Eu³⁺/Tb³⁺-co-doped ZrO₂ nanocrystal rod is a potential phosphor for white light application using UV as an excitation source. Thermoluminescence measurements show that the inclusion of Tb³⁺ ion increases trap depths in the host zirconia.     

Use of the EUR LiB 15/5 data set for radiation shielding calculations in iron

Abstract

The shielding effect of an iron sphere assembly has been tested for a Pu-α-Be neutron source placed in the center of the shield assembly. Emergent neutron and gamma spectra were measured with a stilbene scintillation counter. Discrimination between neutrons and gammas was achieved by the pulse shape discrimination technique based on the zero crossing method. Calculations have been made using the one-dimensional transport code ANISN-Westinghouse version (ANISN-W) and the EUR LiB 15/5 cross section data set. The agreement between measurements and calculations indicates that the cross section set and the calculation model are suitable for studying the iron shielding experiments over the neutron energy range 1.35–10 MeV and the gamma energy range 0.3–6 MeV. Total macroscopic cross sections for fast neutrons, linear attenuation coefficients for gamma rays and half-value thicknesses for neutrons and gammas for the whole energy range and at different energies have been obtained.     

Inverse Quasifission in <Superscript>156,160</Superscript>Gd + <Superscript>186</Superscript>W Reactions

To investigate the impact shell effects have in the formation of neutron-rich fragments in multinucleon transfer reactions, a series of experiments to explore the binary channel in ^156,160Gd + ¹⁸⁶W reactions at energies near and above the Coulomb barrier is performed at the Flerov Laboratory’s U-400 accelerator using the CORSET setup. These experiments are aimed mainly at obtaining the production cross sections of leadlike fragments in the process of inverse quasifission. The mass, energy, and angular distributions of the binary reaction products are measured at energies of 860 and 935 MeV of ¹⁶⁰Gd ions and 878MeV in the case of ¹⁵⁶Gd ions. The excitation energies of primary fragments are estimated using their measured mass–energy distributions. Enhanced yields of products with masses of 200–215 amu are observed for both reactions. At energies above the barrier for side-to-side collisions (935 MeV), the yield of lead-like fragments is an order of magnitude larger than at energies near the Coulomb barrier, due possibly to the influence of orientation effects. The enhancement observed in the yield of reaction products with masses heavier than the target mass confirms that multinucleon transfer reactions can be used to obtain new neutron-rich isotopes, and to synthesize new superheavy elements.