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.     

Fusion cross section at sub-Coulomb energies

The effective barrier at low energies is approximated by a Coulomb and parabolic barriers. A simple closed formula is obtained for the total reaction cross section which is the natural extension of Wong's results into the low energy domain. Theoretical predictions of averaged energy behaviour agree fairly well with various experimental results. The relation?ω ₀≧π(2kh ²Z₁Z₂ e ²/μ)^1/2 R ₀ ^?3/2 between the curvature and the extremum point of the nucleus-nucleus potential is suggested, based on low energy fusion experiments. By assuming equality sign, it is possible to extract the parameters of the barrier directly from experiment. Incidentally it is found that?ω ₀ remains persistently arround 4.5 MeV for all the reactions which have been investigated.     

Non yrast spectroscopy with the (α, n) reaction at the coulomb barrier

About 50 states with spins ≦8? in¹²⁸Xe have been populated by means of the¹²⁵Te (α, n) reaction at projectile energies near the Coulomb barrier. In particular eight 2⁺ or (1,2⁺) states have been observed. The (α, n) reaction at particle energies near the Coulomb barrier seems to be a powerful tool to by-pass the strong selectivity for the yrast states in normal (HI, xn) reactions.     

The 12C+24Mg elastic scattering: An example of anomalous transparency at coulomb barrier energies

Fifteen complete angular distributions of the elastic scattering of ¹²C+²⁴Mg were measured at energies around the Coulomb barrier (E_cm = 10.67–16 MeV). The angular distributions are strongly oscillating and could be well described by an optical potential family, whose real part was determined without continuous ambiguity. The imaginary part of this optical potential is very shallow. At four energies the inelastic scattering angular distributions leading to the 2⁺ state of the ²⁴Mg were also measured and analysed with coupled-channels calculations. The volume integrals of the optical potentials used in the coupled-channels calculations present the threshold anomaly in their energy dependence, with a clear Q-value dependence.     

Time-dependent Hartree-Fock calculation of 12C + 12C with a realistic potential

We have used a realistic single-panicle K-matrix model to compute the head-on scattering of ¹²C + ¹²C at incident projectile lab energies of 3.2, 6.4, 12.8, 19.2, 25.6, 32, 51.2 and 64 $\text{MeV}\text{nucleon}$, above the Coulomb barrier, in the time-dependent Hartree-Fock approximation. Direct and exchange Coulomb forces as well as spin-orbit forces are included. A large deformed harmonic oscillator basis is used. Spatial density and current distributions at various times are shown. The outgoing energy is found to be E₀ = 0.8E_in?28 (MeV), in the c.m. system. Fusion and fully relaxed scattering are observed at low energy. Some compression is seen at higher energies but no shock waves can be detected. Consequences for heavy-ion reactions are discussed.     

Elastic scattering of 40Ar on 40Ca at Elab = 191, 236 and 272 MeV

Cross sections for elastic scattering of ⁴⁰Ar on ⁴⁰Ca have been measured at energies E_lab = 191, 236 and 272 MeV employing position-sensitive detectors and the method of kinematical coincidences. The experimental data are first compared with the ordinary and the generalized Fresnel models. Only the generalized Fresnel model describes the experimental data well. An optical model analysis with a Woods-Saxon potential yields an energy independent set of parameters (V_R = ?21.76 MeV, r_OR = 1.37 fm, a_R = 0.45 fm; W₁ = ?13.69 MeV, r₀₁ = 1.40 fm, a₁ = 0.36 fm) very similar to the one found in ⁴⁰Ca-⁴⁰Ca scattering at corresponding energies. Values deduced for the total reaction cross sections for the three energies are in good agreement with those predicted by the generalized Fresnel model. The data are also compared with optical model calculations with the real part of the potential replaced by various microscopically determined potentials. The proximity, Fleckner-Mosel and the Krappe-Nix-Sierk potentials like the phenomenological optical model potential reproduce the measured data fairly well over several orders of magnitude.     

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.     

Optical potential for 12C + 28Si scattering

¹²C + ²⁸Si elastic scattering angular distributions are smooth functions, relatively easily fit by optical potential predictions, at laboratory bombarding energies, E_1ab, within ≈ 10 MeV of the Coulomb barrier (i.e. up to E_1ab ≈ 27 MeV). Between E_1ab = 27 and 36 MeV the angular distributions show pronounced oscillatory structure which is not easily fit with an optical potential. No optical potential has been found to give a very good account of all the angular distributions simultaneously; the best simultaneous fit to all the data was achieved with a surface-transparent potential whose real and imaginary well depths are energy dependent.     

Heavy ion reactions around the Coulomb barrier

The angular distributions of fission fragments for the ³²S+¹⁸⁴W reaction near Coulomb barrier energies are measured. The experimental fission excitation function is obtained. The measured fission cross sections are decomposed into fusion-fission, quasi-fission and fast fission contributions by the dinuclear system (DNS) model. The hindrance to completing fusion both at small and large collision energies is explained. The fusion excitation functions of ³²S+^90,96Zr in an energy range from above to below the Coulomb barrier are measured and analyzed within a semi-classical model. The obvious effect of positive Q-value multi-neutron transfers on the sub-barrier fusion enhancement is observed in the ³²S+⁹⁶Zr system. In addition, the excitation functions of quasi-elastic scattering at a backward angle have been measured with high precision for the systems of ¹⁶O + ²⁰⁸Pb, ¹⁹⁶Pt, ¹⁸⁴W, and ^154,152Sm at energies well below the Coulomb barrier. Considering the deformed coupling effects, the extracted diffuseness parameters are close to the values extracted from the systematic analysis of elastic and inelastic scattering data. The elastic scattering angular distribution of ¹⁷F+¹²C at 60 MeV is measured and calculated by using the continuum-discretized coupled-channels (CDCC) approach. It is found that the diffuseness parameter of the real part of core-target potential has to be increased by 20% to reproduce the experimental result, which corresponds to an increment of potential depth at the surface region. The breakup cross section and the coupling between breakup and elastic scattering are small.     

Cross sections for nuclear reactions in collisions of238U+238U and238U +197Au near and below the coulomb barrier

Cross sections for nuclear reactions at beam energies near and below the spherical Coulomb barrier V _c were measured in the very heavy collision systems²³⁸U +²³⁸U and²³⁸U +¹⁹⁷Au. The most probable reaction channel with mass transfer is the one-neutron transfer. Its excitation function is understood in terms of Rutherford trajectories together with the quantal process of neutron tunnelling over large distances. In addition, the exchange of up to 15 nucleons is observed down to 0.90 V _c . The excitation functions for the multi-nucleon transfer products have much steeper slopes than that for one-neutron transfer, and are steeper for²³⁸U +¹⁹⁷Au than for²³⁸U +²³⁸U, suggesting that nuclear contact is established in the associated collisions. The angular distribution for one selected multi-nucleon transfer product,²²⁷Th, shows that its formation occurs in more central collisions within contact times shorter than about 10^?21 s. There is no evidence for very longlived di-nuclear systems in the these reactions.     

Interpretation of auger energy shifts of silicon in solid silicon compounds

Chemical shifts of Auger transitions and photoelectron binding energies of silicon have been measured and interpreted using the quasi-atomic approach. The Si KL_2,3L_2,3 and L_2,3V₁V₁ Auger transitions and the binding energies of Si 2p and of the valence electrons at the maximum of the density of states V₁ have been investigated in solid silicon and in the compounds SiC, Si₃N₄, SiO₂, Na₂SiF₆ and T₃Si (T = V, Cr, Mn, Fe, Co, Ni). The relaxation-energy shift ΔR^ea_S(2p, 2p) describing the polarization effect (final-state effect) has been evaluated by AES and XPS measurements. Furthermore, the extra-atomic relaxation energy R^ea_D(2p) of the 2p electrons has been determined experimentally for silicon atoms in differing environments. This allows estimation of the potential parameter V(2p) describing the potential effect (initial-state effect). In general R^ea_D(2p) was found to be more sensitive to changes in chemical bonding than V2p). The behaviour of the quasi-atomic Si V₁ electrons seems to be the converse.     

Dynamic model for alpha particle emission during fission

We investigate the motion of anα-particle in the average time depencent potentialV(R _α ,t) of a fissioning nucleus. The emission process is treated quantum mechanically via a numerical solution of the one-body Schroedinger equation withV(R _α ,t). This solution yields the distribution of initial conditions for classical trajectories describing theα-particles outside the Coulomb barrier. The time and shape dependence ofV(R _α,t) is shown to have significant influence on the observable angle and energy distribution of theα-particles emitted during fission.     

The12C+12C reaction at subcoulomb energies (I)

The reaction¹²C+¹²C has been studied in the energy rangeE _cm=2.45–6.15 MeV byγ-ray spectroscopy. Gamma-ray transitions from a large number of excited states in²⁰Ne,²³Na and²³Mg were observed, which show strong and rapid yield variations. When the influence of the Coulomb barrier is removed, these structures appear superimposed on a flat reaction yield, which does not show a strong increase at low energies, in contrast to previous work. These results obviate the need for the hypothesis of absorption under the barrier at least down toE _cm=2.45 MeV. The nuclear and astrophysical aspects of the data are discussed.     

Mechanism of formation and decay of the compound nuclei150Gd produced by two entrance channels (16O+134Ba) and (40Ar+110Pd)

Excitation functions have been drawn for evaporation residues issued from¹⁵⁰Gd, a compound nucleus produced either by¹⁶O ions on¹³⁴Ba, or by⁴⁰Ar on¹¹⁰Pd. Absolute cross sections were measured for^145–147Gd,^145–147Eu,^141m Sm,^143m Sm,^140m Pm and^139m Nd. Complete fusion cross sections have been obtained and compared to calculated estimations based, at low energies, on the simple expressionσ _CF=πR _Fuss ² (1?V _Fus/E) whereR _Fus andV _Fus are respectively the distance and the potential for fusion atdV/dr=0. For high energies σ_CF=πR _cr ² (1?V_cr/E) with the model of critical distanceR _cr and critical potentialV _cr. Thresholds energies have been determined with a particular care. The analysis of the shape and the width of excitation functions, particularly for the emission of 4 and 5 neutrons, has been carried out and compared to calculated values. The difference between argon and oxygen induced reactions has been attributed to the difference inl population in the entrance channel.     

Fission in238U+238U collisions below the Coulomb barrier

Integral fission cross sections in the system²³⁸U+²³⁸U were measured at beam energies below the interaction barrierV _C. Scattering angle dependent probabilities and integral cross sections for Coulomb fission were calculated. It is concluded that earlier observed discrepancies between measured and calculated angular distributions for the one-neutron transfer product²³⁹U cannot be explained by sequential fission. Multi-nucleon transfer induced fission is observed down to energies (0.90±0.02)×V_C.     

Nuclear deformations from inelastic α-scattering on rare earth nuclei at energies near the Coulomb barrier

Alpha particles in the energy range of 10–20 MeV and scattered at various angles were used to excite the 0⁺, 2⁺, 4⁺ members in the ground state bands of ¹⁵²Sm, ¹⁵⁴Sm and ¹⁸⁶W. The measured excitation probabilities for bombarding energies below the Coulomb barrier were analyzed in the framework of Coulomb excitation theory. The resulting matrix elements of the E2 and E4 multipole operators were interpreted in terms of charge deformation parameters β^cλ = 2, 4. The cross sections for higher energies were analyzed in terms of the deformed optical potential and resulted in potential deformation parameters β^pλ = 2, 4. The two sets of deformation parameters show the same general trend of variation with target mass number. Still, significant differences are observed in some particular cases.     

Nucleon transfer in highly mass-asymmetric reaction systems between197au and relatively light projectiles in the energy region below 10 MeV/u

Mechanisms of heavy-ion induced nuclear reactions in mass-asymmetric systems were studied by focusing on the nucleon transfer reaction. Excitation functions and projected mean recoil ranges for the target-like products in³⁷-,¹⁶O-,¹⁴N-, and¹²C-induced reactions on¹⁹⁷Au were measured by means ofγ-ray spectrometry. The energy range studied was near the Coulomb barrier of the systems with incident energies below 10 MeV/u. Nucleon transfer reactions were discussed by distinguishing the products from quasi-elastic transfer (QET) and those from deep inelastic transfer (DIT). The tendency towards equilibration of theN/Z value and the energy damping, which are the characteristic features of DIT, were found in the production of Au isotopes. Observations are consistent with the generally accepted concept that QET takes place along a trajectory near the Coulomb trajectory. QET was made to be connected with the interaction radius and most of the cross section ratios were reproduced well by an extended tunneling model.     

Analysis of current–voltage and capacitance–voltage characteristics of perylene-monoimide/n-Si Schottky contacts

The electrical and interface state properties of Au/perylene-monoimide (PMI)/n-Si Schottky barrier diode have been investigated by current–voltage (I–V) and capacitance–voltage (C–V) measurements at room temperature. A good rectifying behavior was seen from the I–V characteristics. The series resistance (R_s) values were determined from I–V and C–V characteristics and were found to be 160 Ω and 53 Ω, respectively. The barrier height (Φ_b) of Au/PMI/n-Si Schottky diode was found to be 0.694 eV (I–V) and 0.826 eV (C–V). The ideality factor (n) was obtained to be 4.27 from the forward bias I–V characteristics. The energy distribution of interface state density (N_ss) of the PMI-based structure was determined, and the energy values of N_ss were found in the range from E_c ? 0.508 eV to E_c ? 0.569 eV with the exponential growth from midgap toward the bottom of the conduction band. The values of the N_ss without R_s are 2.11 × 10¹² eV^?1 cm^?2 at E_c ? 0.508 eV and 2.00 × 10¹² eV^?1 cm^?2 at E_c ? 0.569 eV. Based on the above results, it is clear that modification of the interfacial potential barrier for metal/n-Si structures has been achieved using a thin interlayer of the perylene-monomide.     

Photoelectron injection at metal-semiconductor interfaces

A modelling of the photoinjection process is developed which permits fitting of the spectral photoresponse of Schottky barriers including the electric field dependence of barrier height and photoresponse by means of two adjustable parameters: the zero field barrier qφ_BO and λ₀ the zero temperature mean free path for optical phonon scattering of high energy electrons. The model assumes an image force potential barrier with Thomas-Fermi screening in the metal. Effects of optical phonon scattering and quantum mechanical transmission are convoluted on the Fowler photoelectron supply function. The effects of phonon scattering are frequently large because the ranges in energies associated with the transverse momentum and normal momentum are approximately the amount by which the quantum energy hv exceeds the barrier energy qφ_B. At high fields, quantum mechanical tunneling dominates the response when hv < qφ_B. At low fields, phonon assisted transmission is appreciable for the same quantum energy range. The calculation of the collection probability includes effects of multiple scattering even for electrons that do not lie initially within the cone of acceptance at the barrier maximum. An approach that considers the probability of collection the same as that of reaching the potential maximum without scattering is found to be acceptable only at high fields. Experimental results are reported from oxide-passivated epitaxial Pt_xSi-〈111〉 n-type Si Schottky barrier diodes with annular Schottky barrier guard rings measured at temperatures of 90 and 298 K for an electric field range from 5 × 10³ to 9 × 10⁴$\text{V}\text{cm}$. The field, spectral and temperature dependences of the photoresponse data are in excellent agreement with theoretical predictions with λ₀ = 110 Å at both 90 and 298 K. The zero field barrier height obtained from fitting photoresponse curves at a number of electric fields is also in excellent agreement with I-V and C-V measurements.     

Pairing effects in quasi-elastic neutron transfer between Tin isotopes

The transfer of neutrons between low-lying states of Tin isotopes has been measured in the reaction¹²⁰Sn→¹¹²Sn at energies below the Coulomb barrier. The transfer probabilities are deduced as function of the classical minimum distance from transfer and scattering cross sections at different angles and two energies. At the smallest distances very large transfer probabilities,p _t =0.5, are observed for one- and two-neutron transfer. The two-neutron transfer is significantly enhanced as compared to a successive independent single-nucleon transfer. The definition of enhancement is discussed.