Coulomb Barrier Height Research Articles

Special Features of Nuclear Reactions Induced by Loosely Bound Nuclei in the Vicinity of the Coulomb Barrier Height

Excitation functions are measured for complete-fusion and transfer reactions induced by the interaction of 6He and 6Li with 206Pb, 209Bi, and Pt. Data obtained for fusion reactions induced by 6He ions deviate from the predictions of the statistical model of compound-nucleus decay at projectile energies in the vicinity of the Coulomb barrier height. A strong enhancement of cross sections for fusion reactions induced by the interaction of 6He with target nuclei is observed. The cross sections for reactions of cluster transfer, neutron transfer from 6He, and deuteron transfer from 6Li at deep-subbarrier energies, are also found to be enhanced. These results are discussed from the point of view of the effect of the cluster structure of nuclei on the interaction probability at energies in the vicinity of the Coulomb barrier height.

Analysis of the nuclear potential for heavy-ion systems through large-angle quasi-elastic scattering

A study on the surface diffuseness parameter of the nuclear potential for the reactions of ${}^{208}$Pb with ${}^{48}$Ti, ${}^{54}$Cr, ${}^{56}$Fe, ${}^{64}$Ni, and ${}^{70}$Zn is performed using large-angle quasi-elastic scattering experimental data. Diffuseness parameters that are considerably lower than the standard value of around 0.63 fm are required in order to fit the experimental data at deep sub-barrier energies, except for the ${}^{54}$Cr+${}^{208}$Pb system, where the required diffuseness parameter is in satisfactory agreement with (but still lower than) the standard value. Furthermore, when the energies of the experimental data used in the fittings are increased from the deep sub-barrier region to the energies closer to the Coulomb barrier height, the best fitted diffuseness parameters also increase. The increase in the obtained diffuseness parameters as the energies are increased also seems to have a possible tendency to be a function of the charge product of the target and projectile nuclei. We find that the phenomenon of threshold anomaly might explain our findings here. The increase in the diffuseness parameters could also due to dynamical effects, for example, due to neutron movements.

Coulomb and Nuclear Breakup at Low Energies: Scaling Laws

We report on a recent work on the low-energy behavior of the breakup cross section in so far as it has important role in the fusion of weakly bound and halo nuclei at near-barrier energies. We assess the way the nuclear component of this cross section scales with the target mass. In complete accord with previous finding at higher energies we verify that the low energy behavior of the breakup cross section for a given projectile and relative center of mass energy with respect to the Coulomb barrier height scales as the cubic root of the mass number of the target. Surprisingly we find that the Coulomb component of the breakup cross section at these low energies also obeys scaling, but with a linear dependence on the target charge. Our findings are important when planning for experiments involving these exotic nuclei.

Study of the universal function of nuclear proximity potential from density-dependent nucleon–nucleon interaction

Abstract The universal function of proximity potential model is systematically studied from the double folding model with density-dependent nucleon–nucleon interaction (CDM3Y6) by analyzing several hundreds of fusion reaction systems with mass numbers from light nuclei to heavy nuclei. The analytical formula of universal function is obtained at s 0 > − 1 by fitting all of the calculated values. Using this formula we calculate the Coulomb barrier heights for fusion reactions in comparison with the values of the experimental data and Prox77. It is shown that presently obtained universal function in proximity potential model can reproduce the Coulomb barrier heights.

Weak channels in backscattering of20Ne onnatNi,118Sn, and208Pb

To further our understanding of the influence of weakly coupled channels on the distribution of Coulomb barrier heights, we have measured transfer cross sections for ${}^{20}$Ne ions backscattered from ${}^{\mathrm{nat}}$Ni, ${}^{118}$Sn, and ${}^{208}$Pb targets at near-barrier energies. The $Q$ value spectrum in the case of ${}^{208}$Pb target has been determined too. The transfer channels appear to be especially important for ${}^{208}$Pb, whose double-closed-shell nature leads to a relatively low level density for noncollective inelastic excitations.

Extended deep level defects in Ge-condensed SiGe-on-Insulator structures fabricated using proton and helium implantations

SiGe-on-Insulator (SGOI) structures were created using the Ge condensation method, where an oxidation process is performed on the SiGe/Si structure. This method involves rapid thermal chemical vapor deposition and H+/He+ ion-implantations. Deep level defects in these structures were investigated using deep level transient spectroscopy (DLTS) by varying the pulse injection time. According to the DLTS measurements, a deep level defect induced during the Ge condensation process was found at 0.28eV above the valence band with a capture cross section of 2.67×10−17cm2, two extended deep levels were also found at 0.54eV and 0.42eV above the valence band with capture cross sections of 3.17×10−14cm2 and 0.96×10−15cm2, respectively. In the SGOI samples with ion-implantation, the densities of the newly generated defects as well as the existing defects were decreased effectively. Furthermore, the Coulomb barrier heights of the extended deep level defects were drastically reduced. Thus, we suggest that the Ge condensation method with H+ ion implantation could reduce deep level defects generated from the condensation and control the electrical properties of the condensed SiGe layers.

The parametrization of Coulomb barrier heights and positions using the double folding model

Abstract The Coulomb barrier heights and positions are systematically shown with mass numbers and charge radii of the interacting nuclei. The nuclear potential is calculated by using the double folding model with the density-dependence nucleon–nucleon interaction (CDM3Y6). The pocket formulas are obtained for the Coulomb barrier heights and positions by analyzing several hundreds of heavy-ion systems with mass numbers from light nuclei to heavy nuclei. The parameterized formulas can reproduce the calculated barrier heights and positions by using the double folding model within the accuracy of ±1%. Moreover, the results are agreeable with the experimental data. The relation between the barrier height and the barrier position is also studied.

Dinuclear systems at energies in the vicinity of the Coulomb barrier height

The problem of constructing a quantum-mechanical description of neutron and collective degrees of freedom of dinuclear systems at the first stage of the near-barrier fusion of heavy ions is investigated. A new method for numerically solving the Schrodinger equation for an arbitrary axially symmetric field with allowance for spin-orbit interaction is proposed. Thismethod is not plagued by restrictions on the distance between approaching nuclei or on the elongation of fused nuclei or nuclei undergoing separation. The method is illustrated by applying it to determining neutron two-center (molecular) states in systems of light and heavy nuclei. The experimentally observed properties of the barrier distribution extracted from the energy dependence of the cross section for nuclear fusion are explained on the basis of an analysis of neutron states and excited vibrational and rotational states of a dinuclear system.

Special features of nuclear reactions induced by loosely bound 6He and 6,7Li nuclei in the vicinity of the Coulomb barrier height

Excitation functions are measured for complete-fusion and transfer reactions induced by the interaction of 6He and 6Li with 206Pb, 209Bi, and Pt. Data obtained for fusion reactions induced by 6He ions deviate from the predictions of the statistical model of compound-nucleus decay at projectile energies in the vicinity of the Coulomb barrier height. A strong enhancement of cross sections for fusion reactions induced by the interaction of 6He with target nuclei is observed. The cross sections for reactions of cluster transfer, neutron transfer from 6He, and deuteron transfer from 6Li at deep-subbarrier energies are also found to be enhanced. These results are discussed from the point of view of the effect of the cluster structure of nuclei on the interaction probability at energies in the vicinity of the Coulomb barrier height.

Extended Defects in Ge-condensed SGOI Structures Fabricated by Using Proton and Helium Implantations

The SGOI structures using Ge condensation method have been fabricated by rapid thermal chemical vapor deposition using H+ and He+ ion-implantations, and deep level defects investigated using the deep level transient spectroscopy (DLTS). According to DLTS measurement, a deep level defect induced during Ge condensation process was found at 0.28 eV above the valence band with capture cross sections of 2.67x1017 cm-2, and two extended deep levels at 0.54 eV and 0.42 eV above the valence band with capture cross sections of 3.17x1014 cm-2, 0.96x1015 cm-2, respectively. Of ion-implanted samples, the densities of the newly generated defect as well as the existing defects were decreased effectively. And, Coulomb barrier height of the extended defect was drastically reduced. Thus, we suggest that the Ge condensation method using the H+ ion implantation could reduce deep level defects generated from the Ge condensation and control electrical properties of condensed SiGe layers.

Electron emission from laser-heated fullerene dianions: Probing the repulsive Coulomb barrier

Electron detachment from fullerene dianions Cm2- (m=76,84) stored in a room temperature Penning trap was probed upon pulsed laser excitation at wavelengths of 355 and 532 nm. The fraction of Cm(2-) surviving trapping times exceeding tens of milliseconds under UHV conditions, as well as the fraction of singly charged anions Cm(-) generated were recorded as a function of the laser fluence. Analysis by means of Poisson statistics yields absolute absorption cross sections and the number of photons necessary to induce the detachment. The cross sections obtained are in good agreement with the literature values. By describing the electron detachment as a statistical unimolecular process, we deduce effective activation energies from the number of photons required. These energies are compared to the sum of the second electron affinity and the Coulomb barrier height as calculated from an electrostatic charging model.

Instability of the Al42- “All-Metal Aromatic” Ion and Its Implications

Al42 - is a prototype structural unit of a new class of "all-metal aromatic" molecules. Without stabilizing counterions this species is unstable with respect to electron autodetachment in the gas phase. We estimated the height of the repulsive Coulomb barrier to approximately 2.7 eV and calculated a lifetime of 9 fs. This is a short lifetime: The only way to study the isolated dianion experimentally is to use electron scattering techniques. Investigations of the validity of bound-state quantum chemical calculations on the isolated species show that the results suffer from significant admixture of continuum states to the bound-state wave function depending on the basis set. Calculations of molecular properties can therefore give essentially arbitrary results for this ill-defined system, as is demonstrated for the energy and nuclear magnetic shieldings. This substantiates that results from calculations on the isolated dianion should be approached with caution.

Effect of the separation of Coulomb centers on the activation energy in electronic hopping transport: A reinterpretation in poly‐N‐vinylcarbazole

AbstractThe effect of the separation of Coulomb centers in the activation energy for the electronic hopping transport of noncrystalline organic materials was studied. The theoretical analysis was carried out in absence of an applied electric field, using a simple one‐dimensional model of two centers separated by a distance R. It was found that the height of the potential well is reduced by an amount e2/4πεε0R due to the overlapping of the Coulomb centers. This reduction defines an effective activation energy given by (ΔE)0 − e2/4πεε0R, where (ΔE)0 is the height of the nonperturbed potential well. As an experimental application of this result, current–voltage characteristic in poly‐N‐vinylcarbazole at different temperatures was obtained, from which the conductivity at zero electric field was estimated by extrapolation to zero field in Poole‐Frenkel plots. The correspondent activation energy was estimated from Arrhenius plots in the order of 0.65 eV. Because the carriers transport in this polymer occurs by hopping between the molecules of carbazole whose mean separation is in the order of 6.5 × 10−10 m, the height of the nonperturbed Coulomb barrier of a single hopping site in this material would be really estimated in 1.18 eV. POLYM. ENG. SCI., 47:545–550, 2007. © 2007 Society of Plastics Engineers.

Photodetachment of fullerene monoanions and dianions in a Penning trap: Probes of delayed electron emission and associated activation barriers

Singly and doubly charged fullerene anions were introduced into gas phase by means of electrospray starting from a solution containing a very efficient electron donor compound. Following isolation and trapping, mass selected anions were irradiated with a ns-pulse from a Nd:YAG laser at the third harmonic ( hν = 3.5 eV) in order to study (delayed) photoemission. We present results on the laser-fluence dependence of photoemission for singly charged C 76 − and C 84 − as well as for doubly charged C 76 2−. Results are discussed in terms of statistical electron emission as opposed to direct detachment. They are further analyzed to give information on the number of photons necessary to induce delayed electron emission on the experimental time scale. The averaged absolute absorption cross-sections obtained were about 0.6–0.8 Å 2 for all species probed. The agreement between experimental data and statistical modelling shows that electronic excitations which rapidly relax to vibrationally hot ions provide the dominant contributions to the 355 nm absorption cross-sections. As a corollary, we show that direct single-electron detachment is a minor process for the singly charged fullerene anions having a cross-section <0.02 Å 2. For C 76 2−, electron emission follows an absorption law in which single-photon excitation is the leading contribution. From the data we deduce a Coulomb barrier height for C 76 2− of 1.20 eV.

Effect of charge polarization on the Coulomb barrier for cold-fusion reactions

We estimate the decrease of the Coulomb-barrier height between colliding partners due to charge polarizations in the entrance channel for cold-fusion reactions. The resulting charge displacements between protons and neutrons are the sum of the surface- and volume-charge components. We show the difference between the charge polarization of light and heavy nuclei and the decrease of the Coulomb barrier height for synthesizing superheavy elements.

Fusion by diffusion. II. Synthesis of transfermium elements in cold fusion reactions

We describe a method of estimating cross sections for the synthesis of very heavy nuclei by the fusion of two lighter ones. The cross section is considered to be the product of three factors: the cross section for the projectile to overcome the Coulomb barrier, the probability that the resulting composite nucleus reaches the compound nucleus configuration by a shape fluctuation treated as a diffusion of probability in one dimension, and the probability that the excited compound nucleus survives fission. Semi-empirical formulas for the mean Coulomb barrier height and its distribution around the mean are constructed. After overcoming the Coulomb barrier the system is assumed to be injected into an ``asymmetric fission valley'' by a rapid growth of the neck between the target and projectile at approximately frozen asymmetry and elongation. Diffusion in the elongation coordinate in this valley can occasionally bring the system over the saddle separating the injection point from the compound nucleus configuration. This is the stage that accounts for the hindrance to fusion observed for very heavy reacting systems. The competition between deexcitation of the compound nucleus by neutron emission and fission is treated by standard methods, but an interesting insight allows one to predict in an elementary way the location of the maximum in the resulting excitation function. Adjusting one parameter in the theory causes the calculated peak cross sections to agree within about a factor of 2 or so with 12 measured or estimated values for ``cold'' one-neutron-out reactions where targets of $^{208}\mathrm{Pb}$ and $^{209}\mathrm{Bi}$ are bombarded with projectiles ranging from $^{48}\mathrm{Ca}$ to $^{70}\mathrm{Zn}$. The centroids of the excitation functions agree with theory to within 1 or 2 MeV for the six cases where they have been determined, and their widths are reproduced. ``Hot'' fusion reactions, where several neutrons are emitted, are not treated, except that a comparison is made between the hindrance factors in cold and hot reactions to make elements with atomic numbers 112 to 118. The calculated diffusive hindrances in the hot reactions are less unfavorable by 4 to 5 orders of magnitude.

Frequency dependent conductivity of aluminium nitride films prepared by ion beam-assisted deposition

The frequency dependent conductivity of different thin AlN films measured in the frequency range 10 Hz to 2 MHz for temperature between 300 and 573 K is reported. It is observed from the experimental data that the AC conductivity in the AlN thin films is proportional to ω s . The value of s is found to be temperature dependent, which shows a decreasing trend with temperature. The correlated barrier hopping model is the most likely mechanism for the electron transport. The polaron binding energy ( W m), the height of Coulomb barrier ( W), and the characteristic relaxation time (τ 0) have been calculated.

Frequency Dependent Conductivity of Thin ZnO Films Prepared by R.F. Sputtering Technique

AC conductivity of different thin zinc oxide films measured in the frequency range of 10 Hz to 2 MHz in the temperature interval of 300 K to 575 K is reported. ZnO films were prepared by reactive r.f. magnetron sputtering from ZnO target. The experimental data reveal that a.c. conductivity is proportional to ω s . The value of s was found to be temperature dependent, decreases with increasing temperature. These observations suggest that correlated barrier hopping model is the most likely mechanism. The temperature dependence of a.c. Conductivity is expressed in power law form as σ(ω) ∝T n . The temperature exponent n is found to be increasing with increasing temperature and decreasing frequency in accordance with the narrow band limit. At high temperature the conductivity variation with frequency is comparatively small. The polaron binding energy (Wm), the height of Coulomb barrier (W) and the characteristic relaxation time (τ0) have been calculated. The values of Wm and W increase as the thickness decreases whereas the values of τ0 decrease with decreasing thickness.

AC conductivity and dielectric constant of poly(vinyl alcohol) doped with MnSO4

AC conductivity of poly(vinyl alcohol) polymer modified by MnSO4 with different concentration from 100Hz to 100KHz in the temperature range 293–343K is reported. Obtained data reveal that AC conductivity is proportional to ωS in all compositions (5, 10, 15, 20% MnSO4 by weight). The value of σAC(ω) depends linearly on the frequency, with slope S ranging between 0.95 and 0.65. The value of S was found to be temperature dependent, which decreases by increasing temperature. This suggests that the Correlated Barrier Hopping model is the most likely mechanism. Moreover, WM the polaron binding energy, W the height of coulomb barrier, R the hopping distance, Rmin the minimum hopping distance, and Ea the activation energy were calculated and found to be composition dependent.

Argon ion reaction studies on tantalum and gold near Coulomb barrier energies

Gold and tantalum targets were bombarded with a 40Ar beam at 183 and 209 MeV, close to the Coulomb barrier height. Angular distributions of recoiling reaction products were measured by collecting on A1 catcher foils. The radioactivity was counted after the irradiation by Ge(Li) detectors. One-neutron and one- and two-proton transfer products were prominent. Simple semiclassical transfer theory adequately fits the backward peaking angular distributions at or below the Coulomb barrier but fails at bombarding energies above the barrier. Yields of some products more than two mass numbers removed from reactants were observed in thicker targets at 235 MeV, and their significance is discussed.