Charge distributions of the isobar pair 40Ar-40Ca from a phenomenological and a model-independent analysis of elastic electron scattering

Cross sections for elastic electron scattering from ⁴⁰Ar have been measured for the momentum transfer range from 0.8 to 1.8 fm^?. The data were analysed by various phenomenological models as well as by a model-independent method. Using the results of Stanford measurements on ⁴⁰Ca analysed in a similar manner, the difference of the charge distributions of ⁴⁰Ar and of ⁴⁰Ca has been calculated. The combined results of the phenomenological analysis are compared with the results of the model-independent analysis.     

Large contribution of deep inelastic processes to reactions of40Ar and48Ca with238U

Differential recoil range distributions have been measured for heavy-reaction products ranging from Te(Z=52) to quasielastic transfer products near the charge and mass of the targets for the reactions of 276 MeV⁴⁸Ca+²³⁸U, 237MeV and 250 MeV⁴⁰Ar+²³⁸U, and 259 MeV⁴⁰Ar+¹⁹⁷Au. The measured recoil range distributions for the⁴⁰Ar+¹⁹⁷Au reaction agree with range distributions calculated from the known projectile-like fragment angular distributions for this reaction. The angular distributions of recoil products formed in the uranium target reactions are deduced and show that the products in the₇₅Re to₈₃Bi region have backward peaked angular distributions characteristic of deep inelastic reactions. The heavy product angular distributions smoothly vary from a (1/sinθ) shape to an exponential shaped backward peak as the atomic number of the product increases from 52 to 83. The trend in the deduced angular distributions for those elements for which recoil range distributions were determined in the⁴⁰Ar+¹⁹⁷Au reaction and the 250 MeV⁴⁰Ar+²³⁸U reaction is similar, suggesting that just as for the Ar+Au system the composite system for the uranium target reaction is also not fully equilibrated along the mass asymmetry coordinate. These conclusions show that the fraction of the total reaction cross section resulting in complete fusion must be re-evaluated for the⁴⁰Ar+²³⁸U reaction and similar heavy-target reactions.     

Evidence for quasi-fission in40Ar+208Pb collisions near the coulomb-barrier

Fission-fragment angular distributions were measured in the reaction of⁴⁰Ar with²⁰⁸Pb near the fusion barrier. For nearly symmetric mass-/charge splits we find angular distributions symmetric around θ=90 degrees, however, with unusually large anisotropies. These develop gradually into forward-backward asymmetric distributions as one moves away from mass-/charge symmetry. This indicates that non-compound fission (‘quasi-fission’) competes with true fusion-fission. The relative contribution of quasi-fission to the total fission cross section is somewhere between 51 and 85%. In the framework of the extra-push model this is equivalent to an extra-extra push energy for compound-nucleus formation inside the true fission saddle point of 4<E _xx <9 MeV in agreement with a recent empirical parametrization of fusion-barrier shifts based on fusion-fission cross sections. On the basis of cross sections for fusion-evaporation residues it had previously been concluded that fusion of⁴⁰Ar with Pb isotopes occurs unhindered. Possible reasons for this apparent discrepancy are discussed.     

氩团簇高信噪比13—23nm软x射线辐射谱实验观察

用150fs的掺钛蓝宝石激光系统, 在功率密度约为5×10¹⁵ W/cm²时 激励氩(Ar)团簇，利用具有空间分辨能力的平场光栅谱仪观察到13—23nm波段Ar的软x射线谱，并观察到Ar的11阶离子谱线.在较宽的激光脉宽和较低的激光功率密度情况下，通过激励Ar团簇，获得 了Ar的高阶电离度的实验结果，且谱线的信噪比明显好于光场感应电离的情况，说明团簇的 形成大幅度地提高了激光能量的吸收效率. 关键词： Ar团簇 超短强激光 软x射线辐射     

Helium and Argon Isotope Abundances in Rocks of Lavozero Alkaline Massif

Abstract

He and Ar isotope abundances in alkaline rocks of the Lovozero layered ore-bearing complex are used to discuss the origin of hydrogen-hydrocarbon gases (HHG).

Three procedures to extract - crushing, milling and melting - and the chemical analysis of these gases have been described, and then the isotope analysis of the ratios of ³He/⁴He and ⁴⁰Ar/³⁶Ar is related. It is suggested, that (⁴He/⁴⁰Ar)_rad ratios and ⁴He concentrations decrease and ³He/⁴He increase with the distance from the ore zone, the former ratio being the most contrast indicator of ore mineralization. A direct correlation has been revealed between helium isotopic ratios and methane concentrations.

The results of measurements confirm the idea on the crustal origin of HHG.     

Dilepton and strangeness production probed with HADES

With the High Acceptance Di-Electron Spectrometer (HADES) at GSI we have studied dilepton production in the few-GeV energy regime in various collisions systems, from elementary NN, over pA, up to the medium-heavy Ar + KCl system. We have thus confirmed the puzzling results of the former DLS Collaboration at the Bevalac. While we have traced the origin of the excess pair yield in CC collisions to elementary pp and pn processes, in our Ar + KCl data a contribution from the dense phase of the collision has been identified. Together with the e ⁺ e ^? pairs, we have also obtained in the Ar + KCl system at 1.76 A GeV a high-statistics data set on open and hidden strangeness, i.e. K ^±, K _s ⁰ , ??, ?, and ??^?, allowing for a comprehensive discussion of strangeness production in this system.     

Inelastic electron scattering from low-lying states in the nuclei 36Ar and 40Ar

The low-lying level structure of ³⁶Ar and ⁴⁰Ar has been investigated using the technique of inelastic electron scattering. Data were collected at the National Bureau of Standards Linear Accelerator with incident electron energies between 65 and 115 MeV and scattering angles of 92.5° and 110°. The data span a range of momentum transfer squared between 0.29 and 0.92 fm^?2. Tassie model and Helm model analyses have been applied to data for levels at 1.97 and 4.18 MeV in ³⁶Ar and at 1.46, 2.52, 3.21 and 3.68 MeV in ⁴⁰Ar. A 2⁺ assignment to the 3.21 MeV state in ⁴⁰Ar is suggested. Transition strengths, transition radii, and mean lifetimes for these states are computed and compared with results of previous experiments.     

Influence of the potential energy surface on the cross sections for deeply inelastic transfer reactions induced by heavy ions

Large differences are observed in the cross sections for deeply inelastic reactions involving the transfer of several charges from ⁴⁰Ca and ⁴⁰Ar projectiles to the same rare-earth targets. The cross sections for four and six charge transfers from projectile to target are enhanced for ⁴⁰Ca incident ions. These differences are qualitatively explained by considering that the drift paths in the (Z, A) plane are governed by a static potential energy surface which describes the composite system. Shell corrections have been included in the calculation of this potential energy surface.     

Evidence for the temporary existence of a “composite” system in deep inelastic nuclear interactions

Masses and charges have been identified for the light fragments (640Ar+⁵⁸Ni), (⁴⁰Ar+⁶⁴Ni), (⁴⁰Ca+⁵⁸Ni) and (⁴⁰Ca+⁶⁴Ni). From the study ofN and Z distributions, it has been shown that the ratioN/Z of thecomposite system has a dominant role in the determination of theN/Z of the product. Then there should be at least a charge equilibration during the damped collision.     

Sequential fission angular distributions from mass-asymmetric heavy-ion reactions

The angular distributions of sequential fission fragments have been measured for the reactions of ⁴⁰Ar with ¹⁹⁷Au and ²³⁸U as a function of reaction Q-value and charge transfer. These angular, distributions are used to study the angular momentum and alignment of the deep-inelastic products which undergo fission. All of the fission fragment angular distributions are strongly focused into the plane defined by the beam and the projectile-like fragment velocity vectors. The in-plane angular distributions from reactions with uranium are isotropic for small energy losses and become anisotropic as the energy loss increases. For large negative Q-values, the in-plane anisotropy increases as the deep-inelastic products become more symmetric. The variation of the in-plane anisotropy with mass asymmetry for the two systems in this work was compared to a compilation of previous work and a consistent pattern was found. These alignment data are compared to equilibrium statistical calculations.     

The structures and stabilities of mixed inert gas cluster ions: NeHe+ n and ArHe+ n

DIM-type matrices for RgHe⁺ _n clusters have been established from the results of ab initio calculations on RgHe and RgHe⁺ (Rg = Ne and Ar). The method has been tested against ab initio calculations on linear and T shaped RgHe⁺ ₂ and found to be satisfactory. Rotational invariance has been established for larger clusters, and the geometries and energies of clusters up to n = 16 have been determined. Ne cluster ions are more stable than Ar cluster ions because of the greater contribution from charge transfer, and are structurally different. The relative stabilities of these cluster ions are consistent with the available experimental data.     

Elastic 3He scattering from even Ar isotopes and backward-angle anomalies in the systematics of elastic 3He scattering

Angular distributions have been measured for ³He elastic scattering from ^{36, 38, 40}Ar at 26.5 MeV and from ³⁶Ar and ⁴⁰Ca at energies between 24.5 and 28 MeV. This scattering clearly shows features of “anomalous” backward-angle scattering, which is discussed in the systematics of ³He scattering from heavier target nuclei. The data for “anomalous” scattering can be described by optical potentials which show features significantly different from those of “normal” scattering. These features are smaller radius parameters for the real optical potential and a strongly reduced volume integral for the imaginary potential.     

Fusion near the threshold: A comparative study of the systems 40Ar + 112, 116, 122sn and 40Ar + 144, 148, 154Sm

Fusion excitation functions for the systems ⁴⁰Ar + ^{112, 116, 122}Sn and ⁴⁰Ar + ^{144, 148, 154}Sm have been determined, covering cross sections ranging from several hundred mb down to the μb level. The data show a pronounced correlation of the subbarrier behaviour with low-energy collective properties of the nuclei involved and are well reproduced by simplified coupled-channel calculations coupling fusion to inelastic channels. The possibilities of parameterizing the data in terms of a simple dynamic barrier-fluctuation phenomenon are discussed and result in the prediction of remarkably diffuse partial-wave distributions above the barrier. This is shown to be important for the analysis of deexcitation phenomena following fusion reactions.     

Local equilibration and diffusive phenomena in40Ar+27Al and14N+27Al heavy ion reactions

Extensive analysis of the energy dissipation and nucleon exchange has been done with heavier systems. Here we choose the two light systems⁴⁰Ar+²⁷Al (E _lab=340 MeV) and¹⁴N+²⁷Al (E _lab=100 MeV) in order to study the correlation of the energy dissipation with the variance of the charge distributions as a function of total kinetic energy loss bins. Considerable energy damping is found to occur in the approach phase which cannot be explained by a simple Fokker-Planck diffusion model. Indeed a model which interprets the collision as a local equilibration followed by diffusive phenomena is more appropriate to fit the data.     

Analysis of the evaporation-corrected mass and charge distributions in the Ar+Mo reactions

Partly and strongly damped fragments from the reactions³⁶Ar+⁹²Mo and⁴⁰Ar+¹⁰⁰Mo are measured atE _Lab=270 MeV. The extracted mass and charge distributions are carefully corrected forγ, n, p andα particle evaporation. The resulting primary distributions are analysed with theoretical models which assume statistical neutron and proton exchange on the potential energy surface of the projectile-target system. Dynamical-deformation effects in the framework of the surface friction model are included.     

Comparative spectroscopy with the (d, τ) reaction on even Ar isotopes

The (d, τ) proton pick-up reactions on ³⁶Ar and ³⁸Ar have been studied at an incident energy of 52 MeV. Differential cross sections were measured and spectroscopic factors were extracted for transitions to 14 states of ³⁵Cl and 13 states of ³⁷Cl, respectively. Many new $\text{5}\text{2}$⁺ states and nearly the complete $\text{1}\text{d}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}$ strength have been observed in both nuclei. The width of the $\text{1}\text{d}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}$ spectral distribution is appreciably smaller for the semi-closed shell nucleus ³⁸Ar than for ³⁶Ar and ⁴⁰Ar. The results are compared with those from the ⁴⁰Ar(d, τ) reaction and proton stripping reactions on each of the three argon isotopes. Recent shell model calculations proved to be well suited to describe excitation energies as well as spectroscopic factors for the low-lying positiveparity states. Mirror relations were established for the nuclei ³⁵Ar and ³⁵Cl. In ³⁷Cl we succeeded in observing a weak component of a 1p proton hole state at a separation energy of only 18 MeV.     

Isospin dependence of the isotopic distributions from 36Ar and 40Ar fragmentation at about 60 MeV/nucleon

Fragments produced in the reactions of 69 MeV/nucleon ³⁶Ar and 55 MeV/nucleon ⁴⁰Ar on a Be target have been measured experimentally. The isotopic distributions from fragmentation reaction for both projectiles are compared with modified statistical abrasion-ablation model calculations. A strong isospin effect exhibits in the fragment distributions produced in ³⁶Ar and ⁴⁰Ar fragmentation. Experimental evidence for the disappearance of the isospin effect in the isotopic distributions produced by projectile fragmentation is observed with the increase of the violence of nuclear reaction. Received: 15 November 2000 / Accepted: 11 April 2001     

Total cross section measurements for charge exchange of He++ ions with He and Ar atoms

Total charge exchange cross sections were measured for He⁺⁺ in He and Ar gas in the energy range from 50 to 540 eV using a single beam apparatus. For He⁺⁺ in He the measured cross section is in agreement with calculations for symmetric resonant charge exchange. For He⁺⁺ in Ar the cross section for charge exchange decreases with decreasing energy below 300 eV. The measured cross section suggests the formation of Ar⁺ ions to be more important at lower energies and the production of Ar⁺⁺ to be dominant at higher energies.     

Elastische Elektronenstreuung am40Ar zur Bestimmung des quadratisch gemittelten Kernradius

The elastic electron scattering cross section of⁴⁰Ar has been measured relative to that of the proton using gas targets. The momentum transfer ranged between 0.28 and 0.50 fm^?1 (40–60 MeV, 90–120 °). From partial wave calculations, anrms charge radius of (3.41±0.04) fm was evaluated. The error includes uncertainties due to the choice of the charge distribution.     

Nuclear reactions of 118Sn, 121Sb,and 127I with argon ions

Excitation functions have been measured for a number of (⁴⁰Ar, xn), (⁴⁰Ar, pxn), (⁴⁰Ar, 2pxn), and (⁴⁰Ar, 3pxn) reactions induced in ¹¹⁸Sn, ¹²¹Sb and ¹²⁷I over the lab. energy interval 150–280 MeV. Values of the total fusion cross section are obtained and J_crit is deduced. The value of J_crit increases with energy and becomes as large as 110–140?, in reasonable agreement with the yrast limit deduced from the ellipsoidal liquid drop model. The competition between proton and neutron emission from the compound nucleus is examined and $\text{Γ}{}_{\mathrm{<mtext>p</mtext>}}\text{Γ}{}_{\mathrm{<mtext>n</mtext>}}$ is found to increase rapidly with the number of emitted nucleons, thereby imposing severe limits on the production of very neutron deficient miclides via compound nuclear reactions. The effect of very high angular momentum on the excitation functions is examined.