Heavy Ion-atom Collisions Research Articles

CONTINUOUS BACKGROUNDS IN PIXE

Continuous X rays produced by light-ion·atom collisions, which mainly form continuous backgrounds and determine the detection limit of PIXE, have been experimentally and theoretically studied, and it is shown that the experimental results over the wide range of projectile-ion energy from 0.5 MeV to 40 MeV can be well explained by three kinds of radiative process: atomic bremsstrahlung (AB), secondary-electron bremsstrahlung (SEB), and quasifree electron bremsstrahlung (QFEB). Results on the X-ray spectra, the projectile-energy dependence and the projectile-charge dependence, and on the angular distribution of these bremsstrahlungs will be summarized and the importance of AB in heavy-ion atom collision is presented. Discussions are also given on the other origins of continuous backgrounds such as the Compton scattering background, cosmic rays, the Rutherford scattering background, piling up of signals and response functions of detector, charge up effect of the target and natural backgrounds. On the basis of analyses of continuous backgrounds, the detection limit of PIXE is estimated.

Electron-positron correlations in very heavy ion-atom collisions

Electron-positron correlations associated with narrow sum-energy lines have been observed in U+Th collisions. The possible explanation of this correlation by a two-body decay of an yet unknown neutral object has attracted substantial experimental as well as theoretical interest. In this article a report will be given of the present status of the respective heavy-ion experiments performed at the UNILAC accelerator of the Gesellschaft für Schwerionenforschung, Darmstadt, FRG.

Kinetics of multicharged recoil ions produced in swift heavy ion atom collisions

Kinetics of multicharged recoil ions produced in swift heavy ion atom collisions

Master equation versus random walk approaches to phenomena far from equilibrium

We study the relation between (discrete time) random walk and (continuous time) master equation approaches to phenomena in non-equilibrium systems. The equivalence of these approaches is proved under very broad conditions. The obtained equivalence theorem includes generalized master equations that can be derived from quantum mechanics in an exact manner. Results are illustrated by and applied to examples from nuclear physics. A certain soluble model example, concerning the Z and N distributions of emergent fragments in heavy-ion collisions, is presented in order to re-interpret earlier results of Hiller et al. seemingly contradicting the equivalence theorem. It is furthermore shown that experimental observables in nuclear physics applications are sensitive only to the mean time between intranuclear collisions, and not to the nature of the distribution of waiting times between these events.

MULTIPLE IONISATION AND COLLECTIVE ELECTRON EMISSION IN MeV/u URANIUM-ION RARE GAS COLLISIONS

The multiple ionisation of Ne and Ar gas targets in collisions with high-energy, highly-charged Uranium projectiles (1.4 MeV/u U32+, 5.9 MeV/u U65+) has been investigated. Applying a recently developed experimental technique, the charge state q as well as the transverse momentum (with respect to the beam axis) [MATH] of the recoiling target ion was measured in coincidence with charge-state analysed projectiles. An accuracy in the determination of [MATH] relative to the momentum of the incoming projectile [MATH] in the order of 10-6 was obtained which is comparable to laboratory projectile scattering angles θ in the microradian regime. Thus, the multiple ionisation process and the collision dynamics could be investigated in a very sensitive manner. The statistics of multiple ionisation for definite momentum transfer to the target nucleus is shown to follow a binomial distribution for the case of a Ne target as expected in the Independent Particle Approximation (IPA). The measured differential (with respect to [MATH]) scattering cross sections were compared with the results of n-body classical trajectory Monte Carlo (nCTMC) calculations, which take into account the correct momenta of all involved particles, including the target electrons. From this joint experimental and theoretical study of the collision dynamics of high-energy heavy-ion atom collisions we conclude, that the target electrons are emitted collectively with a mean high energy (? 300 eV) in a cone around 60° with respect to the beam axis as was predicted by theory. The trajectories of the heavy nuclei are influenced considerably by the momenta of these ejected electrons. The strong polarisation of the target atom during the collision is found theoretically to lead to negative deflection angle scattering by the projectile at large impact parameters.

Correlations between high energy gamma rays and light fragments in heavy ion collisions.

Light fragments (H, $^{2}\mathrm{H}$, and $^{3}\mathrm{H}$) were detected in coincidence with high-energy (${E}_{\ensuremath{\gamma}}>20$ MeV) gamma rays from collisions of $^{14}\mathrm{N}$ on Zn at a beam energy of $\frac{E}{A}=40$ MeV. The light fragment coincidence energy spectra and angular distributions were similar to those from inclusive measurements. The charged-particle fold distribution measured in coincidence with either gamma rays or with light fragments are nearly the same. These results suggest that high-energy gamma-ray production occurs over a broad range of impact parameters.

Electron-Positron-Paircreation in Heavy Ion Collisions

Recent experiments to study e+-e- paircreation in heavy ion atom collisions at energies close to the Coulomb barrier are reviewed. For high combined charges of the collision system Zu = Z1 + Z2 one finds pairs produced by the strong time changing Coulomb field with cross sections rising proportional to Z16u. The characteristics of the e+ production line Zu and scattering angle dependence as well as their spectral distribution is well understood theoretically. Superimposed on the e+ continua, e+ lines were discovered with energies independent on Zu but with cross sections which rise with Z22u.The line energies are grouped around 250 and 340 keV for all systems with 164 < Zu < 184. For heavy systems (U-Th) and U-U) a third line at about 400 keV is seen.There is strong indication that these positron lines have a common source namely neutral particles produced in the high Coulomb fields, which decay into e+-e- pairs. This is indicated by e+-e- coincidence experiments which show evidence for energy and angle correlated e+-e- emission expected for a particle decay.

Multiple ionization and capture in relativistic heavy-ion atom collisions

We show that in relativistic heavy-ion collisions the independent electron model can be used to predict cross sections for multiple inner-shell ionization and capture in a single collision. Charge distributions of 82- to 200-MeV/amu Xe and 105- to 955-MeV/amu U ion beams emerging from thin solid targets were used to obtain single- and multiple-electron stripping and capture cross sections. The probabilities of stripping electrons from the K, L, or M shells were calculated using the semiclassical approximation and Dirac hydrogenic wave functions. For capture, a simplified model for electron capture was used. The data generally agree with theory.

Instabilities in hot nuclear matter and the fragmentation process

Fragmentation in heavy-ion collisions is studied theoretically by examining the conditions under which density fluctuations may grow during the expansion of a sphere of heated or superdense nuclear matter. Matter is described by the realistic equation of state of Friedman and Pandharipande. The growth rates and accumulated growth factors depend sensitively on the wavenumbers of the fluctuations. Growth rates are examined in the limiting regimes of hydrodynamic and of collisionless motion, and the effects of viscous damping and thermal conduction are also considered.

The Ar16+ 23 S 1 lifetime from a measurement on recoil ions

Slow highly-charged Ar recoil ions have been produced by heavy-ion atom collisions. A time-of-flight measurement of 3 keV x-rays in coincidence with recoil ion charge state has lead to a value of (203±12)ns for the mean life of the Ar16+ 1s2s3S1 state. The experimental procedure is described and compared to earlier measurements of the same entity using fast ion beams.

Positron lines from subcritical heavy ion-atom collisions

Positron lines were observed in heavy ion-atom collisions at bombarding energies close to the Coulomb barrier in subcritical systems with the sum of the atomic numbers of the colliding nucleiZu=Z1+Z2 being smaller thanZu=172. Each collision system, studied,208Pb +208Pb(Zu=164),238U+181Ta(Zu=165), and238Au(Zu=171), exhibits the emission of two positron lines withZu-independent c.m. energies of ∼ 258 keV and ∼ 340 keV, and with widths of about 30 keV, superimposed on continuous positron spectra from nuclear pair decay and pair emission induced by the time changing Coulomb field of the collision. The production cross section of thee+-lines rises with a high power ofZu(ocZu22), which is comparable to theZu20-dependence for the collision induced positrons.

Coupled state calculations of vacancy transfer probabilities from 2p and 3d rotational coupling in Ar-Mg collisions

For heavy ion-atom collisions at low energies, rotational coupling of the 2pπ-2pσ molecular Orbitals is known to be responsible for K-vacancy production in the lighter collision partner, while 3dδ-3dπ-3dσ rotational coupling is responsible for L-vacancy production in both collision partners. We use the screened classical-path impact parameter formalism to set up and solve the coupled-state equations for the vacancy transfer probability amplitudes in these two cases.

K x-ray emission in gas- and solid targets in close heavy ion-atom collisions

Impact parameter dependentK x-ray emission from both collision partners in208Pb+26 →120Sn and Xe collisions at 3,6 MeV/N projectile has been investigated. In solid target collisions the Sn-K emission yield shows impact parameter dependent structures. These structures are interpreted in terms of post-collisional capture processes. TheK x-ray emission from the heavier collision partner related to the 1s σ vacancy production, is found to be dependent on the primary projectile outer-shell configuration.

Expectation values and fluctuations in a time-dependent mean-field approach

Time-dependent expectation values of many-body nuclear observables and their quantum fluctuations are considered within the framework of functional integration. We propose two quantization procedures through saddle-point approximations to a functional integral representation of the real-time generating function. The first method, referred to as Derivation After Variation, leads to the well-known TDHF approximation obtained also in Alhassid, Muller, and Koonin, Phys. Rev. C 23 (1981), 487, with a different functional integration method. The resulting saddle-point approximation of the time-dependent quantum fluctuations is found to include the contribution of the collective phonons of the corresponding time-dependent R.P.A. in addition to the bare contribution of the TDHF mean-field. Direct comparison is made with the time-dependent mean-field approach of Balian and Veneroni, Phys. Lett. B 136 (1984), 301, obtained by application of the variational principle proposed in Balian and Veneroni, Phys. Rev. Lett. 47 (1981), 1353, 1765(E). The second method, referred to as Variation After Derivation, reproduces in a much simpler formulation the results of Troudet and Koonin, Phys. Rev. C 28 (1983), 1465, where the mean-field is found to be dependent upon the observable being measured. We propose this mean-field theory as a possible alternative to the previous mean-field approaches for the evaluation of many-body measurements that are known to be poorly described within the bare TDHF approximation, such as widths of mass distributions of final fragments in heavy-ion collisions. A comparative illustration of these various methods is proposed through the numerical evaluation of the time-dependent expectation values of a one-body observable for a simple shell-model hamiltonian.

Observation of Radiative Capture in Relativistic Heavy-Ion—Atom Collisions

X-ray studies of relativistic heavy-ion-atom collisions allow the direct observation of the radiative-electron-capture photons. The angular distribution of these photons is approximately ${sin}^{2}{\ensuremath{\theta}}_{\mathrm{lab}}$, because of the cancellation of electron retardation effects when the cross sections are subjected to Lorentz transformation into the laboratory frame. Comparison of measured and calculated cross sections reveals the number of equilibrium projectile $K$ vacancies present in the solid targets, which can be compared with charge-state measurements behind the target.

Anisotropy of LlX-ray transition observed in 1.4 MeV N-1heavy ion-atom collisions

The anisotropy of the Ll transition emission following heavy ion-atom collisions has been measured at a projectile velocity corresponding to 1.4 MeV N-1 kinetic energy. The atomic numbers of the collision partners range up to Z=92 covering a wide range of Z1/Z2 values. A discrepancy between the experiment and direct Coulomb excitation predictions has been observed and is considered to be due to quasimolecular excitation processes as well as to satellite line structures.

Projectile ionization in fast heavy-ion—atom collisions

Electron emission following the ionization of projectile ions has been investigated systematically in collisions with ${\mathrm{Ne}}^{q+}$ and ${\mathrm{Ar}}^{q+}$ ions at several hundred MeV incident on different target gases. The projectile electrons are concentrated within one maximum, the electron-loss peak (ELP). The variation of the shape and intensity of the ELP with the projectile energy, its charge state, the observation angle, and the target gas has been measured. Theoretical predictions which are based on the binary-encounter approximation show, in general, good agreement with the experimental data. The contributions of the different subshells to the ELP are deduced. It is shown that electronic screening of the target nucleus plays an important role in the ionization process of the projectile ions.

Statistical treatment of the inner M-shell excitation in heavy ion-atom collisions

A statistical treatment has been applied to interpret the experimental data on the Xe M-shell vacancy production in slow 1.05 MeV XeXe collisions and is shown to give better agreement with experiment than that of the molecular-orbital models.

Direct pair production in heavy-ion—atom collisions

Direct pair production in approx.5-MeV/amu heavy-ion--atom collisions with uranium target atoms is calculated with the plane-wave Born approximation and the semiclassical approximation. Briggs's approximation is used to obtain the electron and positron wave functions. Since pair production involves high momentum transfer q from the moving projectile to the vacuum, use is made of a high-q approximation to greatly simplify the numerical computations. Coulomb deflection of the projectile, the effect of finite nuclear size on the elec- tronic wave functions, and the energy loss by the projectile exciting the pair are all taken into account in these calculations.

Lifetimes and Fluorescence Yields of States Produced in Heavy Ion-Atom Collisions

Recent theoretical results for few-electron metastable states are presented along with the experimental data. Average fluorescence yields are given for different atomic numbers.