Increasing Projectile Energy Research Articles

Semiclassical transition probabilities for the electron-impact excitation of hydrogenic ions in dense plasmas

Plasma-screening effects are investigated on semiclassical transition probabilities for electron-impact excitation of hydrogenic ions in dense plasmas. The electron–ion interaction potential is obtained by a nonspherical Debye–Hückel model of the screened Coulomb interaction for the interesting domain of the Debye length Λ/aZ≥10. Semiclassical straight-line trajectory method is used to describe the behavior of the projectile electron. The screening effect is increased with increasing the projectile energy. The plasma-screening effect on the optically allowed (dipole) transition (1s→2p) is found to be more effective than that on the optically forbidden transition (1s→2s). For 1s→2s transition, the maximum position of the transition probability is almost unchanged with increasing or decreasing the projectile energy and plasma-screening effect. However, for 1s→2p transition, the maximum position of the transition probability has receded from the nucleus as the projectile energy increases.

Systematics of the power law parameter and minimum angular momenta for fragment production

The element yield distributionsσ(Z) of intermediate mass fragments from proton, helium, and heavy-ion induced reactions exhibit a universal behaviour as a function of the bombarding energy. The power law parameterτ, obtained from fits withσ(Z)∝σ -τ, falls monotonically with increasing projectile energy Elab and reaches a saturation valueτ≃2.0 atE lab≃2GeV. It is suggested that the behaviour at low bombarding energies is caused by a dynamical hindrance of the fragment emission at low angular momentum.

Velocity dependence of one- and two-electron processes in intermediate-velocity Ar16++He collisions.

We report investigations of one- and two-electron processes in the collisions of 0.9-keV/u to 60-keV/u (${\mathit{v}}_{\mathit{p}}$=0.19--1.55 a.u.) ${\mathrm{Ar}}^{16+}$ ions with He targets. The cross sections for these processes were measured by observing the final charges of the Ar ions and the recoiling target ions in coincidence. The average Q values for the capture channels were determined by measuring the longitudinal momenta of the recoiling target ions. Single capture (SC) is the dominant process and is relatively independent of the projectile energy. The two-electron transfer-ionization (TI) process is the next largest and slowly increases with projectile energy. The Q values for both SC and TI decrease with increasing projectile energy. Our data thereby suggest that electrons are captured into less tightly bound states as the collision velocity is increased. Both double capture and single ionization are much smaller and fairly independent of the projectile energy. The energy independence of SI is somewhat surprising as our energy range spans the region of the target electron velocity where ionization would be expected to increase. Our analysis suggests that the ionization process is being suppressed by SC and TI processes.

Electron capture in H++N2 collisions.

Electron capture in H[sup +]+N[sub 2] collisions is studied at small scattering angles and energies in the range from 0.5 to 3.0 keV. The important collision processes are identified using time-of-flight techniques for energy analysis. Our results show that the quasiresonant H[sup +]+N[sub 2][r arrow]H(1[ital s])+N[sub 2][sup +]([ital X]) channel dominates the electron capture only at the smallest angles. As an example, at 1.0 keV, capture to this channel occurs with a probability less than 0.5 for scattering angles beyond 0.7[degree]. The reduced cross section for excitation of the quasiresonant channel shows a maximum which moves to larger reduced scattering angles with increasing projectile energy. A second important process, populating H(1[ital s])+N[sub 2][sup +]([ital C]) is found. Although the excitation of this channel involves a multielectron rearrangement it is found to dominate over the one-electron'' H[sup *]([ital n]=2)+N[sub 2][sup +]([ital X]) channel which lies close in energy. We also find that the electron capture, even at small scattering angles, can generate highly excited N[sub 2][sup +] states.

Transition from quantum to quasi-classical behaviour of the binary encounter peak in collisions of 0.6 to 3.6 MeV amu-1 I23+ and Xe21+ with He and Ar

Double differential cross sections are reported for the production of binary encounter electrons in collisions of 0.6 MeV amu-1 I23+ and 1.4, 2.4, and 3.6 MeV amu-1 Xe21+ projectiles incident on He and Ar targets. Electron energy spectra were measured between 0 degrees and 45 degrees in the case of the two lower projectile energies, and between 17.5 degrees and 60 degrees for the two higher projectile energies. The data are compared with quantum mechanical impulse approximation and classical trajectory Monte Carlo calculations. While the quantum model calculation predicts a rapid disappearance of diffraction effects in the binary encounter peak with increasing projectile energy, these remain visible in the experimental results up to the highest energy measured. The necessity of including multiple target ionization involving inner shell electrons in the theoretical description of the collision process is demonstrated by the classical trajectory Monte Carlo calculation, which accounts well for the shape of the 2.4 and 3.6 MeV amu-1 cross sections, except at angles where diffraction effects are manifest. Systematic shifts of the binary encounter peak position towards lower energies with increasing emission angle were observed for all projectile energies.

Continuum electrons in coincidence with the charge state of target ions.

We have measured the continuum electron yield in coincidence with the charge state of the recoiling target ion for the collision systems ${\mathrm{He}}^{+}$\ensuremath{\rightarrow}Ne,Ar (170\ensuremath{\le}${\mathit{E}}_{\mathit{p}}$\ensuremath{\le}330 keV/u). The target ions were detected with a standard time-of-flight spectrometer employing a pulsed electric field to avoid distorting the trajectory of the continuum electron. Comparing our results with cross sections for single- and multiple-target ionization given by DuBois [Phys. Rev. A 39, 4440 (1989)] we find a similarity in the relative proportions of the recoiling charge states for argon at lower energies, but not for neon. Additionally, the yield of continuum electrons associated with each particular charge state of the recoil target ions falls off more rapidly with increasing projectile energy than might be expected.

Tracking the dissipation of energy and angular momentum in central collisions between Ag and 40Ar of 7, 17, 27, and 34 MeV/nucleon.

A 4\ensuremath{\pi} charged-particle multidetector has been used to study the reaction $^{40}\mathrm{Ar}$${+}^{\mathrm{nat}}$Ag from 280--1356 MeV. Charged-particle multiplicity distributions show a low-multiplicity group associated with peripheral collisions and a high-multiplicity group associated with central collisions. Average multiplicities for central collisions increase with increasing projectile energy, indicating ever-increasing collision violence. Angular distributions of emitted protons are essentially isotropic for \ensuremath{\theta}\ensuremath{\ge}80\ifmmode^\circ\else\textdegree\fi{} in a reference frame characterized by the empirical systematics of linear momentum transfer (i.e., \ensuremath{\approxeq}100% to \ensuremath{\approxeq}70% from 7--34 MeV/nucleon). Spectra of these protons at side angles are evaporationlike in shape and indicate relative effective temperatures of 3, 6, 8, and 12 MeV for beam energies of 7, 17, 27, and 34A MeV, respectively. Azimuthal angular correlations between various particle pairs are consistent with spin-driven emission from emitter sources of reasonable spin values. In short, these results support a classical picture of extensively thermalized emitter nuclei even for initial excitation energies of \ensuremath{\approxeq}5 MeV per system nucleon and spins of \ensuremath{\ge}100\ensuremath{\Elzxh}.

Neutralization of fast protons in grazing collisions with a clean Al(111) surface.

We investigate the neutralization of protons scattered from a clean Al(111) surface under grazing incidence for projectile energies ranging from 50 keV to 1.25 MeV. The neutral fraction of the scattered beam strongly depends on the state of preparation of the target surface and monotonically decreases with increasing projectile energy. The fractions are generally smaller than observed after transmission through thin aluminun foils. Our experimental data do not agree with recent theoretical predictions claiming dominant contributions of second-order Thomas scattering to neutralization.

Energy dependence of heavy ion reactions: Systematics

With increasing projectile energy the heavy ion reactions exhibit systematic changes. The 3otal cross section increases almost linearly with energy, the elastic events diminish and the probability for multiprong events is enhanced. The energy and angular distribution of reaction products is also affected with the change in the injection energy. Within a narrow energy region around 10 MeV/u, these changes are not so marked. However, a large number of heavy ion reactions performed in this range and studied with SSNTD indicate features which are described here as the systematics of energy dependence in heavy ion reactions.

Charge asymmetry effects in inner-shell alignment induced by ion-atom collisions

Charge asymmetry effects in an alignment of inner-shell vacancies induced by ion-atom collisions are studied theoretically. The corrections to the first order calculations of the alignment which depend on the sign of the projectile charge are discussed. The effect of these corrections on the differential alignment is large and can be studied in coincidence experiments with proton and antiproton beams of low energies. The calculations show that with increasing projectile energy the charge asymmetry effects decrease and become practically negligible when the projectile velocity is comparable with the velocity of the electron in the shell under consideration. The total alignment depends only weakly on the charge of the projectile. The calculated differences of the integralA g (2p 3 2/−1 ) alignment produced by protons and antiprotons are not greater than 20% in the whole energy range under study.

Changes in target fragmentation mechanisms with increasing projectile energy in intermediate energy nuclear collisions.

We have measured the target fragment production cross sections and angular distributions for the interaction of 16 MeV/nucleon {sup 32}S, 32 MeV/nucleon {sup 40}Ar, and 44 MeV/nucleon {sup 40}Ar with {sup 197}Au. We have deduced the fragment isobaric yield distributions and moving frame angular distributions from these data. The fission cross sections decrease with increasing projectile energy and the heavy residue cross sections (which are much larger than previous counter measurements) increase. There is an unusual change in the fragment isobaric yield distributions in the reactions induced by 32 MeV/nucleon {sup 40}Ar and 44 MeV/nucleon {sup 40}Ar. We have used the symmetry properties of the moving frame distributions to show the relative time scale of the reaction mechanisms involved. The fission fragments associated with the peripheral collision peak in the folding angle distribution originate in a normal, slow fission process in which statistical equilibrium has been established. At the two lowest projectile energies, the fission fragments associated with the central collision peak in the folding angle distribution originate in part in fast, nonequilibrium processes. At the highest projectile energies, there are no fission fragments associated with high-momentum-transfer events. The intermediate mass fragments originate primarily in events in which statistical equilibriummore » has not been established.« less

Yields of sputtered excited atoms versus the projectile energy

Solid elemental targets of Ti, Ni, Cd, and Ta have been bombarded with argon ions, and relative yields of excited, sputtered atoms have been measured with optical spectrometry versus the projectile energy in the range 10–160 keV. Most of the excited levels studied have a binding energy smaller than the work function of the corresponding solid, implying that they can become excited only in endothermic (nonresonant) processes. For such levels we observe that the yield curves (i.e. photon yield per incoming projectile flux unit versus the projectile energy) for different levels within one and the same element and charge state are proportional. Also, they increase steadily with increasing projectile energy, contrary to the total sputter yield curves, which are slightly decreasing in our projectile energy range. This indicates that such excitation features are not related only to the evolution of the collision cascade, which accounts for the total sputter yield. An exception is the 5p3P level in neutral cadmium, for which the yield curve comes closer in shape to the total sputter yield curve. Thus, excitation of this particular level can presumably be partly related to the collision cascade. This is understandable because the level has a binding energy larger than the work function of cadmium metal, implying that it can be populated through a resonant process and can therefore be one of the different exits of the collision cascade.

Fission: A filter for the study of nuclear reaction mechanisms

The use of nuclear fission as a reaction filter to study the dynamics of low-to-intermediate energy nucleus-nucleus collisions has been widely exploited over the past decade. In particular, the fission fragment angular correlation (folding-angle) technique has been utilized to derive an understanding of linear momentum and energy transfer in projectile-nucleus interactions. These results illustrate the systematic evolution in reaction mechanisms from relatively simple processes such as complete fusion and peripheral transfer reactions at near-barrier bombarding energies to highly complex interactions dominated by precompound processes at intermediate energies. Studies of light charged particles, neutrons and complex fragments in coincidence with fission-tagged events demonstrate the growing influence of nucleon-nucleon collision mechanisms relative to the dinuclear mean field in determining the reaction dynamics with increasing projectile energy. The application of these same techniques to the investigation of the inverse fission process--damped collisions--has also yielded important new insights into this class of reactions and recent results for nuclide distributions and excitation energy sharing are described.

Mass distribution of fission fragments in the19F+197Au reaction

Mass distributions of fission fragments have been measured for the reaction of19F on197Au as a function of scattering angle. The mean fragment mass is dependent on the scattering angle, the asymmetry increasing with increasing projectile energy.

Energy dependence of 238U interactions with gold target studied with a CR-39 track detector

An important aspect of the studies of heavy ion interactions is the understanding of how reaction mechanisms change with increasing projectile energy. Such an investigation is plausible only when a complete knowledge of the kinematically relevant quantities can be obtained for a particular reaction at a particular energy. Using CR-39 as the nuclear track detector, we have studied the etched tracks produced in the reaction of uranium incident on gold targets, at two different energies, i.e. 8.7 MeV/u and 16.7 MeV/u.

Atomic collisions with relativistic heavy ions. IX. Ultrarelativistic collisions.

The behavior of projectile electron capture and ionization cross sections at ultrarelativistic energies (g10 GeV/amu) is discussed. Three mechanisms contribute to electron capture at these energies: radiative and nonradiative capture and the capture of the electron following electron-positron pair production. The radiative and nonradiative cross sections both fall off as ${\ensuremath{\gamma}}^{\mathrm{\ensuremath{-}}1}$ at high energies, (\ensuremath{\gamma}-1)${\mathrm{Mc}}^{2}$ being the ion energy. The vacuum capture cross sections increase as the ln\ensuremath{\gamma}. Projectile 1s ionization cross sections also increase as ln\ensuremath{\gamma} at high energies if target screening is negligible. However, when the projectile atomic number is smaller than the target atomic number, the transverse interaction giving the ln\ensuremath{\gamma} term is screened and reduced. Therefore, the total 1s ionization cross sections are dominated by Coulomb ionization, and are nearly constant with increasing projectile energy. Tables of reduced capture and ionization cross sections are given, and are applied to calculating the charge states of ions in matter and the lifetime of ions in storage rings.

Peripheral Like Interaction Model of Spectator Residue with Central Fireball in High Energy Heavy Ion Collisions

The experimental data of differential cross sections of 80 productions in P+ Au, He+ Au and Ne + Au collisions at high energies are analyzed in terms of the peripheral like interaction model of spectator residue with central fireball. The results show that the interaction between the central fireball and the spectator residue gradually becomes peripheral or diffractive as the projectile energy increases. The study of high energy heavy ion reactions has been pursued for both peripher­ al and central collisions of projectile and target. In central collision there are two parts 1H ) of the interaction region, in one of which the overlapping part of the projectile and the target becomes hot and thermalized at a high temperature (called the central fireball) and subsequently decays by emitting fast nucleons and possibly light fragments. The non-overlapping region (called the spectator residue) being formed simultaneously along with the formation of the central fireball remains cold and promptly decays into relatively large and slow fragments. 3 ),4) In this paper we study the nature of the interaction between the central fireball and the spectator residue and also the fragmentation mechanisms of the spectator residue based on a nova mode1. 5

The re-emission of K- meson in the interaction of 1.5 GeV/c K- meson with light emulsion nuclei

Abstract This paper is about a study of the re-emission of K - meson following the interaction of K - meson in the momentum range of 1.5 GeV/c with C 12 , N 14 , and O 16 nuclei in emulsion. Based on the analysis of 1214 light nuclear events the re-emission probability of K - meson has been found to be, 0.18±0.02, as against the expected value of 0.21±0.03. The experimental probability seems to increase with increasing projectile energy. The low energy tail in the energy spectrum has been interpretted to have occured due to the elastic scatterings within the nuclei. Some discussion to search for the resonant states has been given.

124Sn target residues in the interaction of heavy ions at intermediate energies

Cross sections, recoil energy distributions for residues produced by14N,20Ne,40Ar on124Sn targets have been measured in the 10–70 MeV/u intermediate energy range. As the projectile energy increases, processes as complete fusion-evaporation, incomplete fusion-evaporation and intranuclear cascade-evaporation participate progressively in the nuclear collisions. Peculiar residues corresponding to a gain of 4, 5 or even 6 protons for the124Sn target have been observed in the 30–50 MeV energy range.

Approach to the limits for massive energy and spin deposition into a composite nucleus.

We report measurements of correlated fission-like fragments from the reaction 1080 MeV (E/A=27 MeV) $^{40}$Ar${+\mathrm{}}^{238}$U and their further correlations with H/He particles. Fusion-like reactions that form highly excited composite nuclei (500--800 MeV) comprise \ensuremath{\approxeq}40% of the reaction cross section. The probability for such fusion-like reactions is dramatically smaller for $^{40}\mathrm{Ar}$ of E/A of 35--44 MeV. We infer that for this reaction the conditions for high probability of such massive energy and spin containment are near to their limits. The integrated multiplicities for H and He emission are both \ensuremath{\approxeq}3 for fission reactions that involve 50--100 % momentum transfer. These multiplicities are about evenly divided between evaporative and forward-peaked emission; comparison with other results indicates that both have increased by about twentyfold for an increase in projectile energy of 340--1080 MeV. The major sources of evaporative He emission are the very highly excited composite nuclei.