Increasing Projectile Energy Research Articles

Excitation functions for production of heavy actinides from interactions of 40Ca and 48Ca ions with 248Cm.

Excitation functions have been measured for production of isotopes of Bk through Fm in bombardments of $^{248}\mathrm{Cm}$ with 234- to 294-MeV $^{40}\mathrm{Ca}$ ions and with 239- to 318-MeV $^{48}\mathrm{Ca}$ ions. The maxima of the isotopic distributions for these elements occur at only 2 to 3 mass numbers larger for $^{48}\mathrm{Ca}$ than for $^{40}\mathrm{Ca}$ reactions. The shapes of the distributions and the half-widths of about 2.5 mass numbers are quite similar to those observed previously for reactions of $^{16}\mathrm{O}$, $^{18}\mathrm{O}$, $^{20}\mathrm{Ne}$, and $^{22}\mathrm{Ne}$ with $^{248}\mathrm{Cm}$. In general, the excitation functions for $^{40}\mathrm{Ca}$ show maxima near the Coulomb barrier while those for $^{48}\mathrm{Ca}$ are about 20 MeV above the barrier. The cross sections decrease rather slowly with increasing projectile energy over the energy range studied, indicating that the additional projectile energy is not manifested as excitation energy of these actinide products.

Uranium target fragmentation by intermediate and high energy 12C and 20Ne ions.

Target fragment formation cross sections for nuclides with 24\ensuremath{\le}A\ensuremath{\le}237 have been measured for the interaction of 1.0, 3.0, 4.8, and 12.0 GeV $^{12}\mathrm{C}$ and 8.0 and 20.0 GeV $^{20}\mathrm{Ne}$ with $^{238}\mathrm{U}$. Fragment isobaric yields were deduced from these data. The light fragment (A60) yields increase rapidly with increasing projectile energy until 4 to 8 GeV with only smaller increases in yield with increasing projectile energy beyond this consistent with the origin of these fragments in a high deposition energy process. The yields of n-rich fragments (80\ensuremath{\le}A\ensuremath{\le}145) are energy independent from 1--20 GeV consistent with their origin in low energy fission of a uranium-like species. The n-deficient fragments (80\ensuremath{\le}A\ensuremath{\le}145) have excitation functions consistent with their origin in either a deep spallation or high energy fission process. (At a $^{12}\mathrm{C}$ projectile energy of 1.0 GeV, the n-deficient fragments appear to originate primarily from a fission rather than a spallation process.) The excitation functions of the heavy fragments with 60\ensuremath{\le}A\ensuremath{\le}200 are similar to those of the light fragments. No large yields of these fragments were observed for any system studied, contrary to a previous report. Both the intranuclear cascade model and the nuclear firestreak model satisfactorily predict the observed yields of fragments with A>60, indicating that the general pattern of yields of these fragments is governed by the excitation energy deposited in the nucleus during the first step of the reaction and the geometry of the collision.

Ion irradiation and thermal cycling tests of TiC coatings

Ion irradiation of TiC coatings prepared by diffusion annealing was performed with 20–40 keV He+ ions for different doses at room temperature. The polished TiC0.99 coatings irradiated with 40 keV He+ ions showed the surface damage and erosion due to blistering and exfoliation above a dose of 1.8 × 1017 ions/cm2, whereas no change in the surface mophology could be detected for the as-prepared coatings up to a dose of 1.4 × 1018 ions/cm2. The results suggested that surface erosion due to blistering can be effectively reduced on the rough surface of the as-prepared TiC coating. The average blister diameter in the polished TiC0.99 coating increased with increasing projectile energy. For the 40 keV He+ ion irradiation of the polished TiC0.5 coatings, general features in blisters were similar to those observed for the TiC0.99 coatings, but the critical dose for blistering shifted to a higher value in comparison with the polished TiC0.99 coating. Thermal cycling between 500 and 1200°C caused serious surface damage for the TiC0.99 coating irradiated with 40 keV He+ ions below the critical dose for blistering, while the coating with surface damage due to blistering showed no significant change in the surface topography after thermal cycling.

Further experimental evidence for fast fission

Fusion-fission products have been studied for three reactions: Ar + Au, Ar + Bi and Ar + U (5.25–7.5 MeV/u). By measuring symmetric fragmentation components (fission-like events), cross sections for fusion were deduced and compared with the predictions of static and dynamic models. With increasing projectile energy, the width of the mass distributions strongly increases for the two lighter systems. By contrast, for Ar + U it remains essentially constant at a very large value. These results clearly demonstrate that the large increase of the width of the mass distribution cannot be attributed simply to large values of the angular momentum. However, they can be explained by the occurence of a different dissipative process, fast fission, which can be expected if there is no barrier to fission. For the reaction Ar + Au, the total kinetic-energy distributions were also studied in detail. In this case fast fission occurs only at high incident energy. The average total kinetic energy (TKE) was found to be constant with increasing energy whereas the widths of the TKE distribution increase.

Neutron-multiplicity distributions for (α, xnγ) reactions withEα=50−120MeV and the pre-equilibrium neutron deexcitation processes

Pre-equilibrium and equilibrium deexcitation processes for ($\ensuremath{\alpha}, x\mathrm{n}\ensuremath{\gamma}$) reactions induced by 50-120 MeV $\ensuremath{\alpha}$ particles were studied. Reaction channels were identified by measuring rotational $\ensuremath{\gamma}$ rays characteristic of the reaction residues. The branching of the reaction channels gave neutron multiplicity distributions. Features characteristic of the pre-equilibrium process were seen in the reaction channels with small neutron multiplicity $x$. An exciton model calculation code was developed so as to incorporate both multiparticle emission at the fast pre-equilibrium stage and multiparticle evaporation at the slow equilibrium stage. The calculation reproduced the neutron multiplicity distributions in the whole range of ${E}_{\ensuremath{\alpha}}=50\ensuremath{-}120$ MeV. The pre-equilibrium fractions and the entry lines from the pre-equilibrium stage to the equilibrium one were deduced. The pre-equilibrium fractions were found to be approximately 40-60%, being rather independent of the individual reaction channel. The entry lines slowly increase from 25 to 35 MeV with increasing projectile energy.

Fission following capture reactions ofS32+Pb208

Fission fragment mass and angular distributions were measured for /sup 32/S induced reactions on /sup 208/Pb at the laboratory energies of 180, 210, 250, and 266 MeV. By measuring the folding angle distributions between coincident fissionlike fragments at several beam energies it is shown that essentially the entire mass of the projectile is transferred to the composite system, which subsequently decays via symmetric fission. With increasing projectile energy the width of the mass distribution increases approximately as the square root of the nuclear temperature. The energy dependence of the angle integrated fusion-fission cross sections can be well described by classical trajectory calculations using the proximity potential and one-body dissipation. The fission fragment angular distributions become more anisotropic with increasing beam energy. Even at the lowest energy of 180 MeV, the angular distributions are more anisotropic than expected from statistical model calculations using saddle point shapes of the rotating liquid drop model. A simple parametrization of the angular momentum dependence of the effective moment of inertia gives a consistent description of the energy dependence of the angular distributions.

Autoionization of Ne projectiles in collisions with surfaces

We have measured the energy distributions of Ne autoionization electrons produced in collisions of 0.5–3 keV Ne + ions with Al surfaces. We have found a strong broadening and a weak energy shift of the lines with increasing projectile energy. We have also identified the autoionization transitions leading to these lines.

Autoionization of Ne projectiles in collisions with surfaces

We have measured the energy distributions of Ne autoionization electrons produced in collisions of 0.5–3 keV Ne + ions with Al surfaces. We have found a strong broadening and a weak energy shift of the lines with increasing projectile energy. We have also identified the autoionization transitions leading to these lines.

Energy dependence ofBi209fragmentation in relativistic nuclear collisions

The results of cross-section measurements for the reactions $^{209}\mathrm{Bi}(^{12}\mathrm{C},X)\mathrm{Au}$, $E=4.8 \mathrm{and} 25.2$ GeV and $^{209}\mathrm{Bi}(^{20}\mathrm{Ne},X)\mathrm{Au}$, $E=8.0$ GeV are reported. The observed yields of the gold isotopes show a similar dependence on mass number for each reaction, differing slightly in the position of the centroid of the distribution. As the projectile energy increases, the inferred excitation energy of the primary residues remains the same or decreases slightly. This observation is in agreement with the predictions of the intranuclear cascade model of relativistic heavy ion collisions.NUCLEAR REACTIONS $^{209}\mathrm{Bi}(^{12}\mathrm{C},X)\mathrm{Au}$, $E=4.8,25.2$ GeV; $^{209}\mathrm{Bi}(^{20}\mathrm{Ne},X)\mathrm{Au}$, $E=8.0$ GeV; measured Au isotopic distributions, relativistic heavy ions, target fragmentation, Ge(Li) spectroscopy.

Density effect inK-shell ionization by ultrarelativistic electrons

Cross sections for $K$-shell ionization by 70- and 230-MeV-electron impact have been measured on the elements Na, Mg, Al, and Cl. Results for the lighter elements---Na and Mg---are smaller than the values predicted from the Kolbenstvedt theory and the plane-wave Born-approximation calculation. The rates of increase in the cross section with the increase in projectile energy from 70 to 230 MeV are also smaller than the predicted ones, and the cross section shows saturation at high bombarding energy. Furthermore, the increase rates of the cross section for lighter elements are compared with those for heavier elements by measuring relative x-ray yields with high accuracy on targets consisting of a pair of elements: (F,Al), (Na,Cl), (Mg,Mn), (Al,Mn), (Si,Mn), (Cl,Mn), and (Ca,Mn), and the increase rate for lighter elements is smaller than that for heavier elements except for the pairs (Cl,Mn) and (Ca,Mn). A similar result was obtained on the $L$-shell ionization of the pair (Zn,Sn). These results reveal the density effect in inner-shell ionization, which was predicted by Dangerfield, while the magnitude of the effect is not as large as that predicted from a simple theory. The discrepancy is discussed.

Comparison of ion-excited Auger electron emission and secondary ion emission from silicon bombarded with noble gas ions

The projectile-energy dependence of Si-LMM Auger electron emission and Si 2+ and Si 3+ secondary ion emission from silicon has been measured in situ for bombardment with 2–30 keV Ne, Ar, Kr, and Xe ions at an angle of incidence of 45°. Si 2+ and Si 3+ emission has also been studied at normal incidence. It was found that all signals increase very rapidly with increasing projectile energy, I ∝ E n , with n up to 5. The low energy threshold, E th, depends upon the maximum energy transfer between primary ion and target atom. The intensities of Auger electrons and multiply charged secondary ions are directly proportional to each other. Si 2+ and Si 3+ are thus produced as a result of Auger deexcitation of sputtered Si ions containing a 2p hole. Excitation is mostly due to symmetric SiSi collisions in the bulk. Additional excitation due to direct projectile-target interaction is observed in the case of Ar impact. The intensity ratios I(45°)/( I(0°) for Ne, Kr, and Xe bombardment increase by a factor of three between 3 and 10 keV.

Projectile-energy dependence and line shape of ArL Auger spectra from argon bombarded silicon

Ar:L Auger electron spectra from argon bombarded silicon have been investigated at projectile energies between 4 and 30 keV. The results indicate that excitation of argon is mostly due to argon-silicon collisions, in particular below 10 keV. Ar:L and Si:L 2 excitation have been found to be closely related. The prominent Auger line at 212 eV is attributed to deexcitation of argon atoms slowing down in silicon. The line width increases with increasing projectile energy (Doppler broadening).

Deuterium and helium ion irradiation effects on TiB 2 coatings

The surface damage and erosion of chemically vapor deposited TiB 2 coatings and commercial grade titanium, caused by 40, 60 and 120 keV D + and 4He + irradiation, have been studied for the as-deposited coatings and for coating surfaces that were mechanically polished prior to irradiation. Scanning electron microscopy analysis of polished TiB 2 samples irradiated with D + and 4He + to a dose of 3.1 × 10 18 ions cm -2 reveals significant surface damage due to blistering and flaking, whereas for identical irradiation conditions the as-deposited TiB 2 coatings show very little damage. For similar irradiation conditions the titanium metal samples showed blistering for the 4He + irradiation case but no significant surface damage for the D + case. Estimates of the erosion yields due to blister exfoliation in polished TiB 2 samples show an increase with increasing projectile energy for the total dose studied.

L-shell ionisation probability of gold by alpha particles of 12-50 MeV

The impact-parameter dependence of the total L-shell ionisation and of the Lalpha , Lbeta , Lgamma and Ll X-ray production probabilities of Au by alpha -particles with energies of 12, 30, 40 and 50.8 MeV have been measured in the alpha -angle region 0.3 degrees -3 degrees . The experimental values are compared with the theoretical predictions of the semiclassical approximation, the binary encounter approximation and the plane-wave Born approximation ('low-energy SCA'). Serious deviations are found for projectile energies E>30 MeV. All three theories underestimate the recorded data. The discrepancy increases with increasing projectile energy, reaching a factor of about five at an energy of 50.8 MeV.

Nonmonotonic Target Dependence of ClKX-Ray-Production Cross Sections in Single Heavy-Ion-Atom Collisions

Chlorine $K$ x-ray-production cross sections have been measured in single collisions of chlorine ions in low incident charge states with gas targets at MeV energies. The cross sections are nonmonotonic as a function of target atomic number, but they approach monotonicity with increasing projectile energy. To explain these results in terms of $2p\ensuremath{\sigma}\ensuremath{-}2p\ensuremath{\pi}$ transitions in nearly symmetric collisions effectively requires $2p$ vacancy production simultaneous with the promotion of Cl $1s$ electrons.

Blistering in helium-ion-implanted (111) niobium monocrystals

The formation of helium blisters on the (111) surface planes of monocrystalline niobium by implantation of 0.5–1.5-MeV helium ions has been investigated for total doses ranging from 0.1 to 1.0 C/cm2 for different target temperatures, angles of incidence of the projectiles, and channeling conditions. The results indicate that the blister shape is strongly dependent on target temperature. At 900 °C and for channeled projectiles, almost all the blisters have a threefold symmetry resembling a ``crow-foot'' shape. The alignment of the crow-foot blisters with respect to each other exhibits an asymmetry in that their prongs are aligned along only one set of 〈112〉 directions of the host niobium lattice such as [12̄1], and [112̄], and [2̄11] directions and not along the other equivalent set such as the [1̄21], [1̄1̄2], and [21̄1̄] directions. As the target temperature is lowered, the tendency to form dome-shaped blisters increases; at room temperature only dome-shaped blisters form. The average blister size appears to increase with decreasing target temperature, with increasing degree of channeling, and with increasing projectile energy; at room temperature the last of these variables is the one with the most pronounced effect on the size. The blister density is more than two orders of magnitude less for channeled projectiles than for unchanneled ones for irradiation at a target temperature of 900 °C. For the unchanneled projectiles, the angle of incidence of the projectiles does not appear to change the morphology of the blisters significantly.

Interactions between Nuclei at 100-500 MeV/nucleon

Nucleus-nucleus interactions produced by heavy cosmic ray primaries in nuclear emulsions have been analysed. The kinetic energy of the incident nuclei is 100-500 MeV/nucleon. Whenever possible the particles leaving the point of interaction have been identified and their emission angles and kinetic energies have been measured. The angular and energy distribution of the protons emitted can be understood if reasonable values of the momenta of the interacting nucleons inside the colliding nuclei are considered, and if it is further assumed that interactions between nucleons and clusters of nucleons occur. The momentum distribution of the emitted protons and alpha-particles is consistent with an average recoil momentum of the target nuclei of about 23 MeV/c per nucleon. It is found that about 30% of the particles with ranges in the interval 10 μm<R<100 μm cannot be explained by ordinary evaporation theory. The spallation of the target and the projectile has been studied. It was found that the disintegration of the two interacting nuclei increases with increasing projectile energy.

Isomer Ratio ofCe137m to Ce137gProduced in Several Charged-Particle Reactions

The ${\mathrm{Ce}}^{137m}\ensuremath{-}{\mathrm{Ce}}^{137g}$ isomer ratio is studied to obtain information on the effect of angular momentum in the de-excitation of a compound nucleus. The ratio of the formation cross section of the high-spin isomer to that of the low-spin isomer is measured as a function of energy for reactions induced by ${\mathrm{He}}^{3}$, ${\mathrm{He}}^{4}$, ${\mathrm{Li}}^{7}$, and ${\mathrm{C}}^{12}$. The ratio is seen to increase with increasing projectile energy and is shown to correspond to an increase in the average angular momentum. The isomer-ratio results are compared with calculations using the Huizenga-Vandenbosch theory. Using the nuclear temperature based on the Fermi-gas model and the spin cutoff factor calculated with 0.5 of the rigid-body moment of inertia, reasonable agreement was obtained with experimental results in all four reactions studied.