Light particle emission inCl35+Mg24fusion reactions at high excitation energy and angular momentum

Measurements of the momentum distribution of high-energy electrons emitted in weakly relativistic ion-solid collisions are compared with a transport theory which is based on the relativistic electron impact approximation for electron production and which accounts for angular deflection and energy loss but also energy straggling of the transmitted electrons. The measurements on electron ejection by 45 MeV u-1 Ni28+ projectiles colliding with carbon foils of thickness 10-8300 µg cm-2 were made on an absolute scale, providing a sensitive test of the transport theory. The theory gives a satisfactory description of earlier (relative) data from 13.6 MeV u-1 Ar18+ impact on carbon. Absolute singly differential cross sections for Ni28+ agree within 10% in the case of thin targets, but the width of the binary encounter peak is underestimated by theory. For the thickest targets the measured doubly differential cross sections are overpredicted by up to one order of magnitude, particularly at forward emission angles, but peak shapes are well reproduced.

Theory and measurement of absolute doubly differential cross sections of binary encounter electron ejection in collisions of swift heavy ions with solids

Large electron yields, compared to theoretical predictions, have often been observed in the high-energy part of the electron spectra in collisions of energetic ions with atomic, molecular or solid targets. The relative enhancement of the electron emission yield at high energies can be especially strong in ion–solid collisions. In this work, following a brief overview, recent experimental evidences are presented for Fermi-shuttle type accelerating electron scattering sequences in ion–atom collisions. Signatures for double (projectile–target, P–T), triple (projectile–target–projectile, P–T–P) and quadruple (P–T–P–T) scattering sequences have been found in different collision systems. Our new results indicate the presence of even higher-order scattering contributions in the collisions of few keV energy N + ions with inert gas atoms. The observations support that high-energy electrons produced by accelerating scattering sequences may play a significant role in ion–solid collisions.

Fermi-shuttle acceleration of electrons in ion-matter interaction

In nuclear and atomic experiments at high incident ion energies, 20 MeV/ u E u , the impact of swift heavy ions on thin solid targets is a source of fast electrons. The knowledge of their spatial and kinematical distributions is very useful for experimental nuclear and radiobiological applications as well as testing atomic ionization theories. An overview on the main mechanisms underlying the production of the electrons is given. Some recent results obtained at the Catania LNS superconducting cyclotron, mainly with a 45 MeV/ u 58 Ni 19+ beam are shown. In particular, the production and the properties of binary encounter-, convoy-, in-flight Auger- and backward emitted electrons are discussed.

Fast electron production in collisions of swift heavy ions (20 MeV/u < E < 100 MeV/u) with foils of solids

In order to study the emission of energetic electrons induced by the impact of swift heavy ions on thin solid targets, we carried out a series of experiments at the superconducting cyclotron of the Catania Laboratori Nazionali del Sud. We report results on a recent experiment where electron–electron coincidences were measured in a forward ring by bombarding a thin carbon target of 7.4 μg/cm2 with 19+ 45 A MeV 58Ni beam. The velocity1–velocity2 bidimensional plot is dominated by events in which the two detected electrons have a velocity close to the beam velocity 9.03 cm/ns (convoy electrons). The remaining small fraction of coincidences has still a convoy electron and a second electron having either a velocity almost twice the beam velocity 16.5 cm/ns (binary encounter, BE electrons) or a velocity of about 12.7 cm/ns intermediate between BE and convoy velocities (IV electrons). We interprete this last intermediate component as due to in-flight de-excitation of highly excited n+58Ni ions by Auger electrons. Although less distinct, we observe also an intense peak close to the convoy velocity peak, centered at ≈9.7 cm/ns, corresponding to electrons emitted with an energy of only about ≈170 eV in the projectile rest frame of reference.

Recent results on fast intermediate velocity electron production induced by 19 + 45 A MeV 58 Ni highly charged ions on thin solid targets

Fast electron velocity spectra have been measured by means of the multidetector ARGOS (actually located at Laboratorio Nazionale del Sud, Catania) in a large angular range for atomic collisions induced by a $77A\ensuremath{-}\mathrm{MeV}$ ${}^{40}\mathrm{Ar}$ beam on an Al target. The results are in a fair agreement with a recently proposed relativistic billiard ball model and quasielastic scattering approximation calculations. However, the experimental binary-encounter peak velocities are found to be significantly lower than theoretically predicted. The data also show evidence for an excess of fast electrons in the backward direction.

A study of fast electrons produced in atomic collisions by gamma gamma MeV/n $^{40}$Ar heavy ions by means of the multidetector ARGOS

Sensitivity of the particle-residue angular correlations to the compound nucleus angular momentum distribution

A simple procedure for evaluating the excitation energy and the spin transfer in heavy-ion dissipative collisions is proposed. It is based on a prediction of the GEMINI evaporation code : for a nucleus with a given excitation energy, the average number of emitted protons decreases with increasing spin, whereas the average number of alpha particles increases. Using that procedure for the reaction $^{107}$Ag+$^{58)$Ni at 52 MeV/nucleon, the excitation energy and spin of quasi-projectiles have been evaluated. The results obtained in this way have been compared with the predictions of a model describing the primary dynamic stage of heavy-ion collisions.

F. Lo Piano

Papers

A study of fast electrons produced in atomic collisions by gamma gamma MeV/n $^{40}$Ar heavy ions by means of the multidetector ARGOS

Sensitivity of the particle-residue angular correlations to the compound nucleus angular momentum distribution

Evaluation of excitation energy and spin from light charged particles multiplicities in heavy-ion collisions