M. Caron

Experimental and theoretical study of target thickness dependent electron yields induced by electrons in carbon

Atomic collisions in solids lead to ionisation and electron ejection from the target atoms. The subsequent migration of electrons through the solid contributes significantly to energy deposition and damage creation. Several groups developed theoretical models on electron transport in solids and electron emission from solid surfaces. Before including ionisation by ions or atoms, such models have to deal correctly with electron induced ionisation. A basic test is the comparison to experimental data on electron emission. We therefore measured the electron yields induced from both surfaces of thin carbon foils (10-2000 nm) induced by electrons (0.2-5 keV) and compared them to simulation results obtained with two Monte Carlo codes developed independently and using different approximations for the electron interactions in the foil.

Heavy ion track potentials in solids probed by electron yield measurements

Abstract We studied the electronic energy deposition by swift heavy ions in matter by means of electron yield measurements as a function of the projectile energy and atomic number. The dependence of electron yields from carbon induced by ions of about 10 MeV/u on the projectile atomic number is incompatible with the predictions of a recent theoretical model dealing with the induced track charge in the ions wake and the influence of the induced track potential on electron emission.

Mobile UHV set-up for the investigation of electron spectra at large accelerators: first results on high charge effects

We measured energy spectra of electrons from sputter cleaned amorphous carbon targets induced by swift ions of constant velocity (Ep/Mp=9 MeV/u) with a new mobile ultra high vacuum set-up designed for large scale accelerators. We varied the projectile charge qp from 6 to 27. The shape of the spectra was found to depend strongly on qp: a qp2 scaling law at large electron energies and a strong saturation with increasing qp for low energy electrons (below the 1 s ionization threshold) are observed. We also observed an additional feature on the high energy side of the KLL Auger peak, for large projectile charges only. This additional peak may be interpreted as a hypersatellite line due to the double ionization of the K shell.

Experimental and theoretical study of the ratio between the electron emission yield and the electronic stopping power for protons incident on thin carbon foils

Abstract It is often assumed that the kinetic electron emission yield γ is proportional to the electronic stopping power dE/dx for ions incident on solid targets. Though these two phenomena are based on the same physical process in which the incident ions lose their energy in the target in electron excitations, there is no reason why the ratio Λ=γ/(dE/dx) should be constant. We present in this paper a comparison between experimental and theoretical results for protons incident on thin carbon foils in a wide energy range (200 keV–9.2 MeV). The ratios ΛB=γB/(dE/dx) and ΛF=γF/(dE/dx) for both the backward (B) and forward (F) emission yields are studied as a function of the incident proton energy as well as the target thickness.

Neutral atom sputtering yields from Gd3Ga5O12 and Y3Fe5O12 garnets: Observation of a velocity effect

Abstract The effects of swift heavy ions in insulators, leading e.g. to the appearance of latent tracks, have been extensively studied. In particular, it has been shown that at a fixed value of electronic stopping power the damage cross-section in the yttrium iron garnet (Y 3 Fe 5 O 12 ) is larger for low energy ions than for high energy ions. This is the so-called “velocity effect” in the damage creation in the electronic stopping power regime. In order to verify if such a velocity effect also exists for atoms sputtering, the total sputtering yields of yttrium iron garnet (Y 3 Fe 5 O 12 ) and gadolinium gallium garnet (Gd 3 Ga 5 O 12 ) irradiated at high energy with a lead beam of 19.3 MeV/u, were measured using the catcher technique. The sputtered particles, were collected upon an aluminum foil placed in front of the target and the impurities deposited on the collectors were analysed by Rutherford backscattering. By comparison with previous measurements of the sputtering yield for the same electronic energy loss value at lower velocity, we clearly observed an important velocity effect: the sputtering yield is higher at low velocity than at high velocity.

Electronic sputtering by swift highly charged ions of nitrogen on amorphous carbon

Abstract Electronic sputtering with heavy ions as a function of both electronic energy loss d E /d x and projectile charge state q was studied at the French heavy ion accelerator GANIL. Amorphous carbon (untreated, and sputter-cleaned and subsequently exposed to nitrogen) was irradiated with swift highly charged ions ( Z =6–73, q =6–54, energy 6–13 MeV/u) in an ultrahigh vacuum scattering chamber. The fluence dependence of ion-induced electron yields allows to deduce a desorption cross-section σ which varies approximately as σ ∼(d E /d x ) 1.65 or σ ∼ q 3.3 for sputter-cleaned amorphous carbon exposed to nitrogen. This q dependence is close to the cubic charge dependence observed for the emission of H + secondary ions which are believed to be emitted from the very surface. However, the power law σ ∼(d E /d x ) 1.65 , related to the electronic energy loss gives the best empirical description. The dependence on d E /d x is close to a quadratic one thus rather pointing towards a thermal evaporation-like effect.

Saturation effects in highly charged ion interaction with thin carbon foils

Abstract We present a set of experimental values for relative cross sections of Auger electron production in carbon foils for projectile charges Q P ranging from 6 to 39 and fixed projectile velocity 19.2 atomic units. We deduce the 1s single and double ionisation cross sections and their relative variation with respect to the projectile charge. The cross section saturates for the largest projectile charges. However, the comparison of the experimental data with continuum distorted wave-eikonal initial state (CDW-EIS) and classical trajectory Monte Carlo (CTMC) calculations shows that this saturation effect is not well reproduced by the calculations. We suggest that the observed saturation effect in 1s-shell ionisation could be due to multiple electron effects not included in these models based on independent electron approximation. We have then simulated the backward electron emission yield γ B taking into account this strong saturation effect for the 1s shell. We obtained a better agreement for both the emission yield γ B and the stopping power. The electron emission simulation gives some insight on the relative contribution of valence and 1s-shell to saturation effects.

Swift heavy ion induced electron emission from solids

We briefly summarize the results of numerous experiments performed at GANIL aimed at measuring electron yields and doubly differential yields (energy or velocity spectra at different ejection angles, angular distributions). These studies, supported by theoretical investigations and numerical simulations, contributed decisively to our understanding of the very first step in energy deposition in matter, i.e. ionization and subsequent electron transport through condensed matter. The emitted electron spectrum contains a rich variety of features including binary encounter electrons (BEE), convoy electrons (CE), Auger electrons (AE) and the low-energy peak of “secondary” electrons (SE)