Karl-Ontjes Groeneveld

Secondary-electron velocity spectra and angular distributions from ions penetrating thin solids☆

Abstract We present velocity spectra and angular distributions of secondary electrons emitted from cleaned surfaces of thin carbon and gold foils penetrated by H+ (Ep = 0.8 and 1.2 MeV), H2+ (Ep/Mp = 0.8 MeV/u) and C+ (Vp = 2.30− and 2.85 VB). Prominent structures in doubly differential velocity spectra d 2 n/ d E d Ω of secondary electrons (SE) are (1) the “true SE” peak at low electron energies Ee

A semiempirical procedure for the simple calculation of the signal intensity in Pixe analysis of thick samples

Abstract Simple analytical approximations are deduced for the variables “X-ray cross section σ” and “stopping power S”. Based on these approximations the characteristics Lα X-ray intensity is calculated including self-absorption of the emitted X-rays in thick samples. In addition, a mathematically simple approximation is obtained, which permits the calculation of the resulting Lα X-ray intensity of thick samples without any summation method. Thus, it is possible to calculate the signal intensity of PIXE studies of thick samples fast and with good accuracy by a semiempirical method. The calculated results are in good agreement with experimental PIXE data of rare-earth elements in different matrices of low atomic number.

Electron yields from solids: a probe for the stopping power of swift charged particles?

Abstract The kinetic emission of electrons from solid surfaces under swift charged particle bombardment is related to the electronic stopping power S c of the projectiles. We briefly discuss the question whether the yield γ of electrons induced by protons, electrons, and in particular, heavy ions, molecular ions and clusters is proportional to the stopping power, γ ∼ S c .

Electron emission and desorption from frozen gases under fast (MeV) ion bombardment

Abstract First results of the adsorbate-thickness dependence of electron yields obtained with thin copper foils covered with solid CO 2 are presented. Absolute values of the CO 2 coverage were determined by Rutherford forward scattering. The erosion of frozen CO 2 by transmission of protons (1 MeV) was observed for the first time. The obtained CO 2 thickness was related to the yield of convoy electrons ( E e ≈ 540 eV) being emitted along the beam direction ( Θ = 0°) with a velocity ν c equal to the projectile velocity ν p and to the intensity of low energy “true” secondary electrons ( E e λ ( E e ). Our alternative interpretation is based on nonuniform erosion, which generates a modificaton of the bombarded and the contaminated target area with increasing irradiation time.

Nuclear track formation related electron production and transport from ion penetration through solids

Abstract Electrons produced in ion penetration through solids contribute to nuclear track formation. The ejection of electrons from atomic collisions in solids is caused by a multitude of superimposed mechanisms. To disentangle the complicated dominant mechanisms, experimental angular and energy distributions are discussed and interpreted in order to reveil possible contributions to nuclear track formation.

Temperature and dimension of nuclear tracks

Abstract From secondary ion mass and energy spectroscopy we can obtain information on the target properties and on track formation mechanisms. We present measurements of initial velocity and mass distributions of organic fragment ions desorbed from C, Al, and Au substrates by fast heavy atomic and molecular ion (e.g. 0.9 MeV N + /1.8 MeV N 2 + ) bombardment. We show that molecular ions can be used to control the local and temporal distance of nuclear track formation processes. Information on the dimensions of infra- and ultratrack is obtained from a comparison between secondary ion emission from binary molecular and single incident ions.

Experimental study of signal enhancement by matrix effects in PIXE analysis

Abstract In PIXE spectra, secondary fluorescence by X-rays, produced in the target itself, may be one of the main reasons for incorrect analytical results. In our study, we determine the X-ray enhancement in PbGe sandwich targets on a thick matrix (Si) in the projectile energy range 2.0 ≤ EH+ ≤ 3.0 MeV.

Strong influence of the solid target material on the convoy electron production

Abstract We have investigated the dependence of the yield of convoy electrons induced by energetic (MeV) light ions (H + , H + 2 ) on the target material. A large variety of target elements (C, Al, Ni, Cu, Pd, Ag, Sm, Gd Au, Bi) has been studied for the first time under controlled surface conditions (ultrahigh vacuum p UHV −7 Pa). The results exhibit a systematic dependence of the convoy electron yield Y C on the Fermi energy E F of the target material in contrast to common theoretical models, predicting Y C to be a monotonie function of the target nuclear charge Z 5 T .

The temperature of the track

Abstract The energy deposition of swift charged particles penetrating solids is accompanied by such processes as particle (electron, atom, ion, molecular ion, photon …) emission and/or a change of the solid along the particle track. The energy, velocity and mass distribution of such secondary particles obtained from thin solids (such as carbon, polyhydrocarbon, isolators and conductors) penetrated by projectiles (e.g. Ar 1.8 MeV) was measured quantitatively. A number of direct and indirect production mechanisms contribute to the internal source of electrons and secondary ions. The analysis of the ejected radicals give information on the emission processes, the temperature, the charge and the time scale of energy deposition near the surface of the solid which, eventually, become responsible for the track formation.

Carbon foil thickness dependent electron yields induced by swift highly charged ions

Abstract We measured electron yields from the beam entrance and exit surfaces of thin carbon foils (d ≈ 4–700 μg/cm2) bombarded with swift ( E p M = 13.6 MeV/u ) highly charged (q = 16–18) Ar ions. The dependence of the electron yields on target thickness d is analyzed within the framework of a phenomenological semiempirical model. We obtained electron transport lengths of high energy (E ⪢ 100 eV) δ-electrons and diffusion lengths of slow electrons (E ⪡ 100 eV). The projectile velocity dependence of these transport lengths is investigated by comparing our present results for 13.6 MeV/u Ar with results obtained with 0.1 MeV/u Ne, 1 MeV/u C, 3.9 MeV/u S and 74 MeV/u Ni. We also discuss the relation between electron yields and the electronic stopping power.