Short communication

The diffusion of copper, nickel and cobalt in silicon single crystals during irradiation with MeV protons, was studied under various experimental conditions. Under the irradiation normally used for charged-particle activation radiation enhanced diffusion negligible. The diffusion of radioisotopes caused by thermal effects however, lead to important errors. It is important to control the temperature of the irradiated samples.     

Determination of oxygen in metals and semiconductors by means of the16O(T,n)18F reaction

Tritons accelerated up to 3.5 MeV by a Van de Graaff, were used to study the¹⁶O(T, n)¹⁸F reaction, which was then applied to the analysis of oxygen in metals and semiconductors. The calculated sensitivity for the determination of oxygen is 0.5 ppb at 3.5 MeV. Si, Ti and Mo were analyzed non-destructively at 3.5 MeV and the results agreed with results obtained by other methods. Pure Ge and AsGa were analyzed non-destructively at 3 MeV and the detection limits were respectively 25 and 6 ppb.     

3.5 MeV triton activation for elements with Z<34

The sensitivities for the determination of 25 elements from Z=4 to Z=33, using 3.5 MeV triton activation, were calculated from experimentally measured yields for 50 radioisotopes obtained via (t, n), (t, d), (t, 2n) or (t, α) reactions. For an irradiation of 1 hour at 1 μA, the calculated detection limits are better than 100 ppb for B, N, O, F, Na, Mg, Al, Si an S. Because of the Coulomb barrier, triton activation is of considerable interest for non destructive analysis of low Z elements in medium and high Z matrices. Finally integrated activation curves of high sensitivity elements were determined.     

Contrôle de la pureté d'échantillons de niobium et de tantale par spectrométrie gamma directe après irradiation au moyen de protons de 10 MeV

Non-destructive analysis of samples of niobium and tantalum has been achieved by activation with 10-MeV protons and γ-ray spectrometry. Niobium and molybdenum have been detected and determined in tantalum, as well as molybdenum and tungsten in niobium. Upper limits of concentration have been established for over thirty other undetected elements; most of these limits are below the p.p.m. level, and some reach the p.p.b. level.     

The study of fundamental aspects of charged particle activation analysis

Two important problems are discussed: equations and data used for quantitation on one hand, diffusion under irradiation on another hand. It is suggested that recent semiempirical stopping power data for hydrogen and helium are sufficiently accurate to be used in any good calibration method, while for heavier ions it is preferable to use the “double reaction method”, which avoids the use of stopping power data. The problem of the additivity of the stopping powers in the case of compounds is also discussed and it is shown in a specific case that BRAGG-KLEEMAN's rule is valid. Deep diffusion under irradiation has sometimes been mentioned (e.g. F in Ge, Cu in Si): it can be an important source of errors in trace analysis. The possible deep diffusion of F in Ge has been studied: it is shown that such a diffusion does not occur, while surface contamination problems can lead to erroneous observations.     

Laser desorption/ablation combined with nuclear analytical methods for materials characterization

Ubiquitous elements like carbon and oxygen always contaminate surfaces and, therefore, are the soucre of important analytical errors at trace level. Even in the case of radioactivation (with charged particles), where the sample can be etched after irradiation, analytical problems exist. In this work, we show that laser desorption/ablation can efficiently clean surfacaes, in the case of GaAs samples, resulting in better analytical conditions. Under ultra high vacuum, these surfaces remain clean long enough, so that the analysis of carbon and oxygen can be carried out using various nuclear methods, according to the needs of the analyst.     

Etude de la reaction 14N(d.n)15O a basse energie et application au dosage de l'azote dans le zirconium

Study of the ¹⁴N(d,n)¹⁵O reaction at low energy and application to the determination of nitrogen in zirconiumThe thick target yield for the ¹⁴N(d,n)¹⁵O reaction weis determined between 1 MeV and 5.5 MeV; the “average energy” and the sensitivity of the analysis for nitrogen are calculated. Nitrogen can be determined in zirconium samples at the level of 35 μg g^?1, by irradiating at 4 MeV and 5.5 MeV in a deuteron beam from a Van de Graaff accelerator. After irradiation, the samples are subjected to reducing fusion in a graphite crucible, to separate ¹⁵O. The results are compared with those obtained by other methods on identical samples; there is a difference of 3–17%.     

Analyse d'echantillons de cobalt par spectrometrie gamma directe apres irradiation au moyen de protons de 10 mev

Proton activation is applied to the analysis of cobalt samples; 41 elements were only occassionally detected. For the other elements, detection limits were irradiation with 10-MeV protons. Samples from two commercial Companies were analysed: Fe and Cu were always detected, while Ca, Ti, Cr, Zn, As, Se and Mo were only occasionally detected. For the other elements, detection limits were calculated; 33 limits are below the part per million level.     

Attachment of alkali cations on beta-D-glucopyranose: matrix-assisted laser desorption/ionization time-of-flight studies and ab initio calculations

In matrix-assisted laser desorption/ionization mass spectrometry, carbohydrates ionize by attachment of an alkali cation, and the ion yield varies with the nature of the cation. In an attempt to contribute to the understanding of the mechanisms involved, we have conducted matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) experiments on the simple glucose molecule with the alkali cations Li+, Na+ and K+, and have also performed ab initio calculations. The calculations show that, for the most stable carbohydrate-cation geometry, the carbohydrate ring is twisted and the cation is coordinated to four oxygen atoms. Calculations also show that in these complexes the positions of the three cations are very similar, and the smaller the cation, the closer it is to the oxygen atoms. Finally, the calculated formation enthalpies of the different complexes agree with the experimental results obtained for the order of stability of these complexes.     

Copper and native defects in zinc telluride

Zinc telluride crystals were grown from tellurium-rich solutions containing 10¹⁷–10²⁰ cm^?3 atoms of copper. The copper concentrations in these crystals were measured by activation analysis. Hall effect and resistivity measurements were performed. Photoluminescence spectra were also determined. Our interpretation of the different results is that copper brings about both acceptor defects Cu_Zn with a 0·12–0·13 eV ionization energy, and donor defects. The second acceptor level of the zinc vacancy was found to be at 0·15 eV.