MeV Heavy Ions Research Articles

Microbeam Line of MeV Heavy Ions for Materials Modification and In-Situ Analysis

A microbeam line for MeV heavy ions of almost any element has been developed for microion-beam processing such as maskless MeV ion implantation and its in-situ analysis. Beam spot sizes of 4.0 µm × 4.0 µm for 3 MeV C2+ and 9.6 µm × 4.8 µm for 1.8 MeV Au2+ beams were obtained. Maskless MeV gold ion implantation to a silicon substrate and in-situ microanalysis before and after ion implantation were demonstrated.

Focused High-Energy Heavy Ion Beams

A focused ion beam line of MeV heavy ions has been developed by combining a focusing system consisting of objective slits and a magnetic quadrupole doublet to the beam line of a tandem-type accelerator. The demagnification factors of this system were determined to be 1/3.4 for the horizontal direction and 1/14 for the vertical direction, and a minimum beam spot size of 5.6 µm×8.0 µm was achieved. This system allows us ion beam processes such as maskless MeV ion implantation and ion beam microanalysis using heavy ions.

Application of heavy ions to high depth resolution RBS

A detailed study has been made of the use of MeV heavy ions (2 ≤, Z 1 ≤, 10) to improve the limits of depth resolution in surface analysis. While there have been detailed studies of the optimization of the use of He projectiles at glancing angles, heavier ions have received limited attention principally because the energy resolution of surface barrier detectors is significantly degraded for heavy ions and this is a major contributor to limiting the depth resolution of a system near the surface. In this paper the surface barrier detector will be compared with the electrostatic energy analyser (EEA) which does not suffer the same limitations in energy resolution (and hence depth resolution). This work extends a number of limited experimental studies of the application of heavy ions and electrostatic analysers to enhanced depth resolution, though these have rarely been used together. It will be shown that exceptional improvements in depth resolution can be achieved, and subject to the limitations of the theoretical models used in the computer simulation of heavy ion backscattering, it is predicted that depth resolutions of as small as 0.5 nm may be achievable. An algorithm for ascertaining the optimum depth resolution conditions for heavy projectiles is also derived which will identify the best angle of incidence, depth resolution and projectile for a given sample to an accuracy of better than 10%.

Energy of secondary ions desorbed from insulating films by MeV heavy ions as a function of projectile velocity

The influence of projectile velocity on the axial energy distribution and mean axial energy of secondary ions was measured with a double-field time-of-flight method at the GSI accelerator-system. The secondary ions were ejected from thin layers ( ~ 150 nm) of CsI, MgO, CaF2, valine and phenylalanine irradiated with Sn and U projectiles. The mean energy of ions like Cs + and Mg + clearly increases in the velocity range from 0.3 to 1.5 cm/ns, whereas the energy of cluster ions remains almost constant. In most cases an influence of the cluster size on the mean ion energy was not observed. The results are discussed in relevance to desorption models and their significance for the optimum design of time-of-flight mass spectrometers.

Total sputtering yield of Nb bombarded by MeV heavy ions

The angular distribution and total yield of atoms sputtered from polycrystalline Nb by 70 MeV 79Br 7+ ions have been measured. For comparison, measurements have also been made using 100 keV 40Ar + and 100 keV 79Br + projectiles. Targets were sputtered in ultra-high vacuum and sputtered material was collected on cylindrical AI foils. The areal density of collected material was subsequently determined by Rutherford backscattering using 850 keV 4He + and 15 MeV 16O 3+ incident ions. The low energy results were consistent with previous measurements in this energy regime where collision cascades are the dominant sputtering mechanism. For the 70 MeV Br irradiation, we measured a total yield, Y = 0.14, which is also consistent with a collisional description of the sputtering process.

Specimen Preparation of Near Surface Ion Irradiated Samples

Beams of heavy ions from particle accelerators are used to produce radiation damage in metal alloys. The damaged layer extends several microns below the surface of the specimen with the maximum damage and depth dependent upon the energy of the ions, type of ions, and target material. Using 4 MeV heavy ions from a Van de Graaff accelerator causes peak damage approximately 1 μm below the specimen surface. To study this area, it is necessary to remove a thickness of approximately 1 μm of damaged metal from the surface (referred to as “sectioning“) and to electropolish this region to electron transparency from the unirradiated surface (referred to as “backthinning“). We have developed electropolishing techniques to obtain electron transparent regions at any depth below the surface of a standard TEM disk. These techniques may be applied wherever TEM information is needed at a specific subsurface position.

Negative temperature coefficient of resistivity in samples of Nb 3Ge irradiated by electrons or heavy ions

Nb 3 Ge samples have been irradiated at low temperature (T∼20 K) by two drastically different irradiation projectiles: 2.5 MeV electrons and ∼100 MeV heavy ions (uranium fission fragments). The resistivity has been measured versus temperature before and after these irradiations. For both projectiles negative temperature coefficients of resistivity dρ/dT were measured and correlated with the residual resistivity ρ. These anomalous transport properties are discussed and compared to the results of the recent theories on the subject. It is qualitatively clear that the change of sign of the temperature coefficient of resistivity, occurring in a system where the electronic mean free path is of the order of the atomic distance, is indeed related to some localization process, helped by electron-phonon and electron-electron interactions. In other words, the deviation of the resistivity from the Boltzmann nearly free electron behaviour is due to some interference effect between impurity scattering and electron-phonon and/or electron-electron scattering. While the theory of defect induced tunnelling in strongly disordered metals developed by Belitz and Schirmacher seems to be the most appropriate for the explanation of the heavy ion irradiation results, although it includes electron-phonon interactions only, electron irradiation results reveal the necessity of including the interplay with Coulomb interactions as in the model of Alt'shuler and Aronov Des echantillons de Nb 3 Ge ont ete irradies a basse temperature (T∼20 K) par deux projectiles aussi differents que des electrons de 2,5 MeV et des ions lourds d'environ 100 MeV. On mesure la resistivite en fonction de la temperature avant et apres irradiation. Les valeurs negatives du coefficient de resistivite avec la temperature sont mesurees et correlees a la resistivite residuelle. Comparaison de ces proprietes anormales aux resultats des theories recentes

Energy and mass analysis of secondary ions sputtered from metallic targets by MeV heavy ions

The success of heavy ion induced desorption mass spectrometry and the discovery of anomalously high sputtering yields in some dielectric substances underscore the need for a more complete understanding of the interaction of MeV heavy ions with surfaces. Here, we report initial results of a study of the interaction of MeV ions with the surfaces of elemental metallic targets. Secondary ion mass and energy spectra have been measured for Cu, Nb, and Ta targets bombarded by 75 MeV 79Br 8+ ions. Sputtered metal ions and surface contaminants have been observed. The energies of the emitted ions are, on average, lower than for keV ion bombardment.

Time of flight spectrometry in heavy ion backscattering analysis

Time of flight spectrometry for the backscattering analysis of MeV heavy ions is proposed. The capabilities and limitations of this method are investigated. Depth and mass resolutions obtained in measurements of oxide film thickness as well as in GaAs layers analysis are presented. The importance of minimizing pile-up without significant loss of resolution by the use of an adequate absorber set just in front of the rear detector is underlined.

Discovery of ions with nuclear charge [formula omitted] stably trapped in the earth's radiation belts

We report observations of MeV heavy ions made with Explorer 45 in the earth's radiation belts during the 7-month period June–December 1972 when four major magnetic storms occurred. Significant fluxes of ions heavier than fluorine (i.e. with nuclear charge Z ≳ 9) were observed stably trapped in the interior of the radiation belts at L ∼ 2–4. These energetic very heavy ions, were found to appear suddenly during the August 1972 magnetic storm period and their fluxes decayed during the following months on time scales typically several tens of days. Simultaneously, strong increases in the geomagnetically trapped MeV helium and CNO ion fluxes were observed, and the post-injection flux decay of these ions was found to be slower than that of the Z ≳ 9 ions. The relative enhancements in trapped fluxes during the storm increased with increasing ion mass and/or increasing ion energy.

Single and Multiple Electron Loss Processes in MeV Heavy Ion-Target Collisions

Experimental data, derived from the study of interactions between MeV heavy ions and gaseous targets, are presented which illustrate important physical aspects such as the dependence of cross sections on projectile velocity, and projectile and target atomic numbers. Included are data which reflect the importance of the electron binding energy, the existence of shell effects, the possibility of target polarizability, and the presence of non-additivity and density effects in electron loss processes. Charge state yield versus scattering angle, a prescription for relating total single electron loss cross section per atom atomic and molecular target data and formulas which predict electron loss cross sections with reasonable accuracy are also given.

Surface sticking probabilities for sputtered atoms of93Nb and103Rh

The sticking probabilities for sputtered atoms of ^(93)Nb and ^(103)Rh incident on Al_2O_3 surfaces have been measured using the backscattering of MeV heavy ions. In the circumstance where the collecting surface has become thickly covered, the sticking probabilities integrated over the energy distribution of sputtered atoms are 0.97 ± 0.01 and 0.95 ± 0.01 for ^(93)Nb and ^(103)Rh, respectively. In the limit of negligible areal coverage of the collector. the accuracy is less; in this case the sticking probabilities are 0.97^+_(0.08) and 0.95^(+0.05)_(0.08).

The central region of the SIN injector cyclotron

The SIN injector cyclotron accelerates: (1) protons in third harmonic mode to a fixed energy of 72 MeV (injector mode), and (2) light and medium heavy ions in a range of energies (variable energy mode). Ions are produced either by an internal ion source or by an axial injection system. The central region configuration is the same in the injector mode and in the variable energy mode. The internal ion source can be easily interchanged with the deflector of the axial injection system. The construction of this central region, including the ion source, is described. The ion-optical design (based on trajectory calculations with field maps obtained by the magnetic analogue method) is discussed in detail. Some experimental data, obtained during operation of the cyclotron, are compared with the theoretical predictions. The equations of motion applied in the calculation are summarized in an appendix.

A high temperature, ultra high vacuum facility for heavy ion simulation of neutron radiation damage to potential reactor materials

The University of Wisconsin-Madison facility for simulating neutron damage to potential reactor materials is described. Heavy metal ions from a tandem accelerator bombard metal specimens heated as high as 1000°C at pressures below 10 −8 torr. Special feautures of the facility include a radiation heater assembly which allows integration of the beam current, an ultra high vacuum environment which is characterized by partial pressure analysis, use of 15–20 MeV heavy metal ions to induce the damage in the specimens, and direct measurement of the beam composition. This facility is suitable for studying radiation effects in the refractory metals as well as other potential reactor materials.

Small angle multiple scattering of 12–40 MeV heavy ions from thin foils

The angular distributions of collimated beams of heavy ions after passing through thin foils have been measured with a position sensitive silicon detector. Beams of O, Cl and Fe from the Harwell tandem generator were used, at energies in the range 12–40 MeV, with scattering foils of C, Al, Ni, and Au of effective thicknesses between 12 and 250 μg/cm 2. Values obtained for the half-angles at half height of the distributions are in fair agreement with the theoretical calculations of Meyer and are much smaller than those computed on the basis of the theory of Nigam, Sundaresan and Wu. The detailed shapes of the multiple scattering angular distributions agree reasonably well with the predictions of the Meyer theory.

A multiple trajectory ion beam analyzer

A large electromagnetic analyzer has been designed and constructed having three distinct ion trajectories within a single set of pole pieces. The first is a Mattauch—Herzog type mass spectrometer, the second is a “cascade” spectrometer consisting of two 30 cm radius, 180° sectors in tandem; the third is an 80°, 84 cm radius sector which can analyze 8 MeV alpha particles or heavy ions with high kinetic energy. This instrument has possibilities as a research tool in general mass spectrometric analysis, the study of ion impact phenomena, and in applications where time dependent data relating to ion charge state, mass, and energy is desired. The facility can also be employed for the controlled “doping” of semiconductors by ion implantation.

Detection of an intense flux of low-energy protons or ions trapped in the inner radiation zone

Employing a set of CdS crystal energy-flux detectors flown in satellite Injun I, we have observed an energy flux in excess of 50 ergs/cm2 sec sterad at approximately 1000 km altitude in the inner Van Allen zone. Through discrimination by magnetic rigidity and with the help of data from other on-board detectors, it was found that the flux is comprised of protons of energy 0.5 kev ≲E≲1 Mev and/or other heavy ions, apparently trapped in the geomagnetic field. So large a flux of low-energy heavy particles seems to be markedly inconsistent with prevailing ideas of the source function and/or the trapped lifetime of such particles.