Mev Ion-Beam Analysis
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Volume 93, Number 3, May-June 1988 Journal of Research of the National Bureau of Standards Accuracy in Trace Analysis Nuclear Methods MeVIon Beam Analysis 2. Advanced Analysis Techniques J. A. Cookson and T. W. Conlon The power of MeV ion beam analysis can be ex- tended in several ways: by the use of heavy ions, by Harwell Laboratory, UKAEA measuring more than one parameter of the interac- Oxfordshire OXiI ORA, UK tion process, and by the application of coincidence techniques. Some examples are given below. 1. Introduction The main techniques traditionally used in MeV 2.1 ERDA Ion Beam Analysis are Particle Induced X-ray The Elastic Recoil technique (ERDA) utilizes Emission (PIXE), Rutherford Backscattering fast heavy ions to produce recoil substrate atoms (RBS) and Nuclear Reaction Analysis (NRA), that are ejected from the substrate and can be sub- which, broadly speaking, utilize light (m <4) ions. sequently analyzed. In contrast to the light ion These techniques can also be applied in the chan- techniques that generally provide only one parame- neling mode to study the structural properties of ter (usually a particle or proton energy), both re- crystalline materials (such as foreign atom location, coil mass and recoil energy can be measured. radiation damage, and interface studies). The tech- ERDA is in routine use as a single parameter niques have the common feature that all are based technique, where an energy spectrum of all the on processes that occur spontaneously in the inter- ions recoiling from the specimen can be interpreted action of light ion beams with solids. Since these to give depth profiles of the various elements (see processes are well understood and are very well fig. 1). There are several ways of developing a two- established in the literature [1-3], this review will parameter version of ERDA so that a separate en- treat them only with regard to improvements in ergy spectrum is produced for each type of their quantification (sec. 3). recoiling ion. One technique carries out mass spec- Heavy ion beams in solids exhibit additional trometry of the ions with electric or magnetic properties that can be utilized in materials. For ex- fields. Another possibility is the application of the ample, they transfer large amounts of energy in stopping power equation EdE/dx =kMQ2 where collision with other nuclei; they slow down rapidly dE is measured in a thin detector and the total en- and in doing so generate regions of very high ion- ergy E in a thick detector. M and Q are the ion ization density. Analytical methods based on the mass and charge. generation of fast recoils, on the nuclear physics Also used is the time-of flight (TOF) technique process of coulomb excitation and on the heavy ion in which the velocity V of the ion is obtained from induced desorption of atoms and molecules are un- its transit time over a fixed distance, so that, if the der development and are described in section 2. ion energy (QMV2 ) is also measured, the mass M Both the light and heavy ion techniques can be can be uniquely determined. applied either in the broad beam or microbeam All of these options, extensively used in nuclear mode. New developments are taking place in the physics experiments, are being investigated in vari- production and focusing of micrometer size beams ous laboratories world-wide for material science for analysis where lateral resolution is important. uses [2,3]. However, no detailed comparison of the These advances, which could revolutionize the respective merits of the various options has yet prospects for microbeam systems, are described in been published. section 4. 2.2 Coulomb Excitation Studies RBS has proved extremely useful in characteriz- ing heavy element distributions in light element 473 Volume 93, Number 3, May-June 1988 Journal of Research of the National Bureau of Standards Accuracy in Trace Analysis substrates but, because it measures only a single when fission fragments are used). The detection parameter (i.e., E) and because of the small change limit was 100 gg/g for lithium in glass, and micro- in mass-energy product between neighboring ele- scopic analysis on areas as small as 11 m diameter ments, it is unable to resolve distributions of ele- was demonstrated. To provide depth information, ments that are similar in mass (e.g., P in Si, Ga in the technique must be combined with surface ero- As compounds). ERDA is very useful for deter- sion. mining multi-light element depth distributions, especially with two-parameter measurement of M and E (as indicated above). However, unless very 3. Advances in IBA Microbeams heavy high energy ions are used, the recoil ener- gies imparted to elements such as P. Si, Ga and As There are now upwards of 30 groups world- are insufficient to enable them to escape from the wide regularly using the various IBA techniques substrate. with finely focused or collimated beams to measure Coulomb excitation in coincidence with RBS the lateral elemental distributions of a very wide (e.g., called CBS) offers a solution to this class of variety of materials [9,10]. The present state-of-the- problem. In this case the two-parameter measure- art beam quality is the 100 pA of beam in a spot ment of RBS energy and gamma-ray energy from with FWHM of 1 lkm beam achieved at Oxford the recoiling nuclei is sufficient to distinguish the and Melbourne. These groups mainly use PIXE, depth profiles of different nuclides even if their which has the highest cross sections of the IBA mass is the same. This is because each nuclide has a techniques, to perform trace element analysis of bi- unique gamma-ray signature. ological and medical thin specimens with 1 lm res- Figure 2 shows a preliminary test of the method, olution. All the other IBA techniques are used for but insufficient data is yet available to determine a very wide range of applications, but usually at the limits of depth resolution and sensitivity. lower positional resolution. Bakir et al. [6] have reported an earlier attempt At present the highest resolution microbeams use to combine RBS and PIXE (i.e., utilizing x-rays Van de Graaff accelerators with standard ion rather than the gamma-rays from Coulomb Excita- sources to illuminate an aperture which acts as the tion discussed above). They used 34 MeV 160 ions object for a single, demagnifying magnetic quadru- and grazing angle RBS to demonstrate a depth pole lens. Although attempts [9] are being made to resolution of 70 A and a sensitivity of 101 atoms/ improve the focusing lens, it seems likely that sig- cm2 from medium mass elements (Cu on Ge sub- nificant reductions of the spot size below I jim, or strate). major increases in the current density at the present spot size, will require improvement of the bright- ness (defined roughly as current per unit area and 2.3 Particle Desorption Mass Spectrometry solid angle) of the accelerator beam. (PDMS) A simplified view of the factors involved indi- The high ionization density of both fission frag- cates that reduction of the spot diameter by a ments and accelerated heavy ions has been utilized factor 10 without loss of current could be achieved to investigate the ion induced desorption of atomic fairly simply if there were an increase in beam and molecular species from surfaces. Some species brightness of 104. This implies that if the extra are often detected by TOF spectrometry as dis- brightness of a field ionization source could be cussed above for ERDA. The phenomenon was utilized, a microbeam size of 0.1 jim should be first observed by MacFarlane and Torgerson [7] possible. and used for the characterization of large, volatile The type of field ionization source developed for biomolecules. low energy scanning transmission microscopy [11] Most recently Schweikert et al. [8] have investi- has an intrinsic brightness about 106 greater than gated the prospects of developing a technique the sources used in Van de Graaff accelerators. based on this process for surface analysis. In con- Legge et al. have demonstrated [12] such a source trast to the routinely used techniques, which re- giving 21 nA at 4X 10' greater brightness than an quire fluences of typically 10"- 10" ions/cm 2 for RF source, and have discussed [13] how the analysis (and appreciably more for the coincidence brightness might be maintained through an MeV techniques), useful information can be obtained accelerator. from PDMS with much lower fluences (especially 474 Volume 93, Number 3, May-June 1988 Journal of Research of the National Bureau of Standards Accuracy in Trace Analysis An adaptation of the field ion source by from the x-ray attenuation coefficients and to a Bbhringer et al. [14] uses a small pimple formed on lesser extent the energy dependence of the x-ray a tungsten tip to give even brighter beams, but with production. The analysis needs to contain some it- only a few hours lifetime so far. eration as the yield of each element depends on the Liquid metal sources [15] can give ion bright- presence of the others. An extra aspect of this in- nesses comparable with those achieved in hydro- terdependence is the fluorescent production of x- gen field ionization source, and have the advantage rays that can occur as PIXE x-rays are absorbed in that longer source life can be anticipated as the the matrix. emission is from a continuously renewed "Taylor For biological materials, where x-ray attenuation Cone" of liquid metal. So far, a gallium source has is not very severe, PIXE measurements of IAEA been shown [16] to work satisfactorily in a Van de H-8 Horse Kidney and NBS 1572 Citrus Leaves Graaff accelerator at Harwell for non-microbeam have been made by Clayton [20], who shows that use.