THE CHARACTERIZATION OF ALPHA PARTICLES MICRO BEAMS OBTAINED WITH TAPERED GLASS CAPILLARIES

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1 NUCLEAR PHYSICS THE CHARACTERIZATION OF ALPHA PARTICLES MICRO BEAMS OBTAINED WITH TAPERED GLASS CAPILLARIES D. DUDU 1, O. MURESAN 1, S. NITISOR 1, I. RUSEN 1, H. SCHUBERT 1 and I. VATA 1 1 National Institute for Nuclear Physics and Engineering-Horia Hulubei, Atomistilor Str. 407, Magurele Romania ddudu@nipne.ro Received January 24, 2012 The alpha particles beams of 3.0 MeV energy, obtained by a particle accelerator of Cyclotron type, U-120, are collimated by tapered glass capillaries (collimated beams). The experiments were performed on capillaries with an outlet diameter of different dimensions in the micrometers range (1 µm up to 200 µm). The beams characteristic parameters (angular divergence, intensities and energetic spectra) were measured using different set-up based on Rutherford Back Scattering (RBS) method. Experimental data have shown that the density of the beam strongly depends on the outlet diameter and the tendency is that the density increases versus diameter. Key words: alpha particles micro beams, Rutherford Back Scattering. 1. INTRODUCTION In the domain of material research, nuclear analytical methods and techniques have nevertheless their well-known place. Among these various methods, the ion beam spectrometry (RBS Rutherford Backscattering Spectrometry, NRA Nuclear Reaction Analysis, PIXE Proton Induced X-ray Emission) compared to optical ones have a special role due to their capacity of probing deeper in the solid, without destroying the sample. In the latest decades, new materials with special properties have been obtained by size minimization; quantum dots, walls and wires being nowadays ordinary references. Therefore, in the case of ion beam spectrometry, the transversal dimension of the beam has to be adjusted to the dimensions of the investigated sample. In this respect, the transversal size dimension of the spot can be reduced by using complicated and rather expensive ion optics [1, 2] or by using mechanical devices. The mechanical devices can be either a sophisticated adjustable collimator or a much simpler device: a conical (tapered) glass capillary [3], attached to the existing set-up. In the present paper, we present results obtained for the characteristic parameters of the energetic charged particles beam which has been collimated by using conical capillaries of different outlet diameters. Rom. Journ. Phys., Vol. 57, Nos. 9 10, P , Bucharest, 2012

2 2 Characterization of alpha particles micro beams 1309 The interaction between energetic ion beams and the solid surface varies in the nature according to many parameters. For light ions incident on smooth solid surfaces at grazing angles of about 10 mrad the interaction is roughly of three types. The ions that approach close enough to the target nuclei are elastically scattered losing significant amount of their energy and changing their direction. Other process is for ions that may be reflected by the surface potential barrier without significant loss in energy. There are also intermediate cases in which ions have energy losses and directional changes by interacting with inner shell electrons. The dominant interaction depends on ion and surface species, the ion energy and its angle of incidence on the surface, the surface geometry and others. The ion beam optics based on these principles are using taper angles designed to be less than critical angle of channeling so that the ion beam can penetrate the inner space in a very good analogy to general channeling phenomena of ions in single crystals. In typical RBS experiments, with α (He) particle of 2 3 MeV energy, the most favorable interactions between the inner surface of the capillary (of few mrad angular divergences) and the incoming charged particles which contribute to the beam are interactions at grazing angles. Thus, a fraction of the initial beam changes the propagating direction and has a different energy. There are particles that are reflected at the inner surface of the capillary, in which case the energy remains unchanged, and only a small fraction of the beam travelling through the capillary succeeds to get out, the rest of it being stopped. The characteristic parameters of the collimated beam strongly depend on the interactions taking place. 2. EXPERIMENTAL SET-UP The experiments were carried out on an existing facility to which several tapered capillaries have been attached. The experiments were done with alpha particles beams of 3 MeV energy, obtained at a U-120 type cyclotron accelerator, working in the third harmonic for this kind of experiments and a modern endstation delivered by NEC. In Fig. 1, (a, b, c) the specific arrangement for collimation is shown. The characteristic parameters of the collimated beams, like: beam intensity, angular divergence, energetic dispersion were done using capillaries of 2 mm at entrance and different outlet diameters in the range of 0.01 mm up to 0.17 mm. A good characterization of the beams is necessary to perform analytical investigation of micro and nanostructures materials, both in the normal plane of the beam and the parallel one (depth profiling) which is equivalent to 3D analysis in the range of 10-6 m 10-9 m.

3 1310 D. Dudu et al. 3 Angular dispersion δ Beam Collimator Tapered capillary (a) (b) (c) Fig. 1 (a) Schematic representation of the collimating device (tapered capillary) for α particle beam at U-120 Cyclotron; (b) the tapered capillary; (c) the tapered capillary in the reaction chamber. 3. RESULTS AND DISCUSSIONS 3.1. MODIFICATION OF THE ENERGY SPECTRA BY CONICAL GLASS CAPILLARY In similar experiments reported in the literature it had been observed that the transmitted beam through such devices has an energetic component with energy smaller than the initial one. We checked this information by analyzing multilayered nanostructures like CrNi deposited on bulk Si. By comparing the typical RBS spectra obtained with the direct beam to the spectra obtained by collimation, one can observe that the general look of the spectrum is different (Fig. 2 and Fig. 3). The shape of the RBS spectra strongly depends on the initial energy of the beam. In Fig. 2, RBS spectra of a SRM (Standard Material Reference) consisting of nine alternating thin film layers, five layers of pure chromium and four layers of pure nickel on a polished silicon substrate are shown. We have to notice that for the sample having the structure shown in Fig. 2 the scattered ions from Cr and Ni layers are combining in rather monochromatic groups like in optical interference phenomena and the corresponding region of the experimental spectrum will have a sinus like shape. The individual layers have thicknesses that are nominally 57 nm for Cr and 56 nm for Ni [4].

4 4 Characterization of alpha particles micro beams mm diameter beam 1000 RBSspectra collimated beam by mm tapered capillary Energy(2.4keV/channel) Fig. 2 Superposed RBS spectra of a SRM multilayered nanostructure CrNi on Si substrate, obtained for 3 MeV α particle direct beam (black) and by collimated beam using a 0.06 mm outlet diameter (red). In the spectrum obtained with a collimated beam of 60 µm smaller interference maxima and a plateau like signal between the last layer and the signal given by Si substrate can be observed. In Fig. 3, the spectra obtained by collimated beams with a 60 µm and 120 µm respectively are shown. One can observe that the energetic degradation is influenced by the outlet diameter of the capillary. The distorted energy fraction of beam increases while smaller outlet capillaries are used. The height of the signals of CrNi layers decreases with the increasing fraction of beam with distorted energy, due to a larger number of α particles being reflected by the walls of the capillary microns N(imp) microns E(keV) Fig. 3 Superposed RBS spectra of a SRM, CrNi on Si substrate, obtained by using a collimated beam through a 0.06 mm conical capillary and a mm diameter respectively.

5 1312 D. Dudu et al. 5 The energetic spectrum of the 0.06 mm collimated beam is obtained by deconvolution of the typical RBS spectrum obtained for a pure gold foil (Au 99,9%) on aluminum substrate, with direct and collimated beams of 3 MeV α particles (Fig. 4). Fig. 4 The energetic spectrum of a 3MeV α particles beam deduced by de convolution of RBS spectrum obtained with a 60 µm collimated beam (logarithmic scale). For de convolution the program SIMNRA [5] was used. The peak represents the contribution of the beam that travels undisturbed; the rest represent the contribution of the scattered particles on the inner walls of the capillary. From the analysis of the energetic spectrum shape the following observations were pointed out: - There is a fraction of the particle beam at the exit of capillary which keeps quasi monochromatic initial energy of the incoming beam. This fraction represents about 70% from the intensity of the initial beam; - There is a fraction of the beam with completely degraded energy that represents about 30% of the entire initial beam; - The fraction of the beam with energetic degradation increases when the size of the outlet diameter decreases THE AMPLIFICATION FACTOR OF THE BEAM DENSITY The beam density (number of particles per unit area) of the collimated beam, σ coll, is higher than the density of the original (non-collimated beam), σ direct by A factor (1). A factor Qcoll * Ain σcoll = = (1) Q * A σ direct out direct

6 6 Characterization of alpha particles micro beams 1313 where Q coll and Q direct are number of particles traveling by tapered capillary and directly and A in and A out are the cross sections characteristic for the capillary. The density amplification is explained as a superposition of the particles that travels without changing direction or energy on the particles which are reflected by the walls of the capillary. Considering two capillaries with different outlet diameters the density of the beam coming out from a smaller outlet diameter is higher. The densities of the beams were determined from two kinds of RBS spectra: one produced with a direct beam, σ direct, and a second with collimated beam, σ coll. Fig. 5 Amplification factor of the beam density versus outlet diameters. In Fig. 5 and Table 1 the results obtained from RBS spectra of an Au foil deposited on Al substrate are given. The conical capillaries used for these experiments are 2 mm wide at the entrance and 0.01 mm up to mm on the out side. Thus, the conicity (the ratio between the subtracted diameters and the height of the truncated cone i.e. 50 mm) is about 40 mrad. The amplification factor, equation (1) and (2), of the beam density depend inversely proportional to the outlet diameter. The exponent equals ( 1.97±0.09), which is very much alike 2.0. A factor = (2) 1.97 D The values of A factor obtained for each capillary are shown in Table 1 and can be also found in the literature [6, 7].

7 1314 D. Dudu et al. 7 Table 1 Amplification factor for tapered capillaries Nr in-diameter out let-diameter Length (mm) (mm) (mm) A-factor ± ± ± ± ± ± ANGULAR DIVERGENCE OF THE BEAM Due to the reflection of α particle on the walls of the capillary, the collimated beam presents an angular dispersion (Fig. 1 (a)). The quantization of this phenomenon is important for practical reasons as the dispersion contributes to the spatial resolution with an amount in the range of microns. In our case, the angular dispersion has been measured by moving a pure gold wire, placed on Zn plate, perpendicular on beam in different positions; the corresponding RBS spectra allowed the determination of the beam cross-section using the height ratio of the Au and Zn signals (Fig. 6). For this experiment a conical capillary with 0.06 mm outlet diameter was used. Results obtained are shown in Fig. 7. The cross-sections were measured for three distances (17 mm, 34 mm and 68 mm) between capillary tip and moving gold wire. In Fig. 6 the results for two distances are shown. Fig. 6 Beam cross-section obtained by a 0.06 mm wide capillary at 17 and 68 mm distance from the tip of the capillary.

8 8 Characterization of alpha particles micro beams 1315 The angular dispersion δ of the collimated beam is δ =9.35±0.02 mrad, being smaller than the conicity of the capillary 1.94/50 =39 mrad. The decrease of the angular dispersion cannot be explained by the simple scatting of the beam on capillary surface. Fig. 7 Angular divergence of the collimated beam obtained with a conical capillary having 0.06 mm outlet diameter. 4. CONCLUSIONS The collimated micro beams of α particles (3MeV energy) obtained by using a mechanical and very simple system as conical capillaries have characteristic parameters that makes them suitable for PIXE and RBS types of analysis, in the limits of a resolution value comparable to the dimension of the outlet diameter. The conical glass a capillary is focalizing charged particle beams. The resulted collimated beam has increased beam densities, up to 100 times as a function of the outlet diameter of the capillary. A significant fraction of the original beam is not energetically distorted and this is promising that micro RBS analysis can be performed. The beams obtained are very suitable for PIXE analysis. The amplification factor greater than unity and the beam divergence smaller than the conicity of tapered capillary cannot be explained by simple scattering processes of the particles on the walls and further investigations are necessary to explain the interactions of the beams inside of the capillary.

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