A detailed ray-tracing simulation of the high resolution microbeam at the AIFIRA facility

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Abstract

The AIFIRA (Applications Interdisciplinaires des Faisceaux d’Ions en Région Aquitaine) ion beam facility at the CENBG (Centre d’Etudes Nucléaires de Bordeaux-Gradignan) is being equipped with a high demagnification focused microbeam line. This beam line, which is under its final stage of development, should allow focusing of protons and alpha particles down to a sub-micrometer resolution, and is therefore referred to as a “nanobeam line” in the following paper.

We present the complete beam transport simulation along the chosen configuration for the nanobeam line, which consists of a long working distance doublet–triplet of Oxford Microbeam Ltd. OM-50® quadrupoles. These simulations have been performed using the Geant4 Monte Carlo simulation toolkit, which has been previously validated for ray-tracing studies in the design of quadrupole microbeam systems. They include a fine modelling of the quadupole magnetic field, based on the analytical Enge’s model, which is compared to a classical square field model and to a high granularity tri-dimensional field map computed with the OPERA3D® software. The beam emittance model is parameterized from experimental measurements performed directly on the HVEE Singletron® at the AIFIRA facility, allowing the computation of the beam transmitted current along the nanobeam line. Grid shadow images, acquired from simulations, for system alignment purposes and beam deflection studies on target are presented as well.

Introduction

The need for more accurate and higher resolution charged particle probes in certain applications, e.g. Ion Beam Analysis at the sub-cellular level, ion beam lithography, ion beam induced charge, etc., presses the technology of today to its limits. The development of a high resolution beamline on a sub-micrometer scale requires the use of a reliable simulation toolkit able to track charged particles in electromagnetic fields. There exist a number of different softwares which are able to simulate beam transportation; among them the Geant4 toolkit [1], [2] provides the flexibility to include all the parameters in full physical detail. This paper is a continuation in the series of papers describing the development and testing of Geant4 for the simulation of ion transport in fully described beam lines [3], [4], [5], [6], [7].

Section snippets

Beam lines

This paper mainly concerns the nanobeam line under development; however, as simulations are based on measurements on the CENBG microbeam line, a short description of it will be given accordingly. The lens configuration of the microbeam line is of the Dymnikov type, i.e. four quadropoles mounted in a series, allowing equal transverse demagnifications Dx = Dy  10 [4]. The microbeam line has been in use since 1989 and was reassembled during 2005 at the new accelerator facility on the 10° exit pole of

Magnetic field models

Modelling of the magnetic field in the quadrupoles is of main importance in a precise simulation as its imperfections and intrinsic aberrations will influence directly on beam resolution. The quadrupole field, used to simulate beam transport in the nanobeam line, has previously been based on a 3D-mesh calculated by the OPERA3D® software [7]. The drawback of such a model is the fact that the resolution has to be limited to a certain point, in our case, a granularity of 1 mm. In the current paper,

Measurement

In order to create a realistic model of the initial beam delivered by the accelerator, the emittance in horizontal (x, θ) and vertical (y, ϕ) planes were measured in the microbeam line. This was done by cutting out angular parts of the beam with slits H1 and V1 simultaneously with the microbeam object and scanning the cut-outs with a beam switch over the diaphragm C2 (see Fig. 1). For a more thorough description of this procedure, consult [7]. The slits after the 90o magnet waist were fully

Beam transmitted current

Estimations of transmitted current have been computed earlier under the assumption of a Gaussian angular distribution with a maximum incoming divergence of 0.09 mrad [7]. To compare the impact of the phase space model presented in the previous chapter, new simulations have been performed and the results are compared to the above in Table 3. Energy distribution is computed from a Gaussian distribution of mean 3 MeV and FWHM of 4 × 10−5 in ΔE/E[7].

The estimations using the hollow core phase space

Grid shadow images

A widely used and relative straightforward method to analyse the aberrations of a certain quadrupole system is the grid shadow technique [10], [11], [12], [13]. Image coordinates are obtained from a grid placed at the image plane and the divergence coordinates are acquired from the corresponding grid shadow pattern, cast on an ion luminescent screen placed a couple of hundred millimetres downstream.

For alignment purposes it would be an advantage to have information on grid shadow patterns

Electrostatic deflection

The electrostatic scanning plates for the nanobeam line are currently being designed with the help of Geant4 modelling. Particle deflection was studied while changing the length ratio between x and y scanning plates in order to find the optimum value in terms of scan size for a given voltage under the condition that the scanning plate system total length equals 52 mm and the gap between plates should be greater or equal to 4 mm. The resulting model of the scanning plates was used to simulate the

Discussion

The modelled emittance produces a higher brightness than the one measured on the HVEE 3.5 MeV Singletron™ [9]. However, as the measurement of the emittance in the vertical (y, ϕ) plane gave an unreliable result, we proceeded by assuming a static phase space in all the planes around the optical axis. This might of course not be the case and the brightness will change accordingly by the change of the emittance. The object box under construction (at C0 in Fig. 1) will provide the possibilities of

Acknowledgements

We wish to thank G.W. Grime and M.J. Merchant for the calculation of the OM-50® quadrupole parameters used in Enge’s model.

This work is supported by the CELLION Marie Curie Research Training Network, MRTN-CT-2003-503923.

References (13)

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