Measurements of the targeting accuracy of the Gray Laboratory charged-particle microbeam
Introduction
The charged-particle microbeam has been designed to enable cells in vitro to be irradiated individually with micron precision, and with a precise number of charged particles delivered to predetermined locations in each cell (such as the nucleus or cytoplasm).
One rationale for this approach is that it is possible to devise experiments that are a more realistic representation of the situation relevant to environmental and occupational exposures to charged-particles (e.g. from exposure to naturally occurring radon, and from neutrons within nuclear power stations) [1]. The situation within the body exposed to environmental condition is that virtually no cell receives more than one charged-particle [2]. Most recently, microbeams have been used to study the so-called `bystander effect' where un-irradiated cells are seen to respond to signals from nearby irradiated cells [3], [4], [5].
Recently several groups in Europe, USA, Japan and China are developing, or planning to develop an installation for the micro-irradiation of cells in vitro using charged particles [2], [6], [7], [8], [9], [10]. The biological interest in the use of microbeam stems from the potential to define the ionizing energy absorbed by a cell, in terms of particle position and number. Therefore, the spatial resolution of the beam and efficiency of the particle detector are central to the performance of a microbeam. This paper reports measurements made using CR39 track-etch plastic to assess the detection efficiency and targeting accuracy of the Gray Laboratory charged-particle microbeam. The overall accuracy will depend on the properties of the collimator assembly, the accuracy to which targets are identified and the stage positioning accuracy.
Section snippets
Methods and materials
Experiments have been performed using ions, accelerated by a 4 MV Van de Graaff accelerator to 3.5 MeV, and collimated by a borosilicate glass capillary to a micron size beam. The layout of the microbeam system is shown in Fig. 1. Prior to the point at which particles exit the vacuum, a 1 mm long borosilicate glass capillary with a 1, 1.5 or 5 μm bore collimator is installed vertically at the end of the beamline, and is motorised to move precisely in the vertical direction. The collimator
The detection efficiency of the microbeam
The ideal particle counting system should respond to every particle that traverses a cell, and ignore all signals not initiated by a traversing particle. Track-etch plastic has again been used to evaluate the detection efficiency of particles. Fig. 2 is an example of the image obtained by irradiating one particle per location on a 20 μm spaced grid pattern. For this experiment, a gap between the CR39 and the collimator enhances the influence of scattering, such that two particles
The targeting accuracy
The targeting accuracy depends on three independent factors: the properties of the collimator, the accuracy to which targets are identified (and their co-ordinates assigned), and the stage positioning accuracy.
Results and discussion
The aiming resolution is decided by three factors: the precision of sample stage movement, the resolution of the target location finding image system and the collimator performance. With regard to target location and stage positioning accuracy, the results show that 98.4% of targeted positions are within of the desired position and 96.4% within . As for the collimator performance, 99.1% of the irradiated particles are located within the radius of of the collimator centre.
Acknowledgements
The financial assistance of the Chinese Academy of Sciences is gratefully acknowledged. The authors would like to thank the Gray laboratory Cancer Research Trust for the opportunity to undertake this work.
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