Traditionally,ion microbeam is produced by focusing or/and collimating to reduce the beam size to submicron level.The traditional setup for producing the microbeam consists of an expensive focusing and collimating system with a large space,based on electromagnetic fields.Meanwhile,the microbeam obtained through pure collimation of metal micro-tubes is limited by the fabrication processing,i.e.the size of beam spot is largely limited to a few microns and its manufacture is not as simple as that of a glass capillary.Inspired by early studies of the guiding effect,the use of inexpensive and easy-to-make glass capillaries as the tool for ion external microbeam production has become a new direction.In this work,we use a glass capillary with an open outlet(108 μm in diameter),which serves as a vacuum differential and collimating component,to produce a 2.5 MeV-proton microbeam directly from the linear accelerator into the atmosphere for measurements.We measure the beam spot diameter and energy distribution of the microbeam as a function of the tilt angle of the capillary.We also conduct calculations and ion trajectory analysis on the scattering process of 2.5 MeV protons on the inner walls.The measurement results show that when the tilt angle is around 0°,there are a direct transmission part that maintains the initial incident energy,and a scattering part with the energy loss in the microbeam.It is found that the proportion of directly transmitted protons and the beam spot size are highest near zero tilt angle.As the tilt angle increases,the beam spot diameter decreases;when the tilt angle is greater than the geometric angle,all the microbeams come from the scattering with the energy loss.The simulation combined with the ion trajectory analysis based on the scattering process can explain the experimental results.It is found that the large angle scattering determines the entire external microbeam spot,and the central region of the beam spot is composed of directly penetrating ions,whose size is determined by the geometric shape of the glass capillary,i.e.the outlet diameter and aspect ratio.The natural advantage of producing external micobeames easily and inexpensively through glass capillaries is their relative safety and stable operation,and the last but not least point is to simply locate the microbeams on the sample without complex diagnostic tools.The microbeams are expected to be widely used in fields such as radiation biology,medicine,and materials.
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