Production and Measurement of MeV Proton Microbeams in Atmospheric Environment Based on Glass Capillary

The production of the ion microbeam is traditionally by focusing or/and collimating to reduce the size of the beam to submicron meter scale. The traditional setup for the production of 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 by metal micro-tubes is limited by the fabrication processing, i.e., the beam spot size is very much limited by it in the scale of several microns and it is fabricated not as simple as that of a glass capillary. The use of inexpensive and easy-to-make glass capillaries as the tool for ion external microbeam production has become a new direction, inspired by early research on guiding effects. In this work, we used a glass capillary with the open outlet (108 μm in diameter), serving as a vacuum differential and collimating component, to produce a 2.5 MeV-proton microbeam directly into the atmosphere from the linear accelerator for the measurements. We measured the beam spot diameter and energy distribution of the microbeam as a function of the tilt angle of the capillary. We also conducted 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 maintaining the initial incident energy, and a scattering part with the energy loss in the microbeam. It is found that the proportion of the directly transmitted protons and the beam spot size are at maximum around the tilt angle of zero. As the tilt angle increases, the beam spot diameter decreases; when the tilt angle is greater than the geometric angle, all the microbeam are from the scattering with the energy loss. The simulation combined with the ion trajectory analysis based on the scattering process explained the experimental results. It is found that the large angle scattering determines the overall external microbeam spot, while the central region of the beam spot is composed of directly penetrating ions, and its size is determined by the geometry of the glass capillary, i.e., the outlet diameter and the aspect ratio. The easy and inexpensive production of the external microbeam by glass capillaries has the nature benefits for its relative safe and stable operation, and the last but not least point is the simple positioning of the microbeam to the sample without the complex diagnostic tools. It is expected to be widely used in radiation biology, medicine, materials and other fields.