The microbeam system at Tohoku University

Abstract

A microbeam system called MB-I was developed at the Dynamitron laboratory at Tohoku University in 2002 for use in biological applications. Although the system was designed to achieve a submicron beam spot size, parasitic field contamination from tungsten carbide slit chips and an annular Si surface barrier detector have limited the beam spot size to 2 × 2 μm2. By replacing these components, parasitic field contamination of the system was reduced and the performance of the microbeam system was remarkably improved. A measured beam spot size of 0.4 × 0.4 μm2 at a beam current of several tens of pA has been achieved. MB-I has been used for simultaneous in-air/in-vacuum particle-induced X-ray emission (PIXE), Rutherford backscattering spectroscopy (RBS), secondary electron (SE), scanning transmission ion microscopy (STIM) analyses, and three-dimensional PIXE micron computed tomography (PIXEμCT), with applications in various fields. To obtain a higher spatial resolution of several hundred nm and a higher beam current with a resolution of several μm, a triplet lens system was designed and installed in MB-I. The triplet system has a larger demagnification than the existing system; however, it also has larger chromatic and spherical aberration coefficients. Therefore, stricter requirements are imposed on the accelerator performance, particularly the beam brightness and energy stability. In addition to the microbeam, the Dynamitron accelerator was also upgraded to obtain a higher beam brightness. The beam brightness is 2.3pA⋅μm-2⋅mrad-2⋅MeV-1, with a half-divergence of 0.07 mrad. The energy resolution of the accelerator was improved by developing a terminal voltage stabilization system (TVSS), to achieve an energy resolution of 1 × 10−5 ΔE/E. Thus, the effects of the increased chromatic and spherical aberration were mitigated by restricting the divergence angle, without reducing the beam current. A beam spot size of 0.6 × 0.8 μm2 was obtained with a beam current of 150 pA. While the analysis system of MB-I can be used for simultaneous in-air/vacuum PIXE, RBS and STIM analyses, as well as 3D PIXEμCT, without changing the target chamber, changes are required in the experimental setup for these techniques, which is time-consuming, Thus, a new microbeam system, MB-II, was developed. The MB-II is a doublet system and is not equipped with a high-resolution energy analysis system. It is connected to a switching magnet. The beam brightness is 2.4pA⋅μ m-2⋅mrad-2⋅MeV-1 at a half-divergence of 0.1 mrad and this does not decrease as the beam divergence increases. This property is matched to obtain a higher beam current using the doublet system. With a current of 300 pA, the MB-II provides a higher beam current for a 1 × 1.5 μm2 beam spot size than is achievable with the MB-I. At present, both microbeam systems are in routine operation at the Dynamitron laboratory.