Effect of wall material and shape on MeV ion focusing ability of tapered capillary optics

The dependence of the MeV ion focusing ability of tapered capillary optics on the capillary wall material was investigated using tapered glass capillaries made of two different glass materials: borosilicate glass and lead glass. The energy spectrum measurement of capillary-focused proton beams showed that the beam focusing ratio for the lead glass capillary was slightly higher than that for the borosilicate glass capillary, but the enhancement was much lower than expected from the target atomic number dependence of the Rutherford scattering cross section. Three-dimensional Monte Carlo simulations were performed to examine the effect of the capillary wall shape as well as that of the wall material. When a constant taper angle capillary was used, the beam focusing ratio was found to be enhanced by a factor of 2 compared to when the conventional tapered capillary having a convex inner wall was used. These results indicated that the probability of the scattered ion escaping from the capillary wall plays a predominant role in the MeV ion transmission in the tapered capillary optics.