Focusing effect of MeV muon beam with a tapered capillary method

Focusing effect of muon beam with a tapered capillary method has been investigated in a range from 4.2 to 9.2 MeV (i.e., from 30 to 45 MeV/c in momentum). We injected the muon beam into a pair of narrowing (tapered) plates and tubes made of glass, copper and gold-coated copper, and measured the energy distribution and the profile of the muon outgoing from the outlet. The plates were tilted from an inlet of 40 mm to an outlet of 20 mm. The density enhancement was more prominent with the plates made of heavier elements. The largest beam density enhancement at 10 mm downstream of the outlet was 1.3 with the gold-coated copper narrowing plates. The enhancement was composed of muons scattered with a small angle. Their energy was slightly lost less than that of the initial beam. This effect did not depend on the surface roughness. The density enhancement with the copper narrowing tube was 1.7, where the inlet and outlet sizes were 40 mm square and 10 mm square, respectively. The spin polarization was conserved between incoming and outgoing muons. The result strongly suggests a simple and effective way to increase the muon beam density for a small target. (author)