Pulsed neutron-beam focusing by modulating a permanent-magnet sextupole lens

Abstract

We have developed a compact permanent-magnet sextupole lens for neutrons that can focus a pulsed beam with a wide wavelength range-the maximum wavelength being more than double the minimum-while sufficiently suppressing the effect of chromatic aberration. The bore diameter is ø15 mm. Three units of a double-ring sextupole with a length of 66mm are cascaded, resulting in a total length of 198 mm. The dynamic modulation range of the unit-averaged field gradient is 1.06 × 104-5.86 × 104 Tm-2. Permanent magnets and newly developed torquecanceling elements make the device compact, its production costs low, and its operation simpler than that of other magnetic lenses. The efficacy of this lens was verified using very cold neutrons. The diameter of the focused beam spots over the wavelength range of 27-55 Å was the same as that of the source aperture (2mm diameter) when the magnification of the optical arrangement was unity. The total beam flux over this wavelength range was enhanced by a factor of 43. The focusing distance from the source to the detector was 1.84m. In addition, in a demonstration of neutron image magnification, the image of a sample mask magnified by a factor of 4.1 was observed when the magnification of the optical arrangement was 5.0.